By modifying the blood supply in the upper part of the dermis, the colour of the skin is changed by any material in contact with it. This is a great difficulty when we want to measure the colour intensity in erythema or blanching. To avoid this problem, we designed and built a device which measures the skin reflectance value without any contact with the skin. The use of this device on man to study the skin blanching after application of dermocorticoids allows us to determine the more potent drug among a series (clobetasol propionate) and, for a given drug (betamethasone), the more efficient preparation (oil-in-water emulsion). The study of the effect of increasing irradiation doses of ultraviolet light shows that the sensitivity of the device is equivalent to that of the clinical assessment and that the relationship between the irradiation dose and the skin reflectance is sigmoidal.