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Point Source vs Extended Source

For a point source, the emitted light intensity or pointance is the same in all directions, or isotropic. It behaves according to the inverse square law.

A flat surface that reflects or emits equal luminance in every direction from the surface is said to be a lambertian surface. For example, an evenly illuminated flat surface such as a sheet of paper is approximately lambertian in that the luminance that you see from any direction is essentially the same. The intensity or pointance from such a surface in any direction is proportional to the cosine of the reflected angle.

Reflection from a surface can be complicated by surface roughness. For an ideal lambertian surface the reflection will follow the cosine law. The ray showing 50% is at the angle 30° from the surface where cos 60° = 0.5. An interesting aspect of lambertian reflection is that although the luminous intensity will be half as much at that angle, the visual area that your eye sees is also half as much, so the surface appears equally "bright" from that angle.


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