RU2316797C1 - Lens objective with changeable focal length for operation within ir spectrum area - Google Patents

Abstract

FIELD: optical engineering.

SUBSTANCE: objective can be used as objective of devices operating within IR spectrum range for detecting, distinguishing and recognizing objects. Objective has two motionless positive components and mo able negative component disposed those two negative components. First component being dispose along path of beams is made in form of meniscus turned with its concavity to plane of image. Last component is made in form of single menisci turned with its concavity to plane of image. Negative component is made in form of single biconcave lens. Positive component is introduced between negative component and last positive component. Positive component is made in form of menisci turned with its concavity to plane of image. Meniscus is tightly connected with aperture diaphragm and is capable of moving together with diaphragm along optical axis. Surface of menisci, which surface comes in front of aperture diaphragm, is made aspherical. The following conditions are met in the system: n₁=n₂=n₃=n₄=4,0, -f'₃<f'₂<-1,1f'₄, 2,7f'₃<f'₁<3f'₃.

EFFECT: higher technical parameters; improved quality of image; reduced sizes.

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Description

The invention relates to the field of optoelectronic instrumentation and can be used as a lens of thermal imaging devices for monitoring and identifying objects by thermal radiation.

A known lens [1], consisting of four components, the third, fourth mounted with the ability to enter and output them from the optical circuit to change the focal length. The first component contains single positive and negative lenses and a double-glued lens consisting of a biconvex and biconcave, the second component contains a positive and negative meniscus facing concavity to each other, the third component includes a biconcave lens and a positive meniscus convex to the object, and the fourth component contains a lens, a double-glued lens, consisting of a negative and a positive lens, and a positive meniscus, convex to the object.

Its disadvantage is the large number of optical elements. As a result, the overall dimensions increase and the absorption of the incident radiation by the lenses increases.

Another lens is known [2], consisting of four groups of components with the ability to input and output the second group from the optical circuit to change the focal length.

Its disadvantage is the increased overall dimensions and, as a consequence, the impossibility of use in equipment, to which strict weight and size requirements are imposed.

Reducing the overall dimensions of the above lenses is impossible because of the need to have additional space to accommodate the components introduced into the optical circuit when changing its focal length.

The closest adopted for the prototype is a lens lens [3] with a discrete change of focal length for working in the infrared region of the spectrum with the first and last fixed positive components installed along the rays and a moving negative component located between them, while the first component is made in the form positive meniscus facing concavity to the image plane.

The disadvantages of this lens are the change in the magnitude of the relative aperture when changing the focal length, large overall dimensions, low image quality and low resolution, which limit the use of this lens with heat radiation detectors, the value of the heat-sensitive element of which lies within 25-35 microns.

The objective of the invention is to provide higher technical characteristics of the lens: improving image quality and reducing overall dimensions.

The problem is achieved by the fact that it is proposed a lens with a variable focal length for operation in the infrared region of the spectrum, containing two fixed positive components and a moving negative component located between them, while the first stationary component along the rays is made in the form of a meniscus facing concavity to image plane, characterized in that the last component is made in the form of a single meniscus facing concavity to the image plane, a negative component theentent is made in the form of a single biconcave lens, between the negative component and the last positive component an additional positive component is introduced, made in the form of a meniscus, with a concavity facing the image plane, rigidly connected with the aperture diaphragm and having the possibility of moving along it along the optical axis, while the surface the meniscus preceding the aperture diaphragm is made aspherical and the following relationships take place in the system:

n ₁ = n ₂ = n ₃ = n ₄ = 4.0

-f ' ₃ <f' ₂ <-1.1f ' ₄

2.7f ' ₃ <f' ₁ <3f ' ₃

Moving the aperture diaphragm makes it possible to keep the value of the relative aperture constant when changing the magnitude of the focal length.

The implementation of the surface preceding the aperture diaphragm in the form of higher-order aspherics makes it possible, due to the introduction of an additional parameter (asphericity coefficient of the refracting surface), to influence the higher-order residual aberrations.

The proposed invention is illustrated by the following graphic materials:

Figure 1 - The position of the components for

a) f '= 20 mm;

b) f '= 40 mm.

Figure 2 - transverse spherical aberration at different focal lengths;

Figure 3 is a graph of the variation of the radius of the spot of scattering across the field at different focal lengths;

Figure 4 is a graph of the frequency response at various focal lengths;

The optical scheme of a fast zoom lens is shown in figure 1, which shows: the first component 1, the second component 2, the third component 3, aperture diaphragm 4 and the last component 5.

The design parameters of this lens are shown in table 1.

Table 1 Lens design Radii mm Thickness mm Material Light diameters, mm R ₁ = 54.2 D ₁ = 6.0 Germanium 31.9 / 56.4 R ₂ = 64.75 D ₂ = 4.2 / 30.0 28.9 / 52.9 R ₃ = -168.2 D ₃ = 4.0 Germanium 27.1 / 27.2 R ₄ = 168.7 D ₄ = 20.46 / 0.76 26.7 / 26.6 R ₅ = 51.2 D ₅ = 6.0 Germanium 30.0 / 26.6 R ₆ = 42.54 * D ₆ = 3 28.1 / 24.7 R ₇ = ∞ (HELL) D ₇ = 17.23 / 11.12 26.0 / 22.4 R ₈ = 27.78 D ₈ = 4.0 Germanium 18.7 / 16.8 R ₉ = 36.31 D ₉ = 10.13 16.7 / 15.1

Fraction numerator for f '= 20 mm, denominator for f' = 40 mm.

* - aspherical surface determined according to the equation:

where Z is the deviation of the surface profile from the reference sphere, mm

C is the curvature of the surface, mm ^-1 ;

Y is the distance from the axis, mm;

The lens has the following characteristics:

Focal length mm 20-40 Relative hole 1: 1,0 Working spectral range, microns 8-12 Linear image field, 2y ′ mm 8

Figure 2 shows graphs of axial aberrations of the proposed lens. The abscissa axis represents the values of the longitudinal aberrations ΔS '(mm), and the ordinate axis represents the coordinate y' (mm) in the plane of the entrance pupil.

Figure 3 shows graphs of changes in the diameter of the scattering spot over the field for the proposed lens with a focal length of 20 mm and 40 mm

Figure 4 shows graphs of the frequency-contrast characteristics of the proposed lens. The spatial frequency N mm ^{-1 is} plotted on the abscissa axis, relative to the image plane of the lens, and on the ordinate axis, the contrast transfer coefficient T in relative units.

From the above graphs, as well as from the analysis of the prototype at frequencies of 100-130 mm ^-1 it can be seen that the proposed lens circuit has higher performance in terms of image quality.

This lens can be used in devices operating in the infrared range for the detection, discrimination and recognition of objects.

Information sources

1. Patent of the Russian Federation No. 2063058 (analogue).

2. US Patent No. 4989962 (analogue).

3. WO No. 99/59015 (prototype).

Claims (1)

A lens with a variable focal length for working in the infrared region of the spectrum, containing two stationary positive components and a moving negative component located between them, while the first stationary component along the rays is made in the form of a meniscus facing concavity to the image plane, characterized in that the last component is made in the form of a single meniscus facing concavity to the image plane, the negative component is made in the form of a single biconcave lens, m I am waiting for the negative component and the last positive component to additionally introduce the positive component made in the form of a meniscus, with a concavity facing the image plane, rigidly connected with the aperture diaphragm and having the possibility of joint movement along it along the optical axis, while the meniscus surface preceding the aperture diaphragm is made aspherical and in the system the following relationships hold: n ₁ = n ₂ = n ₃ = n ₄ = 4.0, -f ′ ₃ <f ′ ₂ <-1,1f ′ ₄ , 2,7f ′ ₃ <f ′ ₁ <3f ′ ₃ .

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