FILM RESPONSE AS A FUNCTION OF EXPOSURE

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP67B00511R000100180001-1
• Release Decision: RIFPUB
• Original Classification: K
• Document Page Count: 5
• Document Creation Date: December 19, 2016
• Document Release Date: March 21, 2000
• Sequence Number: 1
• Publication Date: October 1, 1961
• Content Type: MAGAZINE

Approved For Release 2006/04/13 : CIA-RDP67B00511 R000100180001-1
Film Response as a Function of Exposure
FRANK SCOTT and MILTON D. ROSENAU, JR.
Reprinted from
PHOTOGRAPHIC SCIENCE AND ENGINEERING
Vol. 5, No. 5, pp. 266-269, September-October 1961
Approved For Release 2006/04/13 CIA-RDP67BOO511 R000100180001-1
 Approved For Release 2006/04/13 : CIA-RDP67B00511 R000100180001-1
PHOTOGRAPHIC SCIENCE AND ENGINEERING
Volume 5, Number 5, September-October 1961
Film Response as a Function of Exposure
FRANK SCOTT AND MILTON D. ROSENAU, JR., Electro-Optical Division,
The Perkin-Elmer Corporation, Norwalk, Conn.
The fact that the image-forming capability of film varies with exposure is of interest when the
film is used as a detector in a photographic system. A technique for determining this capability
as a function of exposure is described in this paper. Sine-wave targets of a given modulation
and of several spatial frequencies are photographed with a high-resolution camera specifically
constructed for this purpose. After processing, the modulation of the light transmission of the
images is measured with a microphotometer. The measured transmission modulation for a given
spatial frequency is modified to account for the instrumentation and is plotted as a function of
exposure. These data make possible the determination of optimum exposure, the loss of image
quality accompanying nonoptimum exposure, and the requirements for exposure control mech-
anisms. The response of the film is determined by comparison of the transmission modulation
of the photographic image with the modulation of the optical image incident on the film.
This technique is shown as applied to an aerial film; therefore the image modulation values
employed are relatively low to simulate the low contrast of aerial scenes. The use of informa-
tion obtained by this method of evaluation improves the designer's ability to predict the per-
formance of a photooptical system.
The light transmitted through a processed negative is
of fundamental importance since it is necessarily
employed in all uses of the negative. The modula-
tion of the transmitted light, or the transmission
modulation, is denoted by MT and is defined by
Tmax - Turin
Tn,ax + Ta?u
where T is the transmittance.
With given processing conditions, the transmis-
sion modulation of an image recorded on film is pri-
marily a function of the exposure, the modulation of
the optical image to which the film is exposed, and
the size of the image structure. This paper de-
scribes a method for determining the transmission
modulation as a function of exposure, for given aerial
image modulations and spatial frequencies.
The evaluation method proposed here is similar to
that proposed by Howlett,' except that objective
measurements have been substituted for subjective
criteria. In the earlier work, resolution targets were
exposed and visually evaluated. This procedure
resulted in some variation due to differences in visual
perception among individuals, and, more important,
provided only an evaluation of the limiting reso-
lution. The new method should permit different
investigators to obtain identical results because it
employs objective measurement. It also accounts
for the object contrast variations that Kardas' ob-
served, except that modulation, rather than contrast,
was chosen to evaluate this effect. Finally, data
obtained indicate that variations due to spatial fre-
Presented at the Annual Conference, Binghamton, N.Y., 25 May 1961.
Received 4 May 1961; revised 26 June 1961.
1. L. E. Howlett, Can. J. Research, 24: 1 (1946); ibid., 26: 60 (1948).
2. R. S. Kardas, Phot. Eng., 5: 91 (1954).
quency can be handled by this evaluation method,
satisfying the requirement, which MacDonald' has
discussed, to "tune" the photooptical system to
match detail size. Even more important is the fact
that this instrumental evaluation describes the
image at all spatial frequencies and not merely the
spatial frequency at the visual limit of resolution.
Procedure
A sample of the film being evaluated is exposed in
an instrument called a microcamera (Fig. 1). In the
Copyright, 1961, by the Society of Photographic Scientists and Engineers, Inc.
Approved For Release 2006/04/13 : CIA-RDP67BOO511 R000100180001-1
 Approved For Release 2006/04/13 : CIA-RDP67B00511 R000100180001-1
P S & E, Vol. 5, 1961 FILM RESPONSE AS A FUNCTION OF EXPOSURE
V
SINE WAVE
TARGET
microcamera, the film is exposed to the image of a
sine-wave target at a reduction of approximately
25X. A sensitometer is used as a controlled light
source.
The exposure time and spectral composition of the
light source can be varied and should be identical to
conditions of the photooptical system for which the
film is being evaluated. Calibrated neutral density
filters are used with the microcamera to vary the
light intensity in increments of 0.1 log meter-candle-
seconds.
Targets for the microcamera were photographi-
cally made on Kodak High Resolution Plates and
processed to yield spectrally nonselective and practi-
cally grainless images. Each target consists of long
lines and spaces of one frequency, the transmission of
which varies sinusoidally. The effective modulation
of the target image is varied by exposing the film
sample twice: first to an open field and then to the
target. Tests show that this procedure is equivalent
to one exposure if the second exposure is made within
a few minutes of the first, and if the time between
exposure and processing exceeds 1 hr. Thus any
sine-wave target of a given modulation can effectively
expose on the film sample an image of any modula-
tion lower than that of the target.
An apochromatic microscope objective, designed
for use without a microscope slide cover glass, images
the target onto the film. The objective is of 8-mm
focal length and the aperture is essentially rectan-
gular (Fig. 2). The original aperture was circular,
f/0.77. Since the film was evaluated for use in an
f/1.42 photooptical system, to simulate the angle of
incidence of light on the film, the short dimension of
the aperture was set equivalent to f/1.42 and is per-
pendicular to the lines of the target.4 The long di-
mension of the aperture, being parallel to the lines of
the target, does not affect the focus or resolution of
the lines, and is thus made as large as possible, equiv-
alent to f/0.77, in order to transmit as much light
as possible.
The emulsion of the film sample is held in the focal
plane of the microcamera, repeatable to within 0.5
?, by a film platen mechanism (Fig. 3). Because the
film is pressed onto the conjugate distance piece,
which is mounted onto the objective cell, variations
KNOB TO RELEASE
PRESSURE DURING FILM
TRANSPORT
ADJUSTABLE
SPRING
of base thickness do not affect the focus position of the
emulsion. The pressure the platen exerts is adjustable
and is such that it is sufficient to hold the film against
the distance piece' but is not excessive, which would
cause the emulsion to bulge into the 1-mm-diameter
aperture of the distance piece. Care is exercised to
maintain the film transport mechanism free from dust
particles and to operate the camera in a reasonably
constant temperature environment to attain cor-
rect placement of the emulsion in the focal plane.
All exposures in a single test are made on a small
area of the film to avoid variations of results which
might otherwise arise due to processing variations.
The exposed film is accompanied during processing
by a sensitometrically exposed film sample.
After processing, the resulting images are scanned
with a microphotometer. A slit is imaged onto the
traveling film with a microscope objective and aper-
ture identical with the objective of the microcamera.
The slit image is 2.5 ? wide and 125 ? long, which is
one-eighth the length of the film image. The output
of the microphotometer is graphically recorded and
represents the photomultiplier output voltage which
in turn represents transmission of the image. The
modulation (MA) of brightness (BA) of the aerial im-
age incident on the film in the microcamera is equal
to the modulation of the target (Mo) as modified by
the objective lens L1 (Fig. 4). As mentioned pre-
viously, lens L1 contains a rectangular aperture, the
transfer function for which is:
TL, = 1-NXv for v