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.
CHAPTER XX
EXPOSURE OF AERIAL NEGATIVES
The principal factors governing the length of exposure in the airplane camera have already been discussed under various headings. These are briefly, the nature of the aerial landscape, the practically attainable lens apertures, the form of the camera support, the speed of the plane, and the characteristics of plates, films and filters. It is convenient however, to re-assemble this information in one place, in such form as to apply to the practical problem of determining the exposure to be given in any specific case.
Limitations to Exposure.—In the ordinary photography of stationary objects, exposure is a variable entirely at the operator's command. Plates of any speed may be selected, so that attention may be focussed on latitude, color sensitiveness, and other tone rendering characteristics. The exposure may be made of a length sufficient to insure all the useful photographic action lying in the “correct exposure” portion of the sensitometric curve. The exposure ratio of any filter it is desired to use is a matter of indifference—its effect on color rendering need alone be considered.
Airplane photography is sharply distinguished from ground “still” photography by its severe limitations as to the amount of the exposure. The actual duration is definitely restricted by the high speed of the plane. In peace work this can be offset in part by using slower planes or by flying against the wind. The practical limitation to 1
100 second, set by war-time requirements as to definition of fine detail, may be increased to 1
50 of a second, or even more, where mapping of grosser features is the object. A common, but entirely avoidable limitation, is that due to vibration of the camera. By proper mounting this may be entirely overcome, leaving the ground speed of the plane the only source of exposure-limiting movement. The amount of light reaching the plate constitutes a primary factor in exposure, and this is a matter of lens aperture. Generally, lens aperture is smaller the larger the plate required to be covered, and the greater the focal length. Because of their larger aperture, the short-focus lenses which will be favored for peace-time large-area mapping will permit more and longer working days than have been the rule in long-focus war photography. The necessary use of filters, particularly at the high altitudes which would be chosen in mapping, in order to economize in the number of flights needed to cover a given area, introduces an inevitable decrease in the amount of light available at the plate, as compared with surface photography under the same illuminations.