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Ophthalmic

Photochromic glass

Non-photochromic spectacle lenses have a constant coloration and can be characterized by the spectral distribution of their radiation transmission. Photochromic spectacle lenses behave differently. They adapt their light transmission to the lighting conditions at the particular time. Extra factors have, therefore, to be taken into account when assessing them, in particular, the time required for changes in transmission properties to take place. Phototropy is understood as the reversible change in transmission in the visible range of the spectrum when subjected to the effects of radiation. When light falls on them photochromic glasses automatically darken and when the light is removed they become clear again. The darkening and clearing of photochromic glasses is dependent on: Influence of glass thickness The degree of saturation transmission tau 15 (tau 1)', which refers to the degree of light transmission in the dark state at 23° after a given exposure, decreases as the thickness of the glass increases, i.e. thicker glass has a higher absorption. Influence of sample temperature The lower the sample temperature, the greater and quicker the darkening, i.e. higher absorption. In the case of clearing, it behaves in the opposite way, in other words the higher the temperature of the sample the quicker it clears. Influence of stimulating radiation The shorter the wavelength of the light the glass is exposed to, the greater the degree of darkening. Influence of the strength of the light The higher the strength of the light, the greater the degree of darkening. All photochromic glasses can also be used as sunglasses because of their transmission properties. Physical and chemical properties according to specification