RU2843624C1 - Athermalized ultra-wide angle lens for infrared region of spectrum - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: invention relates to infrared optics and can be used in thermal imagers. Athermalized ultra-wide angle lens comprises four components arranged in housing made of steel, first of which is negative meniscus, by concave surface facing image plane, second component is a negative meniscus, third component is a positive meniscus whose convex surface faces the image plane. First component is made of germanium and has an aspherical convex surface, second component is made of germanium, has an aspherical concave surface and faces the image plane with a concave surface, third component is made of oxygen-free glass IKS-25, and the fourth component is a biconvex lens made of germanium, the second surface of which is aspherical.

EFFECT: high manufacturability of the lens while maintaining diffraction quality of the image on the entire field of vision and reduced weight and size.

1 cl, 3 dwg

Description

The invention relates to the field of infrared optics and can be used in thermal imagers built on the basis of microbolometric matrix photodetectors sensitive in the spectral range of 8-12 µm.

The 640×480 pixel microbolometric matrices used in the photodetectors have a pixel size of 17×7 µm, which allows obtaining thermal imaging ultra-wide-angle channels while maintaining high geometric resolution. If the scattering circle exceeds the pixel size, this leads to a decrease in pixel illumination, which in turn is comparable to a decrease in the relative aperture of the lens. The effectiveness of a high relative aperture of the lens is possible only with the simultaneous achievement of high image quality.

A wide-angle lens for the IR spectrum region is known according to patent No. 2385475. The lens according to claim 1 of the formula contains five components sequentially located along the beam. The first and second components are negative and positive meniscus, respectively, located with concave surfaces to the image plane. The third component is a negative meniscus facing the image plane with a convex surface. The fourth component is a biconvex lens. The fifth component is a positive meniscus facing the image plane with a concave surface. All components are made of germanium. Between the fifth component and the image plane, in the air gap, an aperture diaphragm is located.

The design parameters of this lens are: angular field of view ±75°, lens length 197.5 mm, focal length 8.2 mm, relative aperture 1:1.1.

The disadvantage of the lens is its large weight and dimensions.

A wide-angle lens for the IR spectrum region is known according to US Patent No. 5502592 of 26.03.1996, IPC G02B. 13/06. 9/62. The lens consists of four components made of germanium. The first component is a negative meniscus with a concave surface facing the image plane, the second and third components are negative and positive meniscus, respectively, with a convex surface facing the image plane. The fourth is a positive meniscus with a concave surface facing the image plane.

The design parameters of this lens (option I): field of view ±135°, lens length 110 mm, lens weight 80 g.

The disadvantages of the lens are:

1. The complexity of manufacturing the first component, which has steep radii of curvature, which forces the lens to be manufactured only in a single method, while the component has a small thickness in the center, which can lead to deformation on the lens blank and, consequently, to the occurrence of large local errors on the lens surfaces;

2. Large amount of distortion (more than 50%), which is a significant factor in deteriorating image quality;

3. Large dimensions and weight of the lens.

The technical problem is to create an athermalized ultra-wide-angle lens with the following technical result: increasing the technological efficiency of the lens while maintaining the diffraction quality of the image across the entire field of view and reducing the weight and size characteristics.

This technical result is achieved as follows.

The athermalized ultra-wide-angle lens for the IR spectrum contains four components housed in a steel housing. The first and second components are negative meniscus made of germanium, with concave surfaces facing the image plane. The third component is a positive meniscus made of oxygen-free glass IKS-25 and with a convex surface facing the image plane. The fourth component is a biconvex lens made of germanium. The first surface of the first component, the second surface of the second component, and the second surface of the fourth component are aspherical. All aspherical surfaces of the lens are higher-order curves. The following relationships are fulfilled:

Where - relative optical powers of the first, second, third and fourth components, respectively;

- equivalent focal length of the entire lens;

- focal lengths of components 1, 2, 3 and 4 respectively.

An example of the implementation of an athermalized ultra-wide-angle lens is shown in the drawings.

Fig. 1 shows the optical diagram of the objective, the ray path is shown. Fig. 2 shows the image contrast (IMC) for three configurations: 20, -40, 50°C and the energy concentration function. Fig. 3 shows the point spread function for three configurations: 20, -40, 50°C.

The ultra-wide-angle athermalized infrared lens (Fig. 1) contains four components installed along the beam path. Protective glass 5 is installed in front of the matrix of photodetector 6 installed in the image plane. Component 1 is a negative meniscus made of germanium, the first surface of which is aspherical. Component 2 is a negative meniscus made of germanium, the second surface along the beam path is aspherical. Component 3 is made of oxygen-free glass IKS-25. Component 4 is a biconvex lens made of germanium, its second surface is aspherical.

The negative meniscus faces the image plane with its concave surfaces. The positive meniscus faces the image plane with its convex surface. The aperture diaphragm is mounted on the first surface of component 3. The design characteristics of the lens are given in Table 1.

The deflection of an aspherical surface is determined by the formula:

where c=1/r0 is the curvature of the surface;

The data of the parameters of the formula that determines the deflection arrow of aspherical surfaces are presented in Table 2.

Optical characteristics of the lens:

1. Focal length 5.01 mm

2. Relative aperture 1:1.36

3. Field of view 97.5° × 84.0°

4. Spectral range 8-12 µm

5. Lens length 45.76 mm

6. Back section 7.25 mm

7. Lens weight 40g

When calculating the lens, the following factors were taken into account: the temperature coefficient of linear expansion of optical materials, the temperature coefficient of expansion of the lens housing material, the change in the refractive indices of the optical materials of the components, the change in the curvature of the surfaces of the components depending on the change in the temperature of the environment.

The design parameters of the lens, indicated in Table 1, provide the following relationships:

The lens operates as follows. The beams of rays pass successively through components 1-4 so that the main beam in the image space goes parallel to the optical axis, i.e. is telecentric.

Below are the characteristics of the lens image quality based on the frequency-contrast characteristic (MCF) and energy concentration function (ECF) graphs.

The MTF graph corresponds (Fig. 3) to a diffraction-limited objective. The image contrast is 0.6 for a spatial frequency of 20 mm ^-1 .

Fig. 2 shows the topology of the scattering circles for 20°C, -40°C and +50°C. The first row shows the scattering circles for the axial point of the field of view, the second - for the zone (at a level of 0.7 from the edge of the field of view), and the third - for the edge of the field of view. The size of the square is 100 μm. Above each circle its size in micrometers is written, corresponding to 80% of the energy concentration.

Looking at these graphs shows that this lens has high image quality in the temperature range from -40°C to +50°C.

Thus, the proposed lens has a wide field of view of 97.5° × 84.0°, a large relative aperture of 1:1.36, while maintaining high image quality across the entire field of view. Also, the proposed lens is more technologically advanced and has better weight and size characteristics compared to the prototype provided.

Claims (6)

An athermalized ultra-wide-angle lens for the IR spectral region, comprising four components housed in a steel housing, the first of which is a negative meniscus with its concave surface facing the image plane, the second component is a negative meniscus, the third component is a positive meniscus with its convex surface facing the image plane, characterized in that the first component is made of germanium and has an aspherical convex surface, the second component is made of germanium, has an aspherical concave surface and faces the concave surface to the image plane, the third component is made of oxygen-free glass IKS-25, and the fourth component is a biconvex lens made of germanium, the second surface of which is aspherical, wherein all aspherical surfaces are higher-order curves and the following relationships are satisfied: Where - relative optical powers of the first, second, third and fourth components, respectively; - equivalent focal length of the entire lens; - focal lengths of components 1, 2, 3 and 4 respectively.