RU2594957C1 - Athermalised lens for infrared spectrum - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: invention relates to infrared optics and can be used in thermal imagers with matrix photodetectors, which do not require cooling to cryogenic temperatures which are sensitive in spectral range of 8-12 mcm. Lens comprises a first negative meniscus, whose concave surface faces image plane, made of germanium, a second positive meniscus whose concave surface faces image plane, made from oxygen-free glass IKS-25, a third negative meniscus, whose concave surface faces image plane, made from zinc selenide or zinc sulphate, and a fourth positive meniscus whose concave surface faces image plane, made of germanium. First surface of first meniscus is coated with diamond-like antireflection coating. Following relationships are satisfied: φ₁:φ₂:φ₃:φ₄=-(0.11÷0.23):(1.44÷1.70):-(0.68÷0.92):(1.04÷1.21), where φ₁, φ₂, φ₃, φ₄ denote relative optical power of first, second, third and fourth menisci.

EFFECT: technical result is providing resistance of lens to effect of external conditions (temperature, humidity), which enables to use lens without additional protective glass.

1 cl, 3 dwg, 3 tbl

Description

The invention relates to the field of infrared (IR) optics and can be used in thermal imagers with photodetectors made in the form of a microbolometric matrix of sensitive elements that do not require cooling to cryogenic temperatures. The spectral range of the lens is 8-12 microns.

Currently, most of the existing observational infrared optical devices are designed to operate in the ranges of 3-5 microns and 8-12 microns, corresponding to the "windows" of transparency of the atmosphere.

IR lenses are made mainly from single-crystal germanium or silicon, as well as from other materials transparent in these spectral regions. These materials, especially germanium, have a significant change in the refractive index from temperature, which causes the lens image to defocus. This leads to a significant decrease in image quality, especially in the temperature range from minus 40 ° C to + 50 ° C.

Another important factor influencing the design and construction of the lens is the condition for its use: air temperature and humidity. So, for example, the use of the lens on an unmanned aerial vehicle will lead to an abrupt change in temperature in conditions of variable humidity. In this regard, special requirements are imposed on the antireflection coating on the first (external) surface of the lens with respect to mechanical, thermal and moisture resistance.

Currently, such coatings are mastered and are called diamond-like, but they can be applied and work effectively on only one infrared optical material - Germany.

A well-known lens for the infrared region of the spectrum of 8-12 microns according to US patent No. 4679891 from 07/03/1985, IPC G02F 1/02, G02B 9/12, G02B 9/34. The lens contains four components, the first of which is a biconvex lens made of zinc selenide (ZnSe), the second is a biconcave lens made of zinc sulfate (ZnS), the third is a negative meniscus facing a concave surface to the image plane, made of germanium, the fourth - positive meniscus facing a concave surface to the image plane, made of germanium.

The design parameters of the lens (option III): focal length 100 mm; relative aperture 1: 1.6; field of view ± 4.5 °. The lens is athermalized, i.e. It has constant image quality in the temperature range from minus 40 ° С to + 50 ° С. All components and the lens itself are motionless.

Our calculation of this lens for the specified parameters of the lens and for the parameters reduced to the parameters of the claimed lens (focal length 75 mm; relative aperture 1: 1.25; field of view 6 ° × 8 °) showed that this lens has the following disadvantage: low image quality by field of view. In addition, the image contrast at the edge of the field of view is reduced by 50% with respect to the axial point of the field of view. The image contrast in the temperature range from minus 40 ° С to + 50 ° С in the center of the field of view at a frequency of 20 mm ^-1 is 0.6, and along the edge of the field of view - 0.3. In addition, since the first lens of the lens is made of zinc selenide, in order to ensure reliable operation of the lens in harsh temperature conditions, it is necessary to use a protective window made of germanium to use a diamond-like coating.This will reduce the transmission of the lens and increase its mass.

The closest in technical essence to the claimed one - the prototype - is a lens for the IR region of the spectrum of 8-12 microns according to the patent of the Russian Federation No. 2538423 of 08/10/2013, IPC G02B 9/38, G02B 11/22, G02B 13/14. The lens contains four menisci located in the housing, the first of which is positive, made of oxygen-free IKS-25 glass and faces with a concave surface to the image plane, the second is negative, is made of zinc selenide and faces with a concave surface, and the third is negative, made from Germany and facing a concave surface to the image plane, the fourth is positive, made of germanium and facing a concave surface to the image plane. Between the relative optical forces of menisci 1, 2, 3, 4 there are the following relationships:

φ ₁ : φ ₂ : φ ₃ : φ ₄ = (0.72 ÷ 0.85) :-( 1.28 ÷ 1.76) :-( 3.00 ÷ 6.00) :( 0.79 ÷ 0.92),

where φ ₁ , φ ₂ , φ ₃ , φ ₄ are the relative optical forces of the first, second, third, and fourth menisci, respectively.

With a focal length of 75 mm and a relative aperture of 1: 1.25, the lens has high image quality in the temperature range from minus 40 ° C to + 50 ° C across the entire field of view.

The disadvantage of the lens is that it does not have the required resistance to external conditions (temperature, humidity), because it is impossible to make a diamond-like antireflection coating on the first (outer) surface of the lens due to the fact that the first lens is made of IKS-25 material. The existing antireflection coating on IKS-25 has a second strength group. Under the influence of external conditions (temperature, humidity), the coating peels off. This leads to the need to install a protective glass made of germanium in front of the lens, on which a diamond-like antireflection coating is applied, which, in turn, leads to a decrease in transmission and an increase in the longitudinal dimensions and mass of the system: the lens and protective glass.

The objective of the invention is to create an athermalized lens, the image quality of which does not depend on changes in ambient temperature, with the achievement of the following technical result: ensuring resistance to external conditions (temperature, humidity), and therefore, eliminating the need to install a protective glass in front of the lens that reduces transmission and increasing the longitudinal dimensions and mass of the optical system of the lens.

The specified technical result is achieved as follows. The thermalized lens for the infrared region of the spectrum, like the prototype, contains four components fixed in the housing, the third of which is the negative meniscus, and the fourth is the positive meniscus made of germany, the menisci being turned with concave surfaces to the image plane. Unlike the prototype, the first component is made in the form of a negative meniscus facing a concave surface to the image plane made of germanium, and the second is in the form of a positive meniscus facing a concave surface to the image plane made of oxygen-free IKS-25 glass, the third meniscus is made of zinc selenide or zinc sulfate, moreover, a diamond-like antireflective coating is deposited on the first surface of the first meniscus, while there are clumps between the relative optical forces of the menisci traveling ratios:

φ ₁ : φ ₂ : φ ₃ : φ ₄ = - (0.11 ÷ 0.23) :( 1.44 ÷ 1.70) :-( 0.68 ÷ 0.92) :( 1.04 ÷ 1 , 21)

where φ ₁ , φ ₂ , φ ₃ , φ ₄ are the relative optical forces of the first, second, third, and fourth menisci, respectively.

Since the first meniscus is made of germanium and a diamond-like antireflective coating is applied to its outer surface, the first meniscus acts as a protective element for the lens. Such a lens can be used without a protective glass, which reduces its transmission and increases the size and weight.

In the spectral region of 8-12 μm, the dispersion of germanium is very small, so the first meniscus is a corrector for the monochromatic aberrations of the lens.

In the vast majority of lenses operating in the spectral region of 8-12 μm, the first lens of the lens is a positive germanium meniscus, which makes the main contribution to the change in focal length with temperature (G.G.Slyusarev "Methods for calculating optical systems", Mash., L ., 1969). The consequence of this is the need for active athermalization due to the movement of the entire lens or its individual components. This implies the presence of an engine, gearbox and power source, which complicates the design of the lens.

In the proposed lens, the first meniscus has a weak negative force and works in the opposite direction, i.e. as a corrector of thermo-optical aberrations of subsequent lens components.

To ensure athermalization in the temperature range from minus 40 ° С to + 50 ° С, the execution of the first meniscus from Germany entailed a change in the shape of the first, second and third menisci and the material of the second and third menisci.

The material of the second, third and fourth menisci of the lens is selected so as to compensate for chromatic aberration. As the material of the third meniscus, zinc selenide or zinc sulfate was chosen. The use of zinc sulfate, known as optical ceramics KO-2 OST3-1652-73, gives better results on achromatization of the lens, but less transmission in accordance with the mentioned standard.

An example of a specific implementation of an athermalized lens is shown in the drawings.

In FIG. 1 shows the optical layout of the lens.

In FIG. Figure 2 shows the point scattering function.

In FIG. Figure 3 shows the contrast of the image and the function of energy concentration during lens operation over the entire temperature range.

The thermalized lens for the infrared region of the spectrum (Fig. 1) contains four meniscus. Meniscus 1 - negative, made of germanium. Meniscus 2 - positive, made of oxygen-free glass IKS-25. Meniscus 3 - negative, made of zinc selenide. Meniscus 4 - positive, made of Germany. All menisci face with a concave surface to the image plane 5. On the first surface of meniscus 1 (the outer surface of the lens), a diamond-like antireflection coating is applied, for example, a DLC coating from IZOVAC (Belarus).

Table 1 shows the characteristics of an athermalized lens, considered as an example of implementation. The entrance pupil of the lens is located on its first surface.

Table 2 shows the optical characteristics of the lens.

The lens design elements shown in Table 1 provide the following relationships between the relative optical powers φ ₁ , φ ₂ , φ ₃ , φ _{4 of} menisci 1-4: φ ₁ : φ ₂ : φ ₃ : φ ₄ = -0.177: 1.538: - 0.751: 1.1,

where: φ ₁ = f ′ / f ′ ₁ , φ ₂ = f ′ / f ′ ₂ , φ ₃ = f ′ / f ′ ₃ , φ ₄ = f ′ / f ′ ₄ ;

f ′ is the equivalent focal length of the lens;

f ′ ₁ , f ′ ₂ , f ′ ₃ , f ′ ₄ are the focal lengths of menisci 1, 2, 3, and 4, respectively.

A thermalized lens works as follows. Beams of rays from an object (an axial beam of rays is shown) sequentially pass through menisci 1, 2, 3, 4 and build an image in the image plane 5.

When calculating the lens, the following factors were taken into account.

1. The temperature dependence of the refractive index.

2. The expansion and contraction of the meniscus material with a change in temperature, which, in turn, leads to a change in the radii and thicknesses of the lenses.

3. Change in the distance between the elements as a result of expansion and contraction of the material of the frames and the lens body.

A traditionally used aluminum alloy with a temperature coefficient of expansion (TCE) of 22 × 10 ^{-6 was} used as the body material. The refractive indices of the selected materials, linear expansion coefficients and the temperature dependence of the refractive index (dn / dt) were selected from domestic standards, and therefore an additional catalog of glasses was created in the ZEMAX program, because IKS-25 glass is absent in it.

The main characteristics of the meniscus materials are presented in table 3.

To calculate the proposed lens in the temperature range from minus 40 ° С to + 50 ° С, the multi-configuration method provided in the ZEMAX program using the "Thermal Pick Up" option was used. With this option in mind, we simultaneously optimized new (relative to the nominal configuration) values of the lens circuit parameters for temperature values from minus 40 ° С to + 50 ° С. For optimization, we used the linear expansion coefficients of materials for the IR spectral region (TCE) and the coefficient of change of the refractive index with temperature (dn / dt), presented in Table 3.

Let us consider the characteristics of the image quality of the lens, namely, the point scattering function (PSF), which clearly demonstrates the topology of scattering spots in the geometric approximation (Fig. 2), image contrast (TFC) and the energy concentration function (PFC), which allows to determine the diffraction scattering circle (Fig. . 3).

In FIG. 2 in the first column gives the topology of the scattering circles for 20 ° C, in the second column for minus 40 ° C, and in the third for 50 ° C. The first line contains scattering circles for the axial point of the field of view, the second for the zone, and the third for the edge of the field of view, i.e. 6.3 ° × 8.3 ° diagonal. The square size is 100 microns. In addition, a diffraction (non-aberrational) Airy circle, 32.3 μm in diameter for a relative aperture of 1: 1.25, is imprinted on each scattering spot. This circle contains 83.4% of energy.

In FIG. Figure 3 on the left shows the contrast of the image at a frequency of 20 mm ^-1 , and on the right - the function of energy concentration for the entire temperature range. The temperature values are printed in the field of the corresponding graphs.

As can be seen from FIG. 3, the image quality of the lens is high, close to diffraction. The size of the matrix element Q is: Q = 0.025 × 0.025 mm. For infrared lenses, the energy density should be at least 80% in the matrix pixel size. For imaging cameras, it is necessary that the value of the image contrast of the sinusoidal world at the Nyquist frequency v = 1 / 2Q = 20 mm ^-1 be at least 0.6. From FIG. Figure 3 shows that the image contrast for the axial point of the field of view approaches 0.7, and for the edge of the field of view it is 0.6.

A detailed review of the FCE graphs in FIG. 3 made it possible to determine the diameter of the scattering circles at 80% energy concentration, the values of which are imprinted in the upper part of the squares in FIG. 2. Consideration of these values allows us to conclude that the proposed lens has diffractive image quality in the temperature range from minus 40 ° C to + 50 ° C.

The calculations show that the technical results are achieved within all of the claimed ranges.

Thus, the present invention provides the resistance of the lens to environmental conditions (temperature, humidity), and therefore, eliminates the need to install a protective glass in front of the lens, which will increase transmission and reduce the longitudinal dimensions and weight of the optical system of the lens.

Claims (1)

An infrared lens for the infrared region of the spectrum containing four components fixed in the housing, the third of which is a negative meniscus and the fourth is a positive meniscus made of germany, the menisci being turned with concave surfaces to the image plane, characterized in that the first component is made in the form of a negative meniscus facing a concave surface to the image plane made of germanium, and the second in the form of a positive meniscus facing a concave surface to a plane of these images made of oxygen-free glass IKS-25, the third meniscus is made of zinc selenide or zinc sulfate, and on the first surface of the first meniscus a diamond-like antireflective coating is applied, with the following ratios:
φ ₁ : φ ₂ : φ ₃ : φ ₄ = - (0.11 ÷ 0.23) :( 1.44 ÷ 1.70) :-( 0.68 ÷ 0.92) :( 1.04 ÷ 1 , 21)
where φ ₁ , φ ₂ , φ ₃ , φ ₄ are the relative optical forces of the first, second, third, and fourth menisci, respectively.

Images