THE MISTING CHARACTERISTICS OF SPECTACLE LENSES

When a spectacle wearer enters a warm environment after having been in a cooler one, his/her spectacles may 'mist up' due to the formation o f condensation on the lens surface. Both the physical properties of th e spectacle lens and a number of environmental factors determine how l ong spectacles take to demist. The relative misting performance of cro wn glass, CR39 and polycarbonate spectacle lenses were assessed using a novel technique, A box in which the environmental conditions could b e controlled was used first to cool the lens and then to warm it. This change in temperature caused condensation to form on the lens surface , By projecting the image of a grating through the lens onto a linear array of photodiodes it was possible to measure the contrast transmiss ion of the spectacle lens as it demisted. Each lens type misted and de misted in a characteristic way. The polycarbonate lens demisted more r apidly than the CR39 lens which, in turn, demisted more rapidly than t he glass lens. This indicates that polycarbonate spectacle lenses shou ld be used in conditions where the adverse effects of spectacle mistin g need to be minimised.