Electromagnetic energy from sunlight is generally divided into ultraviolet, visible and infrared energy. Figure 12.1 shows the spectral energy distribution (SED) of midsummer noon sunlight. Infrared energy (not shown) consists of longer wavelengths than visible red wavelengths, starting above 760 nanometers (nm). Visible light is defined as radiation between 400 and 760 nm. Ultraviolet light consists of radiation below 400 nm. As shown, the International Commission on Illumination (CIE) further subdivides the ultraviolet part of the spectrum2 into UV-A, UV-B and UV-C.
sunshine change
Natural exposure to UV rays varies widely in amount and spectrum, as UV rays are easily filtered by air masses, cloud cover, pollution, and other factors. Figure 12.2 compares the solar UV regions measured at noon in Cleveland, Ohio:
summer solstice
winter solstice
spring equinox
These measurements are generally consistent with data reported by other researchers because during winter, when the sun is lower in the sky, it is filtered by larger air masses. This creates two important differences between summer and winter sunlight -- changes in light intensity and spectrum. Remember: Most of the damaging short-wave UV rays are filtered out in winter. For example, the UV intensity at 320 nm varies by about 8 to 1 from summer to winter. In addition, the short-wave solar cutoff shifts from about 295 nm in summer to about 310 nm in winter. Materials that are sensitive to UV rays below 320 nm are therefore only slightly degraded in winter.
