The present invention provides a method of monitoring performance of a discharge lamp. The discharge lamp includes electrodes and a discharge vessel filled with gas and equipped with a luminescent layer, wherein the gas is intended to emit a first ultraviolet light in a first spectral range when the gas is excited by an electric field produced by the electrodes, and at least part of the first ultraviolet light is intended to be changed into a second ultraviolet light in a second spectral range of longer wavelength than the first spectral range by the luminescent layer. The method comprises the steps of finding the value of a first intensity of the first ultraviolet light; finding the value of a second intensity of the second ultraviolet light; and determining the conversion efficiency of the luminescent layer for converting the first ultraviolet light into the second ultraviolet light on the basis of the ratio of the value of the second intensity to the value of the first intensity.