Method of producing photochromic multifocal spectacle lenses

The method of manufacturing a fused multifocal spectacle lens having a reading lens segment portion fused to a major lens portion having photochromic properties and formed from a glass containing silver salts and requiring heat treatment in a tempering range of temperature from 550.degree. to 650.degree.C. to improve its photochromic properties, the glass of said major lens portion having a tendency to develop turbidity when maintained at a temperature in said tempering range and also to develop turbidity when maintained at a temperature in a turbidity range above said tempering range, the glass of said major portion requiring heating to said tempering range in order to condition it for fusion, said method comprising the steps of elevating the temperature of said major portion relatively rapidly through said tempering range and through said turbidity range to a temperature not less than approximately the upper limit of said turbidity range, heating said reading segment portion to an elevated temperature below said turbidity range, cooling said major portion from its elevated temperature, bringing the hot reading segment portion and the hot major portion into fusion position relative to each other while the major portion is being cooled and is still at least as hot as said tempering range and fusing the two portions to each other, and further cooling the two fused portions, the cooling of said major portion downwardly through said turbidity range and through said tempering range being accomplished relatively rapidly to control turbidity.

The method of manufacturing a fused multifocal spectacle lens comprising a major lens portion composed of a glass having photochromic porperties and at least one reading lens segment portion fused to the major lens portion, said method comprising the steps of:

• a. forming a partially finished major lens portion of a glass containing silver salts potentially capable of being converted by heat to an improved photochromic condition;
• b. forming a partially finished reading lens segment portion;
• c. subjecting the partially finished major lens portion to a heat treatment at a temperature and for a time sufficient to at least initiate conversion of said silver salts to a photochromic condition;
• d. combining the major lens portion and the reading lens segment portion in position for subsequent fusing;
• e. heating the combined portions in the tempering range of softening of one of said portions while further developing and maintaining the photochromic porperties of the major lens portion, whereby the softened portion fuses to the other portion;
• and
• f. subsequently cooling the fused portions downwardly from fusing temperature at rapid rate to avoid production of turbidity in the spectacle lens and to avoid detrimental effect on any photochromic condition already produced.