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Bifocals are eyeglasses with two distinct optical powers. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia , hyperopia , and/or astigmatism . History
Didymium glasses. Didymium (Greek: δίδυμο, twin) is a mixture of the elements praseodymium and neodymium.It is used in safety glasses for glassblowing and blacksmithing and filter lenses for flame testing, especially with a gas ()-powered forge [why?], where it provides a filter that selectively blocks the yellowish light at 589 nm emitted by the hot sodium in the glass without having a ...
It can be used to establish whether a patient has the ability for the eyes to fuse the light that is received from each eye into 4 lights. The test is indicated with the use of a presence of a prism in individuals with a strabismus and fusion is considered present if 4 lights are maintained, with or without the use of a prism. The W4LT can also ...
The same principle has also been applied as an alternative to corrective lenses: a screen of pinholes is mounted on an eyeglass frame and worn as pinhole glasses. Besides pinholes made by the point of a pin, precision commercial pinholes are often made by laser drilling through a thin foil.
A pair of horn-rimmed glasses. Horn-rimmed glasses are a type of eyeglasses. Originally made out of either horn or tortoise shell, for most of their history they have actually been constructed out of thick plastics designed to imitate those materials. They are characterized by their bold appearance on the wearer's face, in contrast to metal ...
Other additives used in crown glasses include zinc oxide, phosphorus pentoxide, barium oxide, fluorite and lanthanum oxide. The crown/flint distinction is so important to optical glass technology that many glass names, notably Schott glasses, incorporate it. A K in a Schott name indicates a crown glass (Krone in German).