Quote:
Originally Posted by pdurrant
Going through the filter twice does reduce the light a little compared to going through once, but it doesn't double. The main attenuation is by restricting the light to just one range of colour (red, green or blue). That only happens once.
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Even for the colours, the attenuation mathematically HAS to be doubled with reflection rather than single pass. That's not saying you lose half the light!
But you are right, the main attenuation is losing the light from most of the spectrum, to leave a narrow band of colour. If you have less saturated colour, you lose less light.
But the red on a red dot, green on a green dot and blue on a blue dot IS attenuated somewhat, and that is doubled for reflective rather than transmissive.
Triple sensor cameras (and maybe triple chip DLP projectors) avoid this extra loss by using three way dichroic splitter prismatic mirrors, no dye filter, though they may have filters to block UV or IR (interestingly most camera phones don't seem to bother, IR or UV invisible to the eye makes a bright dot, point your TV remote at phone, or UV invisible security marker lamp).