Light and Colour Theories, and their relation to light and colour standardization

by Joseph W. Lovibond

Constants.—The first requirement in establishing a scale of light and colour units is a means of co-relating visual sensations to a scale of physical colour constants, in order to secure a power of record and recovery. There is no natural scale available for quantitative colour work, but artificial scales can be constructed, and made constants by co-relation at different points with physical colour constants, and by cross-checking the intervals between these.

The scales used in the “tintometer” consist of red, yellow and blue glass, so graded in equivalents that combinations of equal units transmit colourless light. Full details of these have already been placed before the Society (see this Jour., 1887, p. 186, and 1908, p. 36).

Scale of Luminous Intensities. The Light Unit.—The natural terminals in a scale of luminous intensities are black and white, and the first question which arises is what is black, and what is white? as when used in a popular sense each term covers a wide range of differences.

In the author’s sense the term black means total absence of light, and the term white means a diffused daylight of given intensity, as reflected from a lime sulphate surface. In this sense black and white are the terminals of a scale of light intensities; the scale is divided into units and fractions of units. The unit itself is physiological, and is not in progressive accord with the mathematical light unit based on the inverse squares of distance.

The Black Unit.—Ideal black is practically obtained under daylight conditions by viewing a hole in a box with blackened interior, so arranged that no entering light can be reflected back to the vision.

The box used for this purpose is illustrated in [Fig. 3], and has one surface covered with standard white for the purpose of easy comparison with the pigments. The standard black aperture (1) is in the middle. The pigmentary blacks (2 to 10) are arranged over this, and the pigmentary whites and greys (11 to 20) underneath, each being numbered in accord with its intensity as tabulated.

Fig. 3.