A light source differs from an illuminant in that a source is
Answer: A
An illuminant is a table of relative spectral power that need not be realisable. Illuminant D65 is defined numerically, whereas a daylight simulator lamp is an attempt to realise it. One is a table of numbers and the other a lamp.
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Consider the following statements about sources and illuminants.
1. An illuminant is a defined spectral power distribution.
2. A source is a physical emitter that may approximate an illuminant.
3. Every CIE illuminant has an exact physical realisation.
Answer: B
The first two are correct and give the essential distinction. The third is not so, and the D series in particular has no exact physical realisation. Simulators approximate it to a graded standard. Grades run from A down to E in the standard.
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Assertion (A): Illuminant D65 has no exact physical realisation.
Reason (R): It is defined as a table of relative spectral power rather than as a lamp.
Answer: C
Both are true and the reason gives the reason, and daylight simulators approximate it imperfectly. How well a simulator matches is itself assessed by a defined metamerism index. Grades run from A down to E in the standard.
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A spectral power distribution describes
Answer: D
Correct answer: the relative power emitted at each wavelength across the spectrum
The spectral power distribution is the complete physical description of a source: how much power it puts out at every wavelength. Everything else - chromaticity, colour temperature, rendering index - is derived from it by weighting and integration, which is why it is the datum from which all colorimetry proceeds.
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A blackbody or Planckian radiator is
Answer: A
Correct answer: an idealised body whose emission depends only on its temperature
A blackbody absorbs all radiation falling on it and re-emits according to Planck’s law, so its whole spectrum is fixed by a single number, its temperature. That is what makes it a useful reference: any thermal source can be described by the blackbody temperature it most nearly resembles.
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Assertion (A): An incandescent lamp approximates a blackbody closely.
Reason (R): It emits by thermal radiation from a heated tungsten filament.
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 tungsten filament emits by being hot, which is precisely the blackbody mechanism, so its spectrum is a smooth continuum closely following Planck’s law. The departure comes only from tungsten’s emissivity varying somewhat with wavelength, which is why the match is close rather than exact.
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The colour temperature of a source is
Answer: C
Correct answer: the temperature of the blackbody whose colour it matches
Colour temperature is a description of appearance rather than of the lamp itself: it names the blackbody whose chromaticity the source shares. Note the inversion that catches people out - a higher colour temperature means a bluer, cooler-looking light, and a lower one a warmer, redder light.
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Consider the following statements about the Planckian locus.
1. It traces the chromaticities of blackbody radiators.
2. Sources near it appear white rather than strongly coloured.
3. Every practical light source lies exactly upon it.
Answer: D
Correct answer: 1 and 2 only
Statements 1 and 2 are correct: the locus is the path traced by blackbody chromaticities as the temperature varies, and sources near it appear acceptably white. Statement 3 is false and is why correlated colour temperature exists at all - most practical sources lie a little off the locus, and how far off must be reported separately.
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A colour temperature of about 2856 kelvin corresponds to which CIE standard illuminant?
Answer: A
Correct answer: Illuminant A
Illuminant A is defined as a Planckian radiator at 2856 K and represents tungsten filament lighting. It is the warm reference in the standard set, against which D65 at 6504 K and D50 at 5003 K represent phases of daylight.
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Match each CIE standard illuminant with what it represents.
List I
A. A
B. D65
C. D50
D. F11
List II
1. a warmer daylight used in graphic arts
2. a tungsten filament lamp near 2856 kelvin
3. a narrow band triphosphor fluorescent lamp
4. average daylight of about 6504 kelvin
Answer: B
Correct answer: A-2, B-4, C-1, D-3
The four cover the range of practical assessment conditions. A is tungsten at 2856 K; D65 is average daylight at about 6504 K and the textile standard; D50 is a warmer daylight at about 5003 K used in the graphic arts; and F11 is a narrow band triphosphor fluorescent, included because so much retail lighting is of that kind.
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