The ultraviolet wavelength region from approximately 2,500 to 3,500 angstroms encompasses the wavelengths found to be most valuable in the prevention and cure of rickets, the production of erythema and tan in human skin, the killing of germs, and various therapeutic applications. However, the effectiveness of equal amounts of energy at various wavelengths in this region varies enormously for each of these reactions. Some studies of the relative effectiveness of energy of different wave-lengths for some of these reactions have been made, notably for the cure of rickets, the killing of germs and the production of erythema.
Several investigators, including ourselves,1 have studied the effectiveness of ultraviolet energy in producing erythema, and an "erythemal effectiveness" curve has been tentatively established by combining all the data available.2 The accepted data for wavelengths longer than 3,150 are of questionable accuracy because the energy of longer wavelengths is relatively low in