DESIGN OF COSMETIC SUN SCREENS SAUL I. K•.• pS* Presented September 3, 1958, New York Chapter Meeting WITHIN THE last two decades our knowledge of ultraviolet screens has made it possible to offer to the public suntan preparations which will effectively prevent painful sunburn and at the same time permit adequate tanning. Any material exposed to radiant energy will absorb, reflect or transmit that energy to a varying degree which depends upon both the nature of the incident energy and the nature of the material. It is the function of the cosmetic sun screen to absorb as completely as possible the sun's erythematogenic ultraviolet radiation while it transmits a max- imum of all the other wavelengths of the sun's radiation. Very few chem- icals come close to meeting this ideal situation. Radiant energy is classified in a broad electromagnetic spectrum. Perhaps the simplest way of considering this energy is that in the absence * Van Dyk & Co., Inc., Belleville, N.J. TklE ELE6TRO-MAGNE TI6 SPEOTRUM I0000 I000 U-V IO 98

DESIGN OF COSMETIC SUN SCREENS 99 of matter it is propagated as a wave form. But when it is absorbed it is in the form of discrete packets--photons--whose energy is a function of the wavelength of the wave form. The relationship between energy and wavelength is expressed by the general equation E = • = •(c/x) where E = energy in ergs h = Plank's constant, 6.6242 X 10 -27 erg-second • = frequency, per second X -- wavelength, centimeters, and c = velocity of electromagnetic radiation, 3 X l0 w cm./sec. E3 E• E• E:• GROUND EXCITED STATE STATE AE = E• - E,= hlJ PHOTON -ATOM INTERACTION When atoms are excited by heat or electrical energy their electrons are raised through one or more steps of a possible series of energy levels, E2, Ea, etc., which may be stable, or may revert to a lower level, or even to the ground state with liberation of the absorbed energy. For every drop in energy level, the difference in the energy content of the atom is in the form of monochromatic radiation of frequency such that, for a drop from E•. to E1 AE = E9. -- E• = h•, (2) This radiation forms the emission spectrum of the atoms characteristically shown in an electric arc or in a flame test. When light passes through an absorbing medium, the reverse process takes place: the energy is absorbed and results in raising the energy levels of the electrons in the absorbing medium. This absorption of specific frequencies causes gaps to appear in the spectrum of the original light source which are shown as dark absorption lines at those wavelengths which are absorbed: AE = E• -- Es = h(c/X) (3)