JPH02284107A - Distributed refractive index type lens system - Google Patents

Abstract

PURPOSE:To well correct an offaxial aberration which is the drawback of a distributed refractive index type single lens by forming the end faces of the single lens as concave faces, i.e. providing a negative refracting power on these faces. CONSTITUTION:The lens length z0 of the distributed refractive index type lens 3 which is highest in the refractive index distribution n(r) on the z-axis, i.e. the optical axis and is lowered in the refractive index with the rotationally symmetrical distribution of n<2>(r, z)=n0<2>(z)x {1-g<2>(z)r<2>+h4(z)g<4>(z)r<4>+h6(z)g<6>+(z)r<6>+...} as removed from the optical axis by a distance r is determined in the range of equation I to form an erecting real image 2 and the concave refracting power is provided on both end faces 4, 4'. The curvature of field at the meridian cross section which is heretofore not corrected is well corrected simultaneously with the spherical aberration in this way and the range of the images usable with the single lens is widened.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a lens system using a gradient index lens, a so-called rod lens.

(Prior art) In the conventional lens of gradient index type, light 11qlI
The so-called 5ELFOC[F] is known as having a refractive index 11 that is rotationally symmetrical with respect to the optical axis. This 5ELFO
The refractive index distribution n (r) of C[F] is expressed by the formula using the distance from the optical axis. Here, no and g are constants representing the refractive index distribution. This gradient index lens is Since it has positive refractive power, if the lens length 2° is set in the range of −〈g 2° × π, it becomes an iE erect real image imaging system.Also, when this lens is arranged in an array, it becomes an erect image system. Since it can form a double imaging system, it is widely used in copying machines, facsimile machines, etc.

(Problem to be Solved by the Invention) However, the conventional single lens described above has the drawback that although axial aberrations can be corrected by adjusting the shape of the refractive index distribution, off-axis aberrations cannot be corrected. was. In other words, the axial aberration is calculated by introducing a higher-order term to the refractive index distribution n (r), such as n(r)-nosech(
It is known that by approaching the shape of gr), it is possible to bring it almost to 0.

On the other hand, the off-axis aberration is n. The curvature of field determined by gZo inevitably remains, and cannot be improved even if the refractive index distribution in the radial direction is corrected, such as on the axis. For this reason, optical systems that use off-axis light rays, or lens arrays that overlap off-axis light rays with adjacent lenses to form images, naturally have a limit to their optical performance.

FIG. 7 is an explanatory diagram showing the performance of the conventional single lens described above. In the figure, in the case of one object, 2 indicates a paraxial image plane, and the broken line 2' indicates a curved image plane. 3, which is a gradient index lens, has a 5ech type distribution constant, as seen from the parameters in Table 3. Therefore, the ray A representing the imaging of the axial ray bundle is focused on the axis without aberration, but the ray B representing the imaging of the off-axis ray bundle is focused at a position close to the object plane due to field curvature. ing. FIG. 8 shows the spherical aberration of the school lens shown in FIG. 7, and FIG. 9 shows the astigmatism and field curvature.

An object of the present invention is to provide a gradient index lens system that can satisfactorily correct off-axis aberration 1, which is a drawback of the conventional single lens described above.

(Means for Solving the Problems) The problem of field curvature of off-axis light beams, which is a drawback of the above-mentioned conventional example of the present invention, is solved by making the end surface of the gradient index single lens concave, that is, having negative refractive power. This is solved by this.

Even if the end surface is made concave as in the present invention, the spherical aberration is still lower than that of the conventional 5.
It is possible to maintain the same performance as that of the ech distribution.

On the other hand, as an effect unique to the concave surface of the present invention, it has become possible to correct field curvature within the meridional section, which could not be achieved conventionally by optimizing only the higher-order constants of the refractive index distribution. Furthermore, if the single lens according to the present invention is applied to a lens array for erecting equal-magnification imaging, image blur caused by field curvature of individual lenses can be corrected. As a result, the MTF as an array is improved for each frequency, making it possible to achieve a higher image quality than ever before.

(Embodiment) FIG. 1 is a schematic diagram of a first embodiment that most clearly represents the present invention.

In the figure, 1 is an object plane, 2 is an image plane, and 3 is a gradient index single lens according to the present invention. As is clear from the figure, the feature of this embodiment is that both end surfaces of the gradient index lens are concave. As in the case of the conventional example, ray A represents the imaging of the axial ray, and ray B represents the imaging of the off-axis ray. This shows that it is now possible to simultaneously correct spherical aberration and curvature of field in the meridional section, which could not be achieved by changing the refractive index distribution in the radial direction.

Table 1 shows the design values of - corresponding to the embodiment shown in Figure 1.
・This is an example. The lens is completely symmetrical, has an aperture diameter of 1.1 mm, and has a refractive index distribution of 5ech type. The feature of the present invention is the concave surfaces at both ends, with a radius of curvature of 2.823 mm.The spherical aberration for this numerical example is shown in Figure 2, and the astigmatism and curvature of field are shown in Figure 3. show.

When compared with FIGS. 8 and 9, which are aberration diagrams of the conventional example of the 5ech distribution shown in FIG. ,
It can be seen that the blur has been corrected.

The application of the lens shown in FIG. 1 is not limited to a single lens, but it is also highly effective when used as an erect equal-magnification lens array in devices such as copying machines and facsimile machines. That is, when used as a lens array, an image is formed as a superposition of images from a plurality of lenses. Therefore, for some lenses that form overlapping images, off-axis light beams are used for imaging. Blur caused by field curvature results in loss of image contrast. Therefore, the effect of improving the image plane according to the present invention is remarkable, and M
An improvement in TF is achieved. As a result, according to the present invention, it is possible to transfer and copy high-quality, high-definition images.

Another embodiment of the invention is shown in FIG. In FIG. 4, the distributed index lens has a conventional planar shape at both ends, but plano-concave lenses 41 and 42 are arranged on the outside as a means for giving symmetrical concave refractive power. Of course, the plano-concave lens may be a gradient index lens, but in this example, a homogeneous lens is used. The effect of this concave lens is similar to that of the first embodiment, and it satisfactorily corrects the curvature of field within the meridional section while keeping the spherical aberration the same as that of the conventional lens.

The marks in Table 2 are numerical examples in the second embodiment. Here, the commonly used 12° type 5ELFOC ■ lens has a radius of curvature of 3.6 mm, JI, f of 3. LOm
It has a configuration in which a plano-concave lens of m is attached. Further, these three lenses are a coaxial system. FIG. 5 shows spherical aberration corresponding to the numerical examples in Table 2, and FIG. 6 shows astigmatism and field curvature. As shown in the figure, it can be seen that exactly the same effect as in the first embodiment is achieved.

In the embodiment shown in FIG. 4, three lenses are closely bonded to each other, but as long as the coaxial system conditions are maintained, a medium of equal refractive index, such as air, may be sandwiched between the individual lenses. Further, in Table 2, the refractive indexes of the concave lenses at both ends are the same as the central refractive index of the single lens of the refractive index distribution type at the center, but they may be configured to be different.

Even if glass materials with different refractive indexes are selected, the effects of the present invention can be achieved as long as the optimum radius of curvature and lens thickness are set.

If the combination of three lenses shown in this embodiment is considered as one single lens unit, a lens array can be constructed by arranging these single lens units. In this case, it is necessary to prevent crosstalk between each single lens unit. As a further development of the lens array, there are three lens arrays with exactly the same planar arrangement: one is a conventional gradient index lens array, and the remaining two are gradient index lens arrays. It is also conceivable to prepare a lens array of concave lenses having exactly the same arrangement and to arrange these in series and in close contact with each other with their optical axes aligned. In this case, individual single lens units whose optical axes coincide when brought into close contact are constructed as shown in FIG. 4.

A feature of the present invention is that concave lenses are added to both ends of the gradient index lens. The concave lens in this case can be said to be a field flattener in a broad sense.

Front lens diameter: 1.1 mm T C=
49.5mm lens length Zo s 19.067m+
n center refractive index no 1.6197 quadratic distribution constant g 0.2:12714th distribution constant h
4・0.801 (constant along the optical axis) end surface R・−2,
82344 R' 2.82344 Table 2 Lens diameter Lens length Z.

1.1 mm T Cg, 49.5 mm-
23,269mm (3,097++7.076+3.097)1.6]9
7 0.23271 0.524 1.6197 Center refractive index n.

Quadratic distribution constant g Quaternary distribution constant h4・Refractive index of end lens R--3,61425 R” 3.81455 Table 3 Lens diameter: 1.1 mm T C=
49.5n+m Lens length 20 -14.804mm Center refractive index no 1.6197 Second-order distribution constant g 0.23271 Fourth-order distribution constant h4-0.667 (Sech distribution) Refractive index difference Δn'-Δn = 0.0132 (Effect of the invention) To express what has been explained above more generally, the refractive index on the Z axis, which is the optical axis, is the highest, and as the distance from the optical axis increases, n2 (r, z) = n02 (z) (+ -g”(z)r
2+h4(z)g'(z)r'◆ha(z)g'(Z)
In a gradient index lens where the refractive index decreases with a rotationally symmetrical distribution called r''''''), the lens length z0
By forming an erect real image in a range such that We have achieved an optical device that is well-corrected.

By applying the present invention, the range of images that can be used with a single lens can be increased. Further, when the configuration of the present invention is applied to a lens array, image blur due to correction of field curvature is reduced, so the MTF as a whole system is improved and a high-resolution lens array can be obtained.

The addition of concave power according to the present invention is easy in terms of processing, and even when applied to lens arrays, parts manufacturing is difficult compared to conventional methods.
The method used in the membrane can be used as is. Nowadays, there is a demand for high-quality, high-definition image formation, and the present invention can be realized by directly applying the conventional technology, and is highly effective.

[Brief explanation of drawings]

Fig. 1 is an optical path diagram of the first embodiment showing the concept of the present invention, Fig. 2 is a spherical aberration diagram of the first embodiment, and Fig. 3 is a diagram showing astigmatism and field curvature of the first embodiment. Aberration diagrams, FIG. 4 is an optical path diagram of the second embodiment of the present invention, FIG. 5 is a spherical aberration diagram of the second embodiment, and FIG. 6 is an aberration diagram showing astigmatism and field curvature of the second embodiment. Figure 7 shows s e which is considered to be the best in the conventional example.
Optical path diagram of a lens with c h (gr) type distribution, No. 8
The figure is a spherical surface and aberration diagram of the lens system of FIG. 7, and FIG. 9 is an aberration diagram showing astigmatism and field curvature of the lens system of FIG. In the figure, 1 is the object plane, 2 is the image plane, 2' is the curved image plane, and 3
is a gradient index lens, 4 and 4' are surfaces with concave refractive power or lenses with concave refractive power, ray A is a ray that indicates the imaging state of an axial ray, and ray B is a condensation of an off-axis ray. This is a light ray that indicates the state of the image. Diagram

Claims (3)

[Claims] (1) Refractive index distribution n(
r) is the highest on the optical axis, and as the distance r from the optical axis increases, n^2(r) = n_o^2[1-g^2r^2+h_4
g^4r^4+h_6g^6r^6+ (higher-order term)] In a gradient index lens that decreases while having a rotationally symmetric distribution, the lens length z_o is expressed as (π/2)<(g
z_o)/(2<π) and negative refractive power is arranged at both ends of the lens. (2) Claim 1 characterized in that the refractive index gradient lens system is used in a state where an erect real image is formed.
The gradient index lens system described above. (3) The gradient index lens system according to claim 1, wherein a plurality of the gradient index lens systems are arranged.