Transmission of Human Epidermis and Stratum Corneum as a Function of Thickness in the Ultraviolet and Visible Wavelengths

Abstract The dependence of radiation transmission on sample thickness was studied in isolated samples of human stratum corneum and full‐thickness epidermis. The investigation also included samples of skin repeatedly exposed to UV‐B. Transmission was measured in the ultraviolet and in the visible from 248–546 nm. Two methods, one microscopic and the other mechanical, were used to measure thickness. There was a good correlation between the results. The dependence of transmission on thickness in these samples could be described satisfactorily by an exponential function, implying that the Lambert‐Beer law is approximately valid. Thus, a single parameter, such as the half‐value layer ( d ½ ), is sufficient to characterize absorption in the skin samples. Water content of the isolated stratum corneum was influenced by maintenance conditions: samples floating on water containing a small amount of NaCl were more hydrated than samples floating on a more concentrated salt solution, or stored in air. Changes in water content of the samples resulted in changes of thickness and, to a lesser extent, of transmission. Approximate in vivo values of d ½ were computed after estimating the in vivo water content of stratum corneum. Differences found in the shape of the transmission spectra of stratum corneum and full‐thickness epidermis may reflect differences in chemical composition. The influence of wetting of the skin on its sensitivity to sunlight is explained in a new way.