The present ink jet technology permits to replace the traditional imaging colour technology for which image permanence is of crucial importance. Several test methods and standards have been developed to judge image permanence on accelerated ways for indoor as well as for outdoor applications. Especially for light stability, these methods are based on the conventional law of timeintegrated illuminance. In this paper we will show that, rather than the light dose, the UV & VIS irradiant exposure energy in combination with the material's action spectrum, image optical density, relative humidity and temperature are the decisive parameters for estimating the life expectancy of printed ink jet media in practical situations.