JP3238479B2 - Imaging optics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an imaging optical system used as an optical system of, for example, an optical information recording / reproducing apparatus.
In particular, the present invention relates to an imaging optical system having a focusing function for correcting defocus due to a change in the position of an object position in the optical axis direction.

[0002]

2. Description of the Related Art As a focusing system of an imaging optical system, generally, an entire extension system for moving an entire lens system is often used. In this method, the size of the image does not change when the object is at infinity, that is, when the incident light beam from the object to the lens system can be regarded as a parallel light beam.
However, in order to move the entire lens system, the driving mechanism is necessarily large.

In a telephoto lens, an inner focus method in which focusing is performed by moving a part of the lens groups, and in a zoom lens, a front lens group in which focusing is performed by moving a lens closest to the object. The method is adopted.

[0004]

However, in the conventional focusing method described above, when focusing is performed to eliminate defocus due to a change in the position of an object, the imaging magnification may change. In an optical system that forms and uses a single imaging spot, a change in magnification has little effect.For example, in a multi-beam optical system that forms and uses a plurality of spots with a plurality of beams, a change in magnification causes The distance between the spots changes, which may cause inconvenience in using the optical system.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to provide an image forming optical system in which the image forming magnification does not change even by focusing accompanying a change in object distance. I do.

[0006]

In order to achieve the above object, an image forming optical system according to the present invention comprises, in order from an object side, an objective lens, a relay lens, and an optical axis centered on a focusing reference position for focusing. A focusing lens movable in the direction is arranged, and a relay lens is provided on the rear side of the objective lens with the distance from the rear principal point of the focusing lens at the focusing reference position to the imaging point being dx. Focus and focusing reference position
Distance dx from the front principal point of the focusing lens at
And having a function of the point of partial object side Conjugate.

[0007]

Embodiments of the present invention will be described below.

Generally, in an inner focus type lens system, the focal length changes due to focusing, and the imaging magnification also changes. In the optical system according to the present invention, a relay lens is provided between the objective lens and the focusing lens, and the distance from the rear principal point of the focusing lens to the imaging point at the focusing reference position is d by the relay lens.
x is the back focus of the objective lens and the focusing
By the point and the Conjugate from front principal point distance dx content object side of the focusing lens at the reference position, thereby suppressing changes in magnification due to focusing.

First, the magnification of an optical system composed of three lens elements will be described, and the principle of the present invention will be described.

In the following description, the power of the first lens is A, the power of the second lens is B, the power of the third lens is C, and the power of the second lens is from the rear principal point of the first lens. The distance from the front principal point to the front principal point of the second lens is dx1, the distance from the rear principal point of the second lens to the front principal point of the third lens is dx2, and the distance from the rear principal point of the third lens to the imaging point is dx1. And Each lens is a finite imaging system, and its powers A and C both take values other than 0.

The composite imaging magnification m of this optical system is X = A−
Assuming ABdx1 + B and D = dx2 + dx, the following equation (1) is obtained.

[0012]

M = ((1-dxC) (1-Adx1- (D-dx) X) -dxX) ^-1 (1)

The change in magnification due to focusing is given by the following equation.

[0014]

[Number 2] δm / δdx = C (1- Adx1- (D-2dx) (A-ABdx1 + B)) m 2 = C (1-Adx1- (dx2-dx) (A-ABdx1 + B)) m 2 = -C ((dx1-1 / A) A- ( dx2-dx) (dx1-1 / A) AB + (dx2-dx) A) m 2 = -AC ((dx1-1 / A) - (dx2-dx) (dx1-1 / A) B + ( dx2-dx)) m 2 ... (2)

In order to eliminate a change in magnification due to focusing, the value of equation (2) may be set to zero. In this equation,
A, C and m are not 0. This is because A and C take values other than 0 as described above, and if m is set to 0, imaging cannot be performed at a finite magnification.

Therefore, in order to suppress the change in magnification,
Except for A, C, and m in equation (2), it is necessary to set so that equation (3) holds.

[0017]

## EQU3 ## (dx1-1 / A)-(dx2-dx) (dx1-1 / A) B + (dx2-dx) = 0 (3)

By solving the value of B from the equation (3), the following equation (4) is obtained.
An expression is obtained.

[0019]

B = (dx1-1 / A) ^-1 + (dx2-dx) ^-1 (4)

From the equation (4), the second lens has the function of conjugate the point (dx1-1 / A), that is, the rear focal point of the first lens, with the point (dx2-dx). Thus, it can be understood that the change in magnification becomes zero.

FIG. 1 is a schematic diagram showing an embodiment in which the image forming optical system of the present invention is applied to a reading device for reading a pattern formed on a cylinder.

The cylinder 10 on which the pattern is formed is rotated around its central axis by a drive mechanism (not shown). The reading optical system 20 includes a first lens 21 functioning as an objective lens, lenses 22 and 23 forming a relay lens as a second lens, a third lens 24 serving as a focusing lens, and , And a line sensor 25 that reads the formed image.

The cylinder 10 and the reading optical system 20 are relatively slidable along the rotation axis of the cylinder.
The pattern on 0 is read in the form of a ring along the circumferential direction.

FIG. 2 is a specific configuration diagram of the lens system, and its numerical configuration is shown in Table 1 below. The present embodiment is designed as a lens using light having a wavelength of 780 nm, and the symbol in the table is that d0 is the distance from the object point to the first surface,
fB is the distance from the final surface to the imaging point, r is the radius of curvature, d is the lens thickness or air spacing, n is the refractive index at the d line,
ν is the Abbe number, and n780 is the refractive index at a wavelength of 780 nm.

The second, third, sixth, and seventh surfaces are aspherical. The aspheric surface has a distance x from the tangent plane of the aspherical vertex of a coordinate point on the aspherical surface whose height from the optical axis is y, and a curvature of the aspherical vertex.
Assuming that (1 / r) is C, the conic coefficient is K, the fourth and sixth order aspherical coefficients are A4 and A6, and are represented by the following equations. The radius of curvature of the aspherical surface in Table 1 is the radius of curvature of the apex of the aspherical surface, and the conic coefficients and aspherical surface coefficients of these surfaces are shown in Table 2.

[0026]

Equation 5] ^{x = Cy 2 / (1 +} √ (1- (1 + K) C 2 y 2)) + A4y 4 + A6y 6

[0027]

[Table 1]

[0028]

[Table 2] Surface 2 Surfaces 3 and 7 Surface 6 K = -0.77200 K = -0.79400 K = -0.79400 A4 = 0.00000 A4 = 0.95400 × 10 ^-5 A4 = -0.95400 × 10 ^-5 A6 = 0.00000 A6 = -0.22000 × 10 ^-5 A6 = 0.22000 × 10 ^-5

Next, the distance between the lenses in the above embodiment will be described with reference to FIG. FIG. 3 shows numerical values when an object is located at the front focal point of the first lens, and only the first lens 21 and the third lens 24 are shown with the lenses 22 and 23 constituting the second lens omitted. ing.

The focal length of the first lens 21 is 20.000 mm, the distance from the front principal point P1 to the entrance end face is 1.175 mm, and the distance from the rear principal point P2 to the exit end face is 0.175 mm. Lens 2
19.825 mm from the exit end face of No. 1 to the rear focal point P3, and the distance from the exit end face of the lens 21 to the entrance end face of the lens 24 is
70.893 mm.

The lenses 22, 23 and 24 are the same lens and have a focal length of 10.000 mm. In FIG.
Lens 24 is located at the focusing reference position,
The distance from the front principal point P6 to the incident end face is 0.205 m
m, the distance from the rear principal point P7 to the emission end face is 1.158
mm. The distance between the front focal point P4 of the lens 24 and the incident side end surface is 9.795 mm.

As shown in FIG . 3, the focusing reference position
In this state, the distance dx from the rear principal point P7 of the lens 24 to the imaging point P8 coincides with the focal length in this state.
0.000 mm. Therefore, this Focusin
Grayed reference point from the front side principal point P6 of the lens 24 in the distance dx component to the object in position coincides with the front focal point P4 of the lens 24, the distance between the front focal P4 side focal P3 and the lens 24 of the lens 21 41. 273 mm.

Lens 2 at Focusing Reference Position
The point on the object side by the distance dx from the front principal point P6 of the lens 4 is the lens 2
Since 4 coincides with the front focus P4, lenses 22 and 23 is a relay lens, before the lens 24 in the back-focal point P3 and the focusing reference position after these lenses 21
There needs to be coupled to the side focal P4, the conjugate distance becomes 41.273Mm. Of course focusing operation
The focusing lens 24 is
To move back and forth in the optical axis direction around the reference position.

In the structure of this embodiment,
Is the rear principal point P7 of the lens 24 at the caching reference position?
From the focal point of the lens 24 to the focal point P8
The point to be conjugated to the point P3 is
At the front focal point P4 of the lens 24 at the reference position.
However, depending on the configuration of the optical system, it may be conjugated to the point P3.
In some cases, the point to be set does not coincide with the point P4.

Table 3 shows changes in the object position and the amount of movement of the lens 24 for focusing according to the structure of the embodiment.
The relationship with the magnification is shown. The sign of the magnification is reversed depending on the presence or absence of the relay lens. However, even if the sign of the magnification changes, there is no substantial difference because the interval between the spots does not change.

[0036]

[Table 3]

For comparison, the same numerical values when no relay lens is provided are shown in Table 4 below.

[0038]

[Table 4]

By comparing Tables 3 and 4, it can be understood that the change in magnification due to focusing can be suppressed by providing a relay lens.

In the above-described embodiment, an example in which the sensor is disposed immediately after the focusing lens has been described. However, a similar effect can be obtained when the image formed by the focusing lens is further transmitted and used. be able to.
Further, even when the arrangement relationship between the focusing lens and the objective lens is opposite to that in the above-described embodiment, an optical system that does not change the imaging magnification can be obtained.

[0041]

As described above, according to the present invention, the relay lens is provided between the objective lens and the focusing lens, and the conjugate distance is appropriately set, thereby suppressing a change in the imaging magnification due to focusing. be able to.

Therefore, by using the present invention in a multi-beam optical recording / reproducing apparatus or the like, it is possible to suppress a change in the interval between spots on the image plane due to focusing, and to perform accurate recording / reproducing. .

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a reading device according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a lens of a reading optical system.

FIG. 3 is an explanatory diagram showing a numerical relationship between an objective lens and a focusing lens.

[Explanation of symbols]

 21: Objective lens 22, 23: Lens forming a relay lens 24: Focusing lens

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-60-90318 (JP, A) JP-A-5-341184 (JP, A) JP-A-3-37610 (JP, A) JP-A-51- 96336 (JP, A) JP-A-1-129239 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (1)

(57) [Claims] 1. An objective lens, a relay lens, and a focusing reference position for focusing in order from an object side.
A focusing lens movable in the optical axis direction is arranged at the center, and the relay lens is provided with the focusing lens.
The distance from the rear principal point of the focusing lens at the focusing reference position to the imaging point is dx, and the distance from the rear focal point of the objective lens and the front principal point of the focusing lens at the focusing reference position an imaging optical system characterized by having the ability to Conjugate a point dx partial object side.