Colour Vision and Observer Questions and Answers

Answer: C

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. When the CIE constructed the XYZ system it chose the transformation so that one of the three functions would coincide with the \( V(\lambda) \) luminous efficiency curve. The Y tristimulus value therefore carries the luminance directly, and chromaticity and lightness can be handled separately.

Answer: D

Correct answer: adjust three primaries until they match a test light

The colour matching experiment is a null method: the observer adjusts the amounts of three primaries in one half of a bipartite field until it is indistinguishable from the test light in the other half. It asks for a judgement of identity rather than of appearance, which is what makes the results reproducible enough to build a system on.

Answer: A

The first two are correct and describe the method. The third describes a naming task, which is quite different and far less reproducible. Matching is a far more reliable task than naming. Reproducibility is what makes the method useful.

Answer: B

Correct answer: a primary must be added to the test field rather than the match

Some test colours are more saturated than any mixture of the three primaries can achieve, so the observer desaturates the test instead by adding one primary to its side of the field. That amount is recorded as negative, since algebraically it has been moved across the equation - and it is why the RGB matching functions dip below zero.

Answer: C

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 real primaries define a triangle in the chromaticity diagram, and additive mixture can only reach points inside it. The spectrum locus is convex and lies partly outside any such triangle, so the most saturated spectral colours are unreachable - the origin of the negative values in the matching data.

Answer: D

Choosing them outside the spectrum locus makes all the matching functions positive. The convenience was deliberate and the primaries were never intended to be realisable. Positive functions throughout was the objective.

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are the two design objectives the XYZ transformation was chosen to meet: all-positive matching functions, and luminance carried by Y alone. Statement 3 is the price paid - the X, Y and Z primaries lie outside the spectrum locus and are more saturated than any real light, so they are imaginary by construction.

Answer: B

Correct answer: two degrees

The 1931 observer was derived from matches made in a two degree bipartite field, which is about the angular size of a thumbnail at arm’s length and corresponds to the foveal region. This is why the 1964 ten degree observer was later added for the larger fields typical of practical colour assessment.

Answer: C

Larger fields involve retinal regions with a different receptor mix. Industrial samples are usually viewed at a size closer to ten degrees than to two. Field size changes the retinal region involved. Ten degrees suits most industrial samples better.

Answer: D

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 ten degree field extends well beyond the fovea, where the macular pigment is thinner and the S cone proportion differs, so the effective spectral sensitivity is not the same. Tristimulus values computed on the two observers differ measurably, particularly in the blue, which is why the observer must always be stated with a result.