Patent ID: 8003186

Claim:
A preform comprising: an almost circular shape having a predetermined diameter in top view, a flattened semicircular shape having a downwardly convex curve and nearly horizontal straight line on an upper end side in side view such that a distance from a lowermost part of the convex curve to a straight line becomes a predetermined height of the flattened semicircular shape, a concave face on a top surface, and a convex face on a bottom face, wherein the concave face on the top surface forms the lowermost part of the concave face at approximately a central position in almost circular shape in said top view; wherein said concave face has, at approximately the central position in almost circular shape in said top view, ratio of the distance of from said lowermost part of the concave face to said straight line of said upper end side in said side view to said diameter of the almost circular shape in said top view is from 0.02 to 0.9; ratio of the wall thickness of the preform that is a distance of from said lowermost part of the convex face to said lowermost part of the concave face in said side view to said diameter of the almost circular shape in said top view is from 0.2 to 0.9; the convex face on the bottom face forms the lowermost part of the convex face at a central position of the bottom face corresponding to said approximately the central position; the ratio of said height of said flattened semicircular shape to said diameter of the almost circular shape in said top view is from 0.2 to 0.9; the predetermined diameter is no less than 5 mm and no greater than 20 mm; curvature radius 1 is between 8 and 60 mm, curvature radius 2 is between 6 to 20 mm and the preform is in a hardenedstate; and the preform is used for press molding to produce an optical glass element.