Photochromic spectacle lenses have advanced considerably since the first ones were developed by Roger Araujo at Corning Glass works in the 1960s. For many years photochromic lenses could only be produced in a glassy material but then plastic versions started to become available, although initially these changed from a light tint to a dark tint very slowly and faded even more slowly. Attempts were made by American Optical in the 1980s but it was not until the 1990s that the Transitions company (created jointly by PPG and Essilor) released the first CR39 photochromic and it was not until 1996 that Transitions introduced their third generation product that the lenses could be made in a grey colour and with a reasonable speed of reaction. The latest versions have even better performance, with short reaction times, and can change from very pale to very dark. Photochromic lenses are now widely available and aim to satisfy the need for a variable darkness tint that responds to lighting levels. However, because the chemical reaction that causes the lenses to darken is mainly controlled by the amount of ultra-violet light rather than the total amount of visible light, they often perform very poorly in situations such as behind a car windscreen which absorbs much of the ultraviolet light. Other factors such as the ambient temperature and the lens material are also important as this affects the amount and speed of darkening. Even more significant is the way these factors reduce the amount and speed of fading.
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