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 (d½), 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 d½ 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.

From an optical point of view the outermost skin layers contain numerous structures by which penetrating radiation may be scattered as well as absorbed. The nature and strength of this scattering may strongly influence the extent of penetration. We illuminated samples of stratum corneum and full‐thickness epidermis with collimated radiation and measured the angular intensity distribution of the transmitted radiation; we did this in the ultraviolet for several angles of incidence, and in the visible for perpendicular incidence only. Skin samples were obtained from the skin of the lower back and upper leg of Caucasian volunteers. Epidermis and subsequently stratum corneum were separated by chemical methods. In the case of stratum corneum, the angular intensity distribution of the transmitted radiation peaks strongly at all wavelengths, in approximately the direction of the incident radiation, that has been refracted at the surface of the sample. With full‐thickness epidermis, the distribution of the transmitted radiation also peaks, though less strongly than with stratum corneum. These features suggest a forward oriented scattering mechanism. Both in the case of stratum corneum and full‐thickness epidermis, the angular distribution flattens towards the shorter wavelengths and with increasing thickness. The wavelength dependence suggests that both scattering and absorption increase towards the shorter wavelengths. The existence of a thickness dependence indicates that volume scattering occurs. Hydration of stratum corneum is found to influence its scattering properties. Dry samples scatter less than hydrated samples. The consequences of our findings for modelling skin optics are briefly discussed.

Abstract— The transmission of the outermost layers of human skin is measured, by use of a diffuser. The sample is transferred to the diffuser and inserted into the measuring beam. The reference beam is also measured after passing through the diffuser. The diffuser transforms the light in both the sample and reference beams into a completely diffuse flux. The radiant flux of light emerging from the diffuser is directly proportional to the radiant flux of the light impinging on the diffuser, but does not depend on the angular distribution of the impinging light. Because of this particular property, a diffuser may be used to measure the transmission of scattering specimens. The analogy between a diffuser and an integrating sphere is pointed out. Deviations of commercially available diffusers from the perfect behaviour lead to deviations in the measured transmissions that are negligible (< 3%) for epidermal and corneal samples. Spectral transmission data from representative skin samples are presented. It is found that correction for fluorescence is necessary. Due to this correction the epidermal UV‐C transmission is lower by a factor of 10–100 than without correction, and the epidermal absorption maximum is shifted from 275 towards 265 nm.