Abstract :
This paper briefly surveys the progress which has been made since the beginning of the century in the methods of deriving and maintaining the unit of light. This work has led to the development of the primary standard of light and the ¿New Candle¿ unit now to be called the Candela. It is shown that the specification by the International Commission on Illumination of the ¿Standard Observer¿ in 1931 has not only facilitated visual photometry and colorimetry, but is the foundation on which physical photometry and colorimetry has been developed. The types of photocells used and the techniques which have ultimately enabled the C.I.E. Standard Observer to be realized in practice, first in photoelectric photometers and later in colorimeters, are described, together with other examples of special forms of photoelectric photometers. The development of electric discharge lamps, particularly the tubular fluorescent types, has led to a requirement for techniques for measuring and specifying not only their colour but also their colourrendering properties. Photoelectric methods have been developed for measuring the colour-rendering properties in terms of the percentage luminance in eight spectral bands, coupled with the chromaticity co-ordinates. These methods have been used by British lamp makers for some years, and in July, 1948, the eight spectral bands were adopted by the C.I.E., with a recommendation that this matter should be further studied.
