CHARACTERISTICS OF PHOTOGRAPHIC EMULSIONS
The purely photographic problem in aerial photography, as distinct from
the instrumental one, is the selection of photo sensitive materials
which will yield useful results under the conditions peculiar to
exposure from the air. After such materials have been found by extensive
field tests, it is preeminently desirable to determine their
characteristics in such terms that the kind of plate or film may
thereafter be specified and selected on the basis of purely laboratory
tests. Specification must be made in terms of the ordinary sensitometric
constants of the photographic emulsion—its speed, contrast, fog,
development factor, its color sensitiveness, its ability to render fine
detail, and its grosser physical properties such as hardness and
shrinkage.
=Sensitometry.=—The most generally used system of sensitometry is that
of Hurter and Driffield, commonly referred to as the “H & D.” By this
system, in order to determine the characteristics of a given
photographic plate, it is necessary to take a series of graduated
exposures, a standard illumination of the plate being varied in known
amount by a rapidly rotating disc cut to a series of different openings,
or by some other suitable means. The negative thus obtained is developed
in a standard developer for a definite time, at a fixed temperature, and
is then measured for transmission on a photometer. The following terms
are defined and used in plotting the results:
intensity of light transmitted _I_
Transparency = _T_ = —————————————————————————————— = ———————
intensity of incident light _I__{O}
intensity of incident light _I__{O} 1
Opacity = _O_ = —————————————————————————————— = ——————— = ———
intensity of transmitted light _I_ _T_
Density = _D_ = -log_{10}_T_ = log_{10}_O_
Hurter and Driffield pointed out that a negative would give a true
representation of the differences in the light and shade of the object
if it reproduced these differences by equivalent differences in opacity.
This is equivalent to stating that if the densities are plotted against
the logarithms of the corresponding exposures, a straight line should be
obtained at 45 degrees to the axis of exposure times. If the line is at
another angle the opacities of the negative will be _proportional_ to
the brightness of the object photographed, but the _contrast_ will be
different.
A typical H & D plot is shown in Fig. 103. It will be noted that two
curves are shown. These are obtained with different developments, and
illustrate the fact that the contrast or proportionality between
exposure differences and opacity differences is a matter of time of
development. Each of these curves exhibits certain characteristics which
are common to all made in this way. There is primarily a _straight line
portion_, where opacities are proportional to illumination. This is
commonly called the region of _correct exposure_. The slope of this
straight line portion—the ratio of density/(log exposure)—is the
_development factor_, commonly denoted by “γ,” a gamma of unity denoting
exact tone rendering. Below the region of correct exposure is a “toe,”
or region of smaller contrast, called the region of _under exposure_.
Above the correct exposure region is another where the opacity
approaches constancy (afterwards decreasing or “reversing”), called the
region of _over exposure_.
The _speed_ of a plate on the H & D scale is given by the intersection
of the straight line portion of the characteristic curve when produced,
with the exposure axis. This intersection point, called the _inertia_,
is the same irrespective of the time of development, as is shown in Fig.
103. The numerical value of the speed is obtained by dividing 34 by the
inertia, when the exposure is plotted in candle-meter-seconds.
If a plate is developed until no more density and contrast can be
obtained, its development factor is then γ_{∞}, (gamma infinity), and
the larger this is the more a plate can be forced in development. If the
plate fogs in its unexposed portions this fog is measured and recorded
in density units along with the other constants. The speed of
development is represented by the _velocity constant_, commonly
symbolized by κ.
[Illustration:
FIG. 103.—Typical characteristic curves of photographic plate.]
The length of the straight line portion determines the _latitude_ of the
plate, or the range of permissible exposures to secure a “perfect
negative.” Thus if we assume that an object has a range of brightness of
1 to 30, then a plate with a straight line characteristic extending over
a range of 1 to 120 would have a latitude of 120/30 or 4. That is, the
exposure could be as much as four times the necessary one, and still
give the same result on a sufficiently exposed print. If the latitude of
the plate is too small, the shadows will fall in the under exposure
region, the high-lights in the over exposure portion of the
characteristic curve, with consequent poor rendering of contrasts.
=Criteria of Speed.=—In airplane photography _speed_ is of paramount
importance, but great care must be exercised to insure that all the
factors are considered which can contribute toward yielding the
desirable pictorial quality in the brief exposure which alone is
possible from the moving plane. A “fast” plate on the H & D scale is not
necessarily suitable for aerial work, when we remember that accentuation
of natural contrast is desirable, particularly under hazy conditions.
For, as is shown in Fig. 104, it is a common characteristic of “fast”
plates to have comparatively small latitude and low contrast at their
maximum development.
It is to be noted that the Hurter and Driffield measure of speed is
bound up with the idea of correct tone rendering and with the use of the
straight line portion of the characteristic curve. Other criteria of
speed exist. For instance, the exposure necessary to produce a just
noticeable action (threshold value); and the exposure necessary to give
a chosen useful density in the high-lights when development is pushed to
the limit set by the growth of fog.
As has already been pointed out, correct tone rendering is not necessary
or even indicated as desirable in aerial views. It is, moreover, a
matter of experience that the majority of aerial exposures with existing
plates fall in the “under exposure” period, where contrasts with normal
development are less than in the subject. This being the case, the
problem is to select not necessarily a fast plate, by the H & D
criterion, but a plate which will develop up workable densities in the
under exposure region. A plate of medium speed will sometimes develop to
greater densities in the short exposure region, if development is
forced, than will a fast plate. The contrast in the normal exposure
region will be excessive, but this is of no significance if no exposure
falling in this region is present on the plate.
[Illustration:
FIG. 104.—Characteristic curves of fast and slow plates, developed to
maximum contrast.]
In addition to its capacity for developing density, the plate should
have as low a threshold as possible, thus meeting to some extent the
requirements of both the alternative criteria of speed given above. At
the same time it is true that low threshold and good density for short
exposures are not to be found in really slow plates. Consequently, while
high speed, as ordinarily understood, is undoubtedly the first
requirement, we may expect the complete specification for the best
aerial plate to be a rather complicated thing, describing the
characteristics of a workable “toe” of the curve, in terms of which
several (_e.g._, contrast and speed) are derived from another and quite
different exposure region.
=Effect of Temperature on Plate Speed.=—It has been found by Abney and
Dewar that very low temperatures materially decrease the speed of
photographic emulsions. This decrease may amount to as much as 50 per
cent. in the temperature range from 30 degrees Centigrade above zero to
30 degrees below zero, which is the range over which aerial photographic
operations will have to be carried on in war-time. This effect has not
been at all fully studied, and it is not known whether it is general or
only found in certain kinds of plates. The remedy indicated is to
provide means for heating the plates or films when low temperatures are
encountered. This is fairly easy in film cameras, or in plate cameras
like the deRam, where the entire load of plates is carried in the camera
body. Plates carried in magazines present a more difficult problem. The
heating coil incorporated in the German cameras is perhaps partly for
this purpose.
=Color Sensitiveness.=—Complete specifications for an aerial plate
cannot be made solely on the basis of its speed, contrast, latitude,
threshold, and other sensitometric values which have to do only with the
intensity of the light acting on it. These in general apply to
photography from low altitudes, where the illumination and natural
contrast of the subject are the only factors to consider. When higher
altitudes are reached the interposition of haze decreases the already
deficient contrast, calling either for the development of more contrast
in the plate, or for the use of color filters to cut out the action of
the blue and violet light predominant in haze. Along the lines discussed
in the last section, it is not surprising to find that some plates are
better than others for bringing out gradations masked by haze, even
though no filters are used and though the plates are similar in color
sensitiveness. But the limitations to securing contrast by manipulating
the characteristic curve of the plate are soon reached, and it becomes
necessary to resort to haze-piercing color filters, used with _color
sensitive_ plates.
Roughly, two general types of color sensitive emulsions may be
distinguished: first, those in which sensitiveness to green and yellow
is added to the natural blue sensitiveness, and second, those sensitive
in a useful degree to all colors of the spectrum. The former are called
_iso-_ or _ortho-chromatic_, the latter _panchromatic_ emulsions.
Spectrograms exhibiting the distribution of sensitiveness throughout the
spectrum for several representative plates are shown in Fig. 105.
Orthochromatic plates are adequate for use with light yellow filters and
have the slight practical working advantage that they can be handled by
red light. Panchromatic plates are necessary for use with dark orange or
red filters. They must be handled in total darkness or in an exceedingly
faint blue-green light, taking advantage of the common drop in
sensibility in that region of the spectrum. Plates can, indeed, be
sensitized for the red alone, leaving a gap of almost complete
insensibility in the green, as shown in the fourth spectrogram of Fig.
105. When used with a yellow filter these plates behave as do
panchromatic plates with a red filter.
[Illustration:
FIG. 105.—Spectrograms of representative photographic plates: _a_,
ordinary plate; _b_, orthochromatic plate; _c_, specially
green-sensitive plate; _d_, red sensitive plate, insensitive to
green; _e_, panchromatic plate; _f_, specially red-sensitive
panchromatic plate.]
A rougher idea of color sensitiveness than is given by spectrograms is
furnished by the _tri-color ratio_, which is the ratio of exposure times
necessary with white light to give equal photographic action through a
certain set of red, green and blue filters, expressed in terms of the
blue exposure as unity. In an excellent panchromatic plate the three
exposures would be equal. In an orthochromatic plate the red exposure
will be too large to be figured. In interpreting either spectrograms or
tri-color ratios care must be taken that the _absolute_ exposures
necessary are known. Thus a relatively high red sensitiveness may mean
merely low absolute blue sensitiveness.
Two methods are used in imparting color sensitiveness. Either the
sensitizing dye is incorporated in the plate emulsion before it is
flowed; or the plate is bathed in a dye solution not long before using.
The latter method gives higher color sensitiveness but poorer keeping
quality, and is not a practical method for field operations. Greatly
enhanced sensibility may be given by treatment with ammonia, but this
again is a method for laboratory rather than field use.
=Resolving Power.=—A question which arises in connection with all
photography of detail is the size of the grain of the photographic
emulsion. Dependent on the size of the grain is the _resolving power_,
or ability to separate images of closely adjacent objects. This varies
with the speed, fast plates being of coarser grain than slow ones; with
the exposure; and with the method and time of development. In general,
it may be said that the resolving power of the plate does not enter
practically into aerial work, because the resolving power of all plates
so far found usable corresponds to a smaller distance than the size of a
point image as limited by the performance of the camera lens and the
speed of the plane. Remembering that ⅒ mm. is a fair value for the size
of a point image as rendered by the lens, the rôle of plate-resolving
power is shown by consideration of the following table. Resolving powers
are given in terms of lines to the millimeter just separable.
Emulsion. Resolving Power.
Seed Graflex 25
Eastman Aerial Film 37
Hammer Ortho 44
Cramer Isonon 48
Cramer Spectrum Process 57
Eastman Portrait Film 61
=Tabulation of Requirements for Aerial Emulsions.=—In terms of the
sensitometric quantities just discussed the general requirements for
aerial plates may be listed as follows:
1. _Speed._ The speed usually connected with the contrast and density
required for the exposure times available is about 150 H & D. Faster
plates in general have too low contrast, but the highest speed that will
give the necessary contrast is desired.
2. _Contrast._ The contrast capable of development without fog should be
from 1.5 to 2. This contrast should be produced by light of daylight
quality, and, in orthochromatic and panchromatic plates, with the yellow
or orange filters intended to be used with them. This contrast means a
gamma infinity approaching 2.5.
3. _Speed of development._ A gamma of nearly 2 should be developed in 2½
minutes at 20 degrees C. in the developers recommended below.
4. _Fog._ Not over .25 for this degree of development, and not over .40
for six minutes development.
5. _Color sensitiveness._ This should in general be as high as possible.
In terms of certain representative filters (described in a subsequent
chapter) color sensitiveness should be such that with the white light
speed above specified the relative exposures through the filters shall
not be greater than as follows:
No filter Aero 1 Aero 2 #21 #23a #25
Panchromatic plate 1 3 4.5 7 9 12
Ortho plate 1 2.5 3.5 6
=Relative Behavior of Plates and Films.=—The advantages of film from the
standpoint of weight and bulk have been discussed in connection with
aerial cameras. Were there no other considerations film would
unquestionably be the most appropriate medium for aerial photography.
There is, however, the question of ease of handling, to be treated in a
subsequent chapter, and the question whether the purely photographic
characteristics of film are satisfactory. Can the same speed, contrast,
and color sensitiveness be obtained on film as on glass? Is the picture
so obtained as permanent or reliable as the plate image?
It must be confessed that up to the present emulsions on film have not
proved the equal of those on glass. It has been found by emulsion
manufacturers that the same emulsion flowed on film and on glass gives
better quality on the glass. Emulsions specially prepared for film fall
somewhat short of the best plate emulsions. It has also been found
harder to color-sensitize film, and to insure good keeping quality in
the color sensitized product.
In addition to the question of photographic quality there arises the
matter of shrinkage and distortion. These are negligible with plates,
but are a more or less unknown quantity in film. Irregular shrinkages of
as much as two per cent. are found on experiment. This defect, of
course, would be an obstacle only in exact mapping work.
=Positype Paper.=—The need sometimes arises in military operations to
secure prints ready for examination within a few minutes after the
receipt of the negatives. Even the 15 or 20 minutes within which a
negative can be developed and a wet print taken may be considered too
long. While such occasions are probably more apt to occur in popular
magazine stories than in actual warfare, it is important to have
available methods of producing prints with an absolute minimum of delay.
This need is met to some degree by a direct print process, commercially
exploited under the name of “Positype.”
In this process the exposure is made directly on a sensitized paper or
card, which is developed, the image dissolved out, the residue exposed,
and again developed; thus furnishing a positive picture (reversed right
and left). The time necessary to develop a print ready for examination
need not be more than three minutes. Only a single print is available,
but this is all that would be called for under the extreme conditions
suggested. If later, copies are desired they may be made by the same
process.
=Plates and Films Found Satisfactory for Aerial Work.=—The following
plates and films have been found particularly good for aerial
photography. The list is not intended to be complete. Furthermore, it
may be expected to be soon superseded, as the efforts of various
manufacturers are directed toward developing special aerial photographic
plates.
Among orthochromatic plates: The Cramer Commercial Isonon, the Jougla
Ortho.
Among panchromatic plates: The Ilford Special Panchromatic, the Cramer
Spectrum Process.
Film: Ansco Speedex, Eastman Aero.