RU2611335C1 - Apochromat lens - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: objective lens consists of ten single lenses where the first and the last lenses are negative meniscus lenses with their convex sides facing an object, second lens is convex-convex, third, sixth and eighth ones are negative lenses with first concave surface, fourth, fifth and ninth lenses are positive lenses with first convex surface. The following ratio is observed for the fourth positive and the last negative lens: β₄=β₁₀<75⋅10^-7 1/K, while the other lenses match the following ratio: |β|<35⋅10^-7 1/K, where β₄, β₁₀, β are temperature factors of material refraction index of the fourth, last and the other lenses, respectively, K is Kelvin degree.

EFFECT: ensured heat stability of objective lens, expanded operation spectre range, field of view and focus distance without deterioration of image quality.

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Description

The present invention relates to optical instrumentation and can be used in various optical systems that require thermal disruption of the lens, in particular in television and photo systems with multi-element radiation detectors.

To create thermally unsettable telecentric lenses, as a rule, multicomponent optical systems with a number of lenses of more than 7 and with a certain combination of lenses and menisci are used, using special brands of glasses.

A known lens (RF patent No. 2308063 C1, publ. 10.10.2007), the first and second components of which are positive menisci facing concave surfaces to the image space, the third component is made in the form of a negative double-glued lens, the fourth, fifth and sixth components are positive lenses, the seventh component is made in the form of a biconcave lens, while it additionally contains an eighth component located on the optical axis after the seventh component and made in the form of a positive meniscus, having a concave surface to the image space, the negative double-glued lens of the third component consists of a biconcave lens and a positive meniscus, the fifth component is made in the form of a biconvex lens, the sixth component is made in the form of a positive meniscus facing a concave surface to the image space, the aperture diaphragm is located between the second and third components, the thickness of the second component is at least 0.15 of the focal length of the lens, the distance between the second and third components Tammy is not less than 0.25 focal distance of the lens, and the optical power of the eighth component is at least 0.3 lens optical power. The lens is used to form an image on a CCD. The lens has an angular field of view 2w = 20 °, but does not provide a telecentric path of the rays.

Known wide-angle telecentric lens (JP patent No. 11084232 A, publ. March 26, 1999) containing the first biconvex lens, the second lens is a negative meniscus convex to the object, the third biconcave lens, the fourth positive lens, the fifth negative lens, and the sixth lens are positive meniscus, seventh biconvex lens, eighth component — gluing of biconvex and biconcave lenses, ninth and tenth component — single biconvex lenses. The lens has an enlarged rear segment (up to 100 mm), the relative aperture is 1: 4. The lens is fixed only for the visible region of the spectrum (0.486-0.656 μm) and does not have thermal disintegration - it does not preserve the position of the image plane when the temperature changes.

Closest to the claimed technical solution is a fast apochromat lens (RF patent No. 2338226 C1, publ. 10.11.2008), consisting of option 4 of the first negative meniscus located along the rays, respectively, convex to the object, the second biconvex positive lens, the third a negative lens with a first concave surface, a fourth positive lens with a first convex surface, a fifth positive lens with a first convex surface, a sixth positive lens with a first convex surface rhnostyu seventh negative lens with the concave surface of the first, eighth positive lens with the convex surface of the first and ninth negative lens with a first concave surface.

The lens has a relative aperture of 1: 1.8, a focal length of 100 mm, an angular field of view of 2ω = 8.4 °, it builds a high-quality image in the spectral range of 0.404-0.766 μm. This, in particular, is achieved by using a material with a special dispersion pattern - a calcium fluoride crystal (fluorite), from which the second, fourth and sixth positive lenses are made. This makes it possible to obtain an apochromatic correction of the system, but it contradicts the condition of thermal disintegration, since this material has a high temperature coefficient of refractive index - TKPP (β≈ -106⋅10 ^-7 1 / K, where K is kelvin). Considering that all fluorite lenses are made positive, in the prototype lens the circuit turns out to be sensitive to temperature changes, that is, it cannot fulfill the task. In addition, the lens does not have a sufficiently large field of view and focal length.

The objective of the invention is to create a thermal lens with enhanced technical and operational characteristics, namely with an increased to the near infrared spectral range of operation, field of view and focal length while maintaining image quality.

The problem is solved in that in the apochromat lens containing, respectively, the first negative meniscus convex to the object, the second biconvex positive lens, the third negative lens with the first concave surface, the fourth and fifth positive lenses with the first convex surface, sequentially located behind it is a positive lens with a first convex surface, a negative lens with a first concave surface, a positive lens with a first convex surface ited and last negative lens, unlike known, the last negative lens is formed as a meniscus facing concave to the image, after the fifth lens installed biconcave negative lens, and the fourth for the positive and the latter negative lens the following relation:

β ₄ = β ₁₀ <75⋅10 ^-7 1 / K,

and for the rest of the lenses the ratio is:

| β | <35⋅10 ^-7 1 / K,

where β ₄ , β ₁₀ , β are the temperature coefficients of the refractive index (TKPP) of the material of the fourth, last and other lenses of the lens, respectively, and K is kelvin.

The change in the optical power of the fourth positive lens made of a material with a significant value of TKPP due to a change in the refractive index during temperature fluctuations is compensated by a change in the optical power of the last negative lens from the same material. The introduction of an additional negative lens into the system after the fifth lens makes it possible to abandon the use of a material with a high dispersion coefficient and significant TKPP (fluorite) in positive lenses and replace it with ordinary glass with a low TKPP | β | <35⋅10 ^-7 1 / К. All this ensures the thermal disruption of the system. The introduction after the fifth lens of an additional negative lens also provides apochromatic correction in a wide spectral range and allows you to increase the field of view and focal length. Giving the last negative lens a meniscus shape makes the system close to telecentric.

The lens is illustrated in the drawing, where:

- FIG. 1 shows an optical diagram of a particular embodiment of the proposed lens;

- FIG. 2 represents the level of design quality of the image created by the lens, which is estimated by the contrast transfer coefficient (CPC) over all spectral ranges.

The lens can be implemented according to the following optical scheme (Fig. 1).

The scheme is made of two clearly defined parts - the head, containing positive and negative lenses, and a separate negative meniscus.

Lens 1 - negative meniscus convex to the object, lens 2 - positive biconvex, lens 3 - negative with the first concave surface, lenses 4, 5 - positive with the first convex surface, lens 6 - biconcave negative, lens 7 - positive with the first convex surface, lens 8 is a negative lens with a first concave surface, lens 9 is positive with a first convex surface, lens 10 is made in the form of a negative meniscus, facing concavity to the image. Between the lenses 3 and 4 there is an aperture diaphragm 11.

The design parameters of the lens embodiment are presented in table 1.

The main technical characteristics of the lens:

- focal length 200 mm;

- relative aperture 1: 3;

- angular field of view (2ω) 13 degrees;

- the spectral range of 0.41-0.90 microns.

To ensure high image quality when working in a wide spectral range, apochromatic correction of the optical system is necessary. It plays a particularly important role in the case of multispectral shooting, for which it is necessary to guarantee the unity of the position of the plane of the best setup in several spectral intervals (0.41-0.70; 0.41-0.51; 0.51-0.58; 0 , 60-0.70; 0.70-0.90 microns) while maintaining the same high quality image in each of them.

Another significant feature of the lens is the provision of thermal stability of the system in the temperature range Δt = ± 15 ° C from the nominal value t = 20 ° C, i.e. in the absence of internal mechanical shifts of individual elements of the optoelectronic system in the operating temperature range, the unity of the plane of the best image (PNI) and the preservation of image quality should be ensured.

The requirement of orthoscopy (the distortion within the entire field should not exceed 0.25%) and the telecentricity of the main rays in the image space are also fulfilled.

In the developed lens, the problem of resistance to temperature changes was solved by introducing an additional negative lens into the system after the fifth lens, which eliminates the use of a material with a high dispersion coefficient and significant TKPP (fluorite) in positive lenses and replaces it with ordinary glass with a low TKPP | β | < 35⋅10 ^-7 1 / K, and the change in the optical power of the fourth positive lens made of a material with a significant value of TKPP (β≈48 ... 72⋅10 ^-7 ), due to a change in the refractive index during oscillation x temperature is compensated by a change in the optical power of the last negative lens of the same material.

All this ensures the thermal disruption of the system. The introduction after the fifth lens of an additional negative lens also provides apochromatic correction in a wide spectral range and allows you to increase the field of view and focal length. Giving the last negative lens a meniscus shape makes the system close to telecentric, which, in particular, makes it possible to install interference filters in the working segment of the lens.

The level of the estimated image quality created by the lens, which is estimated by the contrast transfer coefficient (CPC) over all spectral ranges, is shown in FIG. 2. The figure shows the calculated level of the polychromatic value of the CPC in a single PNI of the lens at a spatial frequency of 100 mm ^-1 for the main channel (Δλ = 0.41-0.7 μm) - 1 (center), 2 (field) and at the spatial frequency 50 mm ^-1 for additional channels (Δλ = 0.41-0.51 μm - 3 (center), 4 (field); Δλ = 0.51-0.58 μm - 5 (center), 6 (field); Δλ = 0.6-0.7 μm - 7 (center), 8 (field); Δλ = 0.7-0.9 μm - 9 (center), 10 (field)).

It can be seen from the drawing that the minimum level of quality is determined by the edge of the field of view in the range of 0.41-0.7 μm (at the spatial frequency N = 100 mm ^-1 - 0.44), in the remaining narrower spectral ranges, the CPC is not less than 0.5 - at a spatial frequency of 50 mm ^-1 .

When the temperature changes by ± 15 ° relative to 20 ° C, the thermo-optical position aberration is respectively -0.006 mm (Т = 35 ° C) and +0.0038 mm (Т = 5 ° C), and the thermo-optical aberrations are based on the full pupil and field of view in a single image plane they do not lead to a decrease in the calculated value of the CCP by more than 0.05 contrast units.

The lens is made with spherical surfaces, which makes it possible to manufacture it industrially. Experimental studies of the fabricated samples confirmed the design characteristics of the lens.

Claims (5)

A lens that contains, respectively, the first negative meniscus convex to the object, the second biconvex positive lens, the third negative lens with the first concave surface, the fourth and fifth positive lenses with the first convex surface, the successive lens with the first convex surface , a negative lens with a first concave surface, a positive lens with a first convex surface and a last negative lens, distinguishing by the fact that the last negative lens is formed as a meniscus facing concave to the image, after the fifth lens installed biconcave negative lens, and the fourth for the positive and the latter negative lens the following relation:

, and for the rest of the lenses the ratio is:

, where β ₄ , β ₁₀ , β are the temperature coefficients of the refractive index (TKPP) of the material of the fourth, last and other lenses of the lens, respectively, and K is kelvin.

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