CIE Colour Systems Questions and Answers

Answer: A

It includes lightness as well as chromaticity, which the two dimensional diagrams omit. Colour differences can be computed as distances within it. Lightness is included, unlike the flat diagrams. Differences become distances in three dimensions.

Answer: B

The a and b axes echo the opponent organisation of the visual system. Chroma and hue angle are the polar form of the same two coordinates. The axes echo the opponent channels. Chroma and hue are their polar equivalents.

Answer: C

Correct answer: the arctangent of b star divided by a star

\[ h_{ab} = \arctan\frac{b^*}{a^*}. \] The hue angle is the direction of the point in the a-b plane, measured anticlockwise from the positive a axis, and chroma is its distance from the origin. The quadrant must be resolved from the signs of both coordinates, which a plain arctangent does not do.

Answer: D

It is simply the distance from the neutral axis in the a b plane. A neutral grey has zero chroma whatever its lightness may be. Distance from the neutral axis is the definition. Neutral samples have zero chroma exactly.

Answer: A

Both are true and the reason gives the arithmetic, and hue angle is undefined there. The neutral axis runs vertically through the centre of the space. Hue angle is undefined on the axis itself. Both chromatic coordinates vanish there.

Answer: B

Correct answer: a value of fifty corresponds to a mid grey in appearance

\( L^* \) is a perceptual scale, so its midpoint is the grey that looks halfway between black and white - which reflects about 18 to 20 per cent, not 50. That gap between appearance and physics is the whole reason for the cube root transformation.

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. Lightness follows roughly the cube root of the luminance factor, and \( 0.18^{1/3} \approx 0.56 \), which places an 18 per cent card near the middle of the lightness scale. This is why photographers use such a card as a mid-tone reference despite its low reflectance.

Answer: D

The cube root relationship gives an L star near seventy six instead. Eighteen per cent reflectance is what corresponds to mid lightness. The cube root is what produces the discrepancy. Eighteen per cent gives mid lightness instead.

Answer: A

Tristimulus values are divided by those of the white before the transformation. Changing the white point changes every coordinate in the space. Normalisation by the white precedes the transform. Every coordinate depends on that choice.

Answer: B

Both are true and the reason gives the dependence, and D65 is a common but not universal choice. The white point must be stated alongside the values. D65 is common but by no means universal. The white point must be stated explicitly.