Abstract: A spectral line absorption method was used to determine the Hg 6¹P₁ excited state densities in low pressure mercury vapour discharge columns of various diameters. Two spectral lines, 576.96nm (transition between 6³D2 and 6¹P₁) and 579.07nm (transition between 6¹D₂ and 6¹P₁), were separately chosen for the measurements. The results obtained were in good agreement. From, the 6¹P₁ state density data, we calculated the 185.0nm irradiance on the inner surface of various fluorescent lamps, using the resonance radiation imprisonment theory. It was concluded that the irradiance on the inner surface of the newly developed compact fluorescent lamps was one order larger than that of conventional fluorescent lamps, which seggested that highly stable phosphors to 185.0nm radiation or some method to re-duce the 185.0nm irradiance on phosphors was preferable in those new lamps.