2. For a surface colour such as that of a
textile fabric
we can determine how much light is
reflected at each wavelength
throughout the visible region .
If we knew the tristimulus values for each wavelength, we could
calculate the tristimulus values for the sample.
The tristimulus values of any one wavelength are
the amounts of the three chosen primaries
required to match the light of the particular wavelength.
Obviously the amounts required depend on
the observer,
and results for an average (or ‘standard’ observer) are required.
3. Imagine a visual tristimulus
colorimeter similar to the arrangement
shown in Figure 3.1,
in which one half of the field of view
consists of a mixture of the [R], [G]
and [B] primaries,
while the colour in the other half is a
single wavelength.
To produce a match experimentally it
may well be necessary
to add some of [R], [G] or [B] to the
wavelength to be matched.
4. This is quite possible experimentally , and
the resultant tristimulus values for the
wavelength will include
at least one negative value.
Such experiments were carried out by
Wright
and by Guild.
They used somewhat different
techniques, and in particular different
primaries.
Both considered light of many
wavelengths
throughout the visible spectrum
and averaged results from a several
observers.
5. 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 for
an 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 (Figure
3.4).
6. 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 denoted
by x–, y– and z– and are always positive.
This ensures that tristimulus values for all real colours are always positive.
