Daylight

Daylight consists of directly incident sunlight and light scattered in the atmosphere, skylight. The mixture of both components derermines the color of illumination which varies depending on time of day and the time of year from 4800 K - 18 000 K.

By using conversion filters, the composition of light can be changed in such a way that daylight film which is exactly balanced at 5500 K renders natural colors. The decision which filter to take in each case is however not as difficult as is generally thought. The two most important criteria are altitude of location and the color of the sky. As altitude increases above sea-level, the proportion of short-wave blue radiation in illumination increases also. It has to be balanced by a reddish filter. In areas where the air is pure, that is, far away from urban centres and industrial plant, illumination follows this tendency. As the atmosphere becomes more clouded, the color temperature of radiation falls, illumination is seen as reddish by the film. Industrial and city haze has the greatest effect on light composition even with a clear-blue sky. With a great amount of haze, when the sky is from dull blue to yellowish-white, a light blue KB 1.5 correction filter should be used. A point of reference for determining illumination is if there are clouds present. If there are few clouds, clearly outlined against a blue sky, a reddish conversation filter is abvisable. If clouds cover more than half the area of sky, a colorless UV haze filter is more to advantage. If the clouds are hardly visible in the haze, noticeably reducing sunlight and preventing the creation of sharp shadow, then it is clear that a light blue filter is required.

Sunlight

The sun radiates at a color temperature of 5800 K. Daylight film is balanced to this temperature. As observers, we always have the impression that sunlight is yellow or gold. In reality, it is more like the flame from an electric welder. The eye is fooled; influenced by the blue of the sky surrounding the sun, the eye assigns the sun a warmer tone than it really has, since it does not have the ability to measure exactly. On the other hand, the eye is superb at comparison, and can adapt to every color temperature which film cannot do.

On its way through the atmosphere, the radiation of the sun is altered in many ways. Ultraviolet radiation below 180 nm cannot penetrate the atmosphere at all. In the infrared range, radiation is weakened by water vapor and absorbed to a greater or less extent depending on wave-length. But the most important change sunlight undergoes is caused by the molecules of the air, see Skylight. A direct shot of the sun is possible using very high denity gray filters which only transmit a ten thousandth of incident light.

Skylight

Sunlight undergoes changes on its way through the atmosphere, changes of intensity as well as changes in composition. Light is despersed even by the gas molecules of the air. Air molecues are a considerable obstacle to blue light, while red oscillations, larger in dimension, are hardly affected by the oxygen and nitrogen in the air.

Thus, with clear skies, red light reaches the ground with relative ease, while short-wave blue, violet and ultraviolet radiation is "scattered" over the whole sky. In this way, we get our radiant blue skies in cloudless weather, having a temperature of 8000 K to 20,000 K. If one relates dispersion of red light to dispersion of blue light, one may conclude that blue light is dispersed 20 times more than red light. The explanation for the sharpness of long-distance shots with red filters in black-and-white photography is to be found in the fact that red light penetrates the atmosphere almost unimpeded, see Rayleigh distribution.

Moonlight

This light is reflected sunlight striking the moon's surface. It has a color temperature of approximately 4000 K and therefore has a warmer color tone than sunlight.

However, at full moon, illumination is so weak that the exposure time is 100,000 times as long as for a sunny summer landscape. With the lens aperture fully open ( 1:2 ) and with a 200 ASA film, the scene requires an exposure time of between 5 and 10 minutes. For color shots, there is the extra difficulty of achieving the correct color rendition. Color illumination and the sensitivity of the eye do not agree at all for night or dusk vision. Daylight film "sees" red, the eye sees blue. If one desires to make shots in such light conditions, it is best to use artificial light film. It is not necessarily possible to use a blue conversion filter for such shots as light loss must be avoided.

Candelight

Open candelight and kerosene lamps have a color temperature of about 2000 K. Thus their light is very red. There is a color temperature difference of 34 decamireds from daylight and of 22 decamireds from artificial light.