w m 2 and even more preferably less than 0 03 w m 2 as shown in fig8 in connection with an exemplary embodiment of the invention the maximum power 0 025 w m 2 of irradiance of the new lamp is significantly lower than the peak power 0 06 w m 2 of the prior art lamp in the uvb range on the other hand the new lamp has a higher output power in the range of 290 nm lt u03bb lt 310 nm which results in a better stimulating effect for producing previtamin d3 the suggested power range makes it possible to select longer exposure times without negative effects and to determine the necessary dosage more precisely as it can be seen best in fig8 the lamp dl has a significantly higher emission in the range of 295 nm to 310 nm resulting in a better stimulating effect for the production of previtamind d3 the effectiveness of the stimulation of the previtamin d3 synthesis is best seen in fig9 which shows the power spectra of the lamp according to the invention and the conventional sun tanning lamp weighted with the previtamin d3 action spectrum relative sensitivity of the skin the weighted diagram of the lamp according to the invention peaks at about 305 nm and as it can be clearly seen in most of the active range of the power spectrum of the lamp according to the invention the output power is higher than that of the conventional lamp hl in consequence the suggested lamp dl has a significantly better previtamin d3 efficacy.