QUANTITY OF LIGHT 12. Intensity and Brightness. The luminous power of the source is determined by its intensity and indirectly by its brightness.
The intensity is measured in decimal candles, or, for short, in candles, of perfectly definite intensity, which differs only slightly from that of the candles ordinarily used for lighting pur poses. As the intensity varies according to the direction of the light, the mean value is generally taken, unless the direction is exactly specified.
The apparent brightness of a source in a given direction is given by its luminous intensity, measured in candles, divided by the apparent area of the source in that direction.
The idea of brightness is of special interest in the case of light-sources used for projection purposes, for in these cases the efficiency depends almost entirely on the brightness.
For general lighting purposes it is usual to avoid using very bright sources of illumina tion. For equal intensity a low-brightness source having a large surface gives a more diffused light which produces less sharply-defined shadows.
When we come to the question of the lighting of subjects to be photographed, the considerable differences in the composition of the light given by different sources, and especially the very variable proportion of the radiations which affect ordinary sensitive emulsions, make practically worthless any comparisons which are based only on the values of visual intensity.
13. Illumination. The illumination of a sur face is measured in lux or candle-metres, and is that produced on a screen, normal to the direc tion of the rays and at one metre from a point source having an intensity of one candle.
Using a given surface, it can be shown that the same illumination is obtained on that surface by placing a point source of one candle at one metre, or a source of four candles at two metres, nine candles at three metres, and so on. This
fact is always expressed by the law which states that the illumination is inversely proportional to the square' of the distance. We shall have occasion to refer to this law 2 in dealing with times of exposure and printing. In practice, this law may, however, be applied with sufficient accuracy for practical purposes in all cases where the dimensions of the source are only a small fraction of the distance from the source to the illuminated surface.' When a large surface is illuminated by a source having small dimensions, the illumination of the surface falls off very rapidly on departing from the point of maximum brightness, at which the rays emitted by the source are incident normally. This decrease is still more rapid if, instead of having a radiation approximately symmetrical in all directions relative to the source, the source radiates mainly in one special direction, as is chiefly the case with an illumin ated plane surface made incandescent.
14. Quantity of Light or The quantity of light received by a surface of unit area, or the exposure received by this surface, is the product of the illumination of the surface and the time of illumination. The unit of exposure is the lumen-second or candle-metre second, viz, the quantity of light received in one second by a surface of one square centimetre, exposed to a source of one candle at a distance of one metre. We shall see later that in the majority of photographic processes, equal ex posures do not produce equal effects if the two factors of the exposure, the illumination and the time, are not the same.
A diffusing surface (not polished) is in a way a source of light, so that a brightness can be assigned to it. For practical purposes this brightness can be considered as proportional to the illumination.