There are three possible causes of the Significant differences sometimes experienced between fading lamp and daylight fastness figures' spectral composition, temperature, and moisture content. Although there are marked differences between the spectral composition of fading lamps and sunlight, on theoretical grounds these would not appear to be the main cause of large discrepencies. The most likely cause is surface moisture content during exposure, and this depends on surface temperature and on the relative humidity of the surrounding air. It will be necessary to investigate these factors in detail before a specification for fading‐lamp operation can be issued.
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K. MoLaren (1954) studied this question.