Understanding Ultraviolet Light
Ultraviolet light light that is invisible because its wavelength is shorter than the violet part of the sunlight spectrum. UV is available year round in the tropics and during part of the year farther north and south. UV is also produced by sunlamps. There are three main varieties of UV: UVA, UVB and UVC. UV wavelengths are measured in nanometers (one billionth of a meter).
a. UVA has a wavelength of 380-315 nanometers and when contacting the skin can penetrate beyond the outer layer (epidermis) into a layer called the dermis. UVA does not stimulate vitamin D production.
b. UVB light has a wavelength of 315-290 nanometers and penetrates only the epidermis when it contacts the skin. UVB stimulates the skin, which then produces vitamin D.
c. UVC light has a wavelength of 280-10 nanometers and is filtered out by the Earth’s outer atmosphere.