JPH04125608A - Image-forming lens large in diameter of finite system - Google Patents

Abstract

PURPOSE:To obtain an image-forming lens with short distance between a material and an image and with satisfactory aberration by satisfying a specific condition by a compound lens formed by joining aspherical layers made of transparent material on two planes at the material and image sides of a biconvex spherical glass lens, respectively. CONSTITUTION:A condition represented in equations I-IV can be satisfied assuming the focal distance of the whole systems as (f), image-forming magnification as (m), the apex radius of curvature of a material side aspherical plane as (r1), the radius of curvature at material side of a spherical glass lens L2 as (r2), that at image side of the lens as (r3), the apex radius of curvature of an image side aspherical plane as (r4), the thickness on axis of a material side aspherical layer L1 as (d1), the thickness on axis of the spherical glass lens L2 as (d2), the thickness on axis of an image side aspherical layer L3 as (d3), the refractive index of the material of the material side aspherical layer L1 as (n1), that of the material of the spherical glass lens L2 as (n2), that of the material of the image side aspherical layer L1 as (n3), the height of an axial edge beam of light in the image-forming magnification (m) from an optical axis on the material side aspherical plane as (h1), and the height of the axial edge beam of light in the image-forming magnification (m) from the optical axis on the image side aspherical plane as (h4). Thereby, the distance between the material and the image can be reduced, and the aberration can be improved.

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present station is an objective lens that forms a reduced image of an object at a finite distance, has diffraction-limited imaging performance, and is suitable for playing video discs. Regarding large aperture lenses.

(Prior art and problem to be solved by the invention) The objective lens for playing back video discs has a numerical aperture (NA) of 0.5.
Although the objective lens for compact disc playback may be NAO145, a larger aperture is required.

In addition, in order to reduce the size of the device, it is advantageous to use a finite system imaging lens that directly reduces and images the finite light from the semiconductor laser without using a collimator, but in this case, the imaging magnification is m. The numerical aperture is NAm.If the numerical aperture converted to an infinite system is NAoO, then the relational expression NAoO=(1-m)NAm(mho) holds true. In order to make the device more compact, it is advantageous to increase the size of 1 ml and reduce the focal length of the imaging lens.

If NAm=0.5, when m=-0,25 NAoo=0.625r
When n = -0, 4, NAoo = O17 and when increasing the imaging magnification 1ml of a finite system, N converted to infinity.
A must be made significantly larger. As a method of manufacturing this large-diameter lens, it is extremely difficult to eliminate spherical aberration and sine conditions to the diffraction limit when using a single aspherical lens made of plastic, which has a low refractive index. In addition, spherical aberration changes due to changes in the refractive index and expansion and contraction of volume due to changes in temperature and humidity of the material. This phenomenon is NA
The larger the OO, the more significant it is, so the practical limit of NAOO remains at 0.55.

On the other hand, aspherical large-diameter single lenses made by glass molding are stable enough that there is no need to consider changes in refractive index due to changes in temperature and humidity, or the effects of expansion and contraction of volume, and the refraction of the material. It is advantageous in that it has a large ratio, and even when NA■ is 0.7 or more, it is possible to achieve good spherical aberration and sine conditions (see Japanese Patent Laid-Open No. 64-25113).

However, mass production of large-diameter aspherical lenses by glass molding is currently difficult due to the precision molding process, short mold life, and high costs.

Composite lenses are made by bonding an aspherical layer made of a transparent material to the convex surface on the object side of a spherical glass lens.If the aspherical layer is made of resin, the mold life is long and molding during mass production is easy. Since only the side surfaces are aspherical, the maximum NA■ is only 0.6, and there is also the drawback that the sine condition cannot be completely corrected (see Japanese Patent Laid-Open No. 1-145615).

When aspherical layers made of transparent material are bonded to both the object side and image side of a biconvex spherical glass lens, the NA (X) can be increased, and spherical aberration and sine conditions should be good. I can do it. However, a compound lens in which aspherical layers are bonded to both surfaces is apt to cause parallel eccentricity in the optical axis direction and tilt eccentricity during bonding, which significantly degrades the original performance and has been considered impossible for t1. Furthermore, it is natural that the aspherical layer suffers from performance deterioration due to changes in temperature and humidity, as described above for the plastic aspherical single lens, which is a disadvantage of this method.

The present invention solves the above-mentioned defects according to the following conditions, so that the NA can be increased to 0.625 or more and 0.7, but the spherical aberration is
In addition to astigmatism, coma aberration is also extremely good, making it possible to create a finite-type large-diameter lens that is mass-producible and has high performance.In addition, the device can be made as compact as possible.

(Means for solving the problem) As shown in FIG.
+ An image is formed on the image plane (2) by a compound lens Ll +L2 +L3 formed by bonding an aspherical layer L3 made of a transparent material to the image side. C is a cover glass. Although it is possible to insert a grating glass or a beam splitter between the object point O and the compound lens, this is omitted in the present invention. where f: focal length of the entire system m: imaging magnification rl: radius of apex curvature r2 of the aspherical surface on the object side; radius of curvature r3 on the object side of the spherical glass lens L2
Radius of curvature r on the image side of the bispherical glass lens L2: Vertex radius of curvature dl of the aspherical surface on the image side: Thickness on the axis of the aspherical layer L1 on the object side d2: Thickness on the axis of the spherical glass lens L2 d3: Image side Aspherical layer L
3 axis thickness nl: refractive index n2 of the material of the object-side aspherical layer L1; refractive index n3 of the material of the spherical glass lens L2
: Refractive index h1 of the material of the image-side aspherical layer L3 : Height of the axial marginal ray from the optical axis on the object-side aspherical surface at the imaging magnification m h4 : Image of the axial marginal ray at the imaging magnification m When the height from the optical axis on the side aspherical surface is 0, 75 < r + / r 2 ≦ 1.0
−(1) 0, 5 < r 4 / r 3 ≦1.0
...(2) 0, 5 < r + /
r s < 0.8 ... (3) 0.8
f <d+ +dz +d3<1.2 f...(4
)0≦T12-nl<0.4. O≦n2−n3<
Q, 4...(5) 1.1<hl/h4<1.4...
It is characterized by satisfying the following conditions (6).

Condition (1) relates to the shape of the object-side aspherical layer L1. When the value of rl/r2 becomes small, the axial thickness dl also becomes large, and the shape of the aspherical layer becomes convex meniscus-like and strong. Spherical aberration changes due to changes in the refractive index of the material of the aspherical layer due to changes in temperature and humidity, and due to expansion and contraction. The thicker the aspherical layer at the periphery, the greater the influence of the amount. Therefore, it is more advantageous to reduce the change in spherical aberration by making the peripheral portion of the aspherical layer thinner. When the lower limit of condition (1) is exceeded, the degree of convex meniscus of the aspherical layer is too strong, the difference in thickness between the upper part of the shaft and the peripheral part becomes large, and it is difficult to ensure the accuracy of the aspherical shape.

When the upper limit is exceeded, the aspherical layer becomes a concave meniscus,
Changes in spherical aberration increase with changes in temperature and humidity.

Condition (2) relates to the shape of the image-side aspherical layer L3. When the value of r4/r3 becomes small, the thickness on the axis d3
Also, the shape of the aspherical layer L3 becomes convex meniscus-like and strong. Changes in refractive index and expansion due to changes in temperature and humidity of the material of the aspherical layer.

Spherical aberration changes due to contraction. As in the case of Ll, the thicker the aspherical layer in the peripheral portion, the greater the influence.

Therefore, it is more advantageous to reduce the change in spherical aberration by making the peripheral portion of the aspherical layer thinner. When the lower limit of condition (2) is exceeded, the degree of convex meniscus of the aspherical layer is too strong, and the difference in thickness between the upper part of the shaft and the peripheral edge becomes large, making it difficult to ensure the accuracy of the aspherical shape. When the upper limit is exceeded, the aspheric layer takes on a concave meniscus shape, and changes in spherical aberration due to changes in temperature and humidity become large.

Condition (3) concerns the ratio of the apex curvature radii of the aspheric surfaces on the object side and the image side, and is used to determine the distribution of refractive power on the object side and image side, and to simultaneously improve the spherical aberration and sine conditions of the large-diameter lens. It is about. If an aspherical surface is introduced, the spherical aberration can be made zero, but in order to make the sine condition zero at the same time, it is necessary to appropriately determine the refractive power sharing between the air contact surfaces on the object side and the image side. When the lower limit of condition (3) is exceeded, the refractive power on the object side is too strong and the sine condition becomes under. When the upper limit is exceeded, the refractive power on the object side is too weak and the sine condition is exceeded, making it impossible to obtain good results.

Condition (4) is for obtaining a flat image surface without astigmatism. When the lower limit is exceeded, the image plane becomes undersized.

When the upper limit is exceeded, the image plane becomes oversized and the lens becomes heavier. Furthermore, when the imaging magnification m is small, the necessary working distance cannot be secured.

Condition (5) is respectively.

Spherical glass lens and object side aspherical layer.

Spherical glass lens and image side aspherical layer This relates to the difference in the refractive index of the materials.

When the aspherical layer and the spherical glass lens are bonded together, the smaller the difference in the refractive index of the two materials, the less the influence on the parallel eccentricity in the optical axis direction and the tilting eccentricity. Since the material used for the aspherical layer cannot have a high refractive index, choosing a material with a low refractive index for the spherical glass lens increases the amount of aspherical surface, and improves performance due to mutual eccentricity of the aspherical surfaces on the object side and image side. The decline becomes larger.

Of course, the refractive index n1 of the material of the aspherical layer on the object side and the refractive index n3 of the material of the aspherical layer on the image side need not be the same. When the lower limit of condition (5) is exceeded, performance degradation due to eccentricity between the aspherical surfaces on the object side and image side increases; when the upper limit is exceeded, performance degradation due to eccentricity between the aspherical layer and the spherical glass lens increases.

Condition (6) concerns the ratio of the height hl of the axial marginal ray from the optical axis on the object-side aspherical surface to the height from the optical axis on the image-side aspherical surface at the imaging magnification m. This is to prevent performance degradation due to eccentricity when a composite lens is made by bonding an aspherical layer to a spherical glass lens. When the lower limit is exceeded, the refractive power on the object side is weak, and the influence of eccentricity of the aspherical surface on the image side becomes large. When the upper limit is exceeded, the refractive power on the image side is weak, and the influence of eccentricity of the aspherical surface on the object side becomes large. The expression of condition (6) has a strong relationship with the imaging magnification m.

In this case, 1.5<(h+/h4)・(1-m)<1
．． 8-C7) becomes the conditional expression. When the lower limit of condition (7) is exceeded, the influence of eccentricity of the image-side aspherical surface is large;
When the upper limit is exceeded, the influence of eccentricity of the object-side aspherical surface increases.

(Example) An example of the finite system large-diameter imaging lens of the present invention will be described with reference to the drawings.

FIG. 1 is a cross-sectional view of the lens according to the present invention in an implemented state, and from the left side of the drawing, O is the object point, L, , L2, . L3 is a cemented composite imaging lens, and Ll is an aspherical layer cemented to the object side of the biconvex spherical glass lens L2.

L3 is an aspherical layer bonded to the image side of L2.

C is the cover glass, ■ is the image plane, and WD is the working distance.

TL is the object-image distance. Object point O and compound lens LL2.
Although it is possible to insert a grating glass or a beam splitter between L3, this is omitted in the present invention. The peripheral ray on the axis from the object point O is an aspheric surface on the object side (rl)
The height of the incident point P1 on the image side aspherical surface (r,) from the optical axis is hl+the height of the incident point P on the image side aspherical surface (r,) from the optical axis.

f: Focal length of the entire system m: Imaging magnification rl: Radius of curvature of the vertex of the aspherical surface on the object side r2 Radius of curvature on the object side of the bispherical glass lens L2 r3
: radius of curvature r on the image side of the spherical glass lens L2, : vertex radius of curvature dl of the aspherical surface on the image side; axial thickness d2 of the aspherical surface layer on the object side: axial thickness d3 of the spherical glass lens L2: image Side aspherical layer L
Thickness nl on the axis of 3: refractive index n2 of the material of the object-side aspherical layer L1 refractive index n3 of the material of the bispherical glass lens L2
: refractive index t of the material of the image side aspherical layer L3 : axial thickness no of the cover glass (image side) : refractive index WD of the material of the cover glass (image side); working distance TL : distance between object and image h+ /ha + Imaging magnification m Imaging magnification Ratio of the height from the optical axis on the object-side aspherical surface of the axial peripheral ray to the height from the optical axis on the image-side aspherical surface The formula for the shape of the aspherical surface is X : Distance from the tangential plane at the apex of the lens surface of a point on the aspherical surface h: Height from the optical axis C; Curvature of the aspherical apex: When conic constant A21 = aspherical coefficient (=1/r). expressed.

In addition, the RMS of the wavefront aberration of the axial light beam at wavelength λ=785 nm is described for each example in the case where the eccentricity is 0 and in the case where there is parallel eccentricity and tilt eccentricity within the range in which an aspherical surface can be manufactured.

(Margin below) f = 2.75 rt = 2.405 Table 1 (1st example) N^ = 0.5 m = -1/3J3 (-0,300
3) d+ =0.03 r2-2.8 d2 =2.77 6.0 d, =0.03 r, =-3,8831 WD=1.461 t=1.25 TL=16.701 Aspheric surface Coefficient n, -1,51550 n, =1.69043 n, =1.51550 Mouth. =1.48600 hl /h4 =1.281 (h+/hs> ・ (1-m) On-axis eccentricity at λ=785nm O rl Parallel eccentricity 0.0 O5 r4 Parallel eccentricity 0. OO5 r1 Inclination eccentricity 3' r , Eccentricity of inclination 3' = 1.666 MS O,OO2λ 0.011 λ 0.023λ 0,040 λ 0,039 λ f= 2.7502 r22.5 Table 2 (Second Example) N^ =0.5 m=-1/3.33(・-0,3
003) r, = 2.5 r, = -4,31 r, = -4,31 11D = 1.486 0.005 d2 = 2.77 d, = 0.005 t = 1.25 TL = 16. 699 Aspheric coefficient = 1.50100 n2 = 1.69043 n3 = 1.501C10 nc = 1.48600 hl / h* = 1.279 (h+/ht) ・ (1-m) On-axis eccentricity at λ = 785 nm O rl parallel eccentricity 0.005 r, parallel eccentricity 0.005 rl tilt eccentricity 3' r4 tilt eccentricity 3' =1.664 MS O,002λ 0,025 λ 0,030 λ 0,036 λ 0.035λ f= 2.75 r=2.73 Table 3 (Third Example) N^=0.5 m=-1/ 3.33 (=-0,
3003) = 0.03 n, = 1.50059 r2 = 3.27 d2 = 3.02 0, = 1.79044 rs = -5,6 ds = 0.02 n3 = 1.50059 rs knee 4.17062 D= 1.453 t4.25 rlc TL= 1 6.867 Aspheric coefficient = 1.48600 11+/ha=1.263 (hl/ha) ・(1-m)=1.642 On axis at λ=785 nm MS Tilt eccentricity 3' 0,040λ th f=2.6 rt = 2.4 r= = 2.8 r, =-4,3 r, =-3,35714 WD= 1.656 4 Table (4th Example) N^=0.5 m=-0,4 =0.025 1.50050 d2"2.65 n2 =1.69043 d, =0.015 1, 50050 t=1.25 nc=1. 48600TL=1
3.895 Aspheric coefficient, /h, = 1.166 (hl/1ta) ・ (1-m) On-axis eccentricity Or, parallel eccentricity at λ=785nm 0. O05 r4 Parallel eccentricity 0. O05 r1 Eccentricity of inclination 3' r, Eccentricity of inclination 3' = 1.632 MS O,001λ 0.046 λ 0.013 λ 0,043 λ 0,050 λ f=2.6 Table 5 (5th implementation Example) N^= 0.5 m=-0,4 2,95 n, =1.79044 HD= 1.571 t=1.25 TL=14.072 Aspheric coefficient n e=1.48600 Inclination eccentricity 3' 0.042% f, 3 rt = 2.6 Table 6 (Sixth Example) N^=0.5 m=-0,25cl, =0.0
3 r, = 3.0 112 = 3.07 r, = -5,45 d3 = 0.01 ra = -4,48042 WD = 1.499 t = 1.25 TL = 20. D47 Aspheric coefficient n, = 1.50050 nz = 1.69043 ns = 1.50050 nc = 1.48600 h, /h, = 1.326 (h, /h,) (1-rn) λ = 785nrn On-axis eccentricity O rl Parallel eccentricity 0.005 r4 Parallel eccentricity 0.005 r, Tilt eccentricity 3' r, Tilt eccentricity 3' = 1.658 MS 0000λ 0.018 λ 0.018 λ 0, O35λ 0. 036 λ fth = 3 r, -3,05 r2-3.65 rs = -5,2 r4 = -4,40316 111D = 1.501 7 Table (7th example) N^ = (1,5m = -0,25 ;00 mouth 3 = 1.50050 d2 = 3.41 n2 = 1.79044 d3 = 0.015 n3 = 1.50050 t = 1.25 TL = 20.221 Aspheric coefficient nc = 1.48600 hI /h, =1.280 (h+/ha) ・ (1-m) On-axis eccentricity at λ=785nm O rl Parallel eccentricity 0. OO5 r, Parallel eccentricity 0.005 r1 Inclination eccentricity 3' r, Inclination eccentricity Eccentricity 3' = L600 MS O,000λ 0.056 λ 0,022 λ 0.023λ 0,042 λ f,3 r, = 2.3 Table 8 (Eighth Example) N^= 0.5 m= -0,25d1=0.03 r, =2.6 d2=2.65 rs =-4,7 d, =0.01 r, =-4,15713 111D=1.628 t=1.25 TL= 19.856 Aspherical coefficient n, =1.5[105(] n2 =1.59302 n3=1.50050 me =1.48600 hI /h4 =1.272 (hl/h4) ・ (1-m) λ = On-axis eccentricity at 785 nm O rl Parallel eccentricity 0. OO5 r, Parallel eccentricity 0. OO5 r1 Tilt eccentricity 3' r, Tilt eccentricity 3' = 1.590 MS O,001λ 0.018 λ 0,043 λ 0 .050 λ 0.038λ The spherical aberration and sine conditions of the first example are shown in Fig. 2 (A), the astigmatism is shown in Fig. 2 (B), and the coma aberration is shown in Fig. 2 (C). The aberrations of the second embodiment to the eighth embodiment are sequentially shown in FIGS. 3 to 9.

As is clear from Tables 1 to 8, in these examples, the focal length is short, 2.6 to 3, and the imaging magnification m is large, 0.4 to 0.25.

The inter-image distance is extremely short at 13.9 to 20.2, and the NAoO is large at 0.7 to 0.625. The limit NAoO that can simultaneously satisfy spherical aberration and sine conditions is 0.
There is also an established theory that it is 7071. It is clear from the aberration diagrams that the embodiment of the present invention with NA■=0.7 is close to that, and that in addition to spherical aberration and sine conditions, astigmatism and coma are also very well corrected.

In order to avoid the complexity of the description, the performance degradation due to eccentricity is described only for the axial light beam, but the RMS of the wavefront aberration at the maximum image height shown in the aberration curves of each example is also within 0.07λ.

(Effects of the Invention) As explained above, the present invention provides an aspherical layer made of a transparent material on both the light source side and the image side of a biconvex spherical glass lens in a lens that reduces and forms an image of an object at a finite distance. By cementing these together to form a compound lens, the NAoO can reach a maximum of 0.7.

The minimum image distance is 13.9, which is close to the minimum, making it possible to miniaturize the device, making it an imaging lens with good aberrations, and little deterioration in performance due to eccentricity.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a state in which the lens of the present invention is implemented. FIG. 2 (A> is an aberration curve diagram of the spherical aberration and sine condition of the first embodiment, FIG. 2 (B) is an astigmatism curve diagram of the first embodiment,
FIG. 2(C) is a coma aberration curve diagram of the first embodiment. FIG. 3 to FIG. 9 are aberration curve diagrams similar to FIG. 2 in the second to eighth embodiments. ■: Object point L2. L3: Cover glass: Image plane: Lens of the present invention

Claims (1)

[Claims] On both the object side and image side of a biconvex spherical glass lens,
In a lens that forms a reduced image of an object at a finite distance, each consisting of a compound lens formed by bonding aspherical layers made of a transparent material, f: Focal length of the entire system m: Imaging magnification r_1: Vertex of the aspherical surface on the object side Radius of curvature r_2: Radius of curvature r_3 on the object side of the spherical glass lens:
Radius of curvature on the image side of the spherical glass lens r_4: Vertex radius of curvature of the aspherical surface on the image side d_1: Thickness on the axis of the aspherical layer on the object side d_2: Thickness on the axis of the spherical glass lens d_3: Axis of the aspherical layer on the image side Upper thickness n_1: Refractive index of the material of the object-side aspherical layer n_2: Refractive index of the material of the spherical glass lens n_3: Refractive index of the material of the image-side aspherical layer h_1: Object of the axial marginal ray at the imaging magnification m Height from the optical axis on the side aspherical surface h_4: Height from the optical axis on the image side aspherical surface of the axial peripheral ray at the imaging magnification m: 0.75<r_1/r_2≦1.0 ...(1)0.5
<r_4/r_3≦1.0...(2) 0.5<r_1
/｜r_4｜<0.8...(3) 0.8f<d_1+
d_2+d_3<1.2f...(4)0≦n_2-n
_1<0.4, 0≦n_2-n_3<0.4...(5
) 1.1<h_1/h_4<1.4...(6) A finite system large-diameter imaging lens characterized by satisfying the following conditions.