RU2851162C1 - Lens for ultraviolet spectrum - Google Patents

Abstract

FIELD: optoelectronic technology.

SUBSTANCE: invention can be used as a receiving lens in optoelectronic devices operating in the ultraviolet region of the spectrum. The lens is made of two optical materials and contains six components, the first of which is a positive biconvex lens, the second is a negative biconcave lens, and the sixth is a negative meniscus with its concavity facing the image. The third and fourth components are positive biconvex lenses, the fifth component is a negative meniscus with the concave side facing the object, and the aperture diaphragm is located between the first and second components, using optical materials MgF₂ and BaF₂ are used, and the following relationships apply: n₁=n₃=n₄; n₂=n₅=n₆, where: n₁…n₆ - refractive index of the corresponding component.

EFFECT: increase in relative aperture and reduction of the lower limit of the ultraviolet range of the spectrum to 155 nm.

1 cl, 1 dwg, 1 tbl

Description

The proposed invention relates to the field of optical-electronic technology and can be used as a receiving lens in optical-electronic devices operating in the ultraviolet region of the spectrum with a lower limit of up to 0.15 μm, and implemented without the use of complex aspherical surfaces.

A UV lens is known for use with a photodetector based on AlGaN with a 320x240 element format and a 30 μm pitch. It was developed at Orion Research and Production Association (Kolesova, A.A., Poleskiy, A.V., Khamidullin, K.A., Yudovskaya, A.D. Development and Research of an Ultraviolet Lens. Applied Physics, No. 5, 2013). The lens consists of two cemented components and has a significant axial thickness of the lenses, which allows for the required degree of aberration correction to be achieved (RU patent No. 127206 U1, published on April 20, 2013). The first component consists of a biconvex lens and a meniscus with its convex side facing the image space, while the second component consists of a meniscus with its convex side facing the object space and a biconvex lens. The materials of the first and fourth, and the second and third lenses are pairwise identical, while the ratio of the thickness of each lens to its light diameter is not less than 0.45, the ratio of the optical power of the first group of lenses to the optical power of the second group of lenses is greater than 1.4, but less than 1.6, the ratio of the dispersion coefficient of the first and fourth lenses to the dispersion coefficient of the second and third lenses is greater than 1.4, but less than 1.6.

The lens has a focal length of 70 mm, a linear field of view of 9 mm, a relative aperture of 1:2.5, a spectral range of 270÷400 nm, and an entrance pupil located on the first surface.

The calculated scattering spot size for a point on the axis at an energy level of 90% in the spectral range of 240÷280 nm is 50.5 µm.

The disadvantage of this ultraviolet lens is the small linear field of view and the limitation of the lower limit of the ultraviolet range of the spectrum, starting at 240 nm.

An anastigmat objective for the ultraviolet region of the spectrum is known (author's certificate No. 905790, published on 15.02.1982), containing two single positive lenses, a negative lens located between them, a positive meniscus with its concavity facing the image space and installed behind the first positive lens, and a negative meniscus with its concavity facing the image space and installed behind the negative lens.

The lens has a focal length of 50 mm, a relative aperture of 1:5, a field of view of 31° and a spectral range of 200÷400 nm.

The disadvantage of this ultraviolet lens is the low aperture value of the lens and the limitation of the lower limit of the ultraviolet range of the spectrum, starting at 200 nm.

A high-aperture lens for the ultraviolet region of the spectrum is known (patent RU No. 2040804 C1, published July 25, 1995), containing seven components sequentially mounted on the optical axis - the first positive meniscus, facing concavely towards the image, the second positive and negative meniscus, facing concavely towards the object, the first positive biconvex lens, the negative biconcave lens, the second biconvex positive lens and the third positive meniscus, facing concavely towards the image.

The lens has a focal length of 40 mm, a relative aperture of 1:1.4, a field of view of 25°, effective scattering circles not exceeding 30 µm, and a spectral range of 330÷400 nm.

The disadvantage of this ultraviolet lens is the small angular field of view, a large number of optical elements and a limited lower limit of the ultraviolet spectrum, starting at 330 nm.

The closest in technical essence is the objective lens for the ultraviolet region of the spectrum (Turbin A.V., Rafailovich A.S., Aldokhin P.A. Objective lens for the ultraviolet region of the spectrum. // GEOSIBERIA - 2011, Vol.5, No.1, pp. 81-84.), made on the basis of two optical materials, KU2 quartz glass and lithium fluoride crystals and containing six components sequentially located on the optical axis. The first and fifth components are positive biconvex lenses, the second component is a negative biconcave lens, the third, fourth and sixth components are negative menisci, with the concavity facing the image. The aperture diaphragm is the frame of the first component of the objective lens.

The lens has a focal length of 100 mm, a relative aperture of 1:3, a linear field of view of 18 mm, a scattering spot in the center of the field of view of no more than 13 µm, at the edge of the field of view of no more than 30 µm and a spectral range of 290÷400 nm.

The disadvantage of this ultraviolet lens is the low aperture value of the lens and the limitation of the lower limit of the ultraviolet range of the spectrum, starting at 290 nm.

The objective of the present invention is to increase the aperture ratio of the lens and reduce the lower limit of the ultraviolet range of the spectrum to 155 nm.

The technical result, determined by the task, is achieved by the fact that in the objective for the ultraviolet region of the spectrum, made of two optical materials and containing six components, the first of which is a positive biconvex lens, the second is a negative biconcave lens, the sixth is a negative meniscus facing concavely to the image, in contrast to the known, the third and fourth components are positive biconvex lenses, the fifth component is a negative meniscus facing concavely to the object, the aperture diaphragm is located between the first and second components, while the optical materials magnesium fluoride MgF ₂ and barium fluoride BaF ₂ are used and the following relationships take place:

n ₁ =n ₃ =n ₄ ;

n ₂ =n ₅ =n ₆ ,

where: n _1…6 is the refractive index of the corresponding component.

The diagram of the lens for the ultraviolet region of the spectrum is shown in Figure 1.

The objective for the ultraviolet region of the spectrum consists, along the path of the rays, of a positive biconvex lens 1, a negative biconcave lens 2, positive biconvex lenses 3 and 4, negative menisci 5 and 6, protective glass of the photocathode 7 and an aperture diaphragm 8.

The design parameters of the lens for the ultraviolet spectrum are given in the table.

The lens parameters for the ultraviolet region of the spectrum are as follows:

- working spectral range: 155…260 nm;

- focal length: 30 mm;

- angular field of view: 33°;

- linear field of view: 18.0 mm;

- relative aperture: 1:2.8;

- distortion: no more than 1.6%;

- vignetting: no more than 25%;

- length along the optical axis: 33.2 mm;

- weight of optical elements: 21 g.

The operating principle of the lens for the ultraviolet region of the spectrum is as follows:

Parallel beams of light pass through the first component 1, made in the form of a positive biconvex lens, and hit the aperture diaphragm 8, which forms the entrance pupil of the lens in the object space. After passing the aperture diaphragm 8, the light passes through the negative biconcave lens 2, positive biconvex lenses 3 and 4, and after the negative meniscus 5 and 6 hit the protective glass of the photocathode 7. The reduction of the lower limit of the ultraviolet spectrum to 155 nm is ensured by the use of two optical materials - magnesium fluoride MgF ₂ and barium fluoride BaF ₂ , the correction of aberrations is achieved by a combination of materials, in which the following relationships hold:

n ₁ =n ₃ =n ₄ ;

n ₂ =n ₅ =n ₆ ,

where: n _1…6 is the refractive index of the corresponding component.

This lens for the ultraviolet region of the spectrum enables imaging with a lower limit of the spectral range of up to 155 nm, and the location of the aperture diaphragm between the first and second components allows for an increase in the relative aperture to a value of 1:2.8.

The image quality of the objective lens for the ultraviolet spectral region was calculated for use with a matrix photodetector with a photocathode diameter of ∅18 mm and a resolution of 40 lp/mm in the spectral range of 155–260 nm. The calculated values of the contrast characteristics—the contrast transfer coefficients (CTCs)—of the objective lens for the ultraviolet spectral region in the plane of best alignment (d14=4.355 mm) for a spatial frequency of 30 lp/mm are as follows:

- for a point on the axis: CPC=33%;

- for a field point of ∅10.5 mm (20°): KPK _m = 22%, KPK _s = 24%.

- for the extreme point of the field ∅17.5 mm (33°): KPK _m = 10%, KPK _s = 14%.

As can be seen from the calculations, the lens for the ultraviolet region of the spectrum provides operation in the spectral range from 155 to 260 nm and an acceptable level of image quality.

Claims (4)

An objective for the ultraviolet region of the spectrum, made of two optical materials and containing six components, the first of which is a positive biconvex lens, the second is a negative biconcave lens, the sixth is a negative meniscus with its concavity facing the image, characterized in that the third and fourth components are positive biconvex lenses, the fifth component is a negative meniscus with its concavity facing the object, the aperture diaphragm is located between the first and second components, wherein the optical materials used are magnesium fluoride MgF ₂ and barium fluoride BaF ₂ and the following relationships hold: n ₁ =n ₃ =n ₄ ; n ₂ =n ₅ =n ₆ , where: n ₁ …n ₆ is the refractive index of the corresponding component.