JPS6111715A - Large aperture diameter single lens - Google Patents

Abstract

PURPOSE:To provide the sensitivity with parallel eccentricity which is not excessively large and to correct satisfactorily sine conditions as well by forming a titled lens in such a way that the aspherical shape of the lens face on the further conjugation point side out of both aspherical faces satisfies the specific conditions and that the central thickness of the single lens satisfies the specific conditions. CONSTITUTION:The two lens faces of the single lens having a biconvex shape are formed of the aspherical faces and the shape of the aspherical face of the lens face on the further conjugation point side of the two aspherical faces is formed to satisfy the conditions 0<K<5, 0<A2<0.13 when said shape is expressed by the equation where the distance of the point on the aspherical face from the tangent plane at the vertex of the lens face is designated, as (x), the height from the optical axis as (h), the curvature at the vertex of the aspherical face as C, the constant of the circular cone as K and the coefft. of the aspherical face as Azi. The conditions 0.3<d/f<0.8 are satisfied where the central thickness of the single lens is designated as (d) and the focal length as (f). The single lens having the aspherical faces of which not only the spherical aberration but also the sine conditions are satisfactorily corrected is thus attained and the sensitivity with the parallel eccentricity is not excessively large.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a large-diameter aspheric single lens having diffraction-limited imaging performance, particularly to an objective lens for optical disks.

(Background of the Invention) Conventionally, as this type of objective lens, many lenses consisting of three or more spherical glass lenses have been used and put into practical use. In this case, it is difficult to make a lens that is small and lightweight, and there is a problem in that it is necessary to increase the processing accuracy of the lens barrel or take time and effort for adjustment in order to suppress eccentricity of the lens. In order to overcome these difficulties, biconvex aspherical single lenses have been proposed.

For example, the aspheric single lens disclosed in JP-A-57-76512 can effectively correct astigmatism by providing conditions regarding the relationship between the center thickness, refractive index, and focal length of the single lens. ing. However, as pointed out in Japanese Unexamined Patent Publication No. 59-23313, this method has a disadvantage in that it is difficult to manufacture because of its extremely high sensitivity to parallel eccentricity. Therefore, the above-mentioned Japanese Patent Application Publication No. 59-
No. 23313 proposes a configuration in which sensitivity to parallel eccentricity is suppressed by providing new conditions for the relationship between the central thickness and focal length of a single lens. However, in the aspherical single lens designed in this way, the sine condition has a significantly negative value, as seen in the example described in the above-mentioned Japanese Patent Application Laid-Open No. 59-23313, and it is difficult to obtain a good coma. It was difficult to say that the aberrations had been corrected.

(Object of the invention) An object of the present invention is to eliminate the above-mentioned drawbacks, and to provide a large-diameter aspherical single lens that is not excessively sensitive to parallel eccentricity and that even the sine condition is well corrected. .

(Summary of the Invention) The large-diameter aspherical single lens according to the present invention is a biconvex single lens, as shown in the optical path diagram of FIG. 1, and both lens surfaces of the single lens are formed of aspherical surfaces. , for the aspherical surface of the lens surface on the far side of the conjugate point of both aspherical surfaces, the shape of the aspherical surface is defined as The curvature of the aspherical vertex: Conic constant Azl: As the aspherical coefficient, when expressed as: 0<K<5 (1) 0 < A2 <0.13 ' (2) The condition of (2) is satisfied, and the single lens When the center thickness is d and the focal length is f, the following conditions are satisfied: 0.3 < d/ r < o, s (3).

The above condition (1) is mainly for properly correcting spherical aberration. When the value of K increases beyond the upper limit of this condition, positive higher-order spherical aberration occurs, and when the value of K exceeds the lower limit,
When the value of is small, negative high-order spherical aberration occurs, and in any case, it becomes difficult to maintain good performance 2. Condition (2), together with condition (1), is for correcting the sine condition favorably. The aforementioned Japanese Patent Application Publication No. 59-2331
In the examples disclosed in Public Book No. 3, −1<K
However, in the index term, it is set as A2-0,
Furthermore, A b , A 1. 'A n, hl!
, h14. h16 Although a design example is presented in which a biproportional aspherical term is introduced, according to the present invention, A h , A
Good spherical aberration correction is achieved by introducing an aspherical term called A2 that is proportional to h2 without introducing a higher-order aspherical term called q... which is expected to be accompanied by processing difficulties. However, it also makes it possible to satisfactorily correct the sine condition. In addition, the conditions according to the present invention (
3) is a condition for not increasing sensitivity to parallel eccentricity. If the value of d/f becomes smaller than the lower limit of this condition, it becomes difficult to secure the edge thickness of the lens, and if the upper limit is exceeded, the sensitivity to parallel eccentricity increases, making it difficult to manufacture lenses from plastic. It becomes an obstacle.

(Example) The specifications of the first to tenth examples of the large diameter single lens according to the present invention are shown in Tables 1 to 10 below.

However, of the two points that are conjugate to each other regarding the lens, the radius of curvature of the lens surface (first surface) on the side of the far conjugate point is R, and the radius of curvature of the lens surface (second surface) on the side of the near conjugate point is R2, and d is the center thickness of the lens, n is the refractive index of the lens, K is the conic constant, and A is the aspherical shape expression described above.
z i is the 21st-order aspheric coefficient, t is the thickness of the parallel plane plate placed between the object surface to be measured and the second surface of the lens, N
G Let the refractive index, W, and D of the plane-parallel plate represent the distance between the plane-parallel plate and the second surface of the lens, that is, the working distance.

In addition, in order to show the sensitivity of parallel decentering, the Strehl intensity when the first surface of the lens is parallel decentered by 5μ with respect to the second surface.
The amount of decrease in was expressed as ΔSD (%). For Strehl strength, for example, Max Horn.

Co-authored by Emil Wurtz, Principles of Optics.
sof 0ptics) 6th edition (published by Bergamon Press), Chapter 9, pages 460-462. Note that the refractive indexes n and N are values for a light beam having a wavelength λ=7800 m.

Not 1 (IJI (I square) f = 4.36 N, A, = 0.5R + = 3
．． 17634 d=3.3 n=1.48
529Rt= 5.37928 t-1,2n+m N= 1.55W,
D, = 1.90 Δ5D = -0.5% aspheric coefficient f = 4.36 N, 8, = 0.5R + -4,0
2409d = 3.3 n = 1.48529R
,=-5,37957 t=1.2mm N=1.55 W, D, = 1.90 Δ5D=-0; 3% aspheric coefficient R2-9,49134 t=1.2mm N = 1.55 W, D, = 2.58 Δ5D = -0.2% aspherical coefficient ↓ ↓ W, D, = 2.37 ΔSD - -0.3% aspherical coefficient Dog 5 (i! Bow (1 fire) 11, D, = 2.80 Δ5D = -Q, 9
%Aspheric coefficient table 6 (6th example) 11, = -7,83168 t = 1.2mm N = 1.55W, D, =
1.90 Δ5D--2,0% Aspheric coefficient table Knee U property) Aspheric coefficient W, D, = , 1.90 ΔSD='-0,4%
Aspheric coefficient table mΔkai) f=4.36 N, A, =0.5R+ =
2.70798 rl = 3. On=1.48
529Rt= 7.84797 t= 1.2+nm N=1.55, D,=
1.90 Δ5D=-1.8% Aspheric coefficient f=4.36 N,^,=0.5 R,=5.32745 d=3.01 n=1
．． 48529Rz= 8.02084 t= 1.2mm N= 1.55W, D, =
1.90 Δ5D=-1.6% Aspherical coefficient Various aberration diagrams for the above-mentioned 1st to 10th embodiments are shown in order, respectively. B
) (C). In each aberration diagram, (A) is spherical aberration,
(B) shows astigmatism, and (C) shows coma aberration, and the sine condition is also shown with a dotted line in the spherical aberration diagram.

From each aberration diagram, it can be seen that although each example has a large aperture of N, A, = 0.5, spherical aberration and astigmatism are well corrected, and the sine condition is also well corrected, and coma aberration is reduced. It is clear that it is well balanced throughout the field of view.

(Effects of the Invention) As described above, the present invention provides the following effects.

(1) Since an aspherical single lens that not only spherical aberration but also the sine condition is well corrected is achieved, it is a general-purpose, high-pressure single lens that can withstand practical use even when a somewhat large angle of view is required. can be provided.

(2) Furthermore, since the sensitivity to parallel eccentricity is not excessively high, the lens is useful in that it is easy to manufacture and adjust when it is manufactured from plastic.

[Brief explanation of the drawing]

Fig. 1 is an optical path diagram showing the configuration of a large diameter single lens according to the present invention, Fig. 2 (A) (B) (C) to Fig. 11 (A) (B)
(C) is a diagram of various aberrations of the first to tenth embodiments according to the present invention, in which (A) represents spherical aberration, (B) represents astigmatism, and (C) represents coma aberration, respectively. [Explanation of symbols of main parts]

Claims (1)

[Claims] In a biconvex single lens, both lens surfaces of the single lens are formed of aspheric surfaces, and the aspheric shape of the lens surface on the far conjugate point side of both the aspheric surfaces is where: √(1-KC^2h^2)]+Σ^5
When expressed as _i_=_1A_2_ih^2^i, the following conditions are satisfied: 0<K<5 (1) 0<A_2<0.13 (2), and the center thickness of the single lens is d, and the focal length is f. A large-diameter single lens that satisfies the following condition: 0.3<d/f<0.8 (3).