RU2578268C1 - Infrared lens with variable focal distance - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: lens includes, arranged on a beam path, a first positive meniscus facing the image plane with the concave surface, a second meniscus capable of moving on an optical axis, an aperture diaphragm and a third positive meniscus facing the image plane with the concave surface. The first and third menisci have an aspherical surface each. The second meniscus is negative and in a first fixed position faces the image plane with the convex surface, and in a second fixed position is turned 180° relative to the first fixed position. The optical power of the movable second component OS₂ is: |OS₂| ≥ OS_lens.min, where OS_lens.min is the minimum optical power of the lens.

EFFECT: reducing the number of lenses and widening the focal distance measurement range while maintaining a high aperture ratio and image quality.

1 dwg, 2 tbl

Description

The present invention relates to the field of optoelectronic technology and can be used as a lens for optoelectronic devices operating in the far infrared region of the spectrum.

Known infrared lens (patent WO 99-59015 A1, publ. 18.11.1999), having two focal lengths, containing located along the rays of the first positive meniscus facing concavity to the image plane, the second biconcave lens, the third positive meniscus facing concavity to image plane, and the fourth positive component, made in the form of a positive meniscus, a biconvex lens and a negative meniscus. The focal length is changed 2.5 times by moving the second and third lenses in the same direction with two fixed positions at aperture ratio of 1: 2.5. The infrared lens contains three aspherical surfaces - the second surface of the first meniscus, the second surface of the second biconcave lens and the last surface of the sixth lens of the lens.

The disadvantage of this lens is a small aperture, a large number of lenses from Germany - six, which further reduces the value of physical aperture (taking into account light transmission), and a small range of focal lengths - 2.5 times.

The closest in technical essence is an infrared lens with two focal lengths for working in optical systems of thermal imagers (patent RU 2348954 C1, publ. 10.03.2009), containing the first positive meniscus located along the rays, facing concavity to the image plane, the second biconcave lens, the third positive meniscus, facing concavity to the image plane, and the fourth positive component, made in the form of negative and positive menisci. The focal length is changed three times by moving the second and third lenses in opposite directions with two fixed positions, at fairly high aperture values (1: 1.1 and 1: 1.5) and with the use of spherical surfaces.

The number of germanium lenses in this lens has been reduced to five, and the range of focal lengths has been increased up to 3 times. But such an improvement is also insufficient and does not solve the task.

The objective of the present invention is to reduce the number of lenses to three while increasing the range of focal lengths up to 4 times while maintaining high aperture and image quality acceptable for working with modern matrix infrared receivers.

The technical result due to the task is achieved by the fact that in the infrared lens with a variable focal length, including the first positive meniscus located along the beam, facing concavity to the image plane, the second meniscus with the ability to move along the optical axis, the aperture diaphragm and the third positive meniscus facing the concavity to the image plane, in contrast to the known first and third menisci contain one aspherical surface, the second meniscus is made negative, while in the first fixed position it is convex to the image plane, and in the second fixed position it is rotated 180 ° relative to the first fixed position and turned concavity to the image plane, and the optical power of the moved second component of OS ₂ is:

where OC _rev.min is the minimum optical power of the lens.

Moving the second component with its rotation through 180 ° provides a stepwise change in the focal length, which allows for two operating modes: the search mode and the recognition mode (with a smaller and larger focal length, respectively), which increases the efficiency of searching for objects by their thermal radiation in the far infrared region spectrum.

A diagram of the lens is shown in the drawing.

The lens contains: the first component along the beam - the positive meniscus 1, facing concavity to the image plane, the second component - the negative meniscus 2, convex to the image plane, the aperture diaphragm 3, the third component - the positive meniscus 4, facing the concavity to the image plane, plane 5. In this case, on menisci 1 and 4, the surfaces R2 and R5 are aspherical, and meniscus 2 has the ability to be installed in positions “a” and “b”, where in position “b” it is rotated 180 ° and convex to the object, a e of the optical power is equal to -41.77 when the optical power of «OC _ob.min" Lens 16.67. All menisci are made of Ge.

Design data for the position of the second component in pos. 2a and with a focal length of the lens of 15 mm are shown in table 1.

Notes:

1. * - aspherical surface, R ₀ = 57.41, conical constant K = 0.07.

3. ** - aspherical surface, R ₀ = 38.46, conical constant K = -0.7.

When you move the second component to position "b" with its simultaneous rotation of 180 °, the focal length of the lens becomes 60 mm. Design data at this position are shown in table 2.

Thus, the parameters of the lens options:

- estimated wavelength 10.6 microns; - working spectral range 8.0 ... 14.0 microns; - focal length 15 and 60 mm; - focal length of the moving component 23.9 mm; - back focal segment 27.7 mm; - linear field of view 9.0 mm; - relative hole 1: 1.32.

The principle of the lens is as follows.

The first component 1, the aperture diaphragm 3, the third component 4 and the image plane 5 are motionless.

The second component 2, installed with the possibility of movement along the optical axis and with the possibility of its rotation through 180 °, has a negative optical power OC ₂ , which is:

where OC _rev.min is the minimum optical power of the lens.

The second component, turning, occupies two fixed positions “a” and “b”, in which the image of the aperture diaphragm 3 installed in front of the third component 4 is constructed in the space of objects as the entrance pupil of the optical system with a different diameter for each fixed position of the second component 2.

In this case, the optical system with two fixed positions of the second component ("a" and "b") has different focal lengths with the same position of the focal plane and with the same aperture. Using the axial movement of the second component 2 with its simultaneous 180 ° rotation allows you to increase the range of changes in the focal length and reduce the number of lenses.

Setting the quality criterion - the value of the polychromatic contrast transfer coefficient (CPC) and taking into account:

- the thickness of the protective glass of the photodetector, equal to 1.0 mm;

- spectral efficiency at wavelengths taking into account the sensitivity of the photodetector and the light transmission of the lens -1.0 at a wavelength of 8 μm, 1.0 at a wavelength of 10.6 μm, 1.0 at a wavelength of 14 μm;

- spatial frequency of 30 lines / mm (Nyquist frequency for a photodetector with a sensor element size equal to 17 μm), we obtain the following calculated values of the qualitative characteristics of the lens.

When the position of the movable lens corresponding to f _rpm = 15 mm:

- for a point on the axis (diffraction quality) PDA = 45.1% - for a point on the axis (aberration quality) PDA = 40.5% - for a field point 3 mm from the center Images CPC _m = 35.4% PDA _with = 39.7% - for a field point of 4.5 mm from the center Images CPC _m = 34.7% PDA _with = 33.1%

With the position of the movable lens corresponding to f _rev.max. = 60 mm:

- for a point on the axis (diffraction quality) PDA = 45.1% - for a point on the axis (aberration quality) PDA = 31.3% - for a field point 3 mm from the center Images CPC _m = 30.2% PDA _with = 30.6% - for a field point of 4.5 mm from the center Images CPC _m = 23.6% PDA _with = 29.2%

As can be seen from the calculations, the infrared lens, with its simplicity of design, provides a stepwise change in focal length and acceptable image quality close to diffraction for optoelectronic devices using microbolometric matrices with pixel sizes up to 17 μm as photodetectors.

Claims (1)

Infrared lens with variable focal length, including the first positive meniscus located along the beam, facing concavity to the image plane, the second meniscus with the ability to move along the optical axis, the aperture diaphragm and the third positive meniscus facing concavity to the image plane, characterized in that the first and the third meniscus contains one aspherical surface, the second meniscus is negative, while in the first fixed position it is convex to the plane images, and in the second locked position turned through 180 ° relative to the first fixed position and a concavity facing towards the image plane, and the optical power of the second movable component ₂ is OS:
| OS ₂ | ≥ OS _rev.min ,
where OS _rev.min is the minimum optical power of the lens.