A long-term time course of colorimetric evaluation of ultraviolet light-induced skin reactions

Many attempts have been made to quantify ultraviolet (UV) radiation-induced erythema and pigmentation. However, most of these studies were concerned with the early changes of reactions and neglected events occurring in later stages. The long-term course of skin colour changes in pigmented skin, induced by broad band UVA and UVB radiation, was evaluated in 30 Korean male volunteers by means of a tri-stimulus colorimeter for 10 weeks. The L*a*b* system recommended by the Commission International de l'Eclairage was used to measure skin colour. The L* value (luminance) gives the relative lightness ranging from total black to total white. The a* value represents the balance between red and green and the b* value the balance between yellow and blue. The mean individual typology angle of our subjects was 47.3 degrees, indicating 'light' group of constitutional skin colour category. One day after UV exposure, the L* and b* values decreased significantly, following the colour direction of persistent pigment darkening. They then changed in opposite directions persistently until week 1, when maximum tanning was obtained. Then, a shift toward the original values was observed parallel to the constitutive melanization axis. The a* index showed a significant increase toward the mean colour of haemoglobin on day 1. It returned to its original value following the pathway of constitutive melanization axis. This promising quantitative method may enable objective measurement of dermatophysiologic changes to be made, and allow evaluation of the efficacy of therapeutic modalities on skin disorders without the inherent errors associated with subjective judgement. Our results would provide standard data for long-term UV-induced skin erythema and pigmentation.