For wide field-of-view pushbroom and whiskbroom sensors, such as some airborne spectrographic imagers, the same type of scene surface may be pictured with a significantly different tone in a given band depending on the along-scanline position of the surface's image. This is so because the instantaneous view direction of the sensor varies along a scanline, and the radiance of many surfaces varies with the emitting direction according to their bidirectional reflectance distribution function (or BRDF). Also, when the spatial resolution of the image data is high relative to the size of the scene elements, different parts of similar scene elements may be observed by the sensor at different scanline positions. For example, a tree crown was sensed from close to directly overhead when it was imaged at the scaline center, but more of a side view of the tree crown was sensed when imaged at the scanline ends. If the line from the scene to the sun is perpendicular to the flightline, then light filtering through the foliage may be observed in a side view but not in the nadir view.
In principle, the along scanline variation in at-sensor radiance for a given surface type in a given band may be valuable information in identifying the type or condition of that surface. In practice, when the image processing and analysis is performed by local operations that are oblivious of instantaneous view direction, or when image strips from different flightlines are to be composed into a mosaic, it may be advantageous to reduce the along-scanline tone variation. BRDFCOR provides a means of doing so that does not require explicit knowledge of instantaneous view direction.
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