In the beginning of photography, daylight, or sunlight, was the only light source suitable for exposing the slow film available at that time. Today, photographic film is not only vastly more sensitive to light, but a wide range of light sources have been developed for the needs of the photographer. These light sources include the following: tungsten lamps, tungsten-halogen lamps, fluorescent lamps, and electronic flash.

DAYLIGHT

Sunlight, of course, is the light photographers are most familiar with and for good reason. It is the light they use the most. Naturally, sunlight is the only practical light source for general outdoor photography. Artificial light sources, however, can provide useful supplementary lighting to sunlight as fill-in for shadows (to make them lighter) and take the place of sunlight entirely for photography of small areas and close-ups. Sunlight is often referred to as daylight. The term daylight, as used in photography, is meant to include all

Figure 1-11. Light, plane polarized by reflection.

Figure 1-12. Effects of sunlight passing through the atmosphere.

forms of light, direct or indirect, that originate from the sun.

Of importance to the photographer is the effect of the atmosphere on sunlight and the amount of atmosphere through which sunlight passes (fig. 1-12).

The shorter wavelengths of light (violet and blue) are scattered by the atmosphere much more than the longer wavelengths. The color composition of sunlight becomes increasingly deficient in blue the further the light has to travel through the atmosphere (early morning and late afternoon). As the sunlight becomes more deficient in blue, it appears more yellow. The amount of scattering also depends on the condition of the atmosphere. When the atmosphere is clean (has little moisture or fine dust in it), there is less scattering than when the atmosphere is hazy or dirty (having a good deal of moisture or tine dust and smoke). The variation in color of sunlight can be expressed as color temperature. Sunlight coming from overhead on a clear day has a color temperature of about 5400 K. Just after sunrise and just before sunset, the color temperature ranges between 2000 K and 4000 K. Not only is the color of sunlight different early in the morning and late in the afternoon, but the intensity is also less. These arc important considerations when taking pictures at these times of day.

Light scattered by the atmosphere, or skylight as it is called, can be regarded as a second source of light. Skylight is different than sunlight because it is caused chiefly from the scattering of the shorter wavelengths. It therefore appears more blue than sunlight. Skylight on a clear day may be as high as 60000 K.