How to convert Wavelength(nm) to RGB values

Maybe you could map the wavelength to the HSV color model first, then convert it to RGB. I know it''s not exactly a linear mapping (I''m pretty sure at least), but it shouldnt be too hard to approximate it fairly well and it would make it more intuitive.

This is what you need to do:

first convert the spectral power distribution (SPD) to CIE XYZ tristimulus values, using the CIE colour matching curves. The process is described here.

Then, you can convert the CIE XYZ values to RGB by multiplying them by an appropriate colour transform matrix. The following one is the classical Rec.709 HDTV matrix, with D65 whitepoint:

R   [ 3.240479 -1.53715  -0.498535 ]  X
G = [-0.969256  1.875991  0.041556 ]* Y
B   [ 0.055648 -0.204043  1.057311 ]  Z
 

It''s a bit complicated to find the exact table values for the CIE curves on the net, if you don''t find them let me know, and I can post them.

/ Yann

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Or are the black/white cells fully saturated at usuall monitor intensities?

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Yep. AFAIK, the brain switches them off at normal intensities. They are only useful at night (that''s why you cannot see colours at very low intensity levels).