A metameric pair is defined as two samples that
Answer: A
The tristimulus values coincide although the underlying spectra do not. Any change of illuminant or observer can therefore break the match. The tristimulus values coincide but the spectra do not. That coincidence is what makes the match fragile.
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Assertion (A): A metameric match is inherently fragile.
Reason (R): It depends on a coincidence of integration rather than on identical spectra.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. A metameric match works because two different reflectance curves, weighted by the illuminant and the observer functions and integrated, happen to give the same three numbers. Change either weighting and the coincidence is gone, because nothing about the underlying spectra was ever the same.
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Consider the following statements about metamerism.
1. The two samples have different spectral reflectance curves.
2. They give identical tristimulus values under one condition.
3. They will match under every illuminant and observer.
Answer: C
Correct answer: 1 and 2 only
Statements 1 and 2 are the definition: different spectral curves, identical tristimulus values under one specified condition. Statement 3 describes the opposite case - a pair matching under every illuminant and observer has the same reflectance at every wavelength, which is an unconditional or spectral match and by definition not metameric.
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Metamerism is possible at all because
Answer: D
The reduction from a continuous spectrum to three numbers discards information. Different spectra that integrate identically are consequently inevitable. Trichromacy makes the phenomenon unavoidable. No instrument can remove it from the world.
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Assertion (A): Metamerism follows directly from trichromacy.
Reason (R): Three receptor types reduce a whole spectrum to three numbers.
Answer: A
Both are true and the reason gives the origin, and no amount of instrumentation removes it. An observer with more receptor types would find fewer metameric pairs matching. More receptors would discard less information.
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A pair described as metameric without further qualification is normally understood to be
Answer: B
Correct answer: metameric with respect to a change of illuminant
Metamerism can be with respect to the illuminant, the observer, the field size or the geometry, but the unqualified word conventionally means the first. It is the kind encountered daily, and it is the kind the special index of metamerism was defined to quantify. Where another kind is meant, it is named.
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Illuminant metamerism is the most commonly encountered kind because
Answer: C
Correct answer: goods are routinely seen under several different light sources
A garment is chosen under shop lighting, examined at the window in daylight and worn under a tungsten lamp in the evening, so the illuminant changes constantly in ordinary use while the observer usually does not. The exposure to change is what makes illuminant metamerism the practical problem.
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Two metameric samples must have spectral reflectance curves that
Answer: D
Three crossovers are the minimum for the three integrals to agree. Fewer crossings would leave one of the tristimulus values unequal. Three integrals must agree simultaneously. Fewer crossings cannot satisfy all three.
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The minimum number of crossover points between the reflectance curves of a metameric pair is
Answer: A
Correct answer: three
For the two curves to give the same value on all three integrals, the areas under the difference curve weighted by each of the three observer functions must each vanish. A difference curve that changes sign only twice cannot make all three integrals zero simultaneously, so at least three crossovers are needed. Real metameric pairs usually show three or four.
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Assertion (A): A pair whose curves cross only twice cannot be metameric.
Reason (R): Three crossings are required for all three tristimulus integrals to agree.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. Three integrals must each come to zero over the difference between the curves, and a difference that changes sign only twice cannot satisfy all three at once. Three crossings is therefore the minimum, and it follows from the arithmetic rather than from observation.
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