The spectral composition of light refers to how much of each wavelength of the visible spectrum, which ranges from 380 nm to 770 nm, a particular light source emits. Figures 4.7 and 4.8 provide a comparison of the spectral compositions of a standard 'cool white' fluorescent light source and daylight. Spectral quality is a complex term that mainly refers to how warm or cool a light appears (correlated color temperature of light, CCT) and the shift of color (Color Rendering Index, CRI) that it may cause. The CCT scale is a color-defining scale developed by William Kelvin in the late 1800s. It indicates the specific hue of a light source. The Color Rendering Index relies on a scale from 0 to 100 that represents how closely a light source depicts or reflects an object's true color. As a general rule,
the higher the CRI, the more accurate the color of an object will appear. Because daylight is dynamic, its color properties change. For example, a cloudy day would have a CCT ranging between 8000 and 10 000 K whereas a clear northern sky at noon would have a CCT ranging from 5000 to 5500 K. In terms of its spectral composition, daylight contains 4.6% UV radiation, 46.4% visible light, and 49% infrared radiation.
The benefits of daylight in terms of its spectral qualities can best be demonstrated by the outcome of research not only on daylight but also on full spectrum lighting sources that attempt to mimic the spectral qualities of daylight. There appears to be a relationship between the spectral quality of light and the transformation of vitamin D in the skin. Bunker and Harris (1937) established that a wavelength of 297 nm is most effective in curing rickets. Knudsen and Benford (1938) found that 280 nm was the most effective in curing rickets and that wavelengths of 265, 289, 302, and 312 nm also had anti-rickets effects. Wavelengths longer than 312 nm had no effects on rickets.
Researchers in the United States Environmental Protection Agency (EPA) investigated the relative mutagenic (capable of causing mutations) effects of sunlight, fluorescent light, and typical tanning bed light on the DNA of Salmonella typh-imurium, a laboratory bacterium often used for preliminary investigations in biomedical research (De Marini et al., 1995). In this experiment researchers exposed four strains of this bacterium to sunlight, cool white fluorescent light, and tanning bed light for the same amount of time. The total radiant exposure received by each set of cultures was reported as the sum of the individual exposures to UV-A (315-400 nm), UV-B (280-315 nm), UV-C (250-280 nm) and the rest of the visible spectrum (400-800 nm). In every case, the cultures exposed to sunlight received the highest amount of radiation; however, the relative amount of UV-A and UV-B exposure varied by light type. Tanning light had a 2:1 constant ratio of UV-A to UV-B, cool white fluorescent light had a ratio of 10:1, for sunlight the ratio was 50:1.
Compared with an irradiated control group, all strains exhibited transformation of the genetic information (DNA). Tanning light, containing 80% of UV light, spurred more mutations than sunlight or cool white fluorescent light which contained no more than 10% of UV light. This experiment shows that the ability to cause mutation (mutagenicity) is strongly dependent on both the total amount of UV radiation contained in the light spectrum and the relative amount of UV-B versus UV-A within that spectrum. These results, though not conclusive, indicate that it is the nature of the light spectrum in sunlight that makes it a unique enhancement to human health. Most electric light sources do not replicate the sunlight spectrum. Furthermore, the spectral composition of sunlight changes according to the time of day and the season. This changing cycle may be the central reason for human circadian rhythms, assuming that chemical reactions to promote or inhibit alertness are initiated by UV levels contained in daylight.
Other health benefits of sunlight are found in its effect on liver metabolite, a substance produced by the liver that acts on our metabolism. Exposure to natural light stimulates the secretion of liver metabolite (Neer et al., 1977), but artificially simulated daylight that mimics the spectral composition of daylight also seems to have positive health benefits. Neer's research indicated that increases in the absorption of intestinal calcium were found among healthy men kept indoors during the winter and exposed to high intensity levels of artificially simulated daylight reaching 5000 lux, a level much above that typically found in building interiors using electrical lighting.
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