The biologically important process of endogenous synthesis of vitamin D under UV solar irradiation is widespread in the biosphere and inherent to most animals and plants. A new method of biological dosimetry of UV radiation based on an in vitro model of vitamin D synthesis ('D-dosimeter') is discussed. Unlike the vast majority of biodosimeters, the action of which depends on the UV sensitivity of DNA and thus reflects damaging effects of UV radiation, the process of vitamin D synthesis is beneficial by its nature. To date, the complex network of photo- and thermoreactions of vitamin D synthesis in vitro is well understood, and an adequate mathematical model is available, ensuring a link between biological and physical units. Original spectral analysis of the multicomponent photoisomer mixture has been specially designed to provide the most effective use of the D-dosimeter in situ. Spectral selectivity (exceptional sensitivity of certain parameters to the spectral composition of UV radiation) extends the usefulness of the method.