Light Sources and Illuminants Questions and Answers

Answer: C

Correct answer: average daylight including an ultraviolet component

D65 represents an average phase of daylight, sky included rather than direct sun alone, and it carries ultraviolet. That last point matters practically: fluorescent whitening agents are excited by ultraviolet, so a simulator deficient in it will make a brightened sample look quite different from how it looks outdoors.

Answer: D

Both are true and the reason gives the necessity, and omitting it would misrepresent brightened materials. Instruments must therefore control the ultraviolet content of their source. Brightened samples respond directly to it.

Answer: A

Equal power at every wavelength makes it convenient mathematically. It has no physical realisation and is not used for practical assessment. Its convenience is mathematical rather than practical. No lamp emits equally at every wavelength.

Answer: B

Illuminant A represents incandescent light rather than daylight. The other three are members of the D series derived from daylight measurements. Illuminant A is a tungsten representation. The D series came from real daylight measurement.

Answer: C

Most practical sources sit slightly off the locus and have no true colour temperature. The nearest point on the locus is used instead, measured along lines of constant perceived colour. Few real sources lie exactly on the locus.

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. Correlated colour temperature reports only the nearest point on the locus, so two sources equally distant on opposite sides - one greenish, one pinkish - carry the same figure while looking visibly different. This is exactly why the distance from the locus must be quoted alongside it.

Answer: A

Correct answer: how far the source sits from the Planckian locus

Duv is the signed distance from the locus, positive towards green and negative towards pink. Together with the correlated colour temperature it fixes the chromaticity, which the temperature alone cannot do - and a Duv large enough to be noticed makes a lamp look tinted however respectable its colour temperature.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 go together: correlated colour temperature names the nearest point on the locus, and because a whole line of chromaticities maps to that same point, the distance from the locus must be reported as well. Statement 3 is exactly the error the pairing exists to correct - one number cannot specify a two-dimensional chromaticity.

Answer: C

Correct answer: 2700 kelvin

The scale runs opposite to intuition: low colour temperatures are warm and reddish, high ones cool and bluish. So 2700 K is the warmest here, corresponding to domestic tungsten or a warm white lamp, while 6500 K is a cool daylight white.

Answer: D

Both are true and the reason gives the inversion, and the vocabulary is cultural rather than physical. It causes persistent confusion among people meeting it for the first time. Warm means low temperature in this usage.