On a new standard of illumination and the measurement of light

Abstract

1. The present position of photometry as practically applied to electric lighting is by no means in a satisfactory condition. Mathe­matical accuracy in the measurement of the intensity of the light emitted by a given source is unobtainable owing to the numerous disturbing influences of quality and quantity which render impossible any graduated scale of light. An absolute standard of light has not yet been obtained. I have long felt that to meet the case of electric light illumination we must not depend upon any direct comparison between the light, emitted by the source to be measured and any given recognised standard of light; but that we should rather make our standard of comparison an area illuminated to a given intensity whatever be the source of light. We do not want to know so much the intensity of the light emitted by a lamp, as the intensity So of illumination of the surface of the book we are reading, or of the paper on which we are writing, or of the walls upon which we hang our pictures, or of the surface of the streets and of the pavements upon which the busy traffic of cities circulates. Illumi­nation of this character depends not on one source only, but on many sources of light distributed in innumerable ways. Hence, I propose to measure the illumination of surfaces quite independent of the sources of light by which they are illuminated. The difficulties in comparing the illumination of spaces have hitherto been insur­mountable, but now, thanks to the beautiful little glow lamps intro­duced by Mr. Swan, these difficulties have disappeared. The standard I propose is the space illuminated by 1 British standard candle at 12.7 inches distance, which is the same as the illumination given by 1 French standard “bec” on the same space at 1 metre distance.