Re: How does a colorimeter work?

Dear Baz,
By searching the MadSci Archives using the keywords "color" + "light" , 
you will find information about the relationship between wavelengths of 
electromagnetic radiation on one hand and color, as perceived by the human 
eye, on the other hand. A colorimeter determines the relative percentages 
of light absorbed at different wavelengths - just like a 
spectrophotometer. Older (or simpler) instruments use filters to divide up 
the visible wavelength range, more sophisticated ones work with gratings. 
In every case, photocells measure the intensity of light transmitted (viz. 
reflected) by the sample at a number of wavelength bands. The intensity is 
related  to the amount of a given colorant in (or on) the sample. The 
higher the colorant concentration, the lower the transmitted intensity 
(The first colorimeters were actually built to analyse concentrations of 
dissolved dyes). The data set obtained, together with the description of 
the type of light used (e.g. daylight or light bulb), is then used to 
compute "objective" measures of color. Various formulae are in use for 
this, e.g. CIE Lab values. A color is thus described by three coordinates. 
A bright yellow for instance has L=90, a=0, b=100. Color differences are 
computed using these coordinates by taking the "distance" like in ordinary 
Euclidean space (x,y,z). A distance ("delta E") of 2 units is usually just 
perceptible by the human eye. Where it starts to turn tricky is when you 
try to compare colors of sparkling lacquer with that of velvety tissue.
Best Regards
Werner Sieber