=The Photographic Requirements Dictated by Brightness and Color Considerations.=—Considering only the demands made by the character of the view presented to the airplane camera, and leaving out of account other limitations to photographic operations in the plane, certain requirements as to sensitized materials may be outlined. First of all, the photographic process must not reduce, but should rather be capable of exaggerating, the range of brightness of the object. Preferably the seven-to-one range of the object photographed should be lengthened out to the full range of the printing paper, which may be two to three times this. With such an increase of range, those minute differences of brightness are accentuated, on which the detection of many objects depends. Next, the plate or film must be sensitive to the portion of the spectrum transmitted by a yellow or orange filter which will cut out the effect of haze. This calls for orthochromatic or panchromatic plates, depending on the depth of filter required. Next, if the objects to be photographed differ little in brightness but are different in color composition, we may have to rely on color filters of peculiar transmissions, capable of translating these color differences into brightness differences. These will, in general, call for fully color sensitive, or panchromatic plates. In conclusion it may be pointed out that the endeavor in ordinary orthochromatic photography—to reproduce the visual brightness of colors in the photographic print—has no real justification in aerial work. Neither in respect to color values nor in respect to brightness range is it the object of aerial photography, especially for war purposes, to present a truthful tone reproduction. Its aim is rather the adequate differentiation of detail, by whatever means necessary. CHAPTER XVIII 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.