Patent GB-636283-A (GB)
Title: Production of optical elements
IPC: B29D11/00
Filed: 1947-12-10
Abstract: Optical elements such as lenses and mirrors which possess an axis of reference such that the thickness of the element measured parallel to the axis at all points in the element at a given distance from the axis is substantially constant are made by a process which comprises subjecting a liquid polymerizable composition comprising a photopolymerizable monomeric compound possessing at least one >C=C< group in the molecule to the polymerizing action of light, in a mould having internal surfaces conforming to the shape of the element, until a solid body is formed, the average quantity of light per unit thickness of the liquid composition, measured in the direction of the incident light, falling per unit time upon unit area of the composition, decreasing in a non-linear manner as the distance from the axis of reference increases up to the edge of the optical element.  The light control is achieved by interposing a screen between the source of light and the mould, which may be made of glass.  The screen may be a suitably prepared photographic filter or a shaped plate of light absorbing or pigmented transparent material or an opaque disc from which mathematically calculated portions in accordance with the above requirement have been removed.  The screen and/or the mould may be rotated.  The term "light" means all light from infra-red to ultra-violet.  The monomeric compound may be acrylic and methacrylic acids, methyl methacrylate, cyclohexyl methacrylate, 2,2,2,-trifluorethyl methacrylate, esters of alpha-fluoroacrylic acid and fluorinated alcohols, styrene, vinyl acetate, allyl methacrylate and diallyl fumarate. Mixtures may be used.  The composition preferably comprises a polymeric material of low molecular weight dissolved in the monomeric compound.  The low molecular weight polymers may be obtained by malaxating polymers of higher molecular weight on heated rolls or by polymerizing the monomer in the presence of a chain-ending compound such as turpentine.  The photopolymerization is preferably conducted in the presence of a photopolymerization catalyst, e.g. glycollic aldehyde, benzoin, acetoin, butyroin, 3-hydroxy-4-methyl-pentanone-2, toluin, tert.-butylbenzoin, 12-hydroxy-13-ketotetracosane, o- and p-tert.-butyltoluin, diacetyl, pentanedione-2,3; octanedione-2,3; 1 - phenyl - butanedione - 1,2; benzil; 2,2 - dimethyl-4-phenylbutanedione-3,4; glyoxal; phenyl glyoxal; diphenyl triketone; 1,2-cyclohexanedione; alpha, alpha1-azodiisobutyronitrile; alpha, alpha1-azobis (alpha, gammadimethylvaleronitrile); dimethyl and diethyl alpha, alpha1-azodiisobutyrates; alpha, alpha1-azobis (alpha - ethylbutyronitrile); alpha, alpha1 - azobis (alpha - ethylbutyronitrile) and alpha, alpha1-azo-diisobutyrocarbonamide.  An organic peroxide such as lauryl, acetyl, butyryl, succinyl peroxides and ascaridole may also be present.  After the photopolymerization step the solid material may be heat-treated to assist removal from the mould. Specification 618,787 is referred to.ALSO:Optical elements such as lenses and mirrors which possess an axis of reference such that the thickness of the element measured parallel to the axis at all points in the element at a given distance from the axis is substantially constant are made by a process which comprises subjecting a liquid polymerizable composition comprising a photopolymerizable monomeric compound possessing at least one group in the molecule to the polymerizing action of light, in a mould having internal surfaces conforming to the shape of the element, until a solid body is formed, the average quantity of light per unit thickness of the liquid composition, measured in the direction of the incident light, falling per unit time upon unit area of the composition, decreasing in a non-linear manner as the distance from the axis of reference increases up to the edge of the optical element.  The light control is achieved by interposing a screen between the source of light and the mould, which may be made of glass.  The screen may be a suitably prepared photographic filter or a shaped plate of light absorbing or pigmented transparent material or an opaque disc from which mathematically calculated portions in accordance with the above requirement have been removed.  The screen and/or the mould may be rotated.  The term "light" means all light from infra red to ultra-violet.  The monomeric compound may be acrylic and methacrylic acids; methyl methacrylate; cyclohexyl methacrylate; 2,22,-trifluorethyl methacrylate; esters of alpha-fluoroacrylic acid and fluorinated alcohols; styrene; vinyl acetate; allyl methacrylate and diallyl fumarate.  Mixtures may be used.  The composition preferably comprises a polymeric material of low molecular weight dissolved in the monomeric compound.  The low molecular weight polymers may be obtained by malaxating polymers of higher molecular weight on heated rolls or by polymerizing the monomer in the presence of a chain-ending compound such as turpentine.  The photopolymerization is preferably conducted in the presence of a photopolymerization catalyst.  An organic peroxide may also be present.  After the photopolymerization step the solid material may be heat-treated to assist removal from mould. Specification 618,787, [Group IV (a)], is referred to.