PHOTOMETRY is, as its name signifies, the measurement of light (Gr. /7.s, OcorOs, light, i.4rpov, a measure). Just as me trology includes the measurement of various related quantities, e.g., length, volume and density, so photometry includes the meas urement of luminous intensity (candle-power), luminous flux, illumination and brightness. It will be convenient first of all to consider the relationships between these four fundamental quan tities and the units in terms of which they are expressed, so that the methods of measurement adopted for each may be more readily understood. Every object which can be seen emits or re flects radiant energy in the form of aether waves which aie ca pable of affecting the retina of the human eye so as to produce the sensation of light. In the case of a self-luminous body (or source of light) such as the sun, a canole or an electric lamp, the light emitted is obtained by the transformation of some other form of energy, generally chemical or electrical. In the case of a body which is not self-luminous the light which it sends to the eye is derived originally from some self-luminous body, and is re flected by it in a manner de pending on the character of its surfaces.
The effect produced on the eye by the reception of a given amount of energy per second de pends on the wave-length (X) of the aether waves by which that energy is conveyed ; in fact only those waves for which X lies between about 400 and 75o millionths of a millimetre m,u can produce the sensation of light at all. As the wave-length changes from the shorter of these limits to the longer, the effect produced on the eye alters in two respects. In the first place the colour of the light changes from violet, through blue, green, yellow and orange to red. In the second place the intensity of the sensation produced by a given rate of influx of energy starts from zero at 400 m,u, rises to a maximum at X = 555 mp., and then gradually falls away again to zero at 75o inp,. The curve of fig. I shows the relative magnitudes of the visual effect produced by the reception of equal amounts of energy per second at different wave-lengths. This curve is called the lumi
nosity curve (sometimes the visibility curve) of radiant energy. It is naturally slightly different for each individual, but the curve shown is that now adopted internationally as representing the ef fect on the "average eye." The scale of ordinates at the left is an arbitrary one so arranged that the ordinate of the curve is unity at the wave-length of maximum luminosity. On this scale the ordinate corresponding to any wave-length is termed the "luminosity factor" of radiant energy at that wave-length.
Luminous Flux.—The light given by every source in common use is composite in character, i.e., the energy is conveyed by waves of various lengths, the distribution of energy among these wave-lengths (or the spectral energy-distribution) depending on the source. It is clear that the total effect produced on the eye by a given rate of emission of energy of known spectral distribu tion can be determined by weighting the energy content at each wave-length in accordance with the luminosity factor for that wave-length. The quantity obtained in this way represents, in fact, the efficacy of the energy for producing the sensation of light, and this quantity is termed luminous flux. The formal def inition of this term is : The rate of passage of radiant energy evalu ated according to the luminous sensation produced by it.
The unit in which luminous flux is expressed can be more sim ply defined later. It is sufficient here to state that it is called the lumen and that it has been found that for the average eye, one watt of radiant power at the wave-length of maximum luminosity (555 m,u) is equivalent to about 600 lumens. The scale of ordin ates at the right of fig. I gives the lumen equivalent of I watt of power at each wave-length. The watt equivalent of I lumen of flux at the wave-length of maximum visibility, viz., o.0016 watt per lumen, is often termed the mechanical equivalent of light.