RU2623819C1 - Lens teleocentric in space of subjects - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: lens contains six lenses and an aperture diaphragm arranged in series on the optical axis: a plano-convex lens located on the plane to the aperture diaphragm, a positive meniscus with an aspherical convex surface facing the object space, a weakly negative meniscus facing the aperture diaphragm, an aperture diaphragm, a glued lens consisting of biconcave and biconvex lenses and located with its concave side to the aperture diaphragm, a biconvex lens and a weakly negative meniscus with a convex side facing the aperture diaphragm.

EFFECT: expanding the field of application, improving the technical characteristics and increasing the manufacturability of manufacture.

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Description

The invention relates to the field of optical instrumentation, and in particular to optical systems of control and measuring devices, and can be used in devices for monitoring objects using the shadow optical method.

One of the urgent and often arising tasks in the measuring technique is the control of the geometric dimensions of products. To solve these problems, various optical-electronic control methods are widely used. One of the commonly used methods is the shadow method, which has high performance and low measurement error. The use of standard lenses for the shadow optical system introduces a significant additional error due to the large influence of subject positioning on the linear increase of the system. In this regard, to implement the control of products using the shadow method, it is necessary to use a specialized telecentric lens in the space of objects, which should be practically free from distortion (less than 0.02%), field curvature and astigmatism (less than 0.1 mm), and have a high telecentricity in space of objects. Moreover, due to the parallel movement of the main rays in the space of objects, the diameter of the first lens, and hence the field of view of the lens, must exceed the linear size of the measured object. Most existing lenses do not quite meet the specified requirements.

The known optical scheme of a telecentric in the space of objects of the lens [A.V. Beloborodov, A.A. Gushchina, P.S. Zavyalov, E.S. Senchenko, L.V. Finogenov, Yu.V. Cast iron. Optoelectronic control of ceramic rings // Sensors and Systems, vol. 4, 2012, p. 25-29], consisting of four spherical lenses and a matrix photodetector sequentially located on the optical axis.

The well-known lens is used in the control system of the geometric parameters of ceramic rings and provides the formation of images of a large range of products. But at the same time, this lens has the following disadvantages.

Firstly, the lens has a small field of view (less than 50 mm), which limits the use of the presented lens for a wide range of products.

Secondly, the lens has significant astigmatism (about 0.2 mm) in combination with insufficient telecentricity of the rays in the space of objects (maximum non-telecentricity 0.12 °), which degrades the technical characteristics of the lens and shifts the center of mass in the image of the edge beams.

Also known is a lens with a telecentric path of rays in the space of objects [Patent RU 2305857 "A lens with a telecentric path of rays", published September 10, 2007], consisting of fourteen elements in series — twelve single and one glued spherical lenses.

The known lens can be used in the photolithographic microelectronic industry, and, in particular, for monitoring photolithographic objects, silicon wafers, etc.

The main disadvantages of the lens are, firstly, a large number of used lenses made from different optical materials, which significantly reduces the manufacturability of its manufacture.

Secondly, low resolution, a large amount of distortion (about 0.3%) and a small field of view of the lens (3.3 mm), which affects the technical characteristics of the lens.

These disadvantages limit the functionality and scope of known technical solutions.

The closest in technical essence adopted for the prototype is a telecentric lens [Utility Model Patent No .: 25798 “Telecentric Projection Lens”, published October 20, 2002], consisting of five lenses arranged in series on the optical axis (two of them are glued with spherical lenses , three - single). The aperture diaphragm is located between the third and fourth lenses. The lens provides a ray-path telecentric in the space of objects, which ensures a certain orthoscopicity of the system and close to diffractive image quality of the axial rays.

The disadvantages of the prototype, limiting the scope of its application, include, firstly, low resolution across the field (Strehl number on the axis of about 0.8, on the edge - 0.43).

Secondly, the lens distortion is not well corrected - about 0.5%.

Thirdly, the small field of view of the lens is less than 50 mm.

In addition, with the total small number of lenses, the prototype uses 6 different brands of glass, which affects the manufacturability of its manufacture.

The technical result achieved by the implementation of the invention consists in expanding the scope, improving the technical characteristics of the claimed invention (increasing resolution throughout the field, obtaining a telecentric ray path in the space of objects with a simultaneous decrease in distortion and astigmatism, increasing the linear field of view) and increasing its manufacturability manufacture.

The inventive telecentric in the space of objects lens (hereinafter the claimed lens) is illustrated by the following graphic materials:

FIG. 1 is an optical diagram of the inventive lens.

FIG. 2 - frequency-contrast characteristic (TSC) of the inventive lens.

FIG. 3 - graphs of distortion and curvature of the field of the inventive lens.

The inventive lens (Fig. 1) comprises a plano-convex lens 1 arranged in series on the optical axis, located flat on the aperture diaphragm 2, a positive meniscus 3 with an aspherical convex surface facing the plane of objects, a slightly negative meniscus 4, convex on the aperture diaphragm 2, glued lens 5, consisting of a biconcave lens 6 and a biconvex lens 7, located with the concave side to the aperture diaphragm 2, the biconvex lens 8 and a slightly negative meniscus 9, is turned convex part to the aperture diaphragm 2. Aperture diaphragm 2 is located between the slightly negative meniscus 4 and the bonded lens 5.

The inventive lens has the following positive features:

- a large linear field of view (therefore, it is possible to use sufficiently large objects to measure);

- only three grades of glass and only one glued lens are used in the design of the lens, which increases manufacturability;

- forms an image with low distortion (less than 0.02%) and high telecentricity in the space of objects (deviation from telecentricity in the space of objects - no more than 5 ');

- provides uniform diffractive image quality over the entire field of view (Strehl number not less than 0.9).

The above technical result is achieved due to the fact that:

- the inventive lens contains five sequentially located on the optical axis of the lens and the aperture diaphragm - common with the prototype signs of the claimed invention.

- distinctive features is that the inventive lens contains an additional sixth lens, and the lenses and the aperture diaphragm are arranged sequentially in the following order along the beam: a plano-convex lens 1 located plane to the aperture diaphragm 2; positive meniscus 3 with an aspherical convex surface facing the space of objects; slightly negative meniscus 4, convex side to the aperture diaphragm 2; aperture diaphragm 2; glued lens 5, consisting of a biconcave lens 6 and a biconvex lens 7, located with the concave side to the aperture diaphragm 2; a biconvex lens 8 and a slightly negative meniscus 9, convex to the aperture diaphragm 2.

The flat-convex lens 1 has a diameter larger than the size of the object, faces a flat aperture diaphragm 2. The flat-convex shape of the largest lens simplifies its manufacture, which increases the manufacturability of the entire lens. Flat-convex lens 1 has only a slight distortion, which is explained by its high telecentricity in the space of objects.

Positive meniscus 3 has a convex aspherical surface, has astigmatism and coma, but is freed from spherical and other aberrations. Serves to bring telecentricity to a predetermined value, and also compresses a converging beam of rays. The use of aspherics can also significantly reduce the diameter of the positive meniscus 3 and slightly negative meniscus 4, so that only one large-sized plano-convex lens 1 remains in the lens.

The slightly negative meniscus 4 is convex toward the aperture diaphragm 2. It is made of a material with a small Abbe number, which is necessary to compensate for chromatic aberrations.

Next is the aperture diaphragm 2. After the aperture diaphragm 2, there is a glued lens 5, consisting of a biconcave lens 6 and a biconvex lens 7. The glued lens 5 is located with the concave side to the aperture diaphragm 2. The glued lens 5 is practically free from chromatic aberrations, but has negative coma, curvature of the field and spherical aberration, introduces significant negative astigmatism.

The biconvex lens 8 compensates for astigmatism, spherical aberration, coma and the field curvature introduced by the previous glued lens 5. In addition, it slightly reduces the angle of incidence of the rays on the photodetector.

The slightly negative meniscus 9 compensates for residual aberrations, in particular field curvature and astigmatism.

A plano-convex lens 1, a positive meniscus 3, a biconvex lens 7 in a glued lens 5, and a slightly negative meniscus 9 are made of glass of one brand, a biconvex lens 8 is made of glass of another brand, and the refractive indices of both materials are less than 1.77, the Abbe number is more than 45; a slightly negative meniscus 4 and a biconcave lens 6 in the bonded lens 5 are made of glass of the third brand with a refractive index of more than 1.6 and an Abbe number of less than 45.

Using an aspherical surface makes it possible to better compensate for spherical aberration, coma and astigmatism when the relative aperture is increased to 1: 7. In this case, the use of aspherics does not significantly increase the cost and complexity of the lens compared to the prototype, since with a general increase in the number of lenses in the lens (from five to six) it is necessary to produce only one glued lens, as well as use only three grades of glass instead of six.

Radiation parallel to the optical axis passes sequentially through a plano-convex lens 1, positive meniscus 3, slightly negative meniscus 4 and focuses in the plane of the aperture diaphragm 2. At the same time, telecentricity in the space of objects is observed, all rays incident on the lens at a large angle do not pass into the optical the system. Then the radiation, passing through the glued lens 5, the biconvex lens 8 and the slightly negative meniscus 9, is focused by them and projects the image of the object on the plane of the photodetector.

The authors are not aware of the optical schemes of telecentric objects in space of lenses that have features similar to those that distinguish the claimed lens from a prototype, therefore, the proposed optical system of a telecentric lens in space of objects has significant differences.

As a specific example of the implementation of the inventive lens in table 1 shows the design parameters of the optical elements of the lens having a focal length of 1500 mm; relative aperture 1: 7; diameter of the field of view is 250 mm; spectral range 0.61-0.66 μm. The length of the lens from the first surface to the image plane is 800 mm. The relative length of the lens along the optical axis, referred to the linear field of view is 3.2; 1/12 ^x linear magnification.

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Table 1. Design parameters of a telecentric lens in the space of objects (relative aperture 1: 7; field of view diameter 250 mm; spectral range 0.61-0.66 μm).

In table 1, the position of the lenses is shown in accordance with FIG. one; No. - surface number along the beam; R is the radius of the surface, d is the thickness between the surfaces, D is the diameter of the surface, the grade of glass is according to GOST 3514-94, K is the conical constant.

From the graph in FIG. 3 it can be seen that the inventive lens has a high orthoscopicity (distortion less than 0.02%), which is better than the prototype lens by more than 2 times. In addition, astigmatism is well fixed. This leads to an increase in the telecentricity of the lens in the space of objects in comparison with the analogue by six times (maximum non-telecentricity of 0.019 °).

From the graph in FIG. 2 it follows that the lens has a good resolution, since the contrast transfer coefficients for the spatial frequency of 100 mm ^{-1 are} not worse than 0.5, and the Strehl number throughout the field is not less than 0.9, which is better than that of the prototype lens (0, 8 for the axial beam and 0.43 for the edge beam). Moreover, the linear field of view of the inventive lens is increased compared to the prototype 8 times.

It is important to note that in the proposed embodiment of the lens, the linear field of view reaches 250 mm while maintaining the diffraction quality of the image over the entire field, and the authors found no analogues of telecentric objects in space of objects with a given image quality.

Thus, the proposed telecentric in the space of objects lens with a combination of these essential features allows you to expand the scope of the claimed invention and provide higher technical characteristics (a larger field of view and telecentricity of rays in the space of objects, less distortion and astigmatism of the resulting image, better resolution throughout field of view), as well as to increase the manufacturability of its manufacture.

The proposed telecentric in the space of objects lens can be used in the shadow channels of vision systems to determine the geometric parameters of objects.

Claims (2)

1. Telecentric in the space of objects lens containing five sequentially located on the optical axis of the lens and an aperture diaphragm, characterized in that it contains an additional sixth lens, and the lens and aperture diaphragm are arranged sequentially in the following order along the beam: a plano-convex lens is located plane to the aperture diaphragm , positive meniscus with an aspherical convex surface facing the space of objects, weakly negative meniscus with the convex side facing the aperture the diaphragm, then the aperture diaphragm, a bonded lens of biconcave and biconvex lenses, located concave side to the aperture diaphragm; then a biconvex lens and a slightly negative meniscus, convex to the aperture diaphragm. 2. A telecentric object space according to claim 1, characterized in that the plano-convex lens, the positive meniscus, the biconvex lens in the glued lens, and the slightly negative meniscus located after the aperture diaphragm are made of glass of the same brand, the biconvex lens is made of glass of another brand moreover, the refractive indices of both materials are less than 1.77, the Abbe number is more than 45; another slightly negative meniscus and a biconcave lens in a glued lens are made of third-grade glass with a refractive index of more than 1.6 and an Abbe number of less than 45.

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