We have developed a photodensitometry method to evaluate the intensity of cutaneous erythema objectively. The method measures the optical density of photographic slides of cutaneous erythema. It combines techniques used commonly but separately by investigators: diffuse transmittance spectroscopy (which is a variant of diffuse reflectance spectroscopy) and photography. We have used this method to study photosensitivity in 22 volunteers who received increasing doses of ultraviolet radiation to the back. Our work confirms the usefulness of an important parameter in photobiology: the regression slope of the curve representing the erythema index, a function of the logarithm of the dose applied.