For a surface colour such as that of a textile fabric we can determine how much light isreflected at each wavelength throughout the visible region .If we knew the tristimulus values for each wavelength, we couldcalculate the tristimulus values for the sample.The tristimulus values of any one wavelength arethe amounts of the three chosen primariesrequired to match the light of the particular wavelength.Obviously the amounts required depend onthe observer,and results for an average (or ‘standard’ observer) are required.
Imagine a visual tristimuluscolorimeter similar to the arrangementshown in Figure 3.1,in which one half of the field of viewconsists of a mixture of the [R], [G]and [B] primaries,while the colour in the other half is asingle wavelength.To produce a match experimentally itmay well be necessaryto add some of [R], [G] or [B] to thewavelength to be matched.
This is quite possible experimentally , andthe resultant tristimulus values for thewavelength will include at least one negative value.Such experiments were carried out by Wrightand by Guild.They used somewhat differenttechniques, and in particular differentprimaries.Both considered light of manywavelengths throughout the visible spectrumand averaged results from a severalobservers.
The results differed from one observer to another (as expected), but when the results from the experiments were converted to a common set of primaries,the agreement was considered to be satisfactory.The results were expressed as the tristimulus values foran equal-energy spectrum,using primaries [R], [G] and [B],And the results were expressed as the amounts(called distribution coefficients) r–, g– and b–required to match one unit of energy of each wavelength throughout the visible region (Figure3.4).
Since [R], [G] and [B] were real primaries(actually 700, 546.1 and 435.8 nm respectively)some of the values were negative, as shown in Figure 3.4.As we will see later, the CIE adopted three unreal primaries [X], [Y] and [Z];the colourmatching functions in terms of these primaries are denotedby x–, y– and z– and are always positive.This ensures that tristimulus values for all real colours are always positive.