LIGHTING OF THE SUBJECT ; DAYLIGHT ; ARTIFICIAL LIGHT 287. Daylight. Daylight is not only of a very variable intensity, according to the geographical position, the season, time of day, and the atmospheric conditions, but its composition also varies over very wide limits, although it is always conventionally called " white light." When referring to daylight, it is necessary to distinguish between direct sunlight and the light diffused by the blue or clouded sky.
On a clear day, the light consists almost exclusively of direct sunlight. A comparison of the intensity of illumination of a white surface exposed to direct sunlight and of a shadow thrown on this surface, in which case the shadow is illuminated only by the light diffused. from the sky, shows that diffused light repre sents only about 5 per cent of the total light. This proportion increases as the sky becomes more clouded, until, when it is completely overcast, there is nothing but diffused light. Setting aside the effect of contrast, shadows are better lighted as the sky becomes less clear, that is, as the diffused light increases. In fact, the luminosity of the sky increases with the cloudi ness and, in the case of large sunlit cumulus, can be as much as eight times the luminosity of ordinary blue sky. If, however, the sky becomes so cloudy that rain is imminent, then the luminosity diminishes, being approximately equal to that of blue sky.
288. Leaving aside for a moment the effect of atmospheric conditions, the intensity of sunlight depends essentially on the height of the sun above the horizon. Thus, in two widely separated places (a tropical and a temperate region), the light will have the same value at those times when the height of sun above the horizon is the same. 1 Thus, at Algiers on 15th January or 1st December, at 9.20 a.m. or 2.4.0 p.m. (local time), the intensity of sunlight will be the same as in Paris on the same days at 11.3o a.m. and at 12.30 p.m., or on 15th June at 6.2o a.m. and 5.4.0 p.m., for in all these cases the sun is above the horizon.
The variations in intensity of the different groups of radiations constituting sunlight are not proportional. The ratio of the intensity of the violet radiations (i.e. the active intensity for ordinary photographic plates) to the in tensity of the green radiations (i.e. the visual intensity) is much greater during the middle hours of the day than in either the morning or evening, when the sun becomes richer in red radiations as it sinks towards the horizon. While the visual intensity of sunlight is usually greater in the afternoon than in the morning, yet, at equal times before and after midday the actinic intensities are approximately equal. As already stated (§ 220) these disproportions are even more marked as regards the intensity of the infra-red radiations that are photographically At equal intensities, diffused light is always richer in violet radiations than direct sunlight, and the ratio increases as the sky becomes clearer. As the sun rises higher above the horizon, however, the composition of diffused light tends to approach that of sunlight.
The chief conclusion to be drawn from these considerations is that, owing to the dispropor tionality between the visual and actinic inten sities of light, any attempts to judge the actinic value merely from the visual intensity may be very misleading.
When considering, however, the action of light on orthochromatic plates instead of ordi rtry plates, this disproportionality is markedly less.
The intensity of the light from the moon is about rl000,000th of that from the sun, assuming the two bodies to be at the same height above the horizon.
289. In landscape photography, the only control the photographer has over the distribu tion of lighting is the choice of the most suitable moment for the exposure. This is done with clue regard to the angle of the direct sunlight (time of day), and to the best proportions of diffused and direct light (choice of atmospheric conditions).