Summary

Small-format aerial photographs typically are taken under clear sky when the sun is relatively high to provide for good toplighting of the Earth's surface, although uniformly indirect lightning from high clouds or overcast sky is preferable in some situations. The position of the camera relative to the ground and sun plays a key role for determining the nature of reflected solar radiation reaching the camera. Anisotropic variations in reflectivity, depending on sun position and angle of viewing, give rise to the bidirectional reflectance distribution function (BRDF). Within the solar plane, two positions yield special lighting effects. Sun glint occurs when the angle of solar incidence is equal to the angle of reflection directly toward the camera. This is common in oblique or wide-angle vertical views looking toward the sun. The hot spot is located at the antisolar point, which is the spot on the ground in direct alignment with the camera and sun. The hot spot is seen often in oblique or wide-angle vertical views in the direction opposite the sun.

Small-format aerial photography is done routinely in the visible and near-infrared portions of the spectrum. Many types of spectral combinations may be photographed, ranging from conventional panchromatic to color-infrared. All objects display spectral signatures, which are the bases for recognizing their compositions. Vegetation is particularly instructive. Active "green" vegetation absorbs blue and red light, weakly reflects green, and strongly reflects near-infrared radiation; this spectral signature is unique among materials at the Earth's surface. In color-infrared photographs, active vegetation appears in pink and red colors.

Achieving the optimum lighting conditions for SFAP may prove difficult or even impossible depending on many factors—latitude, time of year, local weather conditions, special mission requirements, logistical limitations, platform capability, etc. The presence of shadows in an image may be desired for interpretation or automatic analysis purposes, but it also may be inconvenient and obscure valuable information. In practice, SFAP must be done often when sun position, cloud cover, and other factors are less than ideal. Even under unfavorable lighting conditions, it is still possible in many situations to acquire effective small-format aerial photographs by carefully selecting viewing directions, time of day, variable cloud cover, or other factors. One of the great advantages of small-format aerial photography is its flexibility for adapting to a wide range of lighting and weather conditions in the field.

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