[Tig] The most out-of-gamut colour

Richard Kirk richard at filmlight.ltd.uk
Tue Jan 31 09:53:48 GMT 2017


Hi.

This comes off the 'Autochromes and Impressionism' topic. People used to 
print onto films with coloured bases, or imbibe dyes into the gelatin to 
give a colour 'mood' to the shots. Dying a film yellow or orange also 
made it very hard to duplicate. There is a peacock blue colour. People 
cannot get this colour on modern RGB displays. In a tiny special case, 
our gamut has actually got smaller since the 1920's.

I have a 2-pixel display. It uses a small filter-wheel projector (I took 
the filter wheel out) and a graded dichroic. This means the two pixels 
(either side of a circle) can have an arbitrary spectrum. The projector 
pixel is less than 2nm on the dichroic, but the dichroic spread is about 
7-8 nm. I experimented with matching the single wavelengths, and it 
pretty much agrees with the theory - if you are mixing with Rec-2020 
primaries, the most out of gamut colour is about 500nm, this blue-green 
colour. It lies off the blue-green edge of the gamut triangle, opposite 
the red corner.

How many wavelengths is enough? Adding a fourth primary at 500nm  would 
help a lot: you get all the blue-greens without losing saturation 
because your green  cross-couples with red. Beyond that, you might have 
near and far versions of red and blue: the far versions are more 
saturated but less efficient, and different people may see them 
differently.

There is even a good reason for adding a broadband white channel. A 
white mixed from three wavelengths will appear different colours to 
people with slightly different vision. However, when you get to six 
bands, you can probably mix a pretty good white from that.

Cheers
Richard Kirk

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