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. ARTIFICIAL  LIGHT The   types   of   artificial   lighting   you   use   in photography  give  you  complete  control  over  the direction, quality, and strength of the light. You can move these light sources around, diffuse them, or reflect them. You can alter their intensity to suit the situation. There are two types of artificial light sources: spotlights  and  floodlights.  Spotlights   provide   a concentrated beam of light. Floodlights give diffused, softer, more even, spread out light. You can add to these two  basic  types  of  artificial  light  sources.  By  using lighting  accessories,  such  as  reflectors,  barn  doors, diffusers, and snoots, you can control the light to provide a variety of lighting effects. Unless special effects are wanted, artificial light sources that are different in color temperature or quality should not be mixed (used together). When you are viewing a scene, your eyes adapt so color differences between two or several light sources are minimized. Color film, however, cannot adapt and shows the color difference  in  parts  of  the  scene  illuminated  by  different light  sources. Tungsten-Filament   Lamps Tungsten light color films are made to be used with tungsten-filament light sources and are color balanced for 3200 K or 3400 K. Tungsten lamps, operated at their rated voltage, produce light of 3200 K and 3400 K. The color temperature of tungsten lamps changes with voltage  fluctuations,  decreasing  with  lower  voltage  and increasing with higher voltage. For example, the color temperature of a tungsten lamp rated for operation at 115 volts increases about 10 K for each 1 volt increase. Usually, a variation of less than 100 K has no adverse effect on the rendering of scene colors. However, a shift as low as 50 K can be noticeable on subjects with important neutral areas, such as white and light shades. When you are using tungsten lamps, the color temperature can shift, depending on the amount of power being drawn on the same circuit. When possible, you should avoid having other equipment on the same circuit. For these lamps to produce light of the correct color, they must be operated at exactly their rated voltage. When it is not possible to operate the lamps at their proper voltage appropriate filters can be used to correct the color of the light reaching the film. 1-8