2.3 Light Emission

Light in the form of photons is generated at light sources. Sources of light include light bulbs and natural sources such as the sun, fire, and biochemical processes.

The intensity of a given light source is often given as the power or the wattage of the source. For a small (point) light source with power ? _s, that emits light uniformly in all directions, we can compute the irradiance, E, at a surface as:

(2.14)

where r is the distance from x to the light source, and ? is the angle between the surface normal and the direction to the light source. This equation is intuitive: imagine a small source sending photons in all directions, where the density of the photons decreases with the distance to the source. The rate at which the photon density decreases is proportional to the surface area of a sphere at the same distance (one can think of each batch of emitted photons as sitting on an expanding sphere). The surface area of the sphere is 4 ?r ². The cosine factor in the numerator is due to the surface orientation. A surface facing the source will receive more photons per area than a surface that is oriented differently.

It is common to refer to the color temperature of a light source. This quantity has an exact physical meaning since it is related to the blackbody radiation. For a blackbody at a given...