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The first contact lenses were made of glass, in 1888. Initially the glass was blown but soon lenses were made by being ground to shape. For the first fifty years, glass was the only material used. The lenses were thin, yet reports of injury were rare. In 1938 perspex (polymethylmethacrylate, or PMMA) began to replace glass in contact lens ...
Grand Vision acquired Vision Express in 1997. The French parent company Grand Vision has clinics under various brands across Europe, and the rest of the world. Under franchise agreement there is a strong Vision Express brand present also in Latvian and Lithuanian markets.
Sleeping in lenses not designed or approved for extended wear is a common cause of complications. Many people go too long before replacing their contacts, wearing lenses designed for 1, 14, or 30 days of wear for multiple months or years. While this does save on the cost of lenses, it risks permanent damage to the eye and even loss of sight.
This list covers optical lens designs grouped by tasks or overall type. The field of optical lens designing has many variables including the function the lens or group of lenses have to perform, the limits of optical glass because of the index of refraction and dispersion properties, and design constraints including realistic lens element center and edge thicknesses, minimum and maximum air ...
A lens may be considered a thin lens if its thickness is much less than the radii of curvature of its surfaces (d ≪ | R 1 | and d ≪ | R 2 |).. In optics, a thin lens is a lens with a thickness (distance along the optical axis between the two surfaces of the lens) that is negligible compared to the radii of curvature of the lens surfaces.
Today the complex surfaces of a progressive lens can be cut and polished on computer-controlled machines, allowing 'freeform surfacing', as opposed to the earlier casting process, thus explaining the difference in price. In short, the price is based on the technology used and the year the lens came to market.