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.
1. The Criterion of Homogeneous Radiant Energy
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 Table 3. 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 mμ in the extreme red (680 mμ), and 1.0 mμ for 588 mμ.
2. Standards of Spectral Energy Distribution
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,
