Relationship between Luminous Intensity and Illumination
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3.    Relationship between Luminous Intensity and Illumination

 

illumination of the surface,

but,

so,

This is known as the Inverse Square Law of illumination.
In case the light falls on a surface at an incidence angle of q as in Figure 4, then the illumination on the surface is given by:

 

This is known as the Lambert's Cosine Rule

Figure 4 Proof of Lambert's Cosine Rule