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What is COSINE LAW?
Cosine Law is the illumination on a plane is proportional to the cosine of the angle that this plane makes with another plane at right angles to the light rays.
In the example it is seen that the same number of rays strike plane #B as are received by plane #A, which is perpendicular to the rays.
But #B surface is larger than #A; therefore the illumination on #B will be less than that on #A, the relative values of illumination vary inversely as the ratio between the areas of planes.
This ratio is equal to the cosine of the angle #a between the planes.
If x1 = illumination on #A, and x2 = illumination on #B, then x2 = x1 cos #a.
reference for this page: COSINE LAW
references to this page: EXITANCE, FOOTCANDLE