How an object appears in an aerial photograph depends on the way in which it is illuminated and the position of the camera relative to the object and source of illumination (Fig. 4-1). Natural sunlight is tremendously diverse in its characteristics, which include direction, diffusion, harshness, and color (Zuckerman, 1996). Thus, natural light varies with season, time of day, latitude, altitude, cloud cover, humidity, dust, and other ephemeral conditions.

Light quality may be described as hard or soft (Defi-baugh, 2007). The former is typical under clear sky during mid-day hours, while the latter is found in cloudy conditions and early or late in the day. To achieve gentle illumination and warm colors, professional landscape photographers prefer the lighting conditions just after sunrise or before sunset, when a diffuse golden glow fills the sky. However, typical aerial photography is taken during mid-day hours when the sun is relatively high in the sky, in order to provide for full illumination or toplighting of objects as seen from above. Under these conditions, more blue light is available, especially in shadows. For purposes of the following discussion, we assume that most SFAP takes place under clear sky between mid-morning and mid-afternoon hours.

Traditional large-format aerial photography is done in the vertical mode; however, many newer remote sensing systems are designed to operate in a variety of vertical and oblique modes. This introduces much greater flexibility for possible viewing angles relative to a given target. In recent years, considerable theoretical and experimental research has been undertaken in order to understand better the phenomenon of multiview-angle reflectance, which is the variation in reflectivity depending on the location of the sensor in relation to the ground target and sun position (Asner et al., 1998). In addition, multiangular reflectance involves interaction of light with three-dimensionally structured surfaces into which light partly penetrates (Lucht, 2004, p. 9).

In principle, multiview-angle reflectance varies in three dimensions around an object. In practice, this situation is often restricted to the solar plane, that is the vertical plane that includes the sun, ground object, and aerial sensor. The bidirectional reflectance distribution function (BRDF) refers to variations of reflectivity with different viewing angles, as demonstrated within the solar plane (Fig. 4-2). Note the position of the sun and differences in reflectivity both toward and away from the sun. Reflection toward the sun is called backscatter, and reflections away from the sun are forward scatter. Of course, considerable radiation is also scattered in the third dimension to either side of the solar plane in both the back and forward directions.

In addition to viewing angle and sun position, many other factors influence lighting and reflectivity typically encountered for small-format aerial photography. These include the nature of objects on the Earth's surface as well as atmospheric conditions. Some of these vary with seasonal regularity; others are temporally irregular or ephemeral in nature. The following sections elaborate various lighting and atmospheric effects for SFAP.

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