FILTERS
=The Function of Filters in Aerial Photography.=—The use of color
screens or filters has been very common in ordinary landscape
photography, for the purpose of securing approximately correct
renderings of the brightnesses of colored objects. Plates of the
non-color-sensitive type have their maximum of sensitiveness in the blue
of the spectrum (Fig. 105) and in consequence blue skies photograph as
white, while other colors are likewise reproduced on a totally wrong
scale. Filters for correct brightness rendering are calculated for a
given color sensitive plate so that the resultant reaction to the light
of the spectrum copies the sensitiveness of the eye, which is greatest
in the yellow-green. Such filters for use with the common orthochromatic
plates are of a general yellow color.
Filters for aerial work are meant to serve quite a different purpose.
Correct tone or color rendering is of quite secondary importance to
another use of filters, namely, to cut or pierce aerial haze. It is
quite a matter of accident that the same general color of filter is
called for both to give correct color rendering and to pierce aerial
haze, namely, _yellow_. Yet on closer analysis it is found that quite
different types of yellow filter are demanded, spectroscopically
considered.
Figure 106 (K_{1} and K_{2}) shows the spectral transmission curves of
the Wratten K_{1} and K_{2} filters, intended for correct color
rendering with orthochromatic plates. The absorption increases
_gradually_ toward the blue. In the same figure is shown on an arbitrary
scale the spectroscopic character of typical haze illumination,
increasing in brightness inversely as the fourth power of the
wave-length, that is, with great rapidity in the blue and violet. It is
evident from this that a much more abrupt absorption than that of the
K_{1} or K_{2} filter is desirable, because in the green of the spectrum
the haze light is comparatively weak, and more will be lost by any
absorption in this region through decreasing useful photographic action
than will be gained by cutting out the haze. This latter consideration
is important. The use of any filter means an increase of exposure; the
use of yellow filters multiplies it several times. Careful experiment
has shown that no filter of depth less than K 1½, to use the Wratten
filters as a basis for discussion, are of real value in haze piercing.
The _filter ratio_, or ratio of exposures with and without filter, is
4.7 for the K 1½ with the Cramer Isonon plate—a figure which shows the
importance of securing the necessary haze-piercing character with the
minimum absorption of useful photographic light.
[Illustration:
FIG. 106.—Characteristics of various filters.]
=Practical Filters.=—Since the character of the absorption of the “K”
filters is not all that could be desired, new filters, both of dyed
gelatin and of glass, have been produced. The glass, a Corning product
having a very sharp-cut absorption, has not yet been produced on a
commercial scale with the high transparency in green, yellow and red
that selected samples have shown. The United States Air Service has
adopted filters of a new dye, called the EK, from the name of the
company in whose laboratory it was produced. These filters are
standardized in two depths of staining, called the “Aero No. 1” and
“Aero No. 2.” Their spectral transmission curves appear in Fig. 106,
along with those of certain darker filters useful only with panchromatic
plates for exceptionally heavy haze. The characteristic of these Aero
filters is their great transparency through all the spectrum except the
blue, whereby the greatest haze-cutting action is attained together with
a low filter factor. The filter factors of the Aero No. 1 and No. 2 with
Cramer Isonon plates are 3 and 5, respectively.
=Effects Secured by the Use of Filters.=—The efficiency of yellow
filters for haze-cutting is best shown by photographs taken at high
altitudes with filters and without. Such illustrations are given in
Figs. 107 and 108, where the first photograph is one taken at 10,000
feet without a filter, the second taken at the same altitude under the
same conditions, but with an orange filter. Both are on panchromatic
plates, and it will be seen that even with these plates the filter makes
all the difference between a useless and a useful picture. But it must
be clearly understood that the difference here lies between a plate
sensitive chiefly in the blue and violet, and a plate affected only by
the yellow, orange and red. The difference is not between what the eye
sees and what a plate with a filter sees, as is sometimes supposed. As
shown in Fig. 108, a filter enables the plate to photograph through the
haze between clouds, but not through the clouds themselves. In general,
no filter and plate combination which is feasible for aerial exposures
is capable of showing more than the eye can see if yellow or orange
goggles are worn. To do this it would be necessary for the photographic
action to take place by deep red or infra-red light, which would demand
exposures now out of the question.
[Illustration:
FIG. 107.—A photograph taken at 10,000 feet, without a filter.]
Filters are almost always necessary in photographing from high altitudes
or in making distant obliques. At times, particularly after a heavy
rain, the air is clear enough so that filters may be dispensed with.
Clearing weather was therefore chosen whenever possible for making
obliques of the battle front.
[Illustration:
FIG. 108.—Photograph taken at same time and over same neighborhood as
Fig. 107, but with an orange filter.]
=Filters for the Photographic Detection of Camouflage.=—In the
photographic as in the visual detection of camouflage, the problem is to
differentiate colors which ordinarily look alike, but which are actually
of different color composition. Particularly important are the
differences between natural foliage greens and the paints used to
simulate them. If these differ in their reflection spectra, a proper
choice of filter will show up the two greens as markedly different. Two
kinds of difference may be produced; either the two colors may be
changed in relative brightness, or they may be altered in hue. Thus
foliage green, due to its possessing a reflection band in the red of the
spectrum, which is absent in most pigments, may be made to appear _red_
while the camouflage remains green or turns black. Filters which cause
changes of color are of course of no use for photographic detection of
camouflage, since the photographic image is colorless. Brightness
differences are alone available.
Those same filters which have been worked out primarily for producing
brightness differences in visual detection of camouflage could be used
photographically, provided the plates employed were color sensitive, and
were as well screened to imitate the sensibility of the eye. But the
most useful visual filters are those causing color differences to
appear; more than this, the visual camouflage detection filters as a
class have low light transmissions, so that their usefulness in
photography is doubtful. Little work has actually been done with
camouflage detection filters for photography. Yet in spite of this
photography has been of real service in this form of detective work. Its
utility for the purpose comes from the fact that the natural
sensitiveness of the plate to blue, violet and invisible ultra-violet
acts to extend the range of the spectrum in which differences between
identical and merely visually matched colors may be picked up.
Consequently the plain unscreened plate has proved a very efficient
camouflage detector—so efficient in fact that all camouflage materials
have had to be subjected to a photographic test before acceptance. Fig.
171 shows how an ordinary photograph reveals the unnatural character of
the camouflage over a battery.
=Methods of Mounting and Using Filters.=—The most primitive way of
mounting a gelatin filter is to cut a disc from a sheet of dyed gelatin
and insert it between the components of the lens. For this purpose the
gelatin must be perfectly flat, which is insured by its method of
preparation and test. One disadvantage of this method is that the filter
can be inserted and removed only upon the ground. It is less
satisfactory the larger the diameter of the lens, and the wastage of
filters due to insertion and removal is apt to be high. The camera
should be refocussed after filters of this kind are inserted.
Glass filters, ground optically true, or gelatin filters, mounted
between optically flat glass plates, are the most convenient and
satisfactory. They may be mounted in circular cells to screw or attach
by bayonet catches to the front of the lens. Or they may be mounted in
rectangular frames to slide into transverse grooves in the camera body.
Fig. 44 shows the mount of this latter form adopted in the larger United
States Air Service cameras. This is particularly convenient if it is
desired to insert or change the filter while in the air—a practice not
generally considered feasible in war work with the photographically
inexperienced observer, but likely to be common with the employment of
skilled photographers for peace-time aerial photography.
German cameras are reported in which the glass filter is carried behind
the lens, on a lever which also carries a clear glass plate of the same
thickness, to be thrown in when no filter is needed, thus maintaining
the focus. The performance of the lens will be impaired by this scheme,
unless it is specially calculated to offset the effect of the glass
introduced in the path of the rays behind the lens—optically true glass
has no effect on definition if placed in front of the lens. Glass
filters may also be placed in close contact with the plate or film, in
which case they must be much larger, but do not need to be of as good
optical quality.
=Self-screening Plates.=—Mention must be made of a quite different mode
of realizing the filter idea, a method available where the sensitive
plate is always to be used with a filter. This is to incorporate a
yellow dye in the gelatin of the plate itself. The dye must be one which
has no direct chemical effect on the plate, but which acts simply as a
coloring agent for the gelatin. “Self-screening” plates, as they are
called, have been produced by the use of the dye called “filter yellow”
and have found some use in orthochromatic photography. They effect a
useful saving of light through the elimination of the reflection losses
at the surfaces of glass and gelatin filters. The filtering action of
the dye in the plate is somewhat different from its ordinary one, since
the deeper portions of the sensitive film are subject to greater action
than the surface, and this tends to diminish contrast.