So its not completely clear what you are trying to do, but its basically either subtractive color mixing or additive color mixing, (google those terms). One thing that seem to be missing from your description is what kind of light source is used, this is vital for all "things color".

I understand the subtractive and additive colour mixing, but it isn't as simple as just adding/subtracting when combining multiple colour values?

It depends on what you mean by combining, if you for instance have several layers of material that light passes through, then the absorbance spectra needs to be multiplied wavelength by wavelength. That will give you the combined spectrum. And then you will have an idea of the color, but you still need a light source to get a meaningful color value.

Hi,

 As a first step, you basically need to calculate how much the spectrum you measured excites the 3 human photoreceptors. In principle, you need the spectral sensitivities of the 3 human photoreceptors, L, M, and S (long-, middle, and short-wavelength). However, depending on what you want to do with your colour specifications, you can also directly calculate the Tristimulus Values (XYZ). XYZ allow you to directly calculate chromaticity coordinates (xyY), which are widely used to communicate colour specifications.

If you want to obtain LMS values you need to get spectral sensitivities (cone fundamentals). If you want to get XYZ you need colour-matching functions (CMF). The CMF are – in principle - a linear transformation of LMS for the purpose that one dimension (Y) corresponds to luminance (i.e. low-level lightness). For further details see e.g. here http://cvrl.ioo.ucl.ac.uk/database/text/intros/introcones_cmfs.htm. You will find cone fundamentals and CMF here http://cvrl.ioo.ucl.ac.uk/cmfs.htm and here http://cvrl.ioo.ucl.ac.uk/cones.htm.

Both consist of 3 columns with the contribution of each wavelength to the cone excitation (LMS) or XYZ, respectively. You just need to calculate the sum over all wavelengths (i.e. the integral) to obtain how much your spectrum excites each of the cones / each of XYZ. This can be done by an inner matrix product (= sum of products). So, in principle you need to do this:

SPECTRA(:,1:3)'*CMF(:,1:3)

where the rows of each matrix (SPECTRA & CMF) are the weights for each wavelength.

 However, you also need to make sure that your spectrum has data for the same wavelengths as the CMF/cone fundamental. If not, you will need to interpolate your spectrum for the respective wavelengths of the CMFs (either linearly or by cubic spline). Moreover, in order to get standardized XYZ you also need to multiply by 683 (=maximum photopic luminous efficacy) and by your wavelength resolution (the wavelength-interval between each data point, e.g. 300:5:800nm à interval = 5nm). Once your CMF and spectrum are matched for wavelengths, the complete formula looks like that:

XYZ = SPECTRA(:,1:3)'*CMF(:,1:3)*683*Interval;

Then you obtain xyY by normalization (x = X./(X+Y+Z); y = Y./(X+Y+Z); Y=Y).

A final complication: All I said above requires your spectra being specified in spectral radiance (w/sr/m2), as measured by a spectroradiometer. However, if you measured photon catches by a spectrophotometer you first need to calculate spectral radiance in order to get XYZ.

All the best,

Christoph

Hi,

You are based in Leeds. Prof. Luo and Prof. Westland have lectures covering these topics. They might be able to share computer code for your specific application.

You can find the conversion equations in the following books:

Roy S. Berns, Billmeyer and Saltzman's Principles of Color Technology, Third Edition, Wiley-Interscience, New York, 2000. ISBN 0-471-19459-X

R.W.G. Hunt, Measuring Colour, Third edition, Fountain Press, Kingston-upon-Thames, England, 1998, ISBN 0-86343-387-1

Best regards,

Patrick

Thank you so much Cristoph and Patrick, both very helpful answers!! I have lots to be getting on with!

Warm regards,

Catriona

Hi,

following on from the above responses, you can find a very useful toolbox for Matlab at http://es.mathworks.com/matlabcentral/fileexchange/40640-computational-colour-science-using-matlab-2e (from the book "Computational Colour Science using MATLAB 2e" by Stephen Westland).

Best,

Juan Luis

Thanks Juan! Again, very helpful!

Kind regards,

Catriona

I thinks the above answers are enough and you study in Leeds, so Prof. Luo is Nr.1 about it. Please find and read his lectures. However, some simple things can be seen and applied quickly. 

1- your spectra can be converted into chromaticity xyY, uv or u'v' or U*V*W* or a*b*L* or a'b'J' in different color spaces.

2- From your chromaticity, you can convert it into RGB based on various RGB working spaces. See here, http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html. It depends on your monitor, display or your mixed lights.

3- Herewith, you should understand about white standard and CMC Adaptation. Read it, http://onlinelibrary.wiley.com/doi/10.1002/col.10005/abstract

4- If you want to mix RGB values to have new ones. I think you should convert from your RGB-values into chromaticity XYZ. Then apply a color mixing law to have mixed XYZ. Then convert back new RGB values. If you want to ask me about it and I understood you correctly, I will supply more about the mixing law. 

Hi again,

I was wondering whether some of my earlier answers to questions could be interesting:

https://www.researchgate.net/post/What_are_the_fundamental_differences_between_color_space_and_color_model

https://www.researchgate.net/post/How_can_we_calculate_CIE_color_coordination_to_find_out_the_color_of_a_compound

 All the best,

Christoph

You may want to look at:

Preprint A Model of Color Vision for Remote Sensing 1