Eden, G., Tronnier, H., Heinrich, U.

Testing of sunscreens using improved reflectance spectroscopy

U. Institute for Experimental Dermatology, University Witten/Herdecke, 58455 Witten, Germany

Background and Objective:

When comparing in vivo tests of sun protection products an the human skin and in vitro transmission analyses, a fundamental problem consists of the difference between the skin and (smooth or roughened) quartz slides and the question of the skin's possible influence an the sun protection products. The reflectance spectroscopy has the advantage of being a noninvasive method and thus, of being ethically unquestioned. Measurements can be easily performed and within short time intervals.

Material and Methods:

Using a reflectance spectrometer (Multiscan OS20, MBR Messtechnik) the spectral reflectance of human skin was measured before and alter application of sunscreen products. Seven sunscreens and one sunscreen base without UV filters were applied an the volar side of the forearm in an even surface density of 2 mg/cmZ, since the published SPF is related to this value. The spectral working range was between 260 470 nm. By means of a novel algorithm developed by us, the spectral reflectance of the skin treated with the sunscreen base was subtracted, and the special irradiance of terrestrial sunlight (E) as well as the erythemal effectiveness (CIE) of skin were multiplied to the absorbance differences. Measurements were made in vivo and in vitro before and immediately after application, and alter elapse of additional 30 minutes, 2, 4 and 6 hours after application. Reflectance spectra were used to determine the SPF for every tested sunscreen product reliant an the whole UV range.

Results:

1. There is a close correlation between biological and reflectionphotometric determination of the SPF.
2. An essential biological influence of the sun protection substances during an expected period of exposure does not occur.
3. The method offers a chance for the testing of water resistance of sun protection products.

Conclusion:

The reflectionphotometric evaluation of sun protection products can complement in vitro transmission measurements and shows values comparable to biological SPF determination.
The reflectance spectroscopy has the advantage of being a non invasive method.
Measurements can be easily performed and within short time intervals.
It provides a very good reproducibility, while taking the physiologically important parameters into account.

References:

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3. Kockott, D.: In vitro Bewertung von Sonnenschutzmitteln; Kosm Med, 1998; 19/5:290 293
4. McKinlay, A.F., Diffey, B.L.: A reference action spectrum for ultraviolet induced erythema in human skin; CIE Journal, 1987; 6:17 22
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6. Sayre, R.M., Agin, P.P., LeVee, G., Marlowe, E.: A comparison of in vivo and in vitro testing of Sunscreening formulas; Photochem Photobiol, 1979; Vol 29:559 566

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