Skin radiance: how to quantify? Validation of an optical method

Background/purpose: The aim of this study was to validate an optic device used to measure the quantity of light reflected by the skin, which depends on the brightness of the skin. Methods: This apparatus gives three parameters, called Complexion/diffusion (Cd), Cr (Complexion/reflection) and Complexion/specular position (Csp). Measurements were carried out on the forehead (slightly rounded surface) and the cheekbone (angular surface). To estimate the repeatability and the reproducibility of each parameter, two female subjects (one with a dull complexion and the other with a glowing complexion) and three operators participated in the study. To investigate repeatability, each operator performed 10 consecutive measurements on both zones. To investigate inter-investigator reproducibility, the operators performed the measurements alternately. To investigate day-to-day reproducibility, these measures were achieved during three visits at a 2-day interval, at D1, D3 and D5. To estimate the sensitivity and the specificity, 30 females participated in the study (15 in the dull complexion group and 15 in the glowing complexion group). Results: Cd was repeatable (variation coefficient < 20%), day-to-day reproducible on both sites (generally P > 0.05) but not inter-investigators reproducible (generally P < 0.05). The sensitivity was 93% on the forehead and 80% on the cheekbone; the specificity was 87% on the forehead and 60% on the cheekbone. Cr was repeatable (variation coefficient < 5%), day-to-day reproducible (P > 0.05) and not inter-investigators reproducible (generally P < 0.05). The sensitivity was 93% on the forehead and 80% on the cheekbone; the specificity was 87% on the forehead and 80% on the cheekbone. Csp was not repeatable (variation coefficient > 5%) on both the studied zones and for the three investigators. Conclusions: The repeatability and the day-to-day reproducibility were satisfying for the Cd and Cr parameters on both the studied sites. The sensitivity and the specificity were satisfying for both parameters on both sites, except for the specificity of the Cd parameter on the cheekbone. Therefore, this apparatus is suitable to follow-up an evolution on both the studied sites, but we recommend: (a) that the same trained investigator always performs the measurements, (b) to privilege the forehead in particular for a classification (dull or glowing).