On 04/10/2019 11:19, Clive Arthur wrote:
On 03/10/2019 17:03, Max Demian wrote:

snip

You can have red, green and blue LEDs to make up white...

They make something which looks white for many or maybe most purposes,
but they don't make white.* They don't make anything above blue, so no
violet, and nothing below 'normal' red, so no deep red, and there are
significant gaps between the colours.

It only really matters if you are doing precise colour matching. Colour
temperature of the incident natural light matters almost as much!

You have to have a material with very peculiar absorbtion band features
before it makes any appreciable difference. A handful of minerals like
Alexandrite and synthetics line didymium doped glass have an apparent
colour to the human eye which depends critically on light quality.

https://en.wikipedia.org/wiki/Chrysoberyl#Alexandrite
https://en.wikipedia.org/wiki/Didymium

Although they don't output violet the mix of blue and red light is good
enough. The eyes sensors are actually blue, green and yellow sensors.
Red is computed in the brain as Y-G which is why didymium doped glass
gives such an interesting effect with out of gamut cartoon colours. It
puts a sharp notch into the yellow wavelengths perfect for glassblowing.

--
Regards,
Martin Brown