FINAL REPORT IMAGE ENHANCEMENT STUDIES USING RING SMEAR TECHNIQUES

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• Document Creation Date: December 28, 2016
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• Publication Date: May 15, 1970
• Content Type: REPORT

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Image Enhancement Studies
Using Ring Smear Techniques
15 May 1970
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date: 11 June 1970
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TABLE OF CONTENTS
Page
SUMMARY
SUBJECT
TASK/PROBLEM
1. (Statement of Problem)
INTRODUCTION
2. - 6.
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DISCUSSION
7. Equipment
8. Procedure
9. Analytical and Subjective Analyses
a. Sine Wave MTF Analysis
b. Subjective Analysis
10. Training
CONCLUSIONS
11. - 12.
13.
REFERENCES
APPENDIX A - Equipment
APPENDIX B - Procedure Development
APPENDIX C - Analytical and Subjective Analyses
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LIST OF ILLUSTRATIONS
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Figure
P
age
1 The Effect of Conventional a
nd Ring Smear
6
Masking on the Amplitude of T
Frequencies
2 Mask Transfer Function (TM) f
3 System Transfer Function (T)
wo Spatial
or Ring Smear
for Ring Smear
10
4 Schematic Diagram of the Proc
edure
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for Ring Smear Enhancement
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Ring smear is a photographic enhancement technique that has
been applied for image enhancement of aerial photographs. The
technique permits selection of the spatial frequency for maximizing
enhancement, plus an adjustment of the amount of enhancement included
in each photograph.
Photographic image enhancement by ring smear has been developed
as a semi-production enhancement technique using the BPE* breadboard
enlarger.
Equipment to perform ring smear enhancement was designed and
tested on the BPE breadboard. Appropriate films to be used in the
ring smear enhancement process were selected, and tolerances were
established for image density ranges and processing gammas.
The enhancement technique was evaluated by both subjective and
sine wave MTF (Modulation Transfer Function) analyses to determine the
most suitable and practical ring smear techniques. An experiment was
included to determine if any real gain in information. was afforded by
ring smear enhancement. It was found that no additional information
could be extracted from ring smear enhanced photographs. However,
subjective evaluations indicated the probability that information is
extracted easier and faster with such enhancement -- especially in the
case of photography from poorer quality systems, and smeared or de-
focused photography from high quality systems.
Selected material from the system was enhanced and submitted
to demonstrate the capabilities on actual mission photography.
recision Enlarger.
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SUBJECT: Image Enhancement. Studies Using Ring Smear Techniques
TASK/PROBLEM
1. Design, fabricate, and mount a ring smear device on the BPE
breadboard enlarger, and using this equipment:
a. Develop equipment necessary to hold enlarged product
and ring smear mask in registration during subsequent printing.
b. Perform image enhancement on selected mission originals.
c. Train selected contractor and customer exploitation
personnel in ring smear enhancement techniques.
d. Study operating parameters of ring smear technique
with the goal of improving the method.
2. A technique known as unsharp masking has been used success-
fully to increase the contrast of fine photographic detail relative to
detail at lower spatial frequencies. This has been accomplished by
generating a "fuzzy," negative copy of the original and superimposing
it with the original to form a composite. Because the overall contrast
of this composite is reduced, compared to the original, high-contrast
printing is used in recording the composite image. The resulting
image will have a lower contrast at, lower frequencies 'than at high
frequencies. The schematic in Figure la"illustrates this form of un-
sharp masking.
3. Armitage, Lohmann and Herrick1 proposed a variation of this
unsharp masking technique that produces a greater relative increase
in the contrast of fine detail. In this technique, known as ring
smear, the MTF (Modulation Transfer Function) used in making the mask
is such that a phase inversion in the fine detail of the image spectrum
takes place. Figure lb illustrates the ring smear enhancement process.
1 See References.
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Figure 1
The Effect of Conventional and Ring Smear Masking
on the Amplitude of Two Spatial Frequencies
(for simplicity, demodulated square waves were drawn as square waves)
a) Usual Unsharp Masking Technique
Frequency Frequency
1 W2
Input
s
Wit
W1 I W2
Distance
Output
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b) Ring Smear Masking Technique
Frequency Frequency
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4. The results of the analysis performed by Armitage, Lohmann
and Herrick show that, under certain conditions, the transfer function
of the unsharp masking process is given by the following relation:
T = 1- YM . TM
where T = the transfer function of the unsharp masking
procedure
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YM = the gamma of the photographic material upon
which the unsharp mask is recorded
(1)
TM the transfer function of the unsharp mask
generation procedure
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The conditions required for this relationship to hold are that the
modulation of the unsharp mask be less than that of the original,
and that the modulation of both be less than some maximum value, approx-
imately 0.3. Both of these conditions can be reasonably well satisfied.
The quantity T, given in the above equation, is the transfer function
corresponding to the processing spread function of the unsharp masking
process. Any desired form of T may be obtained, provided the appropriate
mask transfer function, TM, can be generated. To obtain enhancement
over some band of frequencies, the value of T must be greater than 1.0.
It can be seen that this condition will occur for the appropriate value
of 1M, whenever TM becomes negative. Such transfer functions are known;
e.g., a defocused lens. However, Armitage, Lohmann and Herrick proposed
another method of generating a strongly negative transfer function.
This method is called "ring smear".
5. To generate an unsharp mask by the ring smear method, every
point in the original photograph is smeared into a ring in the mask. The
transfer function of this process is given by:
TM = Jo (2Trpu)
(2)
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p = the radius of ring smear
v = the spatial frequency
a
J
t
1
t
where: J = a Bessel function of the first kind, of zero
0
This transfer function, plotted in Figure 2, goes very strongly negative
to a maximum of about -0.4. The resulting enhancement function, T,
given by Equation (1), is plotted in Figure 3 for yM = 1.0. Note that
T goes above 1.0 to a maximum value of 1.4.
6. The unsharp mask is generated by rotating, during the exposure,
a tilted glass plate approximately 4.0mm thick placed in the optical
path of the enlarger system. Figure 4 outlines a typical procedure for
making and using the mask in ring smear enhancement.
7. Equipment
a. The ring smeared mask is generated by smearing each point
of the aerial image into a ring. The image is smeared by rotating a
piece of tilted glass in the converging, image-forming rays. The angle
at which this glass is tilted with respect to the optical axis determines
the ring size which in turn determines the frequency of maximum enhance-
ment. nConsequently, the glass must be essentially free of wedging so
that the ring size is formed as calculated.
b. A minimum number of rotations of the glass during. exposure
is required so that the entire ring receives an acceptably uniform
exposure. Any density variations in the ring would make the degree of
enhancement a function of direction.
c. Area of the glass must be great enough to prevent any
vignetting of the image forming beam at the largest tilt angle anticipated.
d. The vacuum register board, which holds the raw stock, must
be capable of precisely repositioning the straight print and ring smear
mask after processing so that near perfect registration will be maintained.
Rigorous requirements are therefore placed on processing and drying
conditions to prevent changes in film dimensions.
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Figure 2
Mask Transfer Function TM Ring Smear
TM
1 - T
YM
YM = 1.0
TM
H
m
A
m
Normalized Spatial Frequency
(2 7r p u)
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Figure 3
System Transfer Function T for Ring Smear
F T
0
I .4
in
X
n
C
T
>1~
Z3
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T = YM T M
YM = 1.0
H
A
m
2 3
1 I
Normalized Spatial Frequency
(2 Tf p U)
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Figure 4+
Schematic Diagram of the Procedure for Ring Smear Enhancement
Original
Negative
Enlarging
Lens
Enlarging:
Lens
Enlarged
Positive
SO-233
Contact Print
To Reverse
Polarity
2+30
Sandwich of Ring
Smear Mask and
Contact Negative
Original
Negative
Glass Plate 4mm Thick
Rotated Many Times
During Exposure
Positive Ring
Smear Mask
SO-233
0
Enhanced
Positive
Print 3556
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e. The final enhanced transparency is made by contact
printing the low-contrast, straight-print/mask sandwich onto a high-
contrast film, using the vacuum register board and the BPE light
source. A more detailed discussion of equipment is given in Appendix A.
8. Procedure
a. The conventional ring smear enhancement technique
requires the production of a negative straight-print enlargement and a
low-contrast, ring-smeared positive enlargement. 'These are registered
and contact printed onto a high contrast film* to yield the enhanced
transparency.
b. To meet this goal with normal photographic films, four
steps are necessary to obtain the polarities required in each step.
By the use of a direct reversal film, the number of photographic steps
can be reduced to three.
c. The major drawback to this technique is the loss in
enhancement from the mask not being in contact with the raw stock during
contact printing. Intimate contact between the mask and raw stock
cannot be obtained since the mask is in registration with the straight-
print which, in turn, is in contact with the raw stock.
d. Two unconventional ring smear enhancement techniques were
examined to circumvent this "lack of contact" problem. This is discussed
in Appendix B. However, the most consistent results, with the least
amount of effort and highest enhancement potential, are obtained with
the conventional technique of making a separate straight-print negative
and ring-smeared mask which are placed in register and contact printed.
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e. The first step in ring smear enhancement is to enlarge
the original negative onto a direct reversal duplicating film to
obtain an enlarged negative. This enlarged negative is considered
the straight print, since it is simply a normal enlargement of the
original. This negative straight print is processed to a gamma of
about 1.3. The second step is to make an enlarged, low-contrast,
ring-smeared mask. The gamma of this step is maintained at 1.0 or
less. Both the straight print and mask are exposed through the base of
the raw stock so that the proper orientation of the original is pre-
served. The straight-print can then be printed emulsion-to-emulsion
with a high contrast duplicating film. Enlargements of 20X to 50X are
used so that limiting resolution is comfortably presented to the eye at
about 5 cycles/mm.
f. The final enhanced transparency is made by reregistering
the straight-print negative and positive ring-smeared mask on the
vacuum, pin-register board, and contact printing this sandwich onto a
high-contrast duplicating film. The BPE breadboard light source is
used.
9. Analytical and Subjective Analyses
a. Sine Wave MTF Analysis. The ring smear enhancement technique
was evaluated quantitatively by sine wave modulation transfer function
analysis.
(1) Images of sine waves photographed onto 3404 film*
were enlarged and enhanced by ring smear. Three ring smear enhancement
techniques were evaluated in the study. These techniques included two
versions of conventional ring smear enhancement (multiplicative and
additive exposures) and the new aerial image masking process. In addition,
a straight-print enlargement of the sine wave image was evaluated. These
techniques are discussed in Appendix C.
Kodak High Definition Aerial Film 3404 (Estar Thin Base)
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(2) The sine waves were evaluated for modulation,
expressed in terms of exposure, and ratioed with the input modulation.
The .input modulation is the actual modulation of the 3404 film image.
Effects of the enlarging lens were not removed since the MTF of this
lens is constant for all enhancement cases.
(3) The measured MTF values are subject to considerable
variability because of tone reproduction discrepancies, and inaccuracy
in registering the masks and straight prints.
(4) Although the ring smear enhancement technique is
not strictly linear in terms of exposure, MTF analysis was useful in
pointing out the relative degrees of enhancement between the different
ring smear techniques. It was partly on this basis that the original,
ring smear enhancement process was selected as the best technique.
The results of this MTF analysis are discussed in more depth in
Appendix C.
b. Subjective Analysis
(1) Although MTF curves show a marked increase in
modulation, they do not indicate whether any real increase in information
extraction is afforded. To determine information content, sets of 2:1
contrast geometric figures photographed onto 3404 film were enlarged
and enhanced. These geometric objects were enhanced under four
separate conditions with enhancement maximized at different frequencies
for each condition.
(2) Each of five observers attempted to identify the
objects in each array. The number of correctly identified objects gives
an indication of the information extractable with each enhancement
process.
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(3) The results of this analysis are listed in
Table 1. There is no significant difference in the number of identi-
fiable objects for each condition. Obviously, the enhanced pictures
did not yield any more real information. However, the observers
commented that the enhanced images were easier to look at, and they
thought they were identifying more objects.
(4+) This ease in evaluation in itself may justify
enhancement even though no increase in extracted information was
obtained.
(5) Further discussion of the subjective analysis
results is given in Appendix C.
10. Training. Customer personnel were given a three-day train-
ing course (May 11-May 13 at the contractor's facility) on the theory
and operation of the BPE breadboard/ring smear enhancement equipment.
This course included the enhancement of several samples of operational
material and an examination of ring smear enhancement potential on
original material with gross amounts of linear image smear. The course
included instructions for selecting the proper ring smear glass tilt
angle, and suggestions on film processing gammas and exposure conditions.
All aspects of the ring smear enhancement technique were demonstrated
under darkroom conditions.
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Table 1
Geometric Figure Identification Analysis
Frequency of Maximum Enhancement Relative to Limiting Resolution
Array Straight Print (A) 150%-(B) 100% (C) 80% (D) 40% (E)
X
s % s X s X s X
1 40.6 3.435 39.8 3.421 40.4 3.362 40.4.
. E 3.782 400. 5.225
2 35.6 3.578 34.0 3.31? 34.0 4.743 34.4 4.159 34.4 7.765
3 34,0 4.69o 33.2 3.768 31.6 4,561 33.2 2.683 31.2 5.541
4 24,4 4.669 25.0 2.345 24,4 2.074 24.8 3.962 23.4 4.099
Parameters of C arison between straight Print and
A and B
Enhanced Prints
v t t ti ana r, v t v t
1 10.0 0.369 9.99 0.093 9.89 0.088 8.37 0.072
2 9.93 0.733 9.16 0.602 9.74 0.488 6.44 0.314
3 9.45 0.297 9.99 0.820 7.55 0.331 9.68 0.862
4 6.85 0.257 6.28 010 9.69 0.146
9.so 0.360
At a 99% confidence level, t must be greater than approximately 2.7 for Xi # X3
average correct identifications over five observers
s = estimate of standard deviation of correct identifications
v = degrees of freedom
t = percentage point of student's t distribution
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11. Ring smear enhancement makes aerial photographs more pleas-
ing to look at and easier to evaluate, but does not appear to increase
the amount of information available from any given photograph. Because
ring smear affects both the signal portion of the photograph and the
noise (i.e., granularity) portion, there does not appear to be a
significant change in the signal-to-noise ratio with treatment.
Benefits of the ring smear technique are related to the size of the
smallest details resolved in the original negative. Consequently, with
photo optical systems limited primarily by film granularity, there is
little benefit from ring smear enhancement to the extracting of inform-
ation, with the exception of possibly increasing the ease and speed of
photointerpretation. If the frequency spectrum of film granularity
shows equal power at all frequencies (which is highly probable) the
enhancement technique will yield equal results -- whether the cause of
the limiting resolution in the original negative is film grain or low
signal modulation.
12. There is a more than adequate tolerance in the frequency
selected for maximum enhancement relative to the cutoff frequency.
This makes plate tilt angle less critical. Also, the processing con-
trasts for both masks and straight prints are not critical as long
as they comply within reasonable limits.
13. That the customer use the ring smear device at his facility
to determine further values of this enhancement technique on oper-
ational imagery.
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1. Armitage, Lohmann and Herrick, Applied Optics, Vol. 4, No. 4,
April 1965.
2. James and Southall, Mirrors, Prisms and Lenses, 3rd Edition,
MacMillan Company, 1954, pp. 101, 102.
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APPENDIX A
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Equipment
1. Tilted Glass Plate
a. The glass plate should be between 4mm and 5mm in thick-
ness. This thickness permits meeting the glass flatness requirements
with reasonable effort. In addition, the necessary ring radii can be ob-
tained only within a range of 0-15? tilt angle. The ring radius is
linearly related to tilt angle as shown by the equation given below2.
With a 4mm thick glass plate, ring radii of 0 - 0.35mm can be obtained
over the 15? tilt range. This range corresponds to a frequency en-
hancement range from infinity to 1.75 cycles/mm.
1
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t sin a (-n cos a ? n12 - n22 sing a)
Vn - n22 sing a
b. Using a glass 4mm thick, the smallest practical angle
used is 2?. This angle produces maximum enhancement at a frequency of
about 12 c/mm. Enhancement maximized at higher frequencies is not
anticipated and not practical. Enlargements which contain information
at more than 10 c/mm are not practical since some magnification must
then be used to see detail at limiting-resolution. Limiting resolving
power is best presented to the eye at no more than 5 c/mm. Furthermore,
registering the straight print and mask becomes increasingly difficult
at higher frequencies.
2 See References on previous page.
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c. If the.glass plate has wedging of more than 2 seconds
of are, the smeared ring will be larger than anticipated. This
problem is worsened by the fact that the ring size then becomes a
factor of magnification with wedged glass. Figure A-1 demonstrates
the cause of this problem. Minimum wedging of 2 seconds of arc was
chosen since this would cause a ring radius of O.Olmm at 00 tilt
using an 80-inch working distance (if the plate were rotated). A
ring radius this small is virtually non-existent. More seriously,
in the presence of wedging, the straight print would appear mis-
registered from the ring-smeared mask. Along this line of thought,
the glass plate must be positioned at 00 tilt during exposure of the
straight print. If the glass were not at 00 tilt position during
exposure of the straight print, the straight-print image would not be
centered on the ring-smeared mask image. This misregistration would
result in white borders on one side of all high contrast edges in
the enhanced print. Figure A-2 illustrates the desired orientation
of the ring-smeared mask and the straight print.
d. Assuming a processing gamma of 1.0 for the straight
print, the ring smear glass plate must make at least 16 revolutions to
produce virtually uniform rings. These 16 revolutions provide a
maximum log exposure variation of 0.03 which is barely measurable as
a change in density with a film gamma of 1.0. Ordinarily the smeared
mask has a processing gamma less than 1.0 which leaves a generous
safety.factor. With a four-second exposure time, the motorized
rotating glass plate assures a uniform ring.
e. The ring-smearing rate should be as uniform as possible.
The glass should be turning immediately before and after the exposure
so that there is no start-up or slow-down period.
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Figure A-1
Effect of Glass Wedging
on Ring Radii
Glas s
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t
t
Incident Ray.
Along Axis of
Rotation
Ray Refracted Due to
Wedging
Ring Produced
By Wedging
When Glass is
Rotated at 00
Tilt Angle
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Figure A-2
Orientation of Straight Print and Ring
Smeared Mask Point Spread Functions
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Straight Print
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f. Since the glass plate is in a converging beam, there
will be some astigmatism, spherical aberration and coma. However,
these deficiencies have been demonstrated and calculated to be
undetectable as they are present only at the long conjugate end.
For a glass plate of 10mm thickness (over twice as thick as that to
be used) and at a 200 tilt, the astigmatism (defined as the difference
between optimum horizontal and vertical focal planes), will be
0.01173 inches for a 50mm lens at f/2.0. This amount is about 20
times less than the detectable change in focus in the image plane at
a 40X enlargement. The effects of coma and spherical aberration are
considerably less.
g. The largest field angle considered was 5? off-axis. Any
larger angles result in images which are larger than the 9.5-inch film
available, so that they were not considered. With a 15? tilt angle,
and at 5? off-axis, the rings are circular. Any eccentricity in the
rings across the field could not be detected.
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2. Vacuum Board
a. The raw stock onto which the original negative is enlarged
is held stationary and flat by a conventional Kodak dye transfer vacuum
register board. This board has a set of pins which permit accurate
registration of the mask and straight prints.
b. Once the ring-smeared-mask/straight-print sandwich is
registered, an enhanced, high-contrast-positive transparency is made
by contact printing. This contact printing stage is accomplished
through use of the vacuum register board. The sole purpose of this
device is to maintain perfect registration between the mask and straight
print, and make certain that intimate contact is maintained over the
whole area of the photograph. No problems with Newton Rings appeared in
this contact printing stage, primarily because of the matte finish on
the Kodalith Ortho film.
c. The ring-smearing device and vacuum register board are
shown mounted on the prototype BPE enlarger~in Figures A-3 and A-4.
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Figure A-3
Vacuum Register Board Mounted on the BPE Breadboard Enlarger
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Figure A-1+
Ring Smear Device Mounted on the RPE Breadboard Enlarger
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Procedure Development
1. One difficulty with the ring smear enhancement technique
is the demodulation of the ring-smeared mask exposure caused by the
.lack of contact between this mask and the emulsion of the high-contrast
copy film. Figure B-1 gives an MTF which shows the modulation losses
in the mask caused by the spacer between it and the raw stock. The
spacer represents the straight print. No estimates were obtained of
modulation losses caused by light diffusion in the straight-print
emulsion.
t
t
2. Two techniques were tried to permit intimate contact between
both the raw stock and the straight print and mask. With the first
technique, the mask and straight print were separately exposed onto
the raw stock so that no sandwich was involved. Registration was
assured by hand registering the mask and straight print before exposing
onto the copy film., and punching registration holes with a standard
Kodak pin register punch. The raw stock was similarly punched and
placed on pins.
3. This technique eliminates any decreases in enhancement from
lack'of contact between the ring-smeared mask and the copy film. However,
the overall enhancement potential is not as great, since the combination
of straight print and mask exposures are now additive rather than
multiplicative. The effect of using at additive technique instead of
multiplicative is shown in Table B-1. Note how the modulation is only
slightly increased when using additive exposures.
4. No enhanced prints of scenes were made using the additive
technique because of the low enhancement potential. The enhancement
of sine waves was accomplished and compared to the multiplicative
exposure technique.
t
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Figure B-1
Modulation Transfer Function of Contact Printing
the Ring Smeared Mask Through the Straight
Print Using a Specular Light Source
1
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i I I rr~-LUi? rTTT-fT'TrI"-l--rt I '~"{ ~ ''_ '' '~ I TTY j~T I f f ' 1 mf-r
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2 3
Spatial Frequency (cycles/ten)
1
27
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Table B-1
Comparison of Ring Smear Enhancement Potential Using
Additive or Multiplicative Mask and Straight Print
Composite Exposures
Low Frequency
High Frequency
Effective
Transmittance Density
Effective
Modulation Transmittance Density Modulati
n
o
NegativeStraight
50%
0.30
50%
0.30
Print
20%
0.70
.43
20%
0.70
. 43
Positive Ring Smear
10%
1.00
20%
0.70
Mask
16%
0.80
16%
0,80
Multiplicative
5%
1.3
10%
1.0
Composite Exposure
.52*
(Mask X Straight Print)
3%
1.5
3%
1.5
Additive Composite
60%
0.22
70%
o.16
Exposures
.25
31 *
(Mask + Straight Print)
36%
0.44
36%
o.44
,
Note the difference between Multiplicative and Additive High
Frequency Modulation where maximum enhancement occurs.
' _ 28 _
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e
I
5. With the second technique, known as aerial image masking,
the step of actually making a negative ring-smeared mask is eliminated.
Instead, the original negative is projected through the positive
straight-print onto a low-contrast-duplicating film. During this
exposure, the projected aerial image of the negative is ring-smeared --
hence the term "aerial image mask." The photographic result is a
predominantly negative image superimposed with a ring-smeared positive
image. The process gamma of the straight print is adjusted accordingly
so that the straight-print positive polarity is dominant over the
projected ring-smear negative. Since a ring-smeared mask is never
actually made, no problems with contact printing are encountered.
6. Figure B-2 is a flow diagram for this procedure as discussed
below. -
7. The original negative is enlarged onto 21+30 duplicating film*
and processed to a gamma of from about 1.3 to 1.5. A gamma greater
than 1.0 is necessary so that the positive straight print will have
more contrast than the negative aerial image to be projected through
it. If the positive straight-print contrast were not greater than
1.0, its positive polarity image would be canceled by the negative
.aerial image.
8. The 21+30 film is exposed through the base so that when it is
.replaced onto the enlarger easel after processing, it can be placed
emulsion to emulsion with the low contrast duplicating film S0-233.**
No losses were experienced by exposing the 2130 film through the
base since the film has no backing or antihalation undercoat.
Kodak Fine Grain Aerial Duplicating Film 21+30 (Estar Base)
* Kodak Special Low Contrast Fine Grain Duplicating Film S0-233
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Figure B-2
Aerial Image Masking Ring Smear Enhancement
Original
Negative
Enlarged Positive
Exposed thru Base
2430 Film
r------ ----- .
Original Negative
Enlarged Aerial
Image
--I
Low Contrast
Ring Smear
Enhanced Negative
30-233 Film
Enhanced Positive
Contact Print
3556 Film
-30-
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9. During processing of the 2430 straight print, the original
negative is not removed from the enlarger film gate. This insures
perfect registration between the processed positive straight print and
the projected aerial image of the original negative when the straight
print is replaced on the easel using the registration pins.
1
1
I
10. The S0-233 raw stock is-located between the positive
straight print and the easel board. While the original negative is
exposed through the straight print onto the S0-233 film, the aerial image
is ring-smeared. The developed film image is a combination straight-
print negative superimposed with the ring-smeared positive. Since the
positive straight-print contrast is higher than the negative aerial
image ring-smeared mask, the processed image is predominantly negative.
11. The final enhanced positive is obtained by contact printing
this low contrast, ring-smear enhanced negative onto a high contrast
film such as Kodalith Ortho Film, Type 3.
12. Enhanced photographs were made with limited success using this
technique. The technique was not developed any further; however, since
it tied up the BPE breadboard for an excessive time period while the
straight print was being processed. Furthermore, there was little
apparent increase in image quality attributable to the elimination of
contact printing difficulties.
GR
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PAR 251,
Analytical and Subjective Analyses
1. Sine Wave MTF Analysis
t
1
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t
a. The ring smear enhancement technique was evaluated
quantitatively by sine wave MTF analysis.
b. Images of sine waves photographed onto 3+01+ film were
enlarged and enhanced by ring smear. Three ring smear enhancement
techniques were evaluated during the study. Two of these techniques
were the two versions of ring smear enhancement (multiplicative and
additive exposures), and the third was the new aerial image masking
process. In addition, a straight-print enlargement of the sine wave
image was evaluated.
c. The sine waves were evaluated for modulation by tracing
the enlarged, enhanced images with a microdensitometer equipped with a
5- x 160-micron slit. The measured film image modulation was
expressed in terms of exposure and ratioed with the input modulation.
The-input modulation is the actual modulation of the 31+04 film image.
Effects of the enlarging lens were not removed since the MTF of this
lens is constant for all enhancement cases.
d. The measured MTF values are given in Figure C-l. These
curves are subject to considerable modulation variability because of
tone reproduction discrepancies and inaccuracy in registering the
masks and straight prints. Since the low frequency, or large area
contrast of the enhanced transparencies is lower than the contrast of
the enhanced higher frequencies, some extrapolation of the large area
characteristic curves was required for analysis. Figure C-2 shows a
typical large area tone reproduction characteristic curve including
a polarity reversal in the highlights. The extrapolation necessary
to evaluate the higher frequencies which have a considerably higher
density difference is shown. This extrapolation is justified by the
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wt
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Figure C-1
Modulation Transfer Functions of Four Ring Smear Fnhanrncmcn+
:3uu tff techniques and One Straight Print Enlargement
250
200
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0
100
50
Original Sine Wave on 340 Film
Conventional Super-
imposition Ring
Smear Enhancement
Conventional Ring Smear
Enhancement, Additive Exposure
(Mask 15%; Straight Print 85%
of total Exposure)
(Mask 33%; Straight Print
67% of Total Exposure)
2.0 3.0 4.0
Spatial Frequency (cycles/mm)
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Figure C-2
Tone Reproduction Calibration Curve for Sine Wave
t
MTF Analysis of Ring Smear Enhanced Prints
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Measured from
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High Frequencies -
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Relative Exposure of Enhanced
High Frequencies
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Relative Log Exposure
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Large Area
anced Print
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assumption that the polarity reversal, and hence the high minimum
density, of the large area patches is caused by the contrast of the
ring-smeared mask being nearly as high as the straight print contrast
in the highlights. Since the ring-smeared image is demodulated,
or may experience a phase reversal at higher frequencies, the straight
print maintains a higher contrast over the mask at these higher
frequencies. The result is that the extrapolation should provide
reasonable or at least conservative (in the event of phase reversals)
MTF values.
e. These large increases in modulation at high. frequencies
are somewhat at the expense of the normal tone reproduction indicated
by the aforementioned partial reversals. Objects which are actually
the darkest objects would appear lighter than surrounding areas in the
enhanced print. Again, this problem would be minimized with dual-gamma
processed original negatives. If the dual-gamma processed original
negative had a density range which was still too high, this problem
could be circumvented by the introduction of another photographic step.
This step would either be used to increase the contrast of the straight
print, or decrease the contrast of the ring-smeared mask.
2. Subjective Analysis
r
a. Although the MTF curves show a marked increase in
modulation they do not indicate whether any real increase in information
is afforded. To determine information content, sets of 2:1 contrast
geometric figures photographed onto 3404 film were enlarged and enhanced.
These geometric objects were enhanced under four conditions, with
enhancement maximized at different frequencies for each condition.
Each condition contained four arrays of geometric figures. This gave
a total of sixteen enhanced arrays, plus one straight print of each
of the four arrays giving a grand total of twenty arrays.
b. Each of the five observers attempted to identify the
objects in each array. The number of correctly identified objects
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gives an indication of the information extractable with each
enhancement process.
c. The results of this study were listed in Table 1 of the
text. There is no significant difference in the number of identifiable
objects for each condition. Obviously, the enhanced pictures did not
yield any more information. However, the observers commented that
the enhanced images were easier to look at and thought that they were
identifying more objects.
d. It is not too surprising that no more information was
obtained from the enhanced transparencies. The increase in signal
modulation shown by the MTF curves is also accompanied by an equally
substantial increase in noise (grain) modulation. This noise is not
shown by the MTF curves. The subjective analysis of the geometric
figures gives an excellent indication of the noise interference since
the eye readily sees this grain. Consequently it is seen that the
signal-to-noise ratio remains fairly constant, permitting no real
increase in available information.
e. The enhancement had the distinct advantage of making
the images easier to look at. If the original negative had been
modulation limited, rather than noise limited, the enhancement may
have been more useful. Consequently, photographs obtained with high
quality (high film resolving power) systems do not lend themselves
very well to ring smear enhancement. However, if such a system were
to be out of focus, or the photographs contained linear smear, the
ring smear enhancement technique might have been far more beneficial
to the interpretation of these photographs. Also, the photographs
from poorer quality systems, at lower resolution values, should be
considered for profitable application.
f. Enhancement of high film resolving power photographs,
however, did make them appear better and hence more easily evaluated.
This ease in evaluation in itself may justify enhancement, although
no provable increase in information was obtained.
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