Three Lights Pretending to Be Every Colour - screens, eyes and cameras

The keyhole marked on that ruler, the narrow band your eyes can see, is all a screen has to fake. This lesson is how it fakes the whole keyhole with three lights.

Before you start. You need Your Remote Throws Invisible Light - the EM spectrum: light is one kind of wave on a long ruler, its wavelength decides its colour, and your eyes see only a narrow keyhole of it, roughly 380 to 700 nanometres. That lesson ended with a phone camera seeing a remote’s invisible beam as a purple flicker. This one explains why your eyes can’t and the camera can.

The idea in 30 seconds

Your eye has only three kinds of colour cones, so it reduces every light to three numbers and throws the rest away.
Two lights with different wavelengths that give the same three numbers look identical: metamerism, which is why red plus green looks yellow.
A screen is millions of red, green and blue sub-pixels, set to different brightnesses and too small to see apart, rebuilding the three numbers for each colour.
A camera sensor is colour-blind silicon under a mosaic of red, green and blue filters, Maxwell’s three photographs tiled.
Silicon sees into the infrared, so cameras carry an infrared-cut filter; it leaks enough for a remote’s beam to show up.
A lens bends light from each point of a scene back to one point on the sensor; focusing moves it by fractions of a millimetre.

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