Illumination:

The colour or shade that a surface appears to the human eye based on primarily on three following factors:

• Colour and strength of incoming illumination
• Colour and texture (rough/smooth) of surface
• Relative positions and orientations of surface, light source and observer

Simplifying Assumptions

Neglect colour - consider Intensity: Now for we shall forget about colour and restrict our discussion just to the intensity of light. We will consider white light where the intensity of all colour components (red, green and blue) is equal. This shall give us a monochrome (black and white) picture.

Intensity and the 1/r Effect: generally a light source is a mixture of diffuse background illumination and one or more point sources of light. For the reason of this discussion (mainly to simplify the mathematics) we will assume that the light source we are considering is at a large distance from the scene.

This contains two following effects:

• All light rays from the source are (virtually) parallel
• There is no modification in the intensity of the light across the scene - i.e. there is no fall off of intensity as a 1/r ² effect.