# RGB to wavelength related problem

Discussion in 'Lighting' started by Lord K., Nov 29, 2009.

1. ### Lord K.Guest

Hi,

I am aware that the "RGB to wavelength" question is widely covered on the
net, and I know that there is no way to perform such a conversion. But my
real need is very specific; maybe some good model can be used for
performing what I want.

My first goal was to perform some "black and white conversion" from a
color picture (I know that I am reinventing the wheel...). To be more
precise, I wanted to simulate some old films. I am aware of the
documentation that can be found on this topic, but here is what I did. I
had a look at the technical specifications of these old films. For
instance, you can find the well-known Tri-X at:
http://motion.kodak.com/US/en/motion/Products/Production/
Black_And_White_Films/7266/tech7266.htm
and the spectral sensitivity curve at:

At that point I searched on the net and discovered that the "rgb to
wavelength" conversion can't be achieved. I found some links where people
describe some more-or-less good "models", but of course it was too much
complicated for my real need.

Suddenly I noticed that my favorite film has a very specific curve:
It looks like the curve is made of two straight lines; we could even say
it is made of a single straight line over the visible spectrum.

For that reason, the classical way of converting the RGB color to black
and light with the channel mixer (by giving some coefficient to the red,
green, and blue) is probably not bad. But can't we do better?

Do you see a good way (probably not perfect I know) of getting the gray-
level corresponding to this "spectral sensitivity curve" from the RGB
values? Would it be a good idea, for instance, to get the CYM (cyan,
yellow, magenta) of the color in order to have SIX values (rather than 3)?

Assuming the curve is a straight line, could something be done by
converting the RGB to HSV and get the Hue which can be mapped to the
wavelength?

I would be very happy to have some "scientifical" filter for this film?

Best regards,

Lord K.