JPS60250321A - Large aperture single lens - Google Patents

Abstract

PURPOSE:To obtain a titled lens which has corrected an aberration on the axis and outside the axis, and has a numerical aperture of >=0.4 by forming an aspherical surface of a positive meniscus single lens whose first surface is positive and an aspherical surface, and whose second surface has a negative refractive power, to a prescribed shape, and also satisfying prescribed conditions. CONSTITUTION:The first surface R1 being an aspherical surface of a lens 1 is formed as shown by an expression I. In this expression, Z:(y):(c):(k): and D, E, F and G denote a distance from a tangential plane of an aspherical surface apex of a point on an aspherical surface whose height from an optical axis is (y), a height from the optical axis, a curvature 1/R of the aspherical surface apex, a cone constant, and coefficients of aspherical surfaces, respectively. Also, this optical system satisfies various conditions of an expression II - an expression V. In this regard, in these expressions, n1:R1:R2: and d1 denote a refractive index in a working wavelength of the lens 1, a radius of curvature of the aspherical surface apex of the first surface, a radius of curvature of the second surface, and a thickness of the center of the lens 1, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an objective lens essential for video discs, audio discs, etc., and particularly relates to a large single lens with one surface being spherical and the other surface being aspheric. It is something.

(Conventional configuration and its problems) Objective lenses used for video discs, audio discs, etc. not only have sufficient correction of aberrations, but also are inexpensive, and are also small and lightweight due to the need for autofocus and tracking servo. It is strongly requested that An objective lens made of a spherical glass lens requires a combination of three to four lenses in order to satisfy the required optical performance, making it difficult to meet the above-mentioned demands.

The inventor of the present invention has published Japanese Patent Application Publication No. 58-68711,
No. 231,256 has been filed for a single-lens objective lens, and the present invention relates to a lens with a different solution from those inventions.

(Objective of the Invention) The present invention solves the above-mentioned drawbacks, and by introducing only one aspherical surface, the numerical aperture is reduced to 0, which corrects not only the axial aberration but also the off-axis aberration with a viewing angle of about +1 degree. This provides a large-diameter single lens with a diameter of .4 or larger.

(Structure of the Invention) The large-diameter single lens of the present invention is a positive meniscus single lens whose first surface is an aspherical surface with positive refractive power and whose second surface is a spherical surface with negative refractive power. Aspherical force S1 Z: Distance from the tangential plane of the aspherical vertex of a point on the aspherical surface with height y from the optical axis, y: Height from the optical axis, C: Curvature of the aspherical vertex (=1 /R), k: conic constant, D, E, F, G: aspheric coefficient, n: refractive index at the operating wavelength of the lens, R1: radius of curvature of the aspheric apex of the first surface, R2 : radius of curvature of the second surface, d, : center thickness of the lens, then the following conditions, nl ) 1.8 ・・・・・・・・・(1) −1,0<
k<O... (2) This is a large-diameter single lens that satisfies (2). The lens is realized.

(Description of Embodiment) Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows the structure of an embodiment of a large-diameter single lens according to the present invention, where 1 is a large-diameter single lens, 2 is a disk cover glass, dl and d2 are center thicknesses of the lens, R1,
R2 represents the radius of curvature, and WD represents the working distance.

Next, conditions (1) to (4) described in the configuration of the invention will be explained.

If the lower limit of condition (1) is exceeded, spherical aberration cannot be corrected satisfactorily under the condition that the second surface has negative refractive power.

If the upper limit of condition (2) is exceeded, an under-high-order spherical aberration will occur, and conversely, if the lower limit is exceeded, an excessive high-order spherical aberration will occur and the spherical aberration cannot be corrected completely.

Condition (3) is a spherical thin lens with a refractive index of n, the diaphragm is the lens itself, and the radius of curvature at which the third-order coma aberration coefficient becomes zero when the object distance is infinite. The range in which the ratio R1/R3 of the radius of curvature of the present invention is shifted from the ratio is given. By satisfying condition (3), the introduction of an aspherical surface has a great effect in correcting spherical aberration and coma aberration. If the lower limit of condition (3) is exceeded, spherical aberration will be overcorrected in the largest annular zone, and high-order comatic aberration will occur in the largest annular zone. If the upper limit of condition (3) is exceeded, spherical aberration will be insufficiently corrected and high-order comatic aberration will occur in the largest annular zone.

If the upper limit of condition (4) is exceeded, spherical aberration will be insufficiently corrected in the largest annular zone, and comatic aberration will increase in the intermediate annular zone. If the lower limit of condition (4) is exceeded, spherical aberration will be overcorrected.

Examples 1 and 2 described below satisfy these conditions.

The operating wavelength in these examples is 800 nm, where f is the focal length, NA is the numerical aperture, WD is the working distance, and R1
is the radius of curvature of the first surface, di is the thickness, k is the conic constant of the first surface, D, E, F, and G are aspherical coefficients of the first surface, and ni is the refractive index at the operating wavelength.

Example 1 f = 4.6 mm NA = 0.47 WD = 2.
4081R1=3.5679, d1=2.453 n
,=1.82361R2=42.2184 R3=(X) ,d2. =1.2 R2””1.571
58R4-ω=-4,94768X10-' D-4,49579×1O-5 E=-1,60811X 1O-6 p=-6,69464X 10-'G=-1,0916'3X10-7 Example 2 f = 4.6mm NA = 0.47 Vl/D = 2.
1325R, = 3.7827 a1 = 2.967 n,
=1.89502R2=29.297°R=cx> d2=1.2 n2=1.57158R4
-o.

k=-4,624-77X10'-' D=5.49251 X 10= E=-5,4,5543X10'F=-3,54253X10' G=-3,26076X 10-9 The figures are characteristic diagrams showing aberration performance in Example 1 and Example 2 of the present invention, respectively, and show spherical aberration, astigmatism, and coma aberration.

In the diagram of astigmatism, the solid line shows sagittal curvature of field, and the dotted line shows meridional curvature of field. In the diagram of coma aberration, the solid line shows meridional coma aberration, and the dotted line shows sagittal coma aberration. As shown in these aberration diagrams, the large-diameter single lens of the present invention has both spherical aberration and comatic aberration well corrected.

(Effects of the Invention) As is clear from the above description, the present invention provides a large-diameter objective that corrects aberrations close to the diffraction limit with a single lens by making one surface spherical and the other aspheric. lenses can be realized.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an embodiment of a large-diameter single lens of the present invention.
FIGS. 2 and 3 are characteristic diagrams showing aberration performance in Example 1 and Example 2 of the present invention, respectively. 1...Large diameter single lens, 2...Cover glass. Fig. 1 Fig. 2 Comatic aberration (mm) Fig. 3 Comatic aberration (mm) -III-A aberration (mm)] F'4
Difference (mm)

Claims (1)

[Claims] A positive meniscus single lens consisting of an aspherical surface whose first surface has a positive refractive power and a spherical surface whose second surface has a negative refractive power, wherein the aspherical surface is Distance from the tangent plane of the aspherical apex of a point on the aspherical surface with height y, i: Height from the optical axis, C: Curvature of the aspherical apex (=1/R), k: Conic constant, D, E, F, G: aspherical coefficients, nl: refractive index at the operating wavelength of the lens, R1: radius of curvature of the apex of the aspherical surface of the first surface, R2: radius of curvature of the second surface, dl : Center thickness of the lens, A large-diameter single lens that satisfies the following condition nl>1.8-1.0<k<,0.