The total solar radiant energy which reaches the earth's surface at normal incidence amounts to about 80 million ergs per square centimeter per minute under optimal conditions. Only about 0.2% of this energy lies within the invisible ultraviolet range of wavelengths which can elicit redness or sunburn in human skin. An exposure to about 3 million ergs per square centimeter of these sunburn-producing wavelengths is required to produce a mild erythema in untanned normal white skin. Therefore, an exposure time of about 20 minutes to direct summer midday sunshine can result in sunburned redness in such skin. Tenfold extension of this exposure time results in a painful blistering sunburn.
Factors Influencing Production of Sunburn and Suntan
The action spectrum for sunburn production by ultraviolet radiation is shown in the figure. The shaded area on this figure represents the short-wavelength end of the spectral distribution of solar energy at the earth's