Light Sources and Illuminants Questions and Answers

Answer: C

The first two are correct and are standard maintenance practice. The third would allow the assessment condition to drift far outside specification long before failure. Scheduled replacement is the only reliable method.

Answer: D

Simultaneous contrast from a coloured surround would shift the apparent colour. The specified grey is itself defined by a reflectance value in the relevant standard. Contrast from a coloured surround would bias judgement.

Answer: A

One assesses the lamp and the other assesses the pair of samples. Confusing the two leads to blaming the lamp for a sample problem or the reverse. One grades the lamp and one the samples. Confusing them misdirects the investigation entirely.

Answer: B

The first two are correct and distinguish the two uses. The third confuses them, and each answers a quite different practical question. Confusing them misdirects the investigation. Each answers a different practical question.

Answer: C

Correct answer: the lighting under which the product will actually be used

A match exists under a stated illuminant, so the illuminant chosen should be the one the product will be seen under - daylight for an exterior finish, store lighting for goods on a shelf, and often both. Convenience, availability and observer preference are not reasons, and choosing the flattering illuminant is worse than any of them.

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. Selecting the illuminant that minimises the measured difference conceals the metameric failure rather than revealing it, and the customer will nonetheless see the product under whatever light they have. It amounts to choosing the test that the batch passes, which is not a test at all.

Answer: A

Correct answer: the electricity tariff paid by the laboratory

What must be specified is everything that changes the result: the illuminant or illuminants, the standard observer, and the geometry of illumination and viewing. The cost of running the lamps is a matter for the laboratory’s accounts and has no bearing on the measurement.

Answer: B

Correct answer: its full spectral power distribution

Everything else is derived from it. Correlated colour temperature reduces the spectrum to one number and cannot distinguish sources that look and render quite differently; efficacy says nothing about colour at all; and physical size affects the fixture rather than the colorimetry. Given the spectrum, every other quantity can be computed.