The Measurement or Gauging of Low Values of Illumination and Brightness

Abstract

Present-day problems in illuminating engineering call for the measurement of very low values of both illumination and brightness. Such measurements are difficult, both on account of the reduced sensitivity of the eye to brightness differences and because of the operation of the Purkinje effect. The need is generally for measurements which will represent what is experienced under practical conditions, and so coloured light is measured by visual comparison with white and the results are termed values of “apparent illumination” or “apparent brightness.” The particular features required in illumination gauges and illuminometers suitable for measuring low values of illumination are described, with special reference to the appropriate British Standard Specifications. For certain purposes a portable standard of brightness is convenient, especially where the use of a gauge is awkward or impossible. In the study of luminescent materials a special type of brightness gauge is used, capable of measuring apparent brightness down to 0.0001 foot-lambert.