The max effect is when the dot is aimed at the source of the reflected light. Not sure why your polarizing filter is having an effect on dust on the window, unless the "dust" has specular reflections. It's possible I guess, and based on your description is must be illuminated from the side.
Actually, polarizer filters block 50% of unpolarized light (not exactly, but close enough for this discussion). For polarized light, the polarizing filter can block anywhere from 0% to 100% depending on the angle of the filter to the angle of the polarization (once again, not exactly but close enough for this discussion). So the amount of light that is blocked depends on how much of the light entering your camera is polarized and what is the angle of the filter to the angle of the light polarization.
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Good post, Mike. Thanks!
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