PURPOSE: To attain size reduction although a single lens and to facilitate adjustment for optical axis alignment by providing the objective of a video disk, etc., with the 1st and the 2nd surface which have positive refractive indexes, and making one surface aspherical according to specific expressions.
CONSTITUTION: The lens consists of a single lens element and the 1st and the 2nd surfaces both have positive refractive indexes. Further, one of those surfaces is made aspherical as shown by the equation I. In this case, X is the quantity of displacement from a lens surface peak reference in the optical-axis direction, H the height from the optical axis, R the radius of paraxial curvature, K the elliptic constant, and A, B, C, and D the coefficients of aspherical surfaces of the 4th, the 6th, the 8th, and the 10th degrees. Further, the lens meets requirements shown by inequalities II and III to obtain proper operating distance and spherical aberration. In the inequalities, d1 is the thickness of the lens, (f) the focal length of the lens, and k1 the conic constant of the 1st surface. Therefore, one surface is made aspherical according to the equation I and the inequalities II and III hold, so the lens is reduced in size, the optical-axis alignment is facilitated, and the loss of the quantity of light is reduced.
UEDA SHUJI
NAKADA KUNIO
