Magazine article American Cinematographer

Cinema Workshop

Magazine article American Cinematographer

Cinema Workshop

Article excerpt

Photographic Daylight

What is the color temperature of daylight? The answer would be simple if there was no atmosphere on our planet, but this might make breathing difficult. As the relatively pure and consistent sunlight passes through the atmosphere it undergoes some pretty strange transformations before it eventually reaches the earth's surface. These transformations depend on time of day, month of the year, weather conditions and latitude. Color temperature can vary anywhere from 5000°K to over 25,000°K. At this point the situation may seem mystically complex and totally unpredictable. Fear not. A few basic facts will shed some light on the subject.

The light coming from the sun is almost a perfectly continuous spectrum of about 5500°K to 6000°K color temperature before it reaches our atmosphere. Molecular and dust partides in the atmosphere act like a dichroic filter and scatter the short wavelengths (blues) while letting the longer wavelengths through (reds). Referring to FIGURE 1, this accounts for the direct sunlight appearing slightly red when compared to the blue sky (scattered light). What we call "photographic daylight" is actually a mixture of the direct sunlight (reddish) and the skylight (bluish). You can gather that the color quality of "daylight" will depend on the relative proportions of these two ingredients.

The direct sunlight is usually about 5000°K to 5500°K, while light from a clear blue sky (no direct sun) Is typically 12,000°K to 15,000°K. The resulting "overall" color temperature will obviously be somewhere between, depending on the ratio of these two elements. On a clear day with no clouds, the skylight will comprise only about 20% of the total illumination. Since the sunlight overpowers the blue sky fourto-one under these conditions, the resulting color temperature of the mixture should not be much greater than that of sunlight alone, and the table of FIGURE 2 bears this out. The color temperature of a clear day is typically 6100°K to 6500°K.

A totally overcast day will reduce direct sunlight by about 75% and cut skylight by only 50%. Under these conditions the skylight accounts for a much greater proportion of the total illumination. …

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