PROTOTYPE ENLARGER EXPOSURE REQUIREMENTS

• Document Type: CREST
• Collection: NGA Records (Formerly NIMA)
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78B04747A002700020032-6
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 6
• Document Creation Date: December 28, 2016
• Document Release Date: June 26, 2002
• Sequence Number: 32
• Publication Date: April 3, 1964
• Content Type: MEMO

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MRMOL N]XJM
Apr- 3, 1964
Tos
From
Subjects Prototype enlarger exposure requirements
STAT
The following notes relate to exposure times and illumination requirements
to be expected with the laser and with the alternate source.
Laser Source
The lateral energy distribution over the unaltered laser beam is very
non-uniform, (probably Gaussian - Ref.l). If a diffuser, wobble plate or other
means of reducing coherence is usedg the evenness of illumination generally
improves over the collimated area, but generally at the expense of intensity at
the film gate. In either case it is necessary to considerably overfill the film
gate in order to obtain sufficient uniformity of illumination.
We can set up a constant, 0A (dimension cm`2) which collects the area
coverage factor, transmission of the diffuser (if used), and the minor factor
of condenser and collimator transmission, the definition being implied from:'
P where $o is the irradiance at the object plane
in watts/cm
P is total power output from one end
of the laser.
The value of C will vary with the type of diffuser used, and possibly
with laser mode in the absence of a diffuser, but in other respects will remain
fixed for any given equipment and will provide a figure for film gate irradiance
from a bolometer measurement of the laser output.
The numerical value of CA can be determined experimentally in the equipment
by substituting an aperture mask of known area at the film gate (object plane,?ig.l),
This aperture should represent the area over which the illumination can be
considered to be uniform, and might for example be the inscribed circle of the
square gate area. The light flux concentrated at the plane of the spatial filter
is then compared to the total flux out of the laser by any convenient photo-cell
means, providing a rati (less than unity), the value of CA being found from:
cA = R /T, A
T1 being the measured transmission (see below) of
A the measuring aperture area in cm`.
Measurements made on the breadboard produced value4 of CA of .0476 c; 2
for the laser source without any diffuser, and .0127 ca: 2 with the combination
of the sandwiched rotating diffuser and clear plastic film which provided suitable
freedom from image noise at about 5% loss of coherence. The mask used a the
time had an aperture diameter of 1,50 inches, and thus an area of 11.4 cm which
was perhaps too large to be representative of the one inch square breadboard gate.
The values will be eomewkkt lower on the prototype due to the greater gate area,
but some improvement in efficiency of the diffuser can be expected.
Declass Review by NIMA / DoD
the projector lens, L1 , and
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An alternate method of obtaining object plane irradiance is to make
a bolometer measurement at the spatial filter position, using the aforementioned
test aperture in place of the film gate. It may be mentioned here that the
laser beam makes transmission measurements of any optical component or combination
extremely easy by comparative photo-cell measurements at convenient points on the
axis, particularly when the condenser and collimator are removed.
Knowing the irradiance $o at the object plane (without film gate glass)
the corresponding irradiance at the camera back is:
7o' where Ts is the transmission of the system including
11o TS ?o lenses L1 and L2,fils gate glass and filter
2 plane glass if any.
To is the transmis"on of the areas of interest
of the object film.
It is system magnification.
The ratio Ts/M2 will be fixed for a given system and can be called K, thus&
f a ~f / O / lD ( 1, 4 O l^ y A
1 7,o )Oe' (1,
The above expressions can be used in determining the required exposure
times when film sensitivity data is known. Since the laser and any other source
usable with the enlarger is essentially monochromatic, the data can be taken
directly from the spectral sensitivity curve for the film used. This curve
provides data in terms of the reciprocal of the flux density times exposure,in ergs,
or energy density required to produce a film density of 1.0 above gross fog
at the wavelength selected. By conversion from ergs to watt seconder
fj ^7
s t = /o where t - exposure time in seconds.
3 e film sensitivity at selected wavelength
Then from l,a and l,b ' o r 7
To Has x o (2,a A.,7-, PcAS k/0 C2,b
Much of the breadboard experimental work was done using Eastman Aerial
Film type No. 4404, selected for suitable' response at the laser wavelength,
very high resolution and availability in standard 35 mm size. The sensitivity
of this film at 6328 Ao as derived from the published curve is 0.2 reciprocal
ergs/cn2, The best available figures as measured from breadboard work or
estimated, for substitution in the above are:
Ks Ts/k = 0,9/16 = .056 To s 0.5
P 0,5 z 10-3 watts CA -05 cm 2 (full coherence)
log S = 1,3 S - 0.2 ca` /erg
Whence,from (2,b
1/t = -056 x 0.5 x 0.5 x 10,3 x .05 x 0,2 x 107 1,4
and t - 0.71 seconds
This figure is compatible with experimental results obtained on the
breadboard, where many satisfactory exposures under conditions as described
were made at an exposure time of one second. Probably the greatest uncertainty
at present is the actual power output of the laser. and arrangements should be
made to procure a bolometer or calibrated cell to resolve this.
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3
cd'nsf ii.4red for the alternate source differs basically from that applicable to
0& laser, The laser affords its maximum energy for film exposure when used at
L.
Source Considerations
T FTe photonetry of tyre conventional types of lamps which might be.
-- p-qf+~~ ~~Yr vvvY
`qui red exposure time. A conventional source, on the contrary, if arranged
;to ~irod~fca a coherence approaching unity, results in impractically long exposure
times ( Ref. 2 ). The exposure time is reduced as the square of the relaxation from
'u2-i coherence.
Incandescent Lamm Source
A possible source of this type may consist of a projection lamp, arranged
for reimaging of the filament at a variable iris aperture, which is then considered
as the secondary (reimaged) source ( see Fig. 2. ). The spectral band width of
the radiation is finite, but due to the lack of chromatic correction in the
optical system as well as for coherency considerations this band width must be
limited by a "spike? filter to a 50% width of 100 A0 or less.
However, the filtered lamp can be considered to be monochromatic from
the standpoint of obtaining radiance conversion data without the necessity
for integration, and the total radiance can be considered to be a linear
function of the limited band width, i.e. : l' Af `aA` / W4 ~f
J 1. C lK y TJ~ t4' /N 'M/trp,)
If the source is a typical projection type lamp with interlaced filaments
and a reflector is provided, the filament packing factor may be about 0.8
From the radiation curve or tables for tungsten (Ref.3, for example) it is
found that the emittance at an operating temperature of 3000. degrees 8
in the region of 6328 A0 is: x41 (nAJ = 8'0 WdZT f/Cyi '7
and the radiance is then
o, ~. X 80 ? ~. v? t W4f ._
Usin coated rsim rr Chi f' ,Iwrtl~~ 1Xer
S aging optics (L in Fig. 2) of adequate numerical
aperture, radiance from the iris is su'stantially that of the lamp, (and
independent of the magnification of the reimaging optics). The iris is at-the
focus of a collimator lens system of focal length Fc, and represents a small
but finite source of area Ai at an effective distance of Fc from the object
plane (film gate), where the irradiance is thent
H. ~e`,4 (3, ,
F
fit N ~o4 3
1 C~
where At= area of the iris image formed
at the spatial filter plans
F -'focal length of projector lens Ll
pIOA s spectral band width
In the design of the prototype alternate source system, in order to
obtain 25 % coherence reduction without excessive iris size it was necessary
to add an auxiliary collimator element ( Fig. 2) not used with the laser source
to reduce the composite collimator focal length to 34 on, the resulting maximum
iris diameter then being 2.36 cm and the area 4.38 cm2, (Ref,4)
Examination of the spectrophotometer trace. of a 6328 A? spike filter
previously used on the breadboard"shows that this filter has an effective band
width as adjusted to 100 % transmission of probably about 30 A0 (,003 microns)
and we might use this figure for typical exposure calculations.
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Substituting the above values in equation (3,a ;
STAT
STAT
In this cape
^ this
ILLEGIB
ra a on
h! = 4.38 x 20. * ,r-,,oo3
Transmission through the object film is essentially specular, with the amount of
diffraction in terms of energy at angles greater than the angular aperture of
lens Ll being negligible. Then, repeating equation (2,a. above;
z ~sT9j
and and if as before X. = .056 ~ To - 0-5 S - 0.2 cr2/erg
then 1/t = ,056 x 0.5 x 0.238 x 10-3 x 0.2 x 107 sec,-Z
1Jt = 13.3 sec. -
and the exposure time t - .075 seconds.
Reducing the iris diameter to the point where a,5 % loss of coherence is obtained
increases the exposure time to 1.87 seconds.
Vapor Lamp Source
Ansodium vapor lamp was finally chosen for the prototype source, as this provides a monochromatic start light of sufficiete#ntensity and
the wave length of 5890 A? is sufficiently close to the laser wavelength so that
the optical corrections in lenses 1 and 2 are not violated.
Catalog information on lamps gives the luminous output in photometric
units, i.e. brightness, but here again the conversion is facilitated by the
monochromatic nature of the output. The brightness of the sodium lamp is given
as 12 candles/cm2 ( 12 lumens/steradian/cam- ). This energy is nearly all at the
double line 58900-5996 A which is isolated by an intergerence TO filter, whose
transmission is given by the manufacturer as 65 STAT
From a curve of the standard visibility function aef
obtains a visibility coefficient of 0.9690 at 5990 A 5. for example), one
given above, the lamp (and iris aperture) ) radiance isp
N r Z h' 016.4' ~y,c t1S _
eYOx 0. *6 - .0132
~ Glar J7`~r4~4a
we have essentially
an co s r a r
t7a = --.~ where A - 4.38 cm2 (for 25 it cohere 1
nc
;
A/
rt
'0 f a 'V /0 _3 cv411/C/fl
Sensitivity of the Type 44()4 film at 5890 A0 is about 0.32 (somewhat
higher than the sensitivity ath 6328 A?) Then, from equation (2.a. using values
as given above:
1/t = 4056 x 0.5 x .050 z 10-3x 0.32 x 107 1/t = 4.5 Sec. and the exposure time t = 0.33 seconds ( for 25 % coherence loss ),
e ors)
2 i
Fc - )4 on (as above)
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r` h 4
f#epa'ft / r',ltrf,a'
,
s1M (Ke AJ ^ CK~ a~ G 8n f~~ ~ vd i f ! d s /~
Coherence and Spatial Filtering
It might be of some possible interest to relate the exposure time
directly to the degree of permissible departure from coherence and source
esittance (for extended souress of the types considered in the last two
sections. From equation Ma ;
TT r., '' N where N - total radiance
F,ez rim radius of the iris image at
and for a Laisbertian source: the filter plane
r
W '-e ni )'4O/vr p/ yte,^ JS/I of
the quantity $ is the departure from coherence
as used in the above.
It will be observed that nothing is mentioned in these Hates with
regard to expected exposures when spatial filtering is used. These will in
general be such greater than the figures giveng and of course dependent on
the type of filtering and also on frequency content of the object, and
s .ould be considered in detail.
References;
dftrf.,r
STAT
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