RU2544269C1 - Optical filter element - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: optical filter element has a positive optical refracting power and has two positions: in and out of the radiation path; the filter element can be placed between a lens and an image and between an object and a lens without changing its focal distance. The focal distance of the filter element is determined by the formula f'_F=(S+d)·(S+d-δ)/δ, where S is the distance on the optical axis between the object or the image and the optical filter element; d is the thickness of the optical filter element on the optical axis, and the value δ is calculated using the formula δ=d·(n-1)/n, where n is the refraction index of the material of the optical filter element for the fundamental wavelength of the operating spectral range.

EFFECT: enabling installation of the same filter element between a lens and an image and between an object and a lens without disrupting the structure of the lens, while maintaining image quality.

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Description

The invention relates to optical instrumentation, specifically to optical filters, and can be used in projection lenses, lenses that form images of objects located at a finite distance, for example, to discretely reduce the amount of light entering an image transfer device formed by an X-ray electron-optical converter (REOP) or another electron-optical converter (EOP), to the CCD.

The invention can also be used for the manufacture of low-pass filters.

Known neutral filter (JP 20110250157 20111115; JP 20120229093 20121016), made of optically transparent material in the form of a plane-parallel disk, on one of the planes of which is coated, reflecting or absorbing radiation. Moreover, the coating is made in such a way that, for each filter point, the reflection or transmission coefficient does not depend on the radius vectors coming from the center of the circle forming one of the disk surfaces, but on the angular coordinate of this radius in the disk plane. Such a filter for solving the problem of the application can be used as follows: place the filter in the plane of the aperture diaphragm of the lens, refocus the lens or recalculate the optical system in order to compensate for the introduction of an additional optical element and, rotating the filter around the center of the circle forming its disk, obtain the required values light reduction. The disadvantages of this design are:

- the inability to combine the simultaneous use of the iris aperture diaphragm and filter without violating the symmetry of light distribution in the image,

- the need to change the design of the lens to ensure the installation of the filter.

Known optical filter element US 2005163501 (A1) - 2005-07-28), installed in the plane of the aperture diaphragm and made of two: a neutral filter and an optically opaque plate having a complex geometric shape and having the ability to rotate and move relative to each other. Such an optical filter element for solving the application problem can be used if, at the same time, the following condition is fulfilled:

at the location of the aperture diaphragm in the lens, the aperture rays do not tilt to the optical axis. In this case, the introduction and removal of the optical filter element does not require refocusing the lens.

The disadvantages of this design are:

- the inability to combine the simultaneous use of an iris aperture diaphragm and an optical filter element without violating the symmetry of light distribution in the image,

- the need to change the design of the lens to ensure the installation of an optical filter element.

A known design of an optical filter element of low spatial frequencies (JP 2002006127 (A) - 2002-01-09). This optical filter element is structurally made in the form of a lens mounted near a discrete image receiver, for example, a CCD. Moreover, the lens has a periodic structure in itself or at its radii, the pitch and shape of which is determined by the spatial-frequency characteristics of the image receiver. The disadvantages of this design are: high requirements for the quality of the lens material (bubbling, wrinkling, inclusions) and high requirements for the quality of polished lens surfaces (scratches, punctures), and the higher the higher the rear aperture of the lens and the smaller the pixel size of the image receiver, with which an optical filter element will be used. Also, such an optical filter element will significantly affect the lens aberration, which will require an aberration calculation for each specific lens, or the optical filter element will be used with non-aperture lenses.

As the closest analogue, the design of the patent for the invention (EP 2511761 (A2) - 2012-10-17) was selected, containing an optical element - a neutral light filter made in the form of a lens with positive refractive power and located between the lens and the image during the rays. Thanks to the formulas contained in the description and, based on the known characteristics of the lens, in the rear segment of which the optical filter element will be placed, it is possible to calculate in advance the optical characteristics of the filter (surface radius). This design can be used with any lens that has the size of the rear segment, sufficient to accommodate the structure. The advantages of the closest analogue are: the ability to use the filter design without interfering with the lens design, the ability to introduce and remove the optical filter element from the path of the rays without refocusing the lens, i.e. while maintaining image quality. However, the disadvantages of this design are: high demands on the quality of the lens material (bubble, toughness, inclusions) and high demands on the quality of polished lens surfaces (scratches, punctures), and the higher the higher the rear aperture of the lens and the higher the spatial frequency of the image. Also, such an optical filter element will significantly affect the aberration of the lens, which will require an aberration calculation for each specific lens or the optical filter element will be used with non-fast lenses.

The present invention is to develop an optical filter element having the following qualities:

- the ability to use the design of the optical filter element without interfering with the design of the lens,

- the possibility of introducing and removing the optical filter element from the beam without refocusing the lens,

- performance with low quality lenses having (vesicity, lumpiness, inclusions), as well as scratches or punctures on the surface,

- universality, providing the possibility of use with various lenses without the need for aberration calculation.

Disclosure of invention

The technical result in the inventive optical filter element having a positive optical refractive power and predetermined parameters, having two positions: is introduced or removed from the radiation path, is achieved by the fact that the optical filter element is arranged to be located both between the lens and the image, and between the object and lens without changing its focal length, which is calculated by the formula

f ′ _F = (S + d) · (S + d-δ) / δ,

where S is the distance along the optical axis between the object or image and the optical filter element;

d is the thickness of the optical filter element along the optical axis,

and the value of δ is calculated by the formula

δ = d · (n-1) / n,

where n is the refractive index of the material of the optical filter element for the main wavelength of the working spectral range.

Achievable technical results of the claimed optical filter element are:

- universality of the installation of the same optical filter element both between the lens and the image, and between the object and the lens. The ability to place an optical filter element during the rays without interfering with the lens design, due to its installation in a larger front or rear working segment of the lens,

- the versatility of installing an optical filter element in any lens, if the design features of the filter (thickness, diameter) and lens characteristics (field of view, the size of the front or rear working section, the working spectral range) allow,

- a small effect on image quality (aberration, light scattering) and the ability to use with fast lenses, due to the possibility of installing the element in the working segment with small apertures and its large focal lengths,

- the ability to simultaneously or alternately use an optical filter element and an iris (other) diaphragm without additional focusing of the lens,

- if the optical filter element is neutral, it is possible to change the distribution of illumination along the image plane due to the positive refractive power of the optical filter element, its different thickness in the center on the edge and, therefore, its different light transmission in height,

- the possibility of using an optical filter element as a spectral, frequency-spatial,

- the ability to create asymmetries in the image formed by the lens without violating its aberration quality, due to the superposition on this image of the properties specified by the element and when placing it close to the object or image.

The list of explanatory drawings

The invention and the possibility of its industrial application is illustrated by examples of specific performance, which are shown in figures 1-6.

Figure 1 shows a schematic optical diagram of a projection lens (described in RF patent No. 2473932) using the inventive optical filter element for two options for installing the filter - between the object and the lens and between the lens and the image.

Figure 2 shows a schematic optical diagram of an example of a specific implementation of an optical filter element with a lens (the lens is described in RF patent No. 2433433).

Figure 3 - graphs of the relative illumination of the image depending on the size of the subject for an example of a specific implementation without the use of an optical filter element and using an optical filter element

Figure 4 - graphs of changes in illumination in the center of the field of view of the lens depending on the diameter of the aperture diaphragm of the image for an example of a specific implementation using an optical filter element and without it.

Figure 5 - graphs of the frequency-contrast characteristic (TSC) of the lens used for an example of a specific implementation without an optical filter element.

6 is a graph of the frequency response of the lens used for an example of a specific implementation with an optical filter element introduced into the rays.

DETAILED DESCRIPTION OF THE INVENTION

As shown in figure 1 along the optical radiation from the object 1 to the image 5, the optical filter element 2 is made in the form of a plano-convex lens, and can be located between the object 1 and the lens 3 containing the aperture aperture 4, or between the lens 3 and the image 5. The optical filter element can be set to various parameters, for example, the following: transmission of optical radiation, depending on the coordinate of the optical rays on the input or output surface of the element, non-uniform axisymmetric, non-uniform nonaxisymmetric; transmission with superimposing on the image of a periodic or other structure, for example, a lattice, pattern - for the case when the filter is close to the object; uniform or uneven spectral transmission of radiation.

Figure 2 shows a schematic optical diagram of an example of a specific implementation of an optical filter element 2 with a projection lens 3. An optical filter element 2 is installed between the object 1 and the aperture diaphragm (entrance pupil) 4 of the lens 3 at a distance S from object 1 of 10.5 mm, and It is formed as a plano-convex lens with a focal length f _'F 232.4 mm, the convexity facing an image having a refractive index of n = 1.5163 for a wavelength of 0.589 microns, which material is neutral glass with the absorption of radiation tions 50% at a thickness of d = 1.9 mm.

Figure 3, for an example of a specific implementation, presents graphs of the relative illumination of the image EY / E0 (the ratio of the amount of illumination at any point in the image to the illumination in the center of the image) depending on the size of the subject Y for the lens without using an optical filter element (Nominal) and using neutral glass as the material of an optical filter element (ND filter). The graphs show that for lens 3 without an optical filter element 2, the difference in image illumination for the center of the field of view Y = 0 mm and for the edge of the field of view Y = 12.5 mm is 1.0-0.75 = 0.25, and for the same fields when rays are introduced optical filter element, the difference is 1.0-0.94 = 0.06, which indicates the introduction into the lens properties of uniform distribution of illumination.

Figure 4, for an example of a specific implementation, presents a graph of changes in the luminance of the image E / Emax in the center of the field of view of the lens depending on the diameter of the aperture diaphragm A-stop. The graph shows two curves: ND - for the lens using an optical filter element and AD - without it. It can be seen from the graphs that the simultaneous use of a variable aperture diaphragm and an optical filter element allows to achieve large changes in illumination with small changes in the diameter of the aperture, which reduces the requirements for aperture control actuators and also slows the theoretical resolution limit of the lens in accordance with the well-known formula for calculating the radius Airy (disc Airy)

R = 1.22 λ λ f / D.

Figure 5 and figure 6 for an example of a specific implementation presents graphs of the frequency-contrast characteristics of the image, respectively, for a lens without a filtering optical element and with a filtering optical element inserted into the front working section. The graphs show that for the selected location S of the filtering optical element, the introduction and removal of it from the radiation path causes a slight change in the frequency response, namely, when the optical filter element is introduced, the absolute modulation values at a spatial frequency of 55 line / mm pairs for the center of the field of view do not change, while for the edge of the field of view they change by 0.05 from the initial contrast coefficient, which is 0.88 for the meridional section and 0.59 for the sagittal section.

Depending on the tasks to be solved, the methods of using the optical filter element can be as follows:

- the element is constantly in the course of the rays;

- the element is used in two states: periodically introduced and removed from the course of radiation.

The principle of operation, when the optical filter element is introduced into the radiation path, is illustrated by an example of a specific implementation and figure 2. The radiation from object 1 to image 5 passes and is refracted by an optical filter element 2 having a material thickness d along the optical axis, made in this case in the form of a plano-convex lens mounted in the front working segment of the lens 3 at a distance S from the object, then the radiation passes through the aperture the aperture 4 of the lens 3, and the diameter of the aperture diaphragm 4 can be changed, for example, using the iris, which affects the amount of illumination of the image, then passes through the lenses of the lens 3 and focuses 5, and due to the positive refractive power of the optical filter element 2, the focal length of which is calculated according to the application formulas, the introduction or removal of it from the path of the rays does not affect the focus and image quality of the lens.

The method of using the optical filter element is as follows.

The average specialist in the calculation of optical systems in accordance with the task set for him to calculate the focal length of the optical filter element selects the position (distance S), thickness d and the type of filter material with the corresponding refractive index n. Then, using the formulas in the description, calculates the focal length of the filter. Then, using any optical program for analyzing aberrations, it determines the best radii of curvature of the filter lens, provided that the calculated focal length of the optical filter element is provided taking into account its location relative to the object.

An optical filter element is claimed in which a new technical result is achieved: it is possible to use it without adjusting the design of the lens with which it will be used, and without compromising image quality.

Claims (1)

An optical filter element having a positive optical refractive power and predetermined parameters, having two positions: is introduced or removed from the radiation path, characterized in that the optical filter element is arranged to be located both between the lens and the image, and between the object and the lens without changing it focal length, which is calculated by the formula
f ′ _F = (S + d) · (S + d-δ) / δ,
where S is the distance along the optical axis between the object or image and the optical filter element;
d is the thickness of the optical filter element along the optical axis,
and the value of δ is calculated by the formula
δ = d · (n-1) / n,
where n is the refractive index of the material of the optical filter element for the main wavelength of the working spectral range.

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