LENS HAVING REFRACTIVE INDEX GRADIENT

PURPOSE:To provide a nearly ideal gradient to a lens by regulating the concn. of Tl2O in glass in the surface parts of the lens to a specified mol% and a specified % of the concn. in the central part and by reducing the concn. from the central part to the surface parts so as to provide a nearly square refractive index gradient. CONSTITUTION:Round rodlike glass having about 2mm. diameter for a lens contg., by mole, 1-30% Tl2O, 35-80% SiO2 and 1-42% Tl2O+alkali metallic oxide is immersed in molten KNO3 contg. Tl so as to make the value of Tl/(K+ Tl) 0.2-5.0% at 505 deg.C for a desired time. The glass is taken out and cut to a suitable length, and both ends are polished to parallel planes to form a lens having a refractive index gradient. The concn. of Tl2O in the central part of the lens is about 1-30%, the concn. in the surface parts is 0.3-7%, and the concn. in the surface parts is 3-60% of the concn. in the central part. The lens has high performance such as large numerical aperture and small aberration.