Multiband Aerial Photography
Multiband Aerial Photography
the photography of terrain from the air simultaneously in several regions of the electromagnetic spectrum. Multiband aerial photography is more effective than aerial photography in a single spectral region, because the reflectance of natural and man-made objects on the earth’s surface depends on the wavelengths used. Moreover, this dependence is qualitatively and quantitatively different for different objects. Consequently, some objects can be recorded in an aerial photograph with the required exaggerated contrast when light rays in one region of the spectrum are used. Other objects can be photographed in other regions. Accordingly, in order to select the most suitable spectral regions for a particular case, it is necessary to know the spectral reflectance coefficients of the objects of interest in the territory being photographed; seasonally variable factors and atmospheric optical conditions must also be taken into account.
The basic type of multiband aerial photography, multiband color photography, was developed during World War II as a means of identifying camouflaged objects from aerial photographs and subsequently came into wide use for various scientific and practical purposes. At the present time, multiband aerial color photography is carried out by means of ordinary aerial cameras equipped with yellow or red filters. The film used has a base coated with two or more emulsion layers differing in spectral sensitivity and containing materials that, during color development, form dyes whose colors are complementary. As a result, a composite color image is obtained in a single aerial photograph without losing the details present in each of the constituent single-region images.
The most widely used two-layer films are the panchromatic-plus-infrared negative films with layers sensitive to radiation in the red (570–690 nanometers [nm]) and near-infrared (670–820 nm) regions of the spectrum. The most common three-layer films are, in the USSR, negative films and, abroad, reversal films (called false-color or color-infrared films). Both types are or-thochromatic-plus-panchromatic-plus-infrared films—that is, they contain an additional layer that is sensitive to radiation in the green (500–600 nm) region of the spectrum. Prints from multiband aerial films are made on ordinary color (multilayer) papers, special multiband (two-layer) papers, or positive films.
Terrestrial objects in prints from films and in original films (reversal films) are rendered in a set of transformed colors that feature greater variety, consistency, and conformity to the objects photographed than do the achromatic tones in black-and-white aerial photographs. Along with its advantages for the interpretation of aerial photographs, multiband aerial color photography is well suited for measurement and thus can be used in photogram-metry. In the Soviet Union, multiband aerial color photography is chiefly employed in forestry, agriculture, and geological and topographical work.
A special type of multiband aerial photography that has been developed and is coming into use involves the taking of photographs synchronously with three or more coupled aerial cameras on several black-and-white films that are sensitive to radiation in different regions of the spectrum. A single camera with multiple lenses may also be used. The films are exposed through a series of different filters. Chosen on the basis of their spectral characteristics, these filters single out or remove particular narrow bands of wavelengths from the aerial photograph. Through this approach, a set of photographs is obtained that provides the greatest amount of information about the territory photographed. This approach is known as multichannel multiband aerial photography. Such terms as “multizonal” and “multispectral” have also been applied to it, particularly abroad.
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L. M. GOL’DMAN