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I was wondering whether or not a slide in a slide projector acts as light diffuser? So when I have a light source that does not have a parallel beam on the slide can I expect that the other side of the slide is illuminated with a Lambertian of similar reflectance?

I am wondering this because I want to know if I have to raytrace the light path of a projector from the slide (assuming Lambertian reflectance of the slide) or from the light source.

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I'm not quite sure what you are asking. Are you trying to figure out where to locate the object plane when designing a projector lens? – Colin K Jul 25 '11 at 16:01
In fact the question is as follows: Do I have to raytrace from the light source through the lenses and slide to the screen, or can I assume that I can raytrace from the slide as starting point. So it the object plane the slide or is it the light source? – avanwieringen Jul 26 '11 at 7:07
Yes, I read the text of the question. In fact, you can ray trace from wherever you want, to wherever you want, and it doesn't matter whether the slide is a diffuser or not. So I was asking for clarification in order to help you. But instead I got snarked at. Thanks – Colin K Jul 26 '11 at 7:14
I didn't snark at you? I just wanted to elaborate on my question. But you already provided a clear answer for which I thank you. – avanwieringen Jul 26 '11 at 11:51
up vote 0 down vote accepted

The slide will not act as a diffuser, but there will often be a diffusing element upstream near or adjacent to the slide, so treating it as a diffuse source is probably a good approximation.

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