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Found this info on another interesting forum:
Color: you see purple not because the bulb is purple, but actually you are seeing dispersion. Dispersion is the effect that different wavelengths bend more or less depending on the index of refraction of the material, in this case the glass of a projector lens. Due to the optical power of most projector lenses, you see significant dispersion at the edges, where the blue light bends more (oddly enough the same reason the sky is blue.)
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