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 These standards consist in certain typical forms of stimuli to color, or in factors or functions contributory to such stimuli. Some of the standards considered below are primarily of theoretical or research interest, while others are essentially of technical significance only.

As defined in a preceding Section, the criterion of homogeneity in a stimulus, for the purposes of colorimetrics, must rest upon wave-length sensibility, and hence upon the facts which are summarized in. In general, in order to be considered homogeneous, a given sample of radiation must have a range of wave-lengths not greater than the threshold for wave-length in the given region (defined by its mid-wave-length). As seen from the Table, this varies widely for different parts of the spectrum, e.g., being 22 m&mu; in the extreme red (680 m&mu;), and 1.0 m&mu; for 588 m&mu;.

Under this caption are included curves and constants indicating the distributions of “intensity” (vide supra) in the physical spectrum of certain frequently encountered or critically important forms of heterogeneous radiant energy. These distributions are all at least approximately of the so-called “black body,” or Planckian type, i.e., they are determined by a general equation of the form,

$$E = \frac{c_1}{\lambda^5(e \frac{c_2}{\lambda \Tau}-1)}$$

where E is the energy per unit wave-length, &Tau; the absolute temperature of the source, e the base of the natural system of logarithms, the wave-length, and c; and c, are constants. When our concern is only with chromaticity, we need consider merely relative energies, and any convenient value may be adopted for c$1$, such as a value which makes the maximum of the function equal to unity. The value of c at present recommended is 14350 micron-degrees. This equation has been found to express very closely the energy distributions for the radiation