PURPOSE: To discharge radiations of the same ultraviolet range as natural light, and enable examination of the deterioration and the discoloration of material to be easily carried out by coating the inner wall of a glass bulb, which has an absorption threshold wavelength of around 300nm, with a specified mixture phosphor film.

CONSTITUTION: 35W75wt% of a phosphor which has over 48.6mW spectral output per 1nm in the wavelength range of 300W320nm according to a spectral output curve given for the wave length of discharge light transmitted by a glass bulb, below 60wt% of a phosphor having a spectral output peak near a wavelength of 350nm, and below 35wt% of a phosphor having a spectral output peak near a wavelength of 370nm are mixed so as to make a phosphor mixture. After that, the above phosphor mixture is applied to the inner wall of the bulb, which is made of glass the absorption threshold wavelength of which is near 300nm, so as to form a phosphor film. As a result, a spectral energy distribution extremely similar to that of natural light can be obtained, and the installation used in performing deterioration test of material under artificial light can be simplified to a large extent.