Simple mixing of the frequencies does not
result in the desired modulation. In AM Radio the signal frequency is first
mixed with the carrier frequency and the mixture is pushed through a
bandpass filter. In Optical AM, both the signal frequency,
and the carrier frequency are first passed through separate
bandpass filters, then both results are mixed to produce the modulation. The
Optical color filters are really bandpass filters as they allow a range of
wavelengths through, rather than a single wavelength of light.
On the right is what happens when you switch
the projector filters. You get an image composed of the original image's
complementary colors.