# Precise Color Communication

Determining the color of an object was described previously. However, there is a difference when a color is created independently such as by a light bulb. This is called source color. The following is a simple explanation of the differences between object and source color.

There are three basic factors involved when a human observes the color of an object. They are illumination, the object, and the perception of the observer. However, when a source is observed, there are only two factors: the spectral distribution of the light source and the perception of the observer. The formulas for these concepts are illustrated below.
For object color, it is necessary to determine and evaluate the spectral distribution of the illuminant. This is because the color appears differently when the light source varies.
The illuminant is not required when light source color is measured because the color of the light source itself is what will be determined.
 where S(λ): Relative spectral power distribution of the illuminant , , : Color-matching functions in the XYZ color space R(λ): Spectral reflectance of specimen where S(λ): Relative spectral power distribution of the light source , , : Color-matching functions in the XYZ color space K: Normalizing factor (The tristimulus value Y is set to conform to the measured light quantity.) Use the following equation to determine the absolute value of the measurement light quantity when S(λ) is the absolute value of the spectral radiation intensity for the XYZ color space. K = 683 lm·w-1

The geometric conditions for illumination and optical reception must be considered because different conditions may result in different measurement values for the object color. Six types of conditions have been defined by the CIE and are described in Part III-1. These conditions do not affect measurements of light source color. However, there are certain angular characteristics in which the hue varies depending on the type of light source and the viewing angle, such as with LCDs. In these cases, the viewing angle must be fixed at a constant value.

There are several common methods to describe light source color numerically. They include xy coordinates, CIE 1960 UCS color intensity (u, v), the CIE 1976 UCS color intensity (u*, v*), and color temperature.* The L*u*v* color space (CIE LUV) is also used. However, a standard light source must be determined when using this color space for light-source color because the L*u*v* color space is based on the standard light source as the graph origin.

* Refer to the following page for information about light source color temperature.
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