The sources covered in this section include the more familiar man-made types, such as incandescent, carbon arc and AC arc discharge lamps as well as an exposition on natural daylight.
The general characteristics of each type are delineated in moderate detail, including spectral energy distributions and electrical characteristics. In addition, any special considerations for the cinematographer are carefully noted. Each sub-section on a particular class of light source closes with detailed information on filtering the source.
Natural daylight, on a clear day, is the sum of sunlight and skylight. The sunlight is directly from the sun, whose surface is about 6,000K. Skylight is from sunlight that has been scattered and filtered in the earth's atmosphere. Since the shortest wavelengths are the ones least filtered by the atmosphere, this results in the blue sky. Figure 11 represents the spectral energy distribution for the sun compared to a 5400K source.
Daylight conditions are highly varied, from a photographic viewpoint, based on the local atmospheric conditions, location on the earth, time of year, hour of the day and the amount of atmospheric pollutants that may be present. A brief summary of some of the possibilities is presented on page 319.
In addition to color temperature variations, the degree of diffusion in daylight varies from the least to the most diffuse lighting conditions that can be experienced.
Least Diffuse — In clear cloudless sunlight, the sun as the main lighting source (key) is truly a point. This produces the hardest, most distinct shadows. The incident light level from the sun on such a day can be as much as 9,500 footcandles. The skylight contribution (fill) is about 1,500 footcandles. This produces a lighting ratio of about 7:1 (key to fill).
Lighting control in these situations may require booster lighting or the use of certain grip devices such as large overhead scrims.
Most Diffuse — A completely overcast day is essentially shadowless lighting. The entire sky, horizon to horizon, becomes the light source. The incident level may be as low as 200 footcandles.
Figure 11. Similarity of sunlight to a theoretical 5400 K light source.
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