The earliest contribution made by Jan van der Leun to the field of photobiology was studying the mechanism of UV-induced erythema in human skin - a subject he chose for his PhD in the 1960s. His contribution to this topic encouraged us to continue this work and over a number of years in the 1980s and 1990s, we carried out several studies on quantitative aspects of UV erythema. A major part of Jan's thesis focused on diffusion processes in UV erythema and his observations led him to conclude that erythema induced by radiation with wavelengths of around 300 nm was due to the actions of a diffusing mediator arising in the epidermis, whereas radiation at shorter wavelengths around 254 nm, caused erythema by exerting a direct effect on the dermal blood vessels. By taking his data and combining them with our own studies on the dose response of UV erythema to radiation of different wavelengths, we were able to show that, contrary to Jan's conclusions, the mediator diffusion theory he developed did indeed predict that both UVB and UVC induced erythema could be explained by the action of diffusing mediators.