As we said in Part I-2, different light sources will make colors appear different. For measuring color, the CIE defined the spectral characteristics of several different types of typical illuminants. Figure 22 shows the spectral power distributions of some of these illuminants. A light source is usually built into the color-measuring instrument. This light source may or may not match any of the CIE illuminants; instead, the instrument determines the data for measurements under the selected illuminant through calculations based on the data actually measured under the instrument's light source and the illuminant's spectral distribution data stored in the instrument's memory.

Standard Illuminant Average daylight (including ultraviolet wavelength region) with a correlated color temperature of 6504K; should be used for measuring specimens which will be illuminated by daylight including ultraviolet radiation.
Standard Illuminant C Average daylight (not including ultraviolet wavelength region) with a correlated color temperature of 6774K; should be used for measuring specimens which will be illuminated by daylight in the visible wavelength range but not including ultraviolet radiation.
Standard Illuminant A Incandescent light with a correlated color temperature of 2856K; should be used for measuring specimens which will be illuminated by incandescent lamps.

F6: Cool white
F8: Daylight
F10: Three narrow band daylight white

F2: Cool white
F7: Daylight
F11: Three narrow band cool white

Let's look at examples of what happens if we measure our specimen (apple) using a spectrophotometer under Standard Illuminant (example 1) and Standard Illuminant A (example 2).
In example 1, is the graph of the spectral power distribution of Standard Illuminant and is a graph of the spectral reflectance of the apple. is the spectral power distribution of the light reflected from the specimen (apple) and is the product of and . In example 2, is the spectral power distribution of Standard Illuminant A and is the spectral reflectance of the specimen (apple), which is the same as in Example 1. is the spectral power distribution of the light reflected from the specimen (apple) and is the product of and . If we compare and , we notice that the light in the red region is much stronger in , meaning that the apple would appear much redder under Standard Illuminant A. This shows that the color of a subject changes according to the light under which it is viewed. A spectrophotometer actually measures the spectral reflectance of the specimen; the instrument can then calculate numerical color values in various color spaces using the spectral power distribution data for the selected illuminant and data for the color matching functions of the Standard Observer.