I recently came across this image: https://i.stack.imgur.com/fNGS4.jpg and I am having trouble understanding why this would work.
My understanding of the combination of a green and red color filter is: the green filter absorbs anything but green light, the red filter absorbs anything but red light so no light should be visible behind this combination of filters.
Obviously for this turn signal this is not the case, but why?
Best Answer
You are right that good red and green filters, like used in fluorescence spectroscopy, should let no ligth pass through.
But coloured plastic usually does not make a perfect narrowband filter: there is probably some spectral overlap between the pass-band of the green plastic and of the red plastic. As a result, a portion of yellow light will come out as turn signal.
Most of the light gets absorbed in either filter. It is energetically wasteful, but cool design features like this tend not to be technically optimum in general.