The UV action spectra of two different biologically weighting UV photofilms (spore films), produced with Bacillus subtilis spores (wild-type and DNA repair-deficient strains), were determined at the Okasaki large spectrograph (OLS) within the level of wavelength range 254-400 nm. The action spectrum of the mutant strain film was modified with a cut-off filter, yielding a sensitivity curve similar to the action spectrum for erythemal induction in human skin. The detector system was tested in a field study and in a study using lamps with different UV spectral compositions. The system demonstrated its applicability over the spectral region lambda = 290 nm to the visible light. The system could be calibrated to give the minimal erythemal dose.