Use a surveillence camera. Same principle as optical mouse, just use a black
& white camera.
Much easier for a tiny digital camera to 'see' different brightnesses of red
(as in optical mouse) than to differentiate between colours and
brightnesses, etc. So use a black and white cam, nice bright red or infrared
lighting for it to "see" better, stick a filter over the camera and it's
even easier, as it's only picking up different shades, not diff colours &
brightness/saturation/whatever for each colour. Just red, shades from black
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