| 摘要 |
PURPOSE:To improve spherical aberration and sine conditions in a lens of 2- element constitution for optical disc by making the exit side face of the 2nd lens aspherical. CONSTITUTION:A titled lens is constituted of the positive 1st lens and the 2nd lenses, and the overall length SIGMAd is determined within the range shown by the equation 1. The face of the incident side of the 1st lens is made of an aspherical surface of which the part near the vertex thereof is approximate. The face on the incident side of the 2nd lens is the aspherical surface expressed by the equation 2 when the distance from the optional point on the face up to the tangential plane at the vertex is defined as x. In the equations, y is the distance from the optional point up to the optical axis, E, F, G are respectively the coeffts. of aspherical surfaces at the 4th, 6th and 8th terms. The lens is thus formed to have overall length (d1+d2+d3) as small as <=0.9f. The equations 3, 4 are further satisfied to decrease the bulge of mainly the spherical aberration and sine condition. The thickness d3 of the 2nd lens is determined to satisfy the equations 5 whereby the working distance WD is increased and various aberrations are compensated. |
