The measurement of the intensity of a source of light is, it is well known, a somewhat unsatisfactory process. The eye cannot estimate light intensity; it can only tell when the illumination of two adjacent surfaces is equal. If, for example, we desire to measure the intensity of a metal filament lamp, we compare it with a Hefner lamp and say that the intensities are inversely as the squares of the distances from the photometer head, when equal illumination is obtained. In strictness, however, this method is applicable only when the colours of the two sources, or more accurately when the distribution of energy in the spectra of the two sources, is exactly the same; for the relative luminosity of the different colours of a spectrum varies with the intensity of that spectrum. Abney has two well-known curves illustratinghis. One, which represents the relative luminosity of the different colours of a spectrum at ordinary intensity, has a maximum in the orange; the other which is for a spectrum with the same distribution of energy, but with an intensity of less than 1/100 candle-foot, has its maximum in the green. If, therefore, we have an extremely long photometer bench, and an experimenter with normal colour vision compares the intensities of the metal filament lamp and the Hefner lamp, at first placing the Hefner lamp one foot from the photometer head and afterwards placing it more than 100 ft. from the latter, he should not obtain the same result both times. In the first case, owing to the reddish tint of the Hefner lamp, the intensity of the metal filament lamp should appear less. If, again, a second observer, whose colour vision is slightly abnormal, compares the lamps at the first distance, he gets a third result. Of course the difficulty does not arise in practice, because the sources to be compared have usually the same colour and the illumination of the field of the photometer does not vary over a wide range. Still, a standard unit of light should meet all conceivable cases, and we are at present unable to state satisfactorily in terms of our standards, once for all, the candle power of, for example, a mercury vapour lamp. In order to be definite we must specify, first of all, normal colour vision on the part of the observer, and then we must state the illumination of the fields he compares. It is, of course, the Purkinje effect, the change from rod to cone vision, that causes all this trouble. And it is precisely within the range of illumination in common use, 1 to 100 metre-candles, that this change from rod to cone vision takes place.
On the absolute measurement of light: A proposal for an ultimate light standard
