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I'm working on ray tracing and I'm trying to understand the impact of the angle at which a light-path intersects the surface of a light source on the amount of light that source delivers to the path (per unit time, of course).

My assumption for an ideal diffuse light source is that light rays leaving a particular point on the surface of the light source are uniformly distributed, in which case the angle of the path is irrelevant. I think that in essence, this assumption models each point on the diffuse surface of the light as an individual point-light source. Is that a fair assumption for something like a frosted light bulb, where I'm modeling the light as coming directly from the surface of the bulb, not the filament? Or alternatively, if I wanted to actually model the filament itself, would the same assumption be relatively valid?

Are there any light sources for which this assumption would not work? I'm assuming lasers would not work this way, with most of the light leaving in one particular direction, but I'm not really worried about modeling laser light. For spot lights, I believe this is still just an ordinary "diffuse" light source which uses mirrors, an aperture, and lenses to direct the light once it has already left the light source so I could model it this way (though I suppose a more efficient model would encapsulate all of this and take the angle of the path into account).

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Yes, you can model a diffuse light source as a collection of point sources. For a frosted light bulb, I would definitely use the surface of the bulb. But even for a non-frosted light bulb, it makes little difference whether you use the bulb or the filament if the light source is far away.

A spotlight is, as you suspect, a diffuse light source with an aperture that limits the angles of light leaving the light source.

This works for any incoherent light source. It does not work for coherent light such as lasers. Even though Huygens' principle states that any wavefront is equivalent to each point on the wavefront being a point source, coherent light requires you to take the relative phase of the point sources into account, so you get constructive and destructive interference.

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Very helpful and informative answer, thanks a lot! –  sh1ftst0rm Dec 4 '12 at 11:50

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