Due to the wavelength specificity of the effect of light on materials, the type of light source used in durability testing significantly influences the stability ranking of materials as well as the mechanism and type of degradation. Tests of the relative stabilities of polysulfone and polyarylate show that fluorescent UVB radiation produces reversal in ranking compared with outdoor exposure while the latter is reliably simulated by borosilicate filtered xenon arc radiation. The importance of simulating the irradiance of terrestrial solar radiation over the full actinic region is illustrated by examples of the antagonistic effects of short versus long wavelength UV and data on distortion in lightfastness ranking of printing inks by either fluorescent UVB or UVA-351 lamps. The former has excessive short wavelength UV radiation and both are deficient in long wavelength UV and visible radiation. Reasons are given for dependence of stabilizer effectiveness on type of light source and illustrated with examples.