Comparison of three techniques for evaluating skin erythemal response for determination of sun protection factors of sunscreens: high resolution laser Doppler imaging, colorimetry and visual scoring

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Abstract
Background/Purpose: Sun protection factor (SPF) measurement is based on the determination of the minimal erythema dose (MED). The ratio of doses required to induce a minimal erythema between product‐treated and untreated skin is defined as SPF. The aim of this study was to validate the conventionally used visual scoring with two non‐invasive methods: high resolution laser Doppler imaging (HR‐LDI) and colorimetry. Another goal was to check whether suberythemal reactions could be detected by means of HR‐LDI measurements.Material and methods: Four sunscreens were selected. The measurements were made on the back of 10 subjects. A solar simulator SU 5000 (m.u.t., Wedel, Germany) served as radiation source. For the visual assessment, the erythema was defined according to COLIPA as the first perceptible, clearly defined unambiguous redness of the skin. For the colorimetric determination of the erythema, a Chromameter CR 300 (Minolta, Osaka, Japan) was used. The threshold for the colorimetry was chosen according to the COLIPA recommendation as an increase of the redness parameter Δa*=2.5. For the non‐contact perfusion measurements of skin blood flow, a two‐dimensional high resolution laser Doppler imager (HR‐LDI) (Lisca, Linköping, Sweden) was used. For the HR‐LDI measurements, an optimal threshold perfusion needed to be established.Results: For the HR‐LDI measurements basal perfusion +1 standard deviation of all basal measurements was found to be a reliable threshold perfusion corresponding to the minimal erythema. Smaller thresholds, which would be necessary for detection of suberythemal responses, did not provide unambiguous data. All three methods, visual scoring, colorimetry and HR‐LDI, produced similar SPFs for the test products with a variability of Conclusion: Both HR‐LDI and colorimetry are suitable, reliable and observer‐independent methods for MED determination. However, they do not provide greater sensitivity and thus do not result in lower UV dose requirements for testing.

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