1. the phenomenon of
two colours
That match
under one set of conditions,
but fail to match
under a different set.

2. The two lights must be physically different.
With coloured lights,
a white light could be a mixture of approximately
equal amounts of all wavelengths
and could be matched by a
mixture of two complementary wavelengths.

3. With surface colours,
a sample dyed with a given set of dyes could be
matched,
under certain conditions,
with a sample dyed with different dyes.
In each case the match would be physically
different, and the match would fail to hold when the
conditions changed.
For the white light a match for one observer would
probably not be accepted as a match by a second
observer.

4. For the dyed samples, the match would
probably not hold if the light source were to be
changed;
the match might be satisfactory in daylight,
for example,
but very poor under fluorescent light.
Again, a good match for
one observer might be perceived
as unsatisfactory by a second.

5. Suppose we have two objects whose
tristimulus values
(for a specified standard illuminant and
standard observer) are identical.
The two objects illuminated by
the specified source
and viewed by the specified standard observer
would look identical, yet
their reflectance curves might differ
significantly.

6. For example,
one object could have constant R values of, say, 30% at all wavelengths
(and would look grey under all normal conditions),
while the other object could be a piece of nylon dyed to match the
first using a mixture of yellow, red and blue dyes.
We can generally produce any particular grey colour using
such a mixture of dyes,
but it is likely that the R values will vary with wavelength.
(The lowest R values would probably be found at the wavelengths
of maximum absorption for each dye, say 450, 520 and 620 nm
respectively.)

7. We can match most coloured objects
(under specified conditions) using
a mixture of three suitable dyes,
but the reflectance curve of the dyed fabric will not be the same as
that of the object to be matched.
In these cases the dyeing will normally not match the target colour if the
conditions are changed, and the two samples are said to be metameric,
or to form a metameric pair.
In terms of colour measurements, the two samples
have the same tristimulus values
for a specific combination of illuminant and observer,
 but the reflectance curves are different.
Usually such pairs of reflectance curves cross at least three times.

8. In calculating the tristimulus values
the effects of higher R values
for the dyeing at some wavelengths
 must be balanced
by the effects of lower R values at other wavelengths.
If the illuminant used in the calculation is changed,
not surprisingly The tristimulus values will usually change
as well.
This is also true if the observer is changed.

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