1922 Encyclopædia Britannica/Illuminating Engineering

ILLUMINATING ENGINEERING. The formation of the Illuminating Engineering Society in England in 1909 gave a great stimulus there to the study of illumination. A body with similar aims, had existed in the United States since 1906. Illuminating engineering societies have also been formed in Germany (1912) and in Japan (1918). These bodies include in their programme the study of illuminants, the influence of light on the eye, the measurement of light and illumination, and practical applications of light. A notable step has been the formation, on the proposal of the British Illuminating Engineering Society, of the International Illumination Commission, with national committees in all the chief countries. Work was in abeyance during the World War but has since been resumed. Agreement on a common unit of light (the &ldquo;International Candle&rdquo;) has been attained in Great Britain, France and the United States. In Germany the Hefner candle (equal to 0.9 &ldquo;international&rdquo; candle) is still used.

Researches have led to a more accurate knowledge of the influence of humidity and barometric pressure on the 10-c.p. Pentane standard (see Trotter, Haldane and Butterfield, Int. Phot. Comm., 1911; C. C. Paterson, Phys. Soc. Lond., June 1909; Crittenden and Taylor, Trans. Ilium. Eng. Soc. U.S.A., 1913; Takatsu and Tanaka, Electrot. Laby. of Dept. of Communications, Tokyo, 1917). Meantime the search for an absolute standard of light continues. Recent work by N. A. Allen (Phys. Soc. Lond., 1920), indicating under favourable conditions the maintenance of a constant current-density in the crater of the carbon arc, suggests the possibility of an arc-standard. At the Reichsanstalt in Charlottenburg experiments on the use of a dark hollow space, forming a &ldquo;black body,&rdquo; maintained at a constant temperature, have been conducted (Müller, Licht und Lampe, April 21 1921). Many instruments for measuring illumination, including forms suitable for direct measurement of brightness, are now available. Some recent types, intended for rapid and approximate work, enable values of illumination to be read off

directly by the inspection of a series of slots of graded brightness without manipulation to obtain balance. An instrument of this type was used to determine the illuminating power of parachute lights, flares, etc., during the World War; and other special apparatus for studying the decay of brightness of radium self-luminous material has been developed (Trotter, Illum. Eng., Nov. 1919). Daylight illumination has also been studied by apparatus relating illumination at any point in the room to the unrestricted illumination from the sky-hemisphere. The &ldquo;daylight-factor&rdquo; thus determined may be of the order of 0.25% in schools, and this has been suggested as a standard minimum of access of daylight (Illum. Eng., Jan., Feb., July 1914).

Greater experience has been gained of the practical operation of the integrating sphere photometer for measuring mean spherical candle-power or total flux of light in lumens. It is now recognized that, in view of the great variations in mode of distribution of light from modern illuminants, comparisons should be made in terms of total light emitted, or average candle-power, and not candle-power in one direction only. Accordingly the integrating sphere has assumed greater importance. Various methods of applying this apparatus to determine absolute coefficients of reflection of surfaces have been evolved (A. H. Taylor, Sci. Paper, Bureau of Standards, 1920; Trans. Ill. Eng. Soc. U.S.A., Dec. 1920). These researches assign to magnesium carbonate the high coefficient of reflection of 99%, and this has been proposed as a standard photometric surface.

Attention has also been devoted to physiological aspects of illumination, such as the avoidance of glare from bright unshaded lights in the range of vision, the effect of infra-red and ultra-violet rays, etc. Problems involved in the lighting of shops, schools, streets, factories, theatres, etc., have been discussed and in some cases recommendations issued by joint committees. Thus a joint committee representing the Illuminating Engineering Society, the Institutions of Gas and Electrical Engineers and the Institution of County and Municipal Engineers prepared a draft standard specification for street lighting, streets being classified in order of importance and appropriate minimum values of horizontal illumination specified in each class (see Trotter, Illum. Eng., May, June 1913). Reports of joint committees on school and library lighting (Illum. Eng., July 1913) and eyestrain in cinema theatres (June 1920) have also been issued.

There has been a growing recognition of the importance of adequate illumination in factories in the interests of health, safety and efficiency. In England, the Departmental (Home Office) Committee on Lighting in Factories and Workshops issued an interim report in 1915 (Blue Book Cd. 8000), recommending that there should be statutory power &ldquo;requiring adequate and suitable lighting in general terms in every part of a factory and workshop. . . .&rdquo; The report contained full data on industrial lighting and results of upwards of 4,000 measurements of illumination in factories, and ranks as one of the most important official documents on this subject. In subsequent years industrial lighting codes have been adopted by six of the American states. It has been suggested that ultimately an international code may be developed.

During the World War the Illuminating Engineering Society exerted its influence in favour of judicious lighting economies and scientific methods of darkening streets. An increase in street accidents was attributed partly to inequalities of lighting prevailing in the streets of London (Illum. Eng., Jan. 1917, p. 38). War conditions imposed a check on the development of spectacular lighting in England, but novel forms of illuminated signs, particularly those of the pictorial variety, have since been developed (E. C. Leachman, Illum. Eng., March 1921). The lighting of the Panama-Pacific Exposition, opened at San Francisco in 1915 (Gen. Electric. Review U.S.A., June 1915), was regarded as marking a new era in spectacular lighting, many striking colour effects being devised.

(J. S. D.)