ITEK FINAL REPORT OF FREQUENCY ATTENUATING FILTERS FOR IMAGE ENHANCEMENT.

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP66B00728R000100010021-5
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 67
• Document Creation Date: December 12, 2016
• Document Release Date: March 28, 2002
• Sequence Number: 21
• Publication Date: September 6, 1961
• Content Type: MF

Approved For Release VIC CIA-RDP66B00728R000100010021-5
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M A t M PM : Chief Nation l Photographic Intelligence Center
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Att%ab.d for your infoz tion a file are five (5) copies
Coutr* o "'s 11w Report coveri the design and cc truction of
$04act filters
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co tit s Contractor's completion of irk Order
Not 1 is contract ACCOr+ia, , your c ntS *ndlor a provaai
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 SPECIAL HANDLING
SHC6I-9019-228
Copy # /
August 25, 1961
LOS-
COPY / OF Z.
Enclosed please find six (6) copies each of the final report
to Task Order No. I to Contract B6-L125, entitled Desin and
Construction of Frequency Attenuating Filters, dated 15 August 1961.
Very truly yours,
STATINTL
Contracts Man ger
HAM:pjf
Enclosures (6)
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 ~dre -/So
COPY / OF ,
DESIGN AND CONSTRUCTION
of
FREQUENCY ATTENUATING FILTERS
FINAL REPORT
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STATINTL
STATINTL
ItekJ
Manager, Optics Department
Graphic Information Research Division
Itek Laboratories
Lexington 73, Massachusetts
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1. Introduction . . . . . . . . . . . . . .
1. 1 Purpose and Scope of the Program . . . . .
1. 2 Summary of this Report . . . . . . . . .
2. Summary of Theoretical Investigations . . . . . .
2. 1 Introductory Considerations . . . . . . .
2. 2 The Photographic Edge . . . . . . . . .
2. 3 Non-sharp, Gaussian Pulse Model . . .
2. 4 Frequency Attenuating, Gaussian Filters . .
2. 5 Image of a Gaussian Pulse through a Gaussian Filter
2. 6 Optimizing Gaussian Filter Constant . . . . .
2. 7 Additional Theoretical Considerations . . . .
2. 8 Summary . . . . . . . . . . . . .
3. Spatial Filter Fabrication . . . . . . . . . .
3. 1 Mechanical Basis . . . . . . . .
3. 2 Sensitometric Basis . . . . . .
3. 3 Filter Preparation . . . . . . . . . .
3. 4 Practical Design of Gaussian Filters . .
4. Testing and Evaluation . . . . . . . . . . .
4.1 Purpose . . . . . . . . . . .
4. 2 Preliminary Materials Evaluation . . . . . .
4. 3 Test Methods and Equipment . . . . . . .
4. 4 Results of the Materials Evaluation . . . . .
4. 5 Evaluation of the Gaussian Filters . . . . .
4. 6 Results of the Filter Evaluation Tests . . . .
4. 7 Additional Testing . . . . . . .
5. , Discussion and Conclusions . . .
5. 1 Gaussian Pulse Model . . .
5. 2 Enhancement with Gaussian Filters
5. 3 Filter Manufacture . . . . .
5. 4 Filter Material Quality . . .
5. 5 The Photographic Process . .
6. Recommendations
7. Acknowledgements
8. References . .
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1. Experimental edge trace approximated by a Gaussian . . . . . . . . . .
5
2. Typical Gaussian spatial pulse . . . . . . . . . . . . . . . . .
6
3. Plots of typical Gaussian high-pass filters, showing the variation
due to the central transmission value . . . . . . . . . . . . . . .
10
4. Image intensity of a Gaussian pulse passed through a high-pass Gaussian
filter, showing the parametric variation due to the 40 c term . . . . . . .
11
5. Diagram showing basic concept for achieving rotationally symmetric
photographic exposure . .. . . . . . . . . . . . . . . . . .
14
6. View of the Exposure Jig, exhibiting the cutout in the turntable, and
showing the counterbore in the center of the upper plate which holds the
exposure mask . . . . . . . . . . . . . . . . . . . . .
17
7. View of the Exposure Jig, exhibiting the turntable details . . . . . . . . .
18
8. Typical High Resolution Plate sensitometric test results with the
masks through which they were exposed . . . . . . . . . . . . . .
24
9. Sensitometric characteristics of Kodak High Resolution Plates developed
in D-76 (2:1) under the specified exposure and processing conditions . . . . .
25
10. Linear sensitometric characteristics of Kodak High Resolution Plates
developed in D-76 (2:1) for a specific exposure condition and one processing time.
26
11. Linear sensitometric characteristics of Kodak High Resolution Plates
developed in DK-60a under the specified exposure and processing conditions . . .
27
12. Outline of exposure :mask aperture for producing Gaussian spatial
filter No. 3, using normalized radius vector. . . . . . . . . . . . .
29
13. Comparison of design and actual Gaussian filter No. 3 cross sections . . . . .
30
14. Calibration.of microdensitometer and recording potentiometer, using
cleared Kodak High Resolution Plate for the zero scale setting . . . . . . .
31
15. Typical high-pass Gaussian spatial filters, with the exposure masks
through which they were produced . . . . . . . . . . . . . . . .
32
16. Photograph of the Cutting Jig. The central shaft is hollow, and directly
over the small hole in the lower plate. A microscope illuminator, set on
the handle over the shaft, provides sufficient intensity to drive the meter
needle off -scale . . . . . . . . . . . . . . . . . . . . .
33
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17. Photograph of Cutting Jig base, showing the sensing device used for
filter alignment prior to cutting. The bolt heads, spaced 120? apart,
serve as locating studs to center Cutting Jig on base . . . . . . . . . . 34
18. Gaussian spatial filters mounted in adapters for installation in the Image
Enhancement Viewer. These are oriented to display the characteristics
of the adapters . . . . . . . . . . . . . . . . . . . . . 35
19. Gaussian spatial filters installed on the Aperture wheel of the Image
Enhancement Viewer: The wheel containing the occluding filters has been
rotated out of normal alignment for this photograph, as evidenced by the
displacement of the notches on the wheels . . . . . . . . . . . . . 36
20. Assumed degradation of sharp spatial pulse train into train of approximately
Gaussian shapes, with the parameters defined in terms of the periodicity . . . . 38
21. Design cross-sections of the fabricated Gaussian filters, showing the frequency
attenuation produced in the optical system of the Image Enhancement Viewer . . . 41
22. Enlargement of the photographic image of a Buckbee Meers bar target
observed on the Spatial Filtering Test Bench. It was imaged through the
maximum system aperture, and the focus obtained with the aid of a 100x
microscope. Scratches and other physical disfigurements are on the bar
target object, and do not result from the imaging optics or subsequent photo-
graphic processes . . . . . . . . . . . . . . . . . . . . 47
23. Enlargement of the photographic image of a Buckbee Meers bar target
observed on the Spatial Filtering Test.Bench . . . . . . . . . . . . 49
24. Enlargement of the photographic image of a Buckbee Meers bar target
observed on the Spatial Filtering Test Bench . . . . . . . . . . . . 51
25. Enlargement of an aerial photograph, the image of which was obtained on
the Spatial Filtering Test Bench. One Kodak High Resolution Plate, emulsion
removed, was placed in the diffraction (filter) plate to provide the basis for
comparing this image with the photographs shown in Figure 26 . . . . . . . 55
26. Enlargement of an aerial photograph, the image of which was obtained on
the Spatial Filtering Test Bench, for two specified Gaussian filters . . . . . . 57
27. Microdensitometer traces of one of the buildings in the photographic images,
the enlargements of which are shown in Figures 25 and 26. The inverted scale
is an approximate measure of aerial image intensity, and the traces can be
compared relatively . . . . . . . . . . . . . . . . . . . . 59
28. Enlargement of the spatially filtered image of an aerial photograph obtained
with the Image Enhancement Viewer. The image passed through a high-pass,
sharp cut-off, occluding filter whose lower cutoff frequency was 3 lines/mm . . . 61
29. Microdensitometer traces of one of the roads in the photographic images of
the aerial photographs, the enlargements of which are shown in Figures 25
and 26. The ordinate is an approximate measure of aerial image intensity;
the traces can be compared relatively . . . . . . . . . . . . . . 64
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1. Computations for Gaussian Filter Mask No. 3 . . . . . . . . . . . . 28
2. Summary of Preliminary Gaussian Filter Constant Computations
on which Final Design Decisions Were Based . . . . . . . . . . . . 40
3. Summary of Final Design Constants for Gaussian Spatial Filters . . . . . . 40
4. Materials Evaluation Data Summary . . . . . . . . . . . . . . . 53
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WO
The aim of these project studies has been to investigate the. feasibility of frequency atten-
uating spatial filters and, should their use prove feasible, to devise a practical method for con-
structing such filters. It is known that the filters presently installed in the Image Enhancement
Viewer (sharp cutoff, occluding filters) do not truly enhance images, but operate on their dif-
fraction patterns in a manner which removes image tone, raises the contrast about an edge, and
gives evidence of its use by leaving ghost images or frequency "ringing". Preliminary theoret-
ical studies have pointed out that attenuating spatial frequencies (rather than eliminating them)
should not only preserve image tone, but also provide true enhancement. Such a filtering meth-
od, frequency attenuation, would obviously be useful for improving the fine detail of aerial pho-
tographs. Here the natural degradation of sharp images which results from the optical and pho-
tographic process could be compensated for. The acutance could be improved: details which
were normally just below the visual threshold could be rendered visible.
The contract was undertaken to consider the following specific areas of study and develop-
ment:
1. Extend previous theoretical studies on frequency attenuating filters to ascertain feasi-
bility, and apply these results to the design and construction of such filters.
2. Develop a method for constructing frequency attenuating filters, build or purchase nec-
essary equipment and establish specifications on materials for making such filters.
3. Construct a set of filters (should feasibility be demonstrated) which could be installed
in the Image Enhancement Viewer. The maximum number of such a set was specified
at four.
4. Report the results and evaluate the enhancement produced by such filters, quantitatively
if possible. Such a report would present specific recommendations for extending the
work.
Many facets of study are possible in such a research program. To concentrate efforts on
feasibility determination and filter fabrication, several restrictions were placed on the program
immediately. It was recognized early in the program that there were three basic methods of
producing continuous attenuation (variable transmission) having rotational symmetry. * These
were 1) mechanical, 2) evaporation of suitable materials in vacuum and 3) photographic.
* This is a basic requirement for non-specialized spatial filters, and is discussed in detail in a
subsequent section.
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The detailed development of the first method would not have allowed time to understand the
basic problems in frequency attenuation. The second method required equipment and techniques
beyond the (then) present capabilities of these laboratories. The third method not only did not
require more than present state-of-the-art developments, but the basic techniques could be ap-
plied to the ultimate solution of the first two. Further, more time and effort could be spent in
developing the underlying concepts of frequency attenuating filters. Therefore, at the onset of
the program, all filter fabrication was restricted to the photographic method.
The second restriction came as a result of the theoretical investigations. Apulse or edge
model was a basic theoretical requirement, and must necessarily reflect the actual, physical
fact. Once established, standard analytical techniques could be applied to ascertain the type of
filter to best enhance this model.
In view of the difficulty of finding a suitable model (it was not until well along in the study
program that such a model was devised), the optimum filter for edge enhancement was not es-
tablished. In choosing a filter shape which past experience had indicated would probably work,
efforts were spent in constructing a set of filters based on an assumed rather than a derived
shape. As the theoretical and experimental developments will show, this filter does enhance.
What has not been established as yet is if it is the best filter for the job. This remains to be
investigated.
Then, within these two basic restrictions, the research was carried out as planned. Sev-
eral necessary items of equipment were designed and constructed, and the spatial filters pro-
duced. Tests indicate feasibility, within certain very specific limitations, all related to the
glass filter material. These are covered in the discussions and conclusions of Section 5.
This report covers the details; and results of the work carried out in performance of the
contract for investigating frequency attenuating filters. The first section summarizes the theo-
retical results. It mathematically describes frequency attenuating filters of a specific kind
(Gaussian), and predicts their effect on a spatial pulse for which a special model has been de-
veloped. The sections following cover the mechanical and sensitometric bases for filter fab-
rication and describe how the spatial filters are made. A section is then discussed which re-
ports tests on the filter material and evaluates the results of filtering a typical aerial photo-
graph.
Finally, the results are evaluated in light of the contractual requirements, and some con-
clusions reached. Recommendations are made, as a result of these conclusions, for the con-
tinued development of the Image Enhancement Viewer.
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For the development of spatial frequency attenuating filters, several theoretical problems
must first be considered. From these analyses will come the required filter shape, perform-
ance and parametric optimization.
Recently completed research,' using a sharp spatial pulse, dealt with filter evaluation.
Enhancement, or image improvement, was not considered since little could be done to improve,
per se, a sharp pulse which had a contrast of unity and an infinite slope on the edge. The first
problem of the current research program was to develop a pulse model which would analytically
show improvement. A pulse was considered for analytical purposes because of its symmetry,
but the results would be applied to edges (which are simply one-sided pulses). The pulse con-
cept is a simplification of what actually occurs in the rendering of fine detail in a photographic
transparency. Analytically, a pulse and its edges should respond the same physically provided
the model is verisimilar. The development of such a non-sharp pulse model will be facilitated
by a brief discussion of the photographic edge where the abstract edge concepts can be more
definitely characterized.
A photographic edge represents, in a general sense, the boundary between contiguous re-
gions of differing densities. There are two important variables which control this boundary:
1) the macroscopic density difference between the contiguous regions, and 2) the size of the
microscopic region or zone of transition from one density to the other. Density difference af-
fects the ability of a human observer or densitometer comparator to discover the existence of
an edge, since there is a difference impossible to detect. This difference will vary for the mode
of observation, but such a threshold does exist. It establishes the basic energy limitation for
subsequent photographic processes or intensifiers to delineate the edge by amplification of the
difference across it.
The size of the transition zone is more immediately useful, since it not only provides a con-
venient basis for differentiating between sharp and non-sharp edges, but will also aid in deter-
mining the general characteristics of edge or pulse models. If the microscopic area of the pho-
tographic emulsion which bridges the gap between two differing densities is called the zone of
transition, the following definitions can be made: when the change of density across the zone of
transition is spatially instantaneous, the edge is said to be sharp; when the change is gradual and
continuous, the edge is said to be non-sharp. The size of the zone is therefore a good measure
of edge non-sharpness and gives an. indication of the degrading effect of the antecedent optical
and photographic processes.
In general, a quality which describes the variation of density across this zone is " smooth-
ness" or lack of definition. The end points are ill-defined, and the transition is continuous.
Thus, any edge or pulse model should have this smooth, continuous characteristic, allow for
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adjustment to various zone sizes, and reduce to a sharp edge or pulse in the limiting case. It
should be kept in mind that such a simplification could not possibly represent an edge modified
by the Eberhardt effect. Such non-linear variations cannot be handled by simple models.
2.3 NON-SHARP, GAUSSIAN PULSE MODEL
The several criteria deduced in the foregoing discussion of photographic edges can lead to
more than one pulse model, but none through an appeal to first principles. The Gaussian form
has found usefulness in several instances2' 2,3 , and appears to be a reasonable fit to a wide range
of photographic edge phenomena. Figure 1 is the microdensitometer trace* of a photographic
edge obtained by photographing a sharp edge with a pinhole camera. This has been fitted with a
Gaussian, shown. by the dashed line. The agreement is very good for edges of this type.
A Gaussian pulse may be written, generally, as
2
f(x) = Ti + (T2 - Ti) a-c X P (1a)
2
f(x)=T;2-(T2-Ti)e-cx
depending on whether the pulse is centered about a low or high transmission value. The edge of
Figure 1 is of the form of equation (1a). For these equations,
Ti = minimum transmission value
T2 = maximum transmission value
c = pulse rise -(or drop-off) coefficient
x = object coordinate
This function satisfies the edge criteria analytically, and has the necessary degrees of free-
dom to be fitted to a wide variety of edges. Figure 2 is a plot of equation (la), using arbitrary
transmission values.
Under the assumption that such a, pulse resulted from the degredation of a sharp pulse, the
purpose of enhancement is to reconstruct (as best possible) the original pulse shape from the
information contained in the degraded, non-sharp pulse.
One of the important constants of this equation is the variance, or abscissa corresponding
to the half-power point, a measure of pulse width. This is useful not only in empirically fitting
edges to the assumed Gaussian form, but also, as will be shown later, in determining the size
of the optimum Gaussian attenuating filter. This quantity is obtained by doubly differentiating
equation (la) or (lb), equating the result to zero, and solving for x. This produces,
2c
(2)
Before considering the imaging of such an object pulse, a discussion of attenuating filters
would seem to be in order. However,, it will be restricted to the type considered in these in-
vestigations.
* The ordinate of this and subsequent traces may be converted to optical density through Figure
14.
V.
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CL
to
-N
Slit scan
25mm
-~ 5? N
Gaussian
approximation
0
v
LO
ti
edge
cross section
Fig. 1 - Experimental edge trace approximated by a Gaussian (portion of
Gaussian to right of centerline is shown only to establish the shape more
clearly)
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I I I I 1
-3.0 -2.0 -1.0 0 1.0 2.0 3.0
xfc - Normalized object coordinate
Fig. 2 - Typical Gaussian spatial pulse
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2.4 FREQUENCY ATTENUATING, GAUSSIAN FILTERS
In view of the lack of theoretical basis for determining the "optimum" attenuating filter,
there is no preferred filter shape. However, past experience has shown that there are two im-
portant considerations which can provide some insight into the problem:
1. Low-pass filtering will not enhance or improve edges, since it removes the requi-
site high spatial frequencies.
2. An occulding filter will "ring", and, in blocking out the center of the diffraction
pattern, will remove the basis of image tone.
From these it may be concluded that any shape chosen for attenuating filters should be high-
pass, while the transmission in the center should not be low enough to occlude.
Such a filter shape, of continuous density variation, is exemplified by a Gaussian which is
dark in the center and changes smoothly to high transmission at the edges.* It can be shown
that the one-dimensional analytical form of such a filter is written
w2 f
G(w) =T2 T
1-e awl
-Ow 2
(T2 T e
1 - e-aw f
(3)
wf = upper cutoff frequency due to the field stop in the diffraction plane, and
a = filter-rise (or drop-off) coefficient, similar to the c for the pulse in equation (1).
Considering filters whose variance is small compared to the extent of frequency space, i. e. ,
(4)
equation (3) may be simplified to
2
G(w) = T2 - (T2 - T1) a-Sw
and is identical in form to equation (lb).
* The following filter shape, based on the preceding criteria was also studied:
(5)
f(w) = T1 + (T2 - T1) [w - 27r sin (27rw) ]2.
It was found to possess sufficient "fitting" properties to cover several edge types. This shape
is sufficiently general to be used as a spatial pulse, in a manner analogous to equations (1a) and
(1b). Such a pulse and filter were studied, but the mathematical tractability was low and they
were discarded in favor of the Gaussian forms.
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For purposes of demonstrating the one-dimensional shape of such a filter, equation (5) is
plotted in Figure 3. T2 is assumed equal to unity, and T1 is varied as aparameter. The obvious
effectis to achieve an "inverted" spatial pulse, but its use in the plane of diffraction changes
both its analytical meaning and physical manifestation. Consider, next, the image of a pulse
passed through such a filter.
2.5 IMAGE OF A GAUSSIAN PULSE THROUGH A GAUSSIAN FILTER
Following the systematic analysis described and utilized in OD-61-61, the spectrum of a
Gaussian. pulse in a linear, coherent optical system may be written
00
00 F(w) _ T1 j e-iwx dx + (T2 - Ti) J e cx 2 a-iwx dx,
(6)
using equation (1.a) for the pulse-form, and considering the pulse in one dimension only.
Under the assumption that the pulse width is small compared to the field stop, xf, in object
space, or
c>>
equation (6) becomes
F'(w) = 2(xf) (Ti) sine (wxf) 4- (T2 - T1) V . e-w2/4c
LC
= Field Stop spectrum 4- Pulse spectrum.
(7)
As in OD-61-6, the field stop spectrum can be eliminated analytically since it contributes
only a bias or base energy term (whose significant contributions occur in the vicinity of the
origin). Subsequent analytical considerations need account for this missing term only for pur-
poses of interpretation of results.
Basing further considerations on, the pulse spectrum alone, the image is given by its Fourier
Transform, and following OD-61-6, the image is written
(T2.- T f G(w) a v' /4C e1WZ dw
h(z) _ ./
9J_-7-
Passing the pulse through a Gaussian filter is equivalent to evaluating equation (8)
using equation (5) as G(w). Then
2
h(z) _ _ f Tr2 a-w /4c cos(wz) dw
(T2 - T1) Y ~` -00
f (7
--y ire1
(8)
e-(a + 4c}w2 cos(wz)dw. (9)
00
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where G(w) is the spatial filter.
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under the assumption that pulse and filter widths are small enough, compared with their respec-
tive field stops, to extend the limits of integration to infinity. * Equation (9) can be evaluated
immediately, in closed form and the image of a Gaussian pulse passed through a high-pass
Gaussian filter is given by
cz2
h(z) = f T2 e -cz2 - f (T2 - T1) a_ (413 -c+ 1 )
P(T2 - Ti) 4
(10)
The first term on the right-hand side can be recognized as the image of the object pulse,
without the bias term.. Then the image shape will differ from the object shape according to the
magnitudes of 40 c and f(T2 - Ti). The observable quantity is intensity. Therefore, before treat-
ing the various filtering possibilities it will be necessary to square equation (10), an amplitude
distribution, to obtain the analytical form of intensity.
In Figure 4, image intensity is plotted against a normalized image coordinate for several
values of the 4/3 c product, at a constant T2 and T1. The ordinate normalizations were obtained
by requiring the areas under the curves to be equal. The curves of Figure 4 demonstrate a
definite improvement in image edge gradient and contrast when the pulse is passed through a
Gaussian filter. ~ The pulse has been narrowed and steepened, although some low-intensity
ringing has occurred. Thus, for the assumed pulse model and frequency attenuating filter com-
bination, it appears possible to materially increase the edge gradient without destroying image
tone and adding large amounts of "ringing" or edge fringing.
The next logical problem is the optimization of the filter constants for specific enhancement
criteria. Such a consideration will determine the basis for selection of practical filters for the
image Enhancement Viewer.
Since the filter variance, 0, and the pulse variance, c, are multiplicatively related, a given
filter will affect pulses whose variances fall within the useful range of the 40 c product. This
would indicate that there is no optimum, as far as 0 is concerned, unless it be for a specific
pulse. The choice of (3's is therefore predicated on the gradients of pulse edges to be encoun-
tered in practice. However, by choice of a suitable criterion, it may be possible to utilize the
f(T2 - T1) quantity for optimizing some image feature.
Figure 4 indicates that a useful basis might be the minimizing of the first zero of the filtered
image intensity. This would have the advantage of steepening the pulse edge and raising the height
of the pulse center. It is equivalent, in effect, to maximizing the acutance. There are other pos-
sible criteria, particularly the maximizing of the slope of the image intensity, at, for example,
the inflection point. However, this would be the result of minimizing the abscissa of the first
image intensity zero, as already proposed.. The first choice, then, is not as arbitrary as itmight
first have appeared.
The basic problem consists in first solving equation (10) for z, when h(z) goes to zero. Then
two partial derivatives of z are taken separately: the first with respect to 40 c, the second with
respect to f(T2 - T1). These two equations are then set equal to zero and solved simultaneously
for 40 c and f(T2 - T1). The solutions constitute the optimum values for the specified criterion.
* The superscripts p and f denote the transmission values of pulse and filter, respectively.
An alternate way to note this enhancement is to state that the acutance has been improved.
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G(w)
Ti = 0. 80
Tl = 0.40
T1 = 0.20
Tz = 1.0
w2 - Normalized filter coordinate
Fig. 3 - Plots of typical Gaussian high-pass filters, showing the variation
due to the central transmission value
3.0 I
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;.
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
Actually, the second of these equations vanishes identically, indicating a functional depend-
ence of 4i3 c and f(T2 - Ti). It can be shown that these quantities are related by the equation
"1 = 1 -( 4(3 c+1) a-213 c.
z
The principal usefulness of this equation is to provide a transmission ratio (once a 40 c
product has been chosen) which places the first zero of the image intensity closest to the origin,
and maximizes the acutance. The relationship will be used later to determine the actual work-
ing filter constants.
There are several analytical problems that were not considered in these investigations, pri-
marily because they depended on a suitable pulse model which was not obtained until late in the
research program. Probably the most important analytical problem remaining is a study of op-
timum filtering. The present analysis assumes a Gaussian filter form in view of the lack of a
definite theoretical basis. An analysis should be carried out through the variational calculus to
determine the optimum G(w) for specific enhancement effects. Now that a suitable pulse model
has been established, the problem should involve only the application of known analytical tech-
niques. A simultaneous study to devise a means of analytically assessing the enhancement ef-
fects of attenuating filters would also be of great value.
A theoretical pulse model has been devised and an analysis made of its imaging through a
linear, coherent optical system, using a spatial filter of Gaussian cross-section. The analysis
indicates that such a shape provides a definite enhancement, and establishes a relationship be-
tween the significant parameters for optimum enhancement. Specific areas for future study are
indicated and their usefulness briefly discussed.
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3. 1. 1 Requirements
The general spatial filter shape characteristics to be considered in these investigations may
be listed as follows:
1. Rotational symmetry about the optical center
2. Continuously varying attenuation (without discontinuities)
3. Non-occluding.
The first consideration is due to the fact that while diffraction patterns of photographic trans-
parencies which contain images of man-made objects generally have a preferred orientation, the
frequency content is symmetric about the frequency axes. When the filter is rotationally sym-
metric, it allows for arbitrary planar orientation of the object transparency. Filters which are
selective with respect to orientation generally violate the second and third shape considerations,
and are designed for specific tasks beyond the scope of these investigations.
The second consideration results from a need not only to eliminate a sharp cutoff which would
result in image "ringing"; but also to allow the widest possible range of filter cross sections.
Previous studies have shown the third consideration necessary to maintain the full image tonal
range. Additionally, by making filters non-occluding, more energy (on the average) should be
available for the formation of the aerial image.
3. 1. 2 Basic Manufacturing Technique
In view of the requirements for rotational symmetry, it will be necessary to provide means
for obtaining a given optical density, over 27T radians, at a specified radius from the filter cen-
ter. This can be achieved by exposing a suitable photographic material which is rotating at a
fixed speed, to a collimated beam, through an aperture whose shape is a function of the required
filter cross section. A diagram showing the basic ideas is shown in Figure 5. The fixed and
rotating planes are parallel to each other and perpendicular to the collimated beam. Such a
scheme would be adaptable for evaporating metal or dielectric films in vacuum. The principle
difference would be in the law of optical density formation.
Another variation of this basic technique would be to insert a rotating aperture directly in
the plane of diffraction and allow the resulting attenuation to produce the filtering, on an integrat-
ing basis. This method would require extensive mechanical modification of the Image Enhance-
ment Viewer. It would tend to add vibrations which could degrade image quality. Finally, it
would require dynamic alignment and continual stabilizing readjustment* as parts respond to
* This is done so that in rotating, the center portion would not block out, or occlude the diffrac-
tion pattern center which would destroy image tone.
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Fig. 5 - Diagram showing basic concept for achieving rotationally symmetric
photographic exposure
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temperature variations and frictional wear. Such a method would have the singular advantage of
no glass (which could introduce wedge or add phase errors to the wavefront) in the optical system.
The rotating plate method was chosen as the basis for filter fabrication because it was rela-
tively simple to realize practically, and because the results obtained through photographics would
apply (with some modification) to vacuum deposition and mechanical methods. It was realized
that commercially available photographic plates (even on special order) were not suitable for
maintaining flat-frequency response over the system passband. It was felt, however, that
through their use the feasibility of such filtering could be established and specifications deter-
mined.
3. 1. 3 Aperture Shape
A method for determining aperture shape must first be established before designing and con-
structing an exposure device. It will be sufficient to establish a mathematical relationship be-
tween the required filter cross-section and exposing aperture shape. This can be carried out in
a polar coordinate system, which will specify a radius and an angle for each point on the aper-
ture contour.
The finished filter will constitute a distribution of optical density (or transmission), with
circular symmetry, on a photographic plate. This distribution will be denoted T(i), where the
bar signifies a radial coordinate normalized to the maximum field stop:
r =
rmax
(12)
Some exposure produced T(r), and it may be described functionally as
T(r) = f [E(r)] . (13)
Now exposure (in the absence of reciprocity failure, intermittency, and other mitigating ef-
fects) can be written for the scheme of Figure 5,
E(Tr) = (Iot) w sr)
Io = illumination intensity
t = exposure time
w = rotational speed of turntable (RPM)
s(r) = arc length of aperture opening, at some r
r = normalized radius to some point on aperture
s(r) = e (r)
and because (Iot) w is constant for a given exposure, equation (14) may be simplified.
(14)
(15)
E(i) = C e (r) , (16)
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C = (lot)w = constant = base exposure.
8(r) = Angular separation of mask opening at a given radius, F.
The E(r) of equation (16) is that produced by the exposure device. The f [E(r)]of equation
(13) is that required to produce the filter. Hence the two exposures must be equal. The latter
may be written inversely as
E(r) = g [ T(r)] . (17)
Such an inversion may be accomplished by a reversion of series [assuming T(r) can be expanded
as a power series in E(r)] , or by graphical means. Then equating the exposures of equations
(16) and (17),
g [T(r)]= C 8(r) (18a)
g' [D(r)]= C8(r) , (18b)
the latter considering density as a result of E(r). Equation (18b) is probably more useful since
sensitometric results are generally displayed as a plot of Density versus a function of Exposure.
For a high-pass filter, D(r) is a maximum at the center, where r = 0. The 8 (r) should
also be a maximum there. The constant, C, is thereby determined,
g~ D(0)] E(r)max
C(r)max e(r)m.ax , . ,
holding the exposure illumination, time, and rotational speed fixed. Combining this with equa-
tion (18b), gives the required relationship for high-pass filters,
E(7)max
f)
'
D
}
(20a)
E Q)
g` [ D(0)]
(
g
[
]
.
8 ()
8 (r) max
j E(r)I.
(20b)
E(r)max
This is the equation which determines the size of the aperture, or exposure mask, given as
the angular separation of each side of the opening in the mask as a function of radius. This mask
must now be incorporated in a device which can use its properties.
3. 1. 4 Exposure Jig
A device which embodies the principles sketched in Figure 5 was designed and constructed.
Photographs of this device are shown in Figures 6 and 7.
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Fig. 6 - View of the Exposure Jig, exhibiting the cutout in the turntable, and
showing the counterbore in the center of the upper plate which holds the expo-
sure mask
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Fig. 7 - View of the Exposure Jig, exhibiting the turntable details
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A Borg No. 1007-4SY Synchronous motor rotates a turntable at 30 RPM, to which the photo-
graphic plate is fastened. The three parallel plates which constitute the three decks of the de-
vice are 121/2 inches in diameter, and the unit stands 71/2 inches high. The upper plate, the
fixed plane of the exposure mask, must be centered exactly over the center of rotation of the
turntable. This was positioned in a Jig-Borer and the plate locked down tightly. In order to
minimize the effects due to imperfect light collimations (which would cause the illumination to
diverge and require impossibly precise placement of the jig), the rotating and fixed planes were
placed as close together as possible. The closeness was consistent with the capability of the
human hand(s) to fit between the two top circular plates and secure the photographic plate on the
turntable. A cutout on the turntable was provided to aid in inserting and removing the plate.
This cutout is clearly shown in Figure 6, through the counterbored opening in the top plate.
The illumination was obtained from an incandescent lamp (GE No. 1238) placed approxim-
ately eight (8) feet above the exposure jig, and collimated through an f/8 achromatic doublet of
24. 5 inch focal length. While a point source at 8 feet would have served the same purpose, the
collimator provides more light intensity per unit area on the exposure mask and relieves the
critical optical axis alignment problem.
In operation, the photographic plate is placed on the turntable and held in place by two gibs
and a locking nut. The appropriate exposure mask is placed in the counterbored opening in the
upper plate. The turntable is then set in rotation by throwing the switch located on the lower
plate of the jig. A gentle stream of air is continuously played over the surface of the photographic
plate to remove dirt and dust particles which tend to settle during long exposures. Finally,
metered voltage is applied to the source lamp for a specified time. This is accomplished through
a Gra-Lab Darkroom timer which shuts off the lamp power at the end of the predetermined expo-
sure period. Upon stopping the turntable rotation, the plate may easily be removed for proces-
sing. The details of this processing are discussed in the next section.
3. 2. 1 Requirements
The restriction of these filter fabrication studies to the photographic process places a pre-
mium on the selection of proper photographic materials. The following are some of the import-
ant considerations which affect this choice:
1. To accommodate a large range of filter types, the achievable density range should be large,
with a very low base-density, to allow transmission approaching unity.
2. A wide range of gammas should be achievable, through the use of standard developing
formulas.
3. Grain size should be small to minimize scattering effects when the developed emulsion
is used as a filter.
4. To reproduce the requisite density cross sections and support them when used as filters,
glass plates are necessary. Such plates provide the best dimensional stability available
for photographic materials.
5. Glass plates should be as flat as possible, have minimum wedge, be strain and striae
free, and be capable of being cut and edge-finished.
6. The emulsion should be as thin and uniform as possible in order not to distort the image
when placed in the optical system. Hardened emulsions should not apply stress to the
glass support and thereby add power to the system.
7. Glass thickness should not exceed 0. 125 inch, since extensive modification of the Image
Enhancement Viewer would be probable for thicknesses exceeding this.
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3. 2. 2 Photographic Plates
Some of the requirements are mutually exclusive, and compromises therefore are necessary.
After considering the available materials, Kodak High Resolution Plates were chosen for these
tests. On special order, they were obtained on Ultra-flat glass which specifies flatness to within
. 0007 inch per linear inch. While this is not considered good optical flatness, it was the best ob-
tainable below the specified thickness. The 0. 060 inch thickness in which these plates are sup-
plied prohibits any but minute improvements on surface flatness, since they would not be rigid
enough to refigure properly.
The emulsion is exceedingly thin (virtually transparent), the maximum density and gamma
very high, clear plate transmission nearly unity, and the granularity (under most processing
conditions) is exceptionally small. The effect of this material in the optical system was tested
thoroughly and the methods and results are reported in a subsequent: section.
3. 2. 3 Processing
To reproduce results, a set of processing conditions were established. Developing solutions,
times and temperatures, were not immediately standardized. Since development constitutes the
major variational method of the process, preliminary tests were necessary to establish density
and gamma characteristics.
For purposes of completeness, so that the results of this report may be reproduced by
others, the standardized detailed procedure is outlined below:
1. Development: Time and Temperature as required, at constant agitation.
2. Stop Bath: 30 seconds at 65 - 72?F, with occasional agitation.
Kodak: Indicator Stop Bath solution, diluted 2 ounces per gallon of water.
3. Fix: 10 minutes, at 65 - 72?F, with occasional agitation.
Kodak Acid Fix, full strength.
4. Wash: In running water, 30 minutes, at 65 - 72?F, with occasional agitation to remove
adhering air bubbles.
5. Wash: 2 minutes in Photo-Flo solution at 65 - 72?F, with occasional agitation.
Kodak Photo-Flo diluted 4 capfuls per gallon of water.
6. Rinse: In distilled water, until previous bath has been removed. Do not wipe or
squeegee.
7. Air dry in rack.
8. Additional considerations and details:
a. Plates are handled in Kodak Film and Plate Developing Hangers, No. 4A, 4 x 5 size.
Plates are removed only for rinsing and drying of steps 6 and 7.
b. Agitation: lift Plate hanger completely out of solution; tilt to drain, on the diagonal;
replace immediately. Lift again; tilt in opposite direction; replace. This cycle
should take approximately 2 seconds, and is carried out continuously througout the
development.
c. Kodak Hard Rubber Tanks are used for all processing, and filled with 62 ounces of
solution. Temperature is maintained by immersing the tanks in a regulated water
bath. Temperature is measured with a thermometer calibrated in degrees, which
permits interpolated readings to the nearest one-half degree.
20
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d. No more than eight 4 x 5 plates are developed in one tankful (62 ounces) of developing
solution. The solution is discarded after use. The remaining processing solutions
(Stop Bath, Acid Fix) are used until exhausted. or until normal wastage depletes the
useful supply.
3. 2. 4 Exposure
The plates were exposed, both for the actual filters and the preliminary sensitometric tests,
in the exposure jig shown in Figure 6 and 7. The light source and collimating system have been
described previously. Thus, any sensitometric testing is directly applicable to filter fabrication
without reciprocity failure correction.
In order to determine the characteristics of the emulsion, so that the requirements of equa-
tion (20) could be met, two exposure masks were constructed. These masks consisted of a
stepped aperture, each step varying in a prescribed fashion. One mask varied the stepped open-
ings geometrically as 2n, so that the ratio of the smallest to largest exposure was 128. Referred
to as the Geometric Mask, it is suitable for ascertaining the sensitometric characteristics over a
wide exposure range. The other mask consisted of 10 steps which varied linearly, so that the
ratio of the smallest to largest exposure was 10. Referred to as the Arithmetic Mask, it is es-
pecially suited for determining the sensitometric characteristics over a short exposure range,
particularly in the region of the toe. Photographs of these masks and the test plates they pro-
duce are shown in Figure 8.
3. 2. 5 Sensitometric Test Results
Kodak High Resolution Plates were tested for several developer types and concentrations.
The results for D-76 and DK-60a are shown in Figures 9, 10, and 11, for the specific conditions
shown on the plots. VHD developer was also tried, but since the film speed shifted with small
processing and temperature variations, it was felt that the loss of reproducibility did not war-
rant further use.
Figures 10 and 11, based on the Arithmetic Mask, use a linear scale. The linearity of the
toe region of the characteristic curve has been mentioned in the literature4, and these figures
corroborate such a linear trend. High-pass filters will have regions of high transmission. This
places the present interest on the toe of the curve. Thus Figures 10 and 11, being more accur-
ately defined, by virtue of the greater number of experimental points, will be used for subsequent
filter manufacture. They represent the graphical form of the g' [D(P)] or E(r) of equation (20).
3. 3. 1 Exposure Mask Computation and Layout
The diameter of the counterbore in the top-most plate of the exposure jig has been machined
to within 0. 002 inch. Metal plates, 0. 062 inch thick were machined to the same nominal diame-
ter, undertoleranced to 0. 002 inch. The exact center is marked with a small conical blind hole
so that a compass may be accurately located at center. These plates are checked to assure a
close fit (by insertion in the counterbore), the loose ones discarded,. and the remainder blued on
the surface bearing the conical indentation.
The mask contour is then layed out on the blank, point by point, with a sharp scribe. These
points are connected by a smooth curve, the actual mask outline. The computation of these points
is based on equation (20), utilizing the photographic characteristics recorded in Figures 10 and 11.
A summary of a typical set of these computations is shown in Table 1 which gives filter size and
photographic data. This Table also summarizes the necessary formulas. It should be noted that
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the choice of 270? for the maximum aperture angular opening is purely arbitrary; any other
angle less than 360? would have sufficed. The choice of rmax is fixed by the size of the filter
adapter and its corresponding frequency cutoff value. For the adapters made to fit these filters
to the Image Enhancement Viewer, the effective diameter is 2. 625 inches, corresponding to 100
lines/'mm in frequency space. The last two columns of the Table are the basic data for mask
layout.
A brief description of the procedure used for the preparation of Table 1 will illustrate how
the mask outline is determined and serve as an example of subsequent mask calculations. T(r),
for the normalized coordinate system, is first computed. This constitutes the required filter
radial transmission cross-section. This column is then converted to optical density, D(r), by
the standard formula shown in the listing below the Table. Using the appropriate curve in Fig-
ure 11, the exposure which produces this required density is ascertained, and becomes E(P), or
in terms of equation (20a), g' [D( r) ]. Upon assumption of a maximum aperture angle, the con-
stant, C, of equation (19) is computed. This is used to convert the exposure values into angles,
O(r). The outline of the exposure mask computed in Table 1 is shown in Figure 12, a plot of
columns 1 and 5 of the Table. The area within the outline is to be removed. The layout of the
mask on the metal blank requires only that column 1 be scaled to the desired size. This is ac-
complished by the formulas listed, and shown in the last column of the Table.
3. 3.2 Cutting out the Mask
In practice, when the mask layout has been completed, an additional 0. 125 inch is added to
the outer aperture edge to insure proper illumination at the filter edge. The mask is cut out
with standard machine tools, and a file is used for finishing the curved portions to the proper
shape. Approximately 0. 004 inch excess is left around the critical central region to allow cen-
tering and adjustment within the exposure jig.
3. 3. 3 Centering and Adjustment
Because of the high cost and special surface of the High Resolution plates, less expensive
Kodak 33 plates were used for the centering tests. These plates require less exposure, and de-
velop to their highest contrast in Kodak D-11, at 69?F for five minutes.
The filter exposure mask is placed in the jig; an exposure is made. After processing, the
center of the "filter" is examined. If a "density hole" appears, the mask shadow has covered
the center of turntable rotation and more metal must be removed from the mask. If a dark "den-
sity spot" appears, the mask has had too much removed and the center of turntable rotation lies
outside the mask shadow. Should a dark spot, surrounded by a ligher "density halo" appear, an
off-center condition is indicated with the center of rotation just outside the shadow and to one
side. This is the most difficult condition to correct since the direction of displacement is ren-
dered ambiguous by the rotation of the plate. By interpreting such centering information, and
removing appropriate thicknesses of metal with a jeweller's file, the mask aperture is ultimately
adjusted to the proper shape. A final test is then made using a High Resolution Plate.
Once a satisfactory plate has been obtained, insofar as the center alignment is concerned, a
microdensitometer trace is made. This checks the shape and helps determine the proper exposure.
Should the trace indicate a deviation from the design shape, the mask may be filed or filled appro-
priately. A typical trace, after several shape corrections, is shown in Figure 13. This is the
cross-section of the filter of Table 1. The ordinates of the trace may be converted to density
with the aid of Figure 14.
The spike in the center of the trace of Figure 13 represents a small spot in the center of the
filter. This is indicative of the size of the uncertainty in aligning the mask, and constitutes the
fundamental limitation of this method of mask and filter manufacture.
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Photographs of two exposure masks and the filters they produced are shown in Figure 15.
The trace in Figure 13 was made of the filter shown in Figure 15a, experimental plate No. 178.
3. 3. 4 Cutting out the Filter
While the filter is made on a 4 x 5 glass plate, the Image Enhancement Viewer accomodates
only a circular format. The circular filter must then be cut out and placed in a suitable adapter.
The presence of an emulsion will complicate cutting the glass, since the stress lines will
not relieve through such a tough film. Further, commercially available cutting fixtures do not
cut precise circles even under extremely well-controlled circumstances. Additionally, a means
for assuring that the photographic and physical centers coincide must be provided. This requires
a device for accurately centering the rectangular glass plate by use of the photographic density
variation it contains. There is no assurance that the plates are located exactly the same on the
exposure jig, so that mechanical centering is, useless.
3. 3. 5 Cutting Jig
A device to accomplish the precise cutting was designed and constructed. The cutter of
this device is a diamond, and can be adjusted for at least 0. 50 inch on either side of the required
filter radius.
Running vertically through the center is a hollow shaft. This is positioned over a small
hole in the bottom plate, upon which the glass rests. Beneath this hole is located a photocell*
whose output can be observed with a microammeter. When a microscope .illuminator is placed
over the hollow shaft, light reaches the photocell through the small centering hole. The illum-
inator is positioned so that the reading is a maximum. When the filter is inserted in the beam,
the meter reading drops, and will be a minimum (for high-pass filters) when the filter is exactly
centered. The filter position is adjusted accordingly, the source removed, and the glass scribed
with the diamond. Photographs of the cutting device are shown in Figures 16 and 17.
After scribing, the now circular filter is taken from the rectangular glass by standard
glass-cutting techniques. Rough edges are removed with a file. Edge polishing is optional. In
practice, the glass used for making the High Resolution Plates is slightly brittle and a cleanly
cut edge is difficult to achieve, especially with the restraining emulsion. Breakage (usually ex-
emplified by the glass cracking across the circular scribe marks) is generally high.
3. 3. 6 Mounting
The filters, after cutting to a circular shape, must be mounted in adapters which can be
installed in the Image Enhancement Viewer. The filters are mounted in adapters with a 60 per
cent Harleco Synthetic Resin Xylene solution which hardens by solvent release. A photograph
of a typical set of filters made for the Image Enhancement Viewer is shown in Figure 18. These
in turn are mounted on the aperture wheel of the Image Enhancement Viewer. This installation
is shown in the photograph of Figure 19.
Since the focus of the instrument will change when glass is inserted in the optical path,
the occluding filters presently installed cannot be used without modification. However, by pro-
viding a disc of High Resolution Plate from which the emulsion has been removed, the focus will
remain reasonably fixed when it is used in conjunction with the occluding filters. The image will
be degraded, however, by the poor optical quality of the glass, and proper focus cannot be
guaranteed.
* International Rectifier-Self-Generating Photocell, Mounted Cell #DP-5.
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24
A. Geometric Mask; Plate developed in D-76, at 70?F, for 16 minutes
B. Arithmetic Mask; Plate developed in DK-60a, at 69?F, for 8 minutes
Fig. 8 - Typical High Resolution Plate sensitometric test results with the
masks through which they were exposed
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5 .
Fig. 9 - Sensitometric characteristics of Kodak High Resolution Plates de-
veloped in D-76 (2:1) under the specified exposure and processing conditions
Kodak High Resolution Plate
Development: D-76 (2:1),
70? continuous
agitation, for times
indicated (in minutes)
Exposed through Geometric
Exposure Mask for 2 minutes
at 15 volts, in Filter Exposure
Jig.
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26
Relative exposure
Fig. 10 - Linear sensitometric characteristics of Kodak High Resolution
Plates developed in D-76 (2:1) for a specific exposure condition and one pro-
cessing time
Kodak High Resolution Plates
Development: D-76 (2:1),
69?F continuous
agitation, 16 minutes
Exposed through Arithmetic
Exposure Mask for 4 minutes
at 15 Volts, in Filter Exposure
Jig.
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Fig. 11 - Linear sensitometric characteristics of Kodak High Resolution
Plates developed in DK-60a under the specified exposure and processing condi-
tions
Kodak High Resolution Plates
Development; DK-60a, at
69?F continuous
agitation,for times
indicated -
Exposed through Arithmetic
Exposure Mask for 2 minutes
at 17 Volts, in Filter Exposure
Jig.
27
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Table 1 - Computations for Gaussian Filter Mask No. 3
For: a=8 T2=0.90
DK-60a, 4 minutes, at 69?F
T1=0.13
(see Figure 11)
r
T(r)
D(r)
E(r)
6(r)
(degrees)
rm
(inches)
0.0
.130
.886
8.15
270
0
.05
.145
.838
7.90
261.7
0.066
.10
.189
.723
7.35
243.5
0.131
.15
.257
.590
6.65
220.3
0.197
.20
.341
.467
6.00
198.8
0.262
.25
.433
.363
5.40
178.9
0.328
.30
.525
.279
4.85
160.7
0.394
.40
.686
.163
3.85
127.5
0.525
.50
.796
.099
2.80
92.8
0.656
.60
.857
.067
1.90
62.9
0.787
.70
. 885
. 053
1. 30
43.1
0. 919
.80
.896
.050
1.00
33.1
1.050
.90
.899
.050
1.00
33.1
1.181
1.00
.8999
.050
1.00
33.1
1.312
Summary of Formulas:
2
T(r) = C1 - C2e ar
C1 = T2-T1 a = 0.90026
1 - e-a
C2 = T2-T
1 - e-a
6(r)max = 270-
28
0 (r) max
E(r)max
D(r) = - Log T(r)
(r) = r/rmax
rmax = 1. 3125 inches
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
30?
200
10?
350?
3400
330?
330?
3400
350?
10?
20?
30?
40?
320?
50?
310`
60'
300?
320?
40?
310?
50?
300?
600
70?
2900
800
2800
900
2700
100?
260?
1100
2500
120?
240?
130?
230?
140'
220?
150'
160?
170'
180?
190'
200?
210?
210?
200?
190?
180?
170?
160?
150?
Fig. 12 - Outline of exposure mask aperture for producing Gaussian spatial
filter No. 3, using normalized radius vector
290?
70?
280?
80?
2700
900
2600
100?
250?
110?
240?
120?
230?
130`
220?
140`
29
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I I
VSL SZ1 001
i i
51.1 09t SE L
1 1 t
01 SL 09 9Z 0
I I I ~
SL1 OS1 set 001 91.
I I I d
30
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
'CS O
O
;4
O
N
O O
0 .i
O N 4
I c
Q)
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32
Fig. 15 - Typical high-pass Gaussian spatial filters, with the exposure
masks through which they were produced
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
Fig. 16 - Photograph of the Cutting Jig. The central shaft is hollow, and di-
rectly over the small hole in the lower plate. A microscope illuminator, set
on the handle over the shaft, provides sufficient intensity to drive the meter
needle off-scale
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33
 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
Fig. 17 - Photograph of Cutting Jig base, showing the sensing device used
for filter alignment prior to cutting. The bolt heads, spaced 120? apart, serve
as locating studs to center Cutting Jig on base
34
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
Fig. 18 - Gaussian spatial filters mounted in adapters for installation in the
Image Enhancement Viewer. These are oriented to display the characteristics
of the adapters
35
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36
Fig. 19 - Gaussian spatial filters installed on the Aperture wheel of the Image
Enhancement Viewer. The wheel containing the occluding filters has been ro-
tated out of normal alignment for this photograph, as evidenced by the displace-
ment of the notches on the wheels.
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
3.4 PRACTICAL DESIGN OF GAUSSIAN FILTERS
3. 4. 1 Preliminary Considerations
The Gaussian spatial filter has been analytically described in equation (3). In a form
suitably modified for analysis and subsequent computation, this equation is written
a -awe
G(w) = T2 Tie _ - (T2 Ti) e
1 a-a 1 - e_a
(22)
Wf
The relationship between 4P c and (T2 - Ti) has already been established in equation (11).
The problem remains of associating the filter constants with pulse sizes to be encountered in
typical aerial photographs so that a useful set of filters can be provided. The solution can be
facilitated by knowledge of the proper values of the constant c, the pulse variance. [See equa-
tion (2)].
To obtain an approximate idea of the magnitude of c, consider the following. A set of square
pulses (like the patters in a bar target) of periodicity k lines/mm has been degraded into a set of
less sharply defined mounds of intensity (or density) of approximately Gaussian shape. Such a
train and its resultant degradation is shown in Figure 20. Since the variance is a sufficient mea-
sure of non-sharp pulse width, the sharp and non-sharp pulse widths are related approximately by
2k ~ V c
c 8k2.
(21)
(23)
(24)
Then, once the spatial frequency, k, is specified, it can be assigned a c which is, for all
practical purposes, a reasonable measure of magnitude. Since the Image Enhancement Viewer
has a practical frequency limit of 100 lines/mm due to the filter adapters, it will probably be
of use to choose enhancement frequencies evenly spaced throughout the spectrum. Then a set
of such frequencies which is by no means unalterable could be
k = 20, 40, 60, 80 (lines/mm).
(25)
Once the c' s have been determined through equations (24) and (25), the a' s may be calcu-
lated from (22), with the aid of
wf
= 2n1f
(26)
kf
= 100 lines/mm.
(27)
37
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A. Spatial pulse train of periodicity k lines/m
B. Degraded pulse train of periodicity k lines/mm, with pulse
shapes approximately Gaussian in shape.
38
Fig. 20 - Assumed degradation of sharp spatial pulse train into train of ap-
proximately Gaussian shapes, with the parameters defined in terms of the peri-
odicity
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
Following this, the transmissirn ratio, equation (11), must then be calculated for each 40 c
product considered. Table 2 summarizes these computations for three values of 40 c, utilizing
the k's of equation (25).
3. 4. 2 Final Design Constants
Since the useful sensitometric limit on low density is 0. 05 (see Figure 11) , the quantity,
T2, is fixed for practical purposes at 0. 90. For 4p c = i , T 1 is then calculated to be 0. 041,
requiring an optical density of approximately 1. 40. This lies outside the useful sensitometric
range prepared for these investigations. This effectively eliminates the column of Table 2
which corresponds to 40 c = z .
From the considerations associated with the development of the optimum constants, a pulse
image which was narrower would be considered more suitable. Then computing the zero-
abscissas for 4/3 c equal to unity and two, respectively, and comparing them, the zero for
the former is smaller than that for the latter. This results in a higher peak value (and there-
fore a greater acutance, taking both considerations into account). This can be deduced from
the behavior of the general curves of Figure 4. These results rule in favor of taking 40 c = 1.
Filters having the characteristics shown in Table 3 have been chosen for fabrication, test-
ing and evaluation in the Image Enhancement Viewer. They were based on the above consider-
ations, but have had the values for a slightly changed from Table 2, to facilitate computation.
The cross-sections of these filters are plotted in Figure 21, the abscissa referred to spatial
frequency and scaled exactly for the Image Enhancement Viewer. These plots show the frequen-
cy attenuation which is to be applied in the plane of diffraction.
39
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Table 2 - Summary of :Preliminary Gaussian Filter Constant Computations
on which Final Design Decisions Were Based
Enhancement c
k (mm,-2)
Frequency
a
,
(lines/mm) x 10 "4
,
4 fl c= 1/2
4 Oc = 1
4 p c= 2
20 . 32
15.6
31.2
62.4
40 1.28
3.91
7.82
15.6
60 2.88
1.74
3.48
6.96
80 5. 12
.98
1.95
3.90
Ii
T2
opt.
Table 3 - Summary of Final Design Constants for Gaussian Spatial Filters
Filter No.
a
T2
T1
1
2.0
0.90
0.13
2
3.5
0.90
0.13
3
8.0
0.90
0.13
4
32.0
0.90
0.13
40
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The tests and evaluation carried out for this research program served two purposes. Of
primary concern, of course, was to achieve enhancement, or at least a demonstration of its
feasibility. Of somewhat lesser importance was a quantitative evaluation of the glass used for
the photographic plates, the effect on image quality of the glass bearing a photographic emulsion,
and the resultant improvement (if any) obtained by laminating over the emulsion. It was known
at the start that the glass would degrade image quality. Of paramount interest then, was the
amount of degradation.
The tests reported here were carried out over the length of the entire program, and the
results of each applied to subsequent stages of the research. For convenience, they have been
combined and condensed in this single section. Additional testing was carried out during the
course of the program. This considered problems of optical system vibration, attainment of
best :focus, temporary adaptation of the Image Enhancement Viewer to a 35mm format, and so
forth. Since they have only a secondary bearing on the major concerns of the program, they
will not be reported in detail.
4.2 PRELIMINARY MATERIALS EVALUATION
The purpose of these tests was to investigate image degradation due to the non-flatness of
the glass used for the Kodak High Resolution Plates, and the non-uniformity of its photographic
emulsion. The effect of lamination to improve these results was also to be considered.
4.3 TEST METHODS AND EQUIPMENT
These tests were carried out on the Spatial Filtering Test Bench, an optical bench developed
at Itek, for precise, controlled experiments in a linear, coherent optical system. This bench
contains an optical system identical to the Image Enhancement Viewer. Test results are directly
applicable. The essential difference between the two is that the Test Bench operates with a
35mm format (using an Exakta V'. body) and is vibration and shock isolated.
The test object was a reproduction of a USAF type, Buckbee-Meers sixth-root-of-two tar-
get, having a measured contrast of 13. 7:1. This target, when examined in bluish light under a
microscope, had a resolution limit of 204 lines/mm. The coherent optical system of the Image
Enhancement Viewer, by virtue of its sharp cutoff at approximately 110 lines/mm, utilizes only
part of the given target information. However, it does not attenuate frequencies below this cut-
off limit. Therefore, use of this target permits testing and evaluation without the necessity of
correcting results for systematic deficiencies. The object was immersed in a fluid gate to
minimize coherence losses due to non-uniform flatness across the surface, and to hold it flat.
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Best focus was obtained in the image (and focal) plane by using a 100x microscope. Each
test was individually focused. The tests were made with the following placed in the diffraction
plane, the ultimate location of the spatial filters:
1. Open aperture.
2. A clear High Resolution Plate, emulsion removed.*
3. Two emulsion-free High Resolution Plates laminated with Canada Balsam.
4. One unexposed High Resolution Plate, Cleared in Acid Fix.
5. An unexposed plate, cleared in Acid Fix, laminated to an emulsion-free plate with
Canada Balsam.
Since the flatness of the plates, while staying within the Kodak specifications, varied from
plate to plate, the test plates were chosen at random. Since this is the way the plates would be
selected for filter fabrication, it was felt that choosing the "best" plates would not reflect the
actual conditions fairly. Each test was made visually and the results recorded photographically
for subsequent analysis. The film used for this record was Kodak High Contrast Copy, 35mm,
processed in Kodak D-11 at 69?F for five minutes. Fixation was accomplished in Kodak Rapid
Fix. Such a film type and processing would easily record as much detail as could be observed
visually, provided the focus was correct.
4.4 RESULTS OF THE MATERIALS EVALUATION
Enlargements of the photographic records of the tests on the glass and emulsion are shown
in Figures 22, 23, and 24. These figures are second-generation photographs of 15x enlarge-
ment, resulting in some loss in fine detail. Table 4 summarizes the photographic and visual
resolution readings for these tests and should be examined in conjunction with the Figures.
The Table and Figures show that there is a definite degeneration of system response due
not only to the glass but also to the presence of a non-uniform emulsion. The degradations of
glass and glass with emulsion, while having approximately the same resolution limit, differ in
their over-all quality. t The addition of an emulsion "smeared" the image considerably more,
yet allowed spurious resolution to occur.
There was some optical power introduced by the glass as seen by the need to focus the hori-
zontal and vertical lines separately to obtain the "best" focus, and evidenced by the resolution
limits which differed according to orientation.
Examination of Figure 24 shows that the "ringing" is suggestive of a double image. This
could have been introduced by a non-uniform phase shift due to the non-uniformity of the emul-
sion layer and/or the lack of parallelism of the two filter surfaces.
Laminating the plates, at least with Canada Balsam, does not contribute a significant enough
over-all improvement to warrant further testing. A rather obvious improvement can be seen
by comparing the bar corners in Figures 24a and 24b. The laminated plate shows much more
sharply defined corners. The fundamental limitation of the lamination process is the glass
quality, and since the flatness varies randomly, too wide a range of possible optical problems
* The emulsion was dissolved in an aequous solution of sodium hydroxide.
t This points up the difficulty of using resolving power, or resolution limit to specify image
quality. Sine-wave response specification of such image degradation would serve a more use-
ful purpose.
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exists. It is entirely possible, for instance, to combine two pieces of glass whose flatness
deviations coincide in such a manner as to introduce a large amount of optical power. While
the effects of the emulsion have been definitely reduced, too many problems remain to consider
lamination as a means for improving image quality with these plates.
4. 5 EVALUATION OF THE GAUSSIAN FILTERS
Since the glass quality will degrade the aerial image, any quantitative evaluation of enhance-
ment would be impossible. Quite obviously, the image passed through the clear aperture will
be of better quality than that passed through the glass containing a filter. As a result, only
qualitative studies were made.
The experiments were conducted on the Spatial Filtering Test Bench, which enabled use of
the uncut filters (the filters still on the 4 x 5 glass plates). The test object consisted of a trans-
parency made from an aerial photograph. The best image was obtained with the aid of a 50x
microscope mounted on the rear of the camera and focused on the camera's focal plane.
Gaussian filters #3 and #4 were tested. In order to evaluate the resultant images, a photo-
graph was taken through a cleared High Resolution Plate (emulsion removed) placed in the dif-
fraction plane. As in previous tests, the image was refocused so that any change in glass or
emulsion thickness would not affect the results. Kodak High Contrast Copy film was used and
the processing carried out as in previous tests.
4.6 RESULTS OF THE FILTER EVALUATION TESTS
An enlargement (5x) of the photographic image obtained with the emulsion-free High Resolu-
tion Plate in the plane of diffraction is shown in Figure 25. Enlargements of the filtered images
obtained with the Gaussian filters are shown in Figure 26.
Comparing Figures 25 and 26, there appears to be a definite enhancement of the fine detail.
Note the details of the buildings along the main road; the shape of the buildings is sharply squared
and the corners rendered more distinctly. Figure 27 contains microdensitometer traces of one
of the buildings in this group, showing the relative contrast and sharpness improvement. These
traces were made on the photographic images of the aerial photographs, enlargements of which
are shown in Figures 25 and 26. 4`
The fine objects along the runway are vividly brought out, although not clearly enough to
identify them. without prior knowledge. It would appear that since the spread of the #3 Filter
is larger than the spatial frequency content of the picture, the effect of enhancement is some-
what lessened. The degradation due to the presence of an emulsion and glass plate is clearly
demonstrated, but does not hide the obvious improvement in acutance.
An enlargement of an image which has been passed through a sharp cutoff, occluding filter
is shown in Figure 28. This affords a direct comparison of the two types of filtering, and
*The enhancement ideas developed in the-theoreticial section considered aerial image intensity.
It has been shown' that two carefully controlled photographic processes are required to render
a photographic image linear with respect to aerial image intensity. Since the traces represent
only one such process, direct linear assessment is impossible. However, since the ordinate
scale is proportional to density, an approximation of image intensity can be achieved by invert-
ing the trace and reading "intensity" for the ordinate.
44
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permits a qualitative assessment of their relative effects. Each filter type has its particular
usefulness, and concentrates its effects on different aspects of the image. Figure 28 exhibits
the typical loss of image tone and shows the edge isolation characteristic of high-pass occluding
filters. The Gaussian filtering of Figure 26 is much more subdued in its action on edges, but
the complete tonal range has been preserved.
During the course of the materials tests, it was discovered that the image plane of the Image
Enhancement Viewer was undergoing vibration which blurred the horizontal groups of a bar target
image at frequencies well below the system diffraction limit. In a successful effort to remove
this undesireable image degredation, two steps were taken. First, the D. C. Power Supply* was
removed from contact with the bench and a special table constructed for its support. Then, vi-
bration damping pads t were inserted between the I-Beam and the wooden bipods, and between
the bipods and the floor. These two steps removed the vibration effects, and it is recommended
that the bench retain these modifications permanently.
An accurate measurement of the Image Enhancement Viewer optical system magnification
was made. This was carried out by dimensional comparison of a test object with its photographic
image on a precision Mann optical comparator. Within the tolerances with which a carefully
handled photographic emulsion can maintain its dimensions, the system magnification is unity.
* This supply contains a cooling fan which is not vibration-isolated.
t" Vibra-Check", Lowell Industries, Inc. , Boston 34, Mass.; supplied by the manufacturer,
gratis.
45
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
u^^ ^
15x
Fig. 22 - Enlargement of the photographic image of a Buckbee Meers bar
target observed on the Spatial Filtering Test Bench. It was imaged through
the maximum system aperture, and the focus obtained with the aid of a 100x
microscope. Scratches and other physical disfigurements are on the bar tar-
get object, and do not result from the imaging optics or subsequent photographic
processes.
47
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
2 3 '
III
1.11
3 = Ii' illE3
MM 4'
4 IH .
5E111
~ E 111 II.I _: I
15x
A. One Kodak High Resolution Plate, emulsion removed, placed in the diffrac-
tion (filter) plane
2. 2 ill
JIM a
5 3
moo
s X11 2
66111
l
won
B. Two laminated Kodak High Resolution Plates, emulsions removed, placed
in the diffraction (filter) plane
Fig. 23 - Enlargement of the photographic image of a Buckbee Meers bar
target observed. on the Spatial Filtering Test Bench
49
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
2 .3
111 NMI
3
3is j ?
5 1 1!11 ~. 2
15x
A. One unexposed Kodak High Resolution Plate, cleared in Acid Fix, placed
in the diffraction (filter) plane
2 3
~_IIIMIII_
. III!
-3 11153
M84.
NMI
one
llt ..~ 2
6 iii III
15x
B. One unexposed Kodak High Resolution Plate, cleared in acid fix, laminated
to one High Resolution Plate with emulsion removed, placed in the diffraction
(filter) plane
Fig. 24 - Enlargement of the photographic image of a Buckbee Meers bar
target observed on the Spatial Filtering Test Bench
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
U ~~p 4
1 '~ RS
o W
UOI
10
41 F-4
C) y o U ~eS O ? n " ~'~ it O-i
cd 0)
'-?? y d o
U 4' 0
C? 0
00 0 0 1Up ' ` bA G~i
O eye
0 w
N Sr
U U
O d! C N N C- m
CV CA
ni C)
L6
m CW CV
L6 L6 Lf; cq
M di M
Cd
U U U
c1"" cd O
y Sr iy
.a
cy O 0 C O
y 0 d O 0
Sa
a a a w
9.1
IM. rX4 rT-t Cu
+, ro
-0 Cd N -0
a wPq r o:a
0
M
0 s.
O O' O Ej ~'' .~1 w O O
ry r, a>
a R R' .- 0 Q' U 4J U ~i V] 0
Cd bA
O
4) cd
d F.
V! N
Cd
Cd ,a Cd ,O L10 '~
N CV CV CV * -!-
53
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
Fig. 25 - Enlargement of an aerial photograph, the image of which was ob-
tained on the Spatial Filtering Test Bench. One Kodak High Resolution Plate,
emulsion removed, was placed in the diffraction (filter) plane to provide the
basis for comparing this image with the photographs shown in Figure 26.
55
40 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
A. Filtered with No. 3 Gaussian filter
B. Filtered with No. 4 Gaussian filter
Fig. 26 - Enlargement of an aerial photograph, the image of which was ob-
tained on the Spatial Filtering Test Bench, for two specified Gaussian filters
57
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25 5(0 75 100 125 150 1 ' 5 200 225
l 1 I ~ I
0),6 0
g .6
25 150 IT 5 200 225
'
.
U)
c o O
to w
U U Cd
U
.ti
25 50 --7- 100 125 150 175 200 225
0
k Cd 0
t
N !/1 ti ~
I.
~I ! I I. I I I ~
I
Cn
a
N
bA
G)
N
V)
U O
M U
a,
U
%all
6m" 6w . ir..d _ bmmW km u4 i mmi ' _ ubm i MOWN il~~l~ r111r~ ^AWd `id
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
5x
Fig. 28 - Enlargement of the spatially filtered image of an aerial photograph
obtained with the Image Enhancement Viewer. The image passed through a
high-pass, sharp cut-off, occluding filter whose lower cutoff frequency was 3
lines/mm
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 Approved For Release 2002/06/11 : CIA-RDP66B00728R000100010021-5
5. 1 GAUSSIAN PULSE MODEL
The theoretical developments show that a Gaussian pulse model can be enhanced (its acu-
tance improved) by use of a high-pass Gaussian spatial filter. Controlled experiments to
verify these predictions were not carried out, primarily due to lack of properly prepared test
objects. However, the aerial photographs of Figures 24 and 25 provide a qualitative verifica-
tion. In the lower left-hand section of Figure 24, there appears a small road which leaves the
large main road at right angles. The small width of this road and the indistinctness of the
edges provide a close approximation to the Gaussian pulse. Figure 29 shows three microdensito-
meter traces of the photographic image transparencies of approximately the same part of this
road. The first of these has not been filtered, and its cross-section looks quite like the non-
sharp pulse of the theoretical studies. The second and third traces were obtained from the
images which had been passed through the Gaussian filters #3 and #4, respectively. Comparison
of these "pulses" shows an increase in contrast in a manner predicted by the plots of Figure 4,
although the center trace appears to be from a section of the road not quite the same as the
other two.
While this is hardly conclusive proof, it does indicate that the nonsharp pulse model has a
reasonable verisimilitude, and warrants further experimental study. Such a model can serve
a useful purpose in that spatial filter types may be tested "against" it, through equation (8), and
enhancement ascertained analytically.
5.2 ENHANCEMENT WITH GAUSSIAN FILTERS
The photographs of Figures 24 and 25 indicate the degree of enhancement which can be ex-
pected with the Gaussian spatial filters. The effects are very subtle, unlike the dramatic
changes effected by the sharp cutoff, occluding filters. The desired image tone has been pre-
served as expected, and the regions of fine detail (edges, building shapes, etc.) have all been
raised in contrast and sharpened. Within the obvious restrictions of the degradations caused
by the glass plates, frequency attenuating filters have definitely proved capable of enhancement.
The method of filter manufacture appears practicable, within the limitations of the inherent
reproducibility tolerances of the photographic process. The average number of plates rejected
for each one accepted was 15. Rejections resulted from improper exposure (usually due to
lamp brightness fluctuations caused by local power drains), pinholes and other emulsion imper-
fections, and minute misalignments of the exposure mask. Because of the required tolerances
on the mask blank, identical replacement of the exposure mask was impossible. This caused
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N
Fig. 29 - Microdensitometer traces of one of the roads in the photographic
images of the aerial photographs, the enlargements of which are shown in Fig-
ures 25 and 26. The ordinate is an approximate measure of aerial image in-
tensity; the traces can be compared relatively
Abscissa scale: 1mm on image = 400mm on chart;
Slit width = 5 microns
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about 30% of the rejections. The largest single cause of rejection was the imperfections of the
emulsions, either inherent or exemplified by "processing" pinholes. Particles of dust, dirt,
glass, and similar materials occasionally adhered to the emulsion prior to or during exposure.
Since their presence prevented exposure at that point, they became transparent holes after fixa-
tion. Pre-cleaning with air and brushing did not always alleviate this condition.
The major difficulty in the entire process is the centering of the mask over the center of
rotation of the turntable. This is a trial and error problem, and its successful solution lies
in the ability of the "centerer" to judge how much (and where) is to be filed off the mask aper-
ture. Interpreting the test results for the next correction is an art.
There exists a physical limit on filter size and shape which is determined by the ability to
cut out and file the aperture in the mask to the desired degree of precision. While this limit
was not actually determined, it is felt that the practical limit for the Gaussian filters is reached
for an a of 128. To obtain a feeling for this, refer to Figure 15b, which shows the mask cor-
responding to an a of 32. However, the Gaussian series could be extended beyond a = 128 if
required, by making the filters in two photographic steps. This would then place the sensitive
filing operation on the other end of the aperture which would make a negative of the desired
filter in the first step. Since the image degrading characteristics of the first plate would carry
over to the second (with its own problems of dust, dirt, non-uniformity) this method was not
deemed sufficiently useful to warrant serious consideration in these investigations.
The filters which were made for installation in the Image Enhancement Viewer represent a
set which was chosen to illustrate the filter shapes over the unmodified system passband, as
detailled in the discussions of Section III leading to Table 3. The glass quality limits the use-
fulness of this set to the two smallest filter sizes (#3 and #4).
5.4 FILTER MATERIAL QUALITY
The glass on which the filters are held serves as the chief practical limitation of these filters.
The unevenness of the emulsion and relative unflatness of the glass distorts the higher frequency
content of the image and thereby destroys the fine detail. The fact that the information content
out to 40 lines/mm is relatively distortion-free is remarkable, considering the 0. 060 inch (nomi-
nal) thickness of the plates. Variation in distortion and focus for different areas of the same
plate limit the usefulness of such filter material in a precision coherent optical system such as
the Image Enhancement Viewer. In a system using heterochromatic illumination, such a slight
focal shift might not be too bothersome. With monochromatic illumination, as in the Viewer, the
shift is disastrous. The depth of focus is exquisitely narrow, and will not absorb even a minute
change in image position, considering the spatial frequency range of interest. Finer focusing
mechanisms must therefore be provided before these particular filters can be properly used.
5. 5 THE PHOTOGRAPHIC PROCESS
While the photographic process, per se, appears adequate for use in such filters, the photo-
graphic plates (at least those commercially obtainable) are definitely not. The glass must be of
sufficient quality so that it does not deform a plane wave passing through it. While flatness is
essential for several reasons, the deformation is minimized by having the plate surfaces parallel.
Since plates of this quality are not generally available, they must first be made and then coated
with an emulsion.
The cost and time required for their manufacture are inversely proportional to the thickness.
Plates 1/8 inch thick would cost about three times plates of 3/8 thickness. Glass plates whose
surfaces have been parallel to within the required tolerance have been made in the Itek optical
shop, in 0. 050 thickness and 10 inches in diameter. Thus, a technological "breakthrough" is
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not necessarily required, insofar as the plates are concerned. However, these laboratories do
not presently possess the capability for the precisely thin emulsion manufacture and coating re-
quired for these filters. Research is presently being carried out in the Photographic Department
which will lead to the required emulsion technology, but this does not lie in the near future. The
cost of coating small lots (24 - 36 circular glass blanks) outside these laboratories is prohibi-
tive, so that if research were continued on the photographic methods for fabricating filters a
research and development study would be necessary to improve and extend the current coating
technology.
The resulting thickness of such filters (conceivably as large as 0. 75 inch with lamination)
would require special supporting structure and a definite modification of the Image Enhancement
Viewer's filter wheels. Further, because of the quality of the glass required, there must be a
minimal manufacturing rejection rate on the finished filters. Thus, the exposing equipment and
techniques must be improved by a whole order of magnitude.
It may be concluded that filters made through the photographic process on available glass
plates are not sufficiently free of image degrading characteristics towarrant further develop-
ment. By bettering the glass and improving the techniques, it -still might be possible to pro-
duce useful filters photographically, but it is felt that there are better and less devious ways to
produce frequency attenuating filters.
From these studies also comes the conclusion that any glass which is placed in the optical
system must be of at least quarter-wave flatness if the full image quality is to be achieved. The
feeling also persists that the less glass in the system, the better, and that methods of filtering
which use no supporting glass are preferred.
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In view of the established feasibility of frequency attenuating filters, and the development of
a basic technique of manufacture, several recommendations for future equipment development can
easily be presented. However, the limitations of the methods and materials disclosed by these
investigations call for a deeper inquiry into the ultimate usefulness and purpose of the Image
Enhancement Viewer.
The occluding filters presently used on the Viewer serve a useful purpose in providing a
definite mensurational capability. They cause no image deformation and the diffraction limit of
the optical system is modified by their use in a known and controllable manner. Their continued
use and installation in the Image Enhancement Viewer is definitely recommended.
Frequency attenuating filters serve a useful, demonstrable purpose in providing an enhance-
ment capability for the Viewer. Were the filter material without image-deforming flaws, these
filters could be used out to the system diffraction limit. As the test results have shown, the
present Gaussian spatial filters destroy image quality in details in excess of 40 lines/mm, even
though they provide the desired enhancement below this value. Thus, regions of fine detail which
lie near the system frequency limit cannot be observed clearly in the image, let alone be enhanced.
It is therefore recommended that the spatial filters developed for this program not be installed
and used in the Image Enhancement Viewer except for photographic transparencies whose frequency
content lies below 40 lines/mm.
An additional consideration which favors non-installation of these filters is the focal shift
resulting from glass and emulsion thickness variations. To fully realize the information contained
in the image, the focus must be constantly checked and adjusted. The Viewer does not presently
have such a capability.
The demonstrated effects of such filters warrant further investigation, in both theoretical and
practical aspects. The photographic method, by virtue of the relatively poor glass quality, is not
suited for high-resolution linear imagery. It is therefore recommended that photographic methods
using the commercially available glass plates of the quality used for these investigations be used
in the future only for development and testing of new filter types. Based on the fabrication princi-
ples developed in this study it is further recommended that the means for producing practical,
high-quality spatial filters be developed through the evaporative or mechanical methods if further
studies in these areas indicate feasibility.
In view of the possibility of obtaining plates with the properly parallel surfaces and of develop-
ing an adequate emulsion technology, the photographic method should not be entirely dropped from
serious consideration. Future studies should continually be measured against these possibilities
since they represent a demonstrably feasible means for manufacturing frequency attenuating filters.
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Because the effects of frequency attenuation are subtle, they must be viewed under suitable
magnification. This naturally varies with the information content (and scale factor) of the trans-
parency under study. Further, in order to realize the benefit of such filtering, photographic re-
cording must be precise, with the focal plane determined accurately. The Image Enhancement
Viewer cannot meet these two requirements without modification in the viewing plane of the sys-
tem..
Therefore, it is recommended that should additional research be carried out with an end to
equipping the Viewer with full-range frequency attenuating filters, it should be carried out in
parallel with an engineering modification program. This program would provide a means for
variable magnification of the image, more precise location of the focal plane, and a more ver-
satile (70mm, Robot) photographic system at the image plane.
Finally, because a full understanding of the process is basic to further development, it is
recommended that additional studies be made with regard to frequency attenuating filters, image
enhancement, and optimum filtering. Such a program is necessary to extend the useful results
reported here and exploit more fully the obvious advantages of image enhancement.
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