LIMITS OF LUMINOSITY IN PHOTOGRAPHIC SUBJECTS 15. Range between Extreme Luminosities in some Common Cases. There is a general ten dency to exaggerate very considerably the ratio of the extreme luminosities of the various sub jects which are photographed. Measurements which have been carried out, either indirectly by means of photographic plates (Hurter and Driffield, 1890) or by direct (visual) photo metric tests of points in a subject (Mees, 1914 ; Goldberg, 1919), have allowed us to assign nu merical values to the luminosities of various parts of photographic subjects, such as a land scape, an interior scene, a portrait, etc.
In a sunlit landscape, without any dense shadows in the foreground, the luminosity of the sky (comparable to that of a white paper receiving an illumination of about 16,000 lux (§ is) ) is not more than about 25 to 30 times that of the deepest shadows. The ratio of the extreme luminosities for certain subjects is indicated roughly in the following table _ The relatively low values of these ratios are due to two facts : firstly, that absolute blacks do not exist in Nature,' and, secondly, that, with the exception of polished objects, even the whitest ones reflect only a part of the light which they receive.
A mass of magnesia, or a block of chalk, the whitest substances that are known, reflect only about 88 per cent of the light which falls on them," even when the surface has been freshly scraped and made perfectly clean. For white paper this value falls to from 6o to 8o per cent according to the texture of the paper, its orienta tion, and the degree of purity. It is only 78 per cent for freshly-fallen snow and 5o per cent for a white-washed wall_ The blackest surface known, black silk velvet, reflects 0-4 per cent ; matt-blackened wood and matt-black cloth 2 to 3 per cent ; whilst black packing paper reflects up to Io per cent of the light falling on it.
In a landscape the ratio of the extreme lumin osities becomes less as the sky gets more covered. In full sunlight and with a very clear sky, the shadows are only illuminated by the diffused light from nearby objects which receive the sun's light directly. When the sky is clouded the whole of it acts roughly as a uniform illum inating surface, and there are no longer any shadows. Between these two extremes, the more intense the diffused light from the sky relative to that coming directly from the sun, the more are the shadows illuminated.
In a landscape, the ratio of the extreme luminosities is less for objects farther away. If the distant parts of a landscape are examined with a telescope (or even with a cardboard tube, so as to isolate part of the field of view), no heavy shadow can be observed ; diffused light from the atmosphere due to dust and water-vapour in suspension is superposed on the direct light from the object observed. At the farthest distance which can be seen in the direction of the horizon, no detail can be observed, all objects having the same luminosity as the sky, and becoming indistinguishable in a kind of bluish mist, called the atmospheric haze. Painters and draughtsmen make use of this fact (known to them as aerial perspective) when they wish to convey the impression of extreme distance.
z6. Sensitivity of the Eye. Thanks to the reflex movements of the pupil, which, by ex panding continuously in a dark place and contracting almost instantaneously in bright light (varying from about 2 to 8 mm. in dia meter), automatically regulates the quantity of light which falls on the retina, and owing to the adaptive power of the retina (246), the human eye can see objects of which the illuminations lie between some millionths of a lux and several million lux. Such extreme differences of lumin osity cannot, however, be perceived simul taneously. In full daylight, the minimum per ceptible is about 20,000 times more luminous than that perceptible during the night.
The presence in the field of view of an object which is brighter than those surrounding it, and especially of an actual source of light, results in a kind of dazzling of the eye, which diminishes its sensitivity considerably and produces fatigue. The less the intensity of the surrounding illumination to which the eye is adapted, the less may be the illumination of the object in order to give the sensation of dazzle.
The ease with which the eye adapts itself to very different illuminations usually results in feeble intensities being evaluated much too highly. At a very rough approximation the following table indicates the average relative values of the luminosity under different condi tions— x7. Perception of Details of Luminosity. In monocular vision (one eye only), we distinguish different objects, or different parts of the same object, only by their differences of colouring, or by the variation of luminosity when we pass from one to another. If one examines a land scape or any other object through a blue, green, or red filter of sufficient strength to destroy practically all differences of colour, then the details are perceived solely because of the variation of luminosity from point to point.
In a good light, which is neither too strong nor too weak, the eye can generally perceive the contrast between two adjacent surfaces when their illuminations differ by from i to 2 per cent (Nutting, 1914 ; Goldberg, 1919).
It should be noted that, as in acoustics, the smallest perceptible interval is determined by a ratio and not by a difference. The ease with which luminosity differences can be perceived becomes less as one of the surfaces becomes smaller, or when the surfaces compared have a coarser structure. Thus it is easy to perceive on a smooth wall a luminosity difference of 2 per cent, and yet it is difficult to see a difference of 5 per cent on a rough-cast wall, or on one of bricks. The sensitivity of the eye to luminosity differences becomes very much less both in a lighting strong enough to cause dazzle and in the case of poorly-lighted surfaces. In the shadows in a sunlit landscape, luminosity differ ences cannot be seen unless they are as much as 20 per cent, or 30 per cent, or even 5o per cent in the case of leaves or other masses of a very pronounced structure.