Figure 25.9 SPDs of blackbody radiators at several temperatures are graphed here. As the temperature increases, the absolute power increases and the peak of the spectral distribution shifts toward shorter wavelengths.

Relative power

1.5

5500 K

4500 K

0.5

4000 K

3500 K

0.0

Wavelength [nm]

Figure 25.10 SPDs of blackbody radiators, normalized to equal power at 555 nm, are graphed here. The dramatically different spectral character of blackbody radiators at different temperatures is evident.

Relative power

9300 K

1.5

6500 K

1.0

5500 K

5000 K

0.5

3200 K

0.0

Wavelength [nm]

The symbol for Kelvin is properly written K (with no degree sign).

To a colour scientist, it’s paradoxical that cold water faucets are colour-coded blue and hot water faucets are colour-coded red!

The 1960 [u, v] coordinates are described in the marginal note on page 281.

power increases and the spectral peak shifts toward shorter wavelengths. If the power of blackbody radiators is normalized at an arbitrary wavelength, dramatic differences in spectral character become evident, as illustrated in Figure 25.10 above.

Many sources of illumination have, at their core,

a heated object, so it is useful to characterize an illuminant by specifying the absolute temperature (in units of kelvin, K) of a blackbody radiator having the same hue.

The blackbody locus is the path traced in [x, y] coordinates as the temperature of a blackbody source is raised. At low temperature, the source appears red (“red hot”). When a viewer is adapted to a white reference of CIE D65, which I will describe in a moment, at about 2000 K, the source appears orange. Near 4000 K, it appears yellow; at about 6000 K, white. Above 10,000 K, it is blue hot.

Colour temperature

An illuminant may be characterized by a single colour temperature number – the temperature of a blackbody radiator that exactly matches the chromaticity of the source. If the match is approximate, the term correlated colour temperature (CCT) is used.

Colour temperature is sometimes augmented by a second number giving the closest distance in the deprecated CIE 1960 [u, v] coordinates of the colour

The CIE D illuminants are properly denoted with a two-digit subscript. CIE Illuminant D65 has a correlated colour temperature of about 6504 K.

Concerning 9300 K, see page 311.

from the blackbody locus – the arcane “minimum perceptible colour difference” (MPCD) units. I consider it more sensible to specify colour temperature in kelvin for intuitive purposes, accompanied by [x, y] or [u’, v’] chromaticity coordinates.

When a blackbody source’s temperature sweeps from a low value (say 1000 K) to a high value (say 20,000 K), the chromaticity coordinate of the source sweeps out a path called the blackbody locus in the chromaticity diagram. (See Figure 25.8, on page 275.) Such a plot distributes temperatures in a highly nonuniform manner.

White

As I mentioned a moment ago, there is no unique definition of white: To achieve accurate colour, you must specify the SPD or the chromaticity of white. In additive mixture, to be detailed on page 288, the white point is the set of tristimulus values (or the luminance and chromaticity coordinates) of the colour reproduced by equal contributions of the red, green, and blue primaries. The colour of white is a function of the ratio – or balance – of power among the primary components. (In subtractive reproduction, the colour of white is determined by the SPD of the illumination, multiplied by the SPD of the uncoloured media.)

It is sometimes convenient for purposes of calculation to define white as an SPD whose power is uniform throughout the visible spectrum. This white reference is known as the equal-energy illuminant, denoted CIE Illuminant E; its CIE [x, y] coordinates are [1⁄3, 1⁄3].

A more realistic reference, approximating daylight, has been numerically specified by the CIE as Illuminant D65. You should use this unless you have a good reason to use something else. The print industry commonly uses D50 and photography commonly uses D55; these represent compromises between the conditions of indoor (tungsten) and daylight viewing. Figure 25.11 shows the SPDs of several standard illuminants; chromaticity coordinates are given in Table 25.1.

Many computer displays and many consumer television receivers have a default colour temperature setting of 9300 K. That white reference contains too much blue to achieve acceptable image reproduction in Europe or