RU2348953C1 - Infrared rapid three-lens objective - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: invention concerns objectives of non-scanning thermal imaging devices with non-cooled matrix receivers. The objective contains the first plus, second subzero and the third plus meniscuses. The first and third meniscuses are converted by an incurvation to a plane of images, second - to space of subjects. All refracting surfaces are executed by the spherical. The length from the first refracting surface to the plane of images does not exceed 1.8 focal distances of an objective. An exponent of a refractive of a material of the first and third meniscuses for the basic wave length of a working spectroscopic range not less than 4.0, and the second meniscus - no more than 2.5. The inventions of the relation specified in the formula between optical forces of meniscuses and an objective as a whole, curvature radiuses, thickness of the second meniscus, and distance between the first and second meniscuses are carried out.

EFFECT: improvement of quality of the image within all angular field at angular aperture maintenance, pinch of concentration of energy in a stain corresponding to the sizes of pixels of matrix thermal imaging receivers for long-range infrared spectrum field.

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Description

The invention relates to the field of optical instrumentation, and in particular to lenses of “looking” type non-scanning thermal imaging devices that use uncooled matrix radiation detectors, for example microbolometers, for recording thermal images.

Known infrared fast lenses operating in the far infrared region of 8-12 microns, having a relative aperture of 1: 1.5 and above, containing two, three or more lenses [Tarasov V.V., Yakushenkov Yu.G. "Looking" type infrared systems. - M .: Logos, 2004. -444 p.]. The image quality of the lenses is indicated for a spatial frequency of 10 mm ^-1 : the contrast transfer coefficient is 0.68 ÷ 0.76 at the center of the field and 0.54 ÷ 0.60 at the edge of the field. The disadvantage of such lenses is their low-tech design, as with a relative aperture of 1: 1 or more, for example 1: 0.7, the lenses contain aspherical surfaces. Information on the image quality of lenses at higher frequencies is not given.

Since modern matrix receivers of the far infrared range of the spectrum (8-14 μm) have a pixel size of 0.025-0.035 mm, lenses that provide image quality consistent with the pixel size of the radiation receiver are needed to create the optimal thermal imaging device. the lens should provide a high value of the function of energy concentration in the spot, the size of which corresponds to the size of the pixel.

The closest in technical essence, taken as a prototype, is an infrared fast three-lens lens [US Patent No. 3363962, a variant presented in figure 1 and claim 1 of the formula of this patent]. The specified lens contains sequentially located along the rays of the first positive meniscus, the second meniscus and the third positive meniscus, with the first and third menisci facing concave to the image plane, the second to the space of objects, all refracting surfaces are made spherical. The length along the optical axis from the first surface of the lens to the image plane is 1.7 of the focal length of the lens. The refractive index for the main wavelength (10 μm) of the working spectral range of the material of the first and third positive menisci is not less than 4.0, and the second not more than 2.5.

The main disadvantage of the prototype is the low image quality, which is unacceptable for working with modern matrix thermal imaging radiation detectors, the working spectral sensitivity range of which corresponds to the far infrared region of the spectrum.

In an infrared fast three-lens lens [US Patent No. 3363962], the design parameters of the lens are shown for a normalized focal length of 1. The ratios of the optical powers of the components to the optical power of the entire lens are: 0.71; 0.02; 0.98. The distance d ₂ along the optical axis between the first and second menisci is 0.43 from the focal length of the entire lens. The ratio of the radius of curvature r _{3 of the} third refracting surface along the rays (first surface of the second meniscus) to the radius of curvature r _{4 of the} fourth refracting surface (second surface of the second meniscus) r ₃ / r ₄ is 0.93. The ratio (r ₃ -r ₄ ) / d ₃ is 0.413 (here d ₃ is the thickness along the optical axis of the second meniscus). The lens has a relative aperture up to 1: 0.75, an angular field of 12 °, and a focal length of 76 mm. The ratio of the radii of curvature of the refracting surfaces to the radius of curvature of the last surface are: 2.11; 3.23; -1.55; -1.67; 0.92; 1. The calculation of this lens at a focal length of 76 mm shows that an energy concentration of 70% is achieved only in a spot with a radius of 0.045 mm, which is unacceptable for working with modern matrix thermal imaging radiation detectors.

The proposed infrared fast three-lens lens solves the problem of improving image quality within the entire angular field while maintaining the relative aperture of the lens, namely: increasing the function of energy concentration (PCE) in the spot corresponding to the pixel sizes of modern matrix thermal imaging radiation detectors, the working spectral sensitivity range of which corresponds to the far infrared spectrum.

To solve this problem, it is necessary to find such a combination of optical forces, radii of refracting surfaces, and distances between menisci that simultaneously satisfies the set of requirements: primary aberrations of wide beams of rays (spherical, coma), field aberrations (astigmatism, image curvature, distortion), chromatic are corrected aberrations to values that ensure the size of the scattering spot in the plane of the radiation receiver, comparable with the size of the diffraction spot and the pixel size of the radiation receiver If the relative aperture of the lens is not worse than 1: 0.75 and the angular field is at least 12 °.

An infrared fast three-lens lens is proposed that contains the first positive meniscus, the second meniscus and the third positive meniscus sequentially located along the rays. The first and third menisci are turned concave to the plane of the images, the second to the space of objects. All refracting surfaces are made spherical. The length along the optical axis from the first refracting surface to the image plane does not exceed 1.8 of the focal length of the lens. The refractive index of the material of the first and third positive menisci for the main wavelength of the working spectral range is not less than 4.0, and the second meniscus is not more than 2.5. The second meniscus is negative. The following relationships occur in the lens:

where f ₁ , f ₂ , f ₃ , f - optical power, respectively, of the first, second, third menisci and the lens as a whole;

r ₃ , r ₄ are the radii of curvature of the third and fourth along the rays of the refracting surfaces;

d ₃ - thickness along the optical axis of the second meniscus;

d ₂ is the distance along the optical axis between the first and second menisci;

f 'is the focal length of the lens.

The proposed infrared fast three-lens lens allows to improve image quality within the entire angular field while maintaining the relative aperture of the lens: to increase the PCE in the spot corresponding to the pixel sizes of modern matrix thermal imaging radiation detectors, the working spectral sensitivity range of which corresponds to the far infrared region of the spectrum.

Improving the image quality in the infrared fast three-lens lens is achieved by a new set of distinctive features:

- the implementation of the second meniscus with negative optical power;

- observance of the relations (1) in the lens.

The execution of the second meniscus in the infrared fast three-lens lens with negative optical power allows you to correct the chromaticity of the position, which is a necessary condition for achieving high image quality.

Observance of the indicated relations (1) allows the distribution of the meniscus optical forces, the shape of the menisci and their relative position in such a way that, while maintaining the relative aperture of the lens no worse than 1: 0.75 and the angular field at least 12 °, residual aberrations (spherical, coma, astigmatism , image curvature, distortion, chromatic) form the size of the scattering spot in the plane of the receiver, comparable with the size of the diffraction scattering circle, which allows the efficient use of modern thermal imaging arrays e uncooled radiation detectors.

The specified solution, in our opinion, has novelty and inventive step. The invention is based on the relationship between the optical forces, the shape, the relative position of the lenses in the infrared fast three-lens lens, which was first established by the applicants.

The authors are not aware of infrared fast three-lens lenses in which these features would be implemented.

The proposed invention is illustrated by the following graphic materials:

Figure 1 - Optical diagram of an infrared fast three-lens;

Figure 2 - Frequency-contrast characteristic (TSC) of an infrared fast three-lens lens;

Figure 3 - FKE infrared fast three-lens.

Figure 1 shows the proposed optical scheme of the infrared fast three-lens. The optical system contains menisci located along the rays 1, 2, 3. The positive meniscus 1 faces the concavity of the image plane. Negative meniscus 2 faces the concavity of the plane of objects. The positive meniscus 3 faces the concavity of the image plane. Menisci 1, 3 are made of germanium (refractive index 4.003), meniscus 2 - of zinc selenide (refractive index 2.406). The refracting surfaces of all menisci are spherical. In the infrared fast three-lens, the above relations (1) are satisfied.

Figure 1 additionally shows a plane-parallel plate 4, acting as a protective glass thermal imaging radiation detector.

Infrared radiation coming from each point of the distant object, passing sequentially menisci 1, 2, 3 of the lens and the protective glass 4, is focused in the plane of the radiation receiver.

As a specific example of execution, table 1 shows the design parameters of the optical scheme of the infrared fast three-lens lens.

Table 1 Design parameters of an infrared fast three-lens lens one one 70,850 5,026 48.0 Ge 2 120,671 19,615 46,4 2 3 -49,209 5,457 35.5 Znse four -61,991 18,997 36.6 3 5 33,614 3,994 25.0 Ge 6 58,933 4,815 23.1 four 7 ∞ one 15.0 Ge 8 ∞ 3.4 14.7

The position of the lenses is indicated in accordance with FIG. 1; No. - number of the refracting surface along the beam; R is the radius of the spherical refracting surfaces; d - the lens thickness and air gap: About _⌀ - light diameters. The first and third positive menisci are made of germanium (the refractive index for the main wavelength is 4.003), the second negative meniscus is made of zinc selenide (the refractive index of 2.406 for the main wavelength). The refracting surfaces of all menisci are spherical.

The focal length f 'of the lens is 35.49 mm, the relative aperture is 1: 0.75, the angular field in the space of objects is 12 degrees, and the image size is 7.3 mm. The working spectral range is 8-12 microns, the main wavelength is 10 microns. The length along the optical axis from the first refracting surface to the image plane is 63.3 mm, i.e. is 1.75 of the focal length of the lens.

As follows from table 1, in a specific example of an infrared fast three-lens lens, the following distribution of optical forces is performed: Ф ₁ = 0.67Ф, Ф ₂ = -0.16 Ф, Ф ₃ = 1.52Ф. At the same time, the following relations between the parameters are fulfilled in the optical system: (r ₃ - ₄ ) / d ₃ = 2.35, r ₃ / r ₄ = 0.79, d ₂ = 0.55f ', i.e. the above relations (1) are observed.

Figure 2 shows a graph of the frequency response of an infrared fast three-lens lens. The ordinate shows the contrast transfer coefficients in relative units, the abscissa shows the spatial frequencies in the range from 0 to 50 mm ^-1 , relative to the image plane of the lens. The upper curve in Fig. 2 corresponds to the diffraction frequency response (diffraction designation), the rest of the curves are for a point on the axis, for tilt angles of 6 ⁰ (field edge) and 4 ⁰ (respectively, designation 0, 6 ° and 4 °) as for meridional ( m) and sagittal (s) sections. From the graphs it follows that the limiting frequency for all points of the field exceeds 50 mm ^-1 , which indicates better image quality compared to the prototype, providing the limiting frequency of up to 33 mm ^-1 .

Figure 3 shows the graphs of the PCE of an infrared fast three-lens lens. The ordinate values of the FCE in relative units are indicated, along the abscissa axis the values of the radius of the scattering spot for which the FEC are calculated. The upper curve in Fig. 3 corresponds to the diffraction FCE (diffraction designation), the rest of the curves are for a point on the axis, for tilt angles of 6 ⁰ (field edge) and 4 ⁰ (respectively, designation 0, 6 ° and 4 °). So, an infrared fast three-lens lens in a spot with a radius of 0.015 mm concentrates at least 70% of the energy coming from anywhere in the space of objects within an angular field; in a spot with a radius of 0.01 mm, the fraction of concentrated energy ranges from 57 to 67% depending on the field points. In the proposed fast three-lens lens, a similar image quality is maintained up to a relative aperture of 1: 0.7, subject to ratios (1).

Calculation of an infrared fast three-lens lens taken as a prototype for a focal length of 35 mm shows that no more than 40% of energy is concentrated in it in a spot with a radius of 0.015 mm, and no more than 22% in a spot with a radius of 0.01 mm.

Thus, the proposed infrared fast three-lens lens having the combination of these distinguishing features, in comparison with the prototype allows to improve image quality within the entire angular field while maintaining the relative aperture of the lens.

The proposed infrared fast three-lens lens can be used in non-scanning “looking” type thermal imaging devices that use uncooled matrix radiation detectors for recording thermal images.

Literature

1. Tarasov VV, Yakushenkov Yu.G. "Looking" type infrared systems. - M .: Logos, 2004 .-- 444 p.

2. US patent No. 3363962.

Claims (1)

An infrared fast three-lens lens containing the first positive meniscus, the second meniscus and the third positive meniscus sequentially located along the rays, the first and third menisci facing concave to the image plane, the second to the space of objects, all refracting surfaces are made spherical, the length along the optical axis from the first refracting surface to the image plane does not exceed 1.8 of the focal length of the lens, the refractive index of the material of the first and third spruce menisci for the main wavelength of the working spectral range of not less than 4.0, and the second meniscus - not more than 2.5, characterized in that the second meniscus is negative and the following relationships occur in the lens:

1.8≤ (r ₃ -r ₄ ) / d ₃ ≤3.3, r ₃ / r ₄ = (0.7 ÷ 0.8), d ₂ = (0.5 ÷ 1) f ',
where f ₁ f ₂ , f ₃ , f - the optical power, respectively, of the first, second, third menisci and the lens as a whole;
r ₃ , r ₄ are the radii of curvature of the third and fourth along the rays of the refracting surfaces;
d ₃ - thickness along the optical axis of the second meniscus;
d ₂ is the distance along the optical axis between the first and second menisci;
f 'is the focal length of the lens.

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