Method of increasing depth of field of imaging setup and diffractive-optical lens attachment to implementation of the method

Abstract

There is disclosed a method for increasing the depth of field of a lens coupled to a digital camera, using an optical cap in the form of an annular diffractive element of transmittance such as an Axicon, Light Sword or bifocal lens with intentionally reduced optical power to reduce imaging chromaticity. The subject of the invention is also a diffractive optical attachment.

Description

[0001] METHOD OF INCREASING DEPTH OF FIELD OF IMAGING SETUP

[0002] AND DIFFRACTIVE-OPTICAL LENS ATTACHMENT TO

[0003] IMPLEMENTATION OF THE METHOD

[0004] The subject of the invention is a method of increasing the depth of field of imaging and a diffraction-optical lens attachment for the implementation of this method intended for lenses of high-resolution digital cameras, used especially in close-circuit television systems using machine vision.

[0005] In known solutions, in order to increase the depth of field, optical elements are used that are permanently integrated with the lens or that are in the form of lens attachments occupying the entire aperture of the lens.

[0006] From the documentation of the international application WO 201 1 / 099003 Al, there is a known way to increase the depth of field by adding a coding phase plate to the imaging lens, whose task is to cause light interference for at least three distinguished wavelength ranges of light. The purpose of interference is to increase the depth of field for at least one wavelength from the above three wavelengths with a negligible disturbance in the propagation of the remaining wavelengths.

[0007] The US patent document US 7,158,317 B2 describes a phase-only diffraction structure applied to a lens containing regularly or randomly spaced phase-modulating fragments so that during tandem imaging the structure and the lens throughout the aperture one obtains the effect of imaging with multiple focal lengths simultaneously, thus contributing to an effective increase in the depth of field with a significant deterioration of the MTF (Modulation Transfer Function) and thus imaging resolution.

[0008] The US patent document US 7.061,693 B2 is known as a method of adding to the lens a binary phase plate with wide zones of thousands of wavelengths, which works on the principle of refraction, not diffraction, thus allowing for broadband imaging. The phase plate contains periodically repeated phase patterns with optimized shapes and must operate with the entire aperture of the lens.

[0009] The US filing document US20190212478 Al describes a method of increasing the depth of field by covering the main imaging lens with an adjacent layer containing a phase diffraction structure with additional optical power. In addition, the other side of the above-mentioned lens is covered with an additional optical layer containing a diffraction structure aimed at increasing the depth of sharp imaging for different wavelengths of light.

[0010] Patent documents US 6,069,738 and US7218448B1 provide a way to increase the depth of field of imaging by inserting a phase plate into the optical path that introduces intentional asymmetric blur spot distortion, which is independent of the defocusing parameter of the imaging system. The method requires digital image processing after acquisition to correct the distortion introduced by the phase plate.

[0011] The international application document WO 03 / 052492 shows how to increase the depth of field of the eye lens by changing the shape of its surface in order to modify the phase transmittance of the above-mentioned lens so as to encode a diffraction structure in it, making the imaging blur spot less dependent on the defocusing parameter.

[0012] In the known solutions discussed above, time-consuming methods related to digital image processing are used, as well as elements permanently integrated with the optical elements of the imaging system. They are characterized by the necessary loss of light energy by deflecting it into angular regions beyond the detector's acquisition capabilities, a decrease in energy efficiency and / or imaging resolution.

[0013] Bearing in mind the above disadvantages, the aim of the invention is to provide a new way to increase the depth of field, which would not interfere with the functioning of the lens in terms of resolution, contrast and brightness of the imaging, and which would allow to increase the depth of field of imaging beyond the native range of the lens.

[0014] The method of increasing the depth of field of imaging in an imaging system containing a digital camera with an optical attachment with a diffraction structure mounted separably on the lens, consisting in transforming the visible light beam through the optical attachment, focusing this beam through the lens of the lens, and then mapping the image on the detection array of a digital camera, is characterized by the invention that the optical attachment is equipped with a photoresist layer, whose phase transmittance is a representation of the transmittance of a diffraction element focusing a plane wave into a focal length or a set of discrete axial foci, corresponding to a given type of objective lens. The phase transmittance of the photoresist layer is described by the formula:

[0015] T(r,0) = exp[-ik Φ (r, 0 )] where: r, 0 - radial and angular coordinates in the polar coordinate system of a given transmittance point,

[0016] Φ (r, 0 ) - phase delay, k = 2π / λ, - wavenumber, λ - average wavelength of light for the operating wavelength of the optical element.

[0017] The thickness of the photoresist layer is determined in a given place, depending on the value of the phase delay, and the difference in the thickness of the photoresist layer between the thickest and thinnest place is λ / (n-l), where n is the refractive index of the photoresist.

[0018] Preferably in the case of a diffraction element of the Axicon type, the phase delay is described by the formula: where: r - radial coordinate of a given transmittance point in the polar coordinate system, x1, x2- minimum desired distance of sharp imaging of the lens, minimum distance to the beginning of the depth of field zone.

[0019] R, R’ - radii of the concentric circles detennining the width of the photoresist layer.

[0020] Preferably in the case of a Light Sword diffraction element, the phase delay is described by the formula: where: r, 0 - radial and angular coordinates in the polar coordinate system of a given transmittance point, k = 2π / λ, - wavenumber, λ - average wavelength of light for the operating wavelength of the optical element, x1, x2- minimum desired distance of sharp imaging of the lens, minimum distance to the beginning of the depth of field zone.

[0021] Preferably in the case of a bifocal lens diffraction element, the phase delay is described by the fonnula: where: r - radial coordinate in the polar coordinate system of a given transmittance point, k = 2π / λ, - wavenumber, λ - average wavelength of light for the operating wavelength of the optical element, x1, x2- minimum desired distance of sharp imaging of the lens, minimum distance to the beginning of the depth of field zone.

[0022] Preferably, the photoresist layer is formed in the form of a strip formed between the concentric circles, whose difference in the length of the rays is within the range of 5 to 20%, on a round flat-parallel glass plate.

[0023] Preferably a fragment of the glass plate without a photoresist layer is removed.

[0024] Preferably the glass plate is covered on both sides with anti-reflection layers for visible light.

[0025] A diffractive optical attachment for increasing the depth of field of imaging containing a fixing ring, inside which a round glass element equipped with a photoresist layer is laid flat, is characterized by the invention that the photoresist layer is in the form of a strip formed between the concentric circles, the difference in the length of the rays of which is within the range of 5 to 20%. The phase transmittance of the photoresist layer is a representation of the transmittance of the diffraction element corresponding to a given type of objective lens and is described by the formula:

[0026] T(r,0) = exp[-ikΦ(r, 0 )] where: r, 0 - radial and angular coordinates in the polar coordinate system of a given transmittance point,

[0027] Φ(r, 0) - phase delay, k = 2π / λ, - wavenumber,

[0028] X - average wavelength of light for the operating wavelength of the optical element.

[0029] The thickness of the photoresist layer is determined in a given place depending on the value of the phase delay, and the difference in the thickness of the photoresist layer between the thickest and thinnest place is λ / (n-1), where n is the refractive index of the photoresist.

[0030] Preferably in the case of the diffraction element of the lens of the Axicon type, the phase delay for the structure is described by the formula: where: r - radial coordinate in the polar coordinate system of a given transmittance point,

[0031] X1, x2- minimum desired distance of sharp imaging of the lens, minimum distance to the beginning of the depth of field zone

[0032] R, R’ - radii of the concentric circles determining the width of the photoresist layer.

[0033] Preferably in the case of the diffraction element of the lens (2) of the Light Sword type, the phase delay for the structure is described by the formula: where: r, 6 - radial and angular coordinates in the polar coordinate system of a given transmittance point, k = 2π / λ, - wavenumber, λ - average wavelength of light for the operating wavelength of the optical element,

[0034] X1, x2- minimum desired distance of sharp imaging of the lens, minimum distance to the beginning of the depth of field zone.

[0035] Preferably in the case of the diffraction element of the objective lens (2) of the bifocal lens, the phase delay for the structure is described by the formula: where: r - radial coordinate in the polar coordinate system of a given transmittance point, k = 2π / λ - wavenumber,

[0036] X - average wavelength of light for the operating wavelength of the optical element,

[0037] X1, X2- minimum desired distance of sharp imaging of the lens, minimum distance to the beginning of the depth of field zone.

[0038] Preferably, the glass element of the attachment is a flat-parallel glass plate.

[0039] Preferably, the glass element of the attachment is in the form of a ring with a width equal to the width of the photoresist layer (16).

[0040] Preferably, the glass element of the attachment is covered on both sides with anti- reflection layers for visible light.

[0041] The invention makes it possible to eliminate lens dysfunction in terms of resolution, contrast and brightness of imaging, thanks to the use of a photoresist layer dedicated to the characteristics of a given lens, covering only partially the lens aperture.

[0042] The subject of the invention is explained in more detail in the embodiment in the drawing in which fig.1 Shows the optical system of acquisition of test images with increased depth of field, fig.2 shows an attachment that increases the depth of field in the front (left) and side (right) projection, fig.3 shows the cross-section of the attachment ring and fig.4 shows a picture of a test object positioned at a distance of 01 taken on the camera's photosensitive array, fig.5 shows a picture of the test object positioned at a distance of 05 obtained on the camera’s photosensitive array, and fig.6 presents photoresist layers with phase transmittances corresponding to selected types of diffraction elements of lens lenses.

[0043] Example I

[0044] An example of the implementation was made using the 65 MP Emergent HZ-65000 camera with 3.2 μm pixels with F / 1.4 Zeiss Otus lens f=85 mm.

[0045] The method of increasing the depth of field of imaging of digital camera lenses is carried out, according to the example of the invention, in an imaging system using a digital camera 1 with a mounted lens 2, whose focus ring 12 is permanently set to the imaging distance at the 03 point, which is the midpoint of the native imaging range. The object to be imaged is test chart 3 located centrally on optical axis 4, located at a known and precisely determined distance from lens 2, measured along optical axis 4. The distances between the nearest point of the extended O1 imaging range and the farthest point of the extended 05 imaging range, determining the EDoF segment, are measured along optical axis 4 measured from the end of the lens housing 2. If test board 3 is placed at any distance within the DoF (Depth of Focus) range between the distances from the nearest point of the native 02 imaging range to the farthest point of the native 04 imaging range, test board 3 is imaged with full focus through lens 2 on the detection array 7 of the digital camera 1 and observed on a computer screen 9 connected to the digital camera 1 by link 8. The course of light rays 6 coming from the central point 5 of test board 3 is bent refractively by focusing lens 17, which is an approximation (idealization) of the optical function of the lens 2. Light rays 6 focus on the detection matrix 7 into a blur spot with a diameter not exceeding the width of a single pixel of the detection matrix 7. In this way, the condition of imaging with full depth of field is met, and the distance range of the DoF segment is called the native depth of field of a given combination of lens 2 and digital camera 1. It is advantageous that the non-diffraction bending light (zero diffraction beam) in ring 13 is still involved in imaging through lens 2.

[0046] When placing test board 3 at 01 or 05 distances outside the range of the native depth of field of DoF, test board 3 is imaged out of focus through lens 2 on the detection array 7 of the digital camera 1. Light rays 6 focus on the detection matrix 7 into a blur spot with a diameter exceeding the width of a single pixel of the detection matrix 7. In this way, the condition of acute imaging is not met.

[0047] The manufacturing of attachment 11 increasing the depth of field along with determi- ning the phase transmittance of the photoresist 16 layer is carried out according to the procedure carried out in the following stages.

[0048] 1. Experimentally, by adjusting the focus ring 12 of lens 2, the position of the 03 point is selected so that the position of the 04 point coincides with the maximum desired focus distance of the lens 2. Thus, the functionality of the 11 overlay increasing the depth of field will be limited to improving the focus only in the 01-02 range, where 01 is the same as the minimum desired sharp imaging distance of the lens 2. The fixed position of focus ring 12 shall be locked. The distance 01 should be remembered as X1, the distance 02 should be remembered as x2. 2. The value of R is selected as the thread radius 10 minus the housing thickness of the

[0049] 14 socket 11. 3 The value of R' shall be chosen as 80 % of the value of R.

[0050] 4. The transmittance of the photoresist 16 layer is: where r and 6 are the coordinates of a given transmittance point in polar coordinates, z is the imaginary uni k=2π / λ, and X is the wavelength of light for which ring 13 is designed. For example, the wavelength of light is λ=530 nm. The value Φ (r, 0) of the phase delay at a given transmittance location takes different values depending on the choice of the type of diffraction element increasing the depth of field from among the following: a. Axicon b. Light Sword c. Bifocal lens

[0051] 5. The photoresist 16 layer is shaped according to the determined transmittance using known photo- or electrolytography techniques so that the target thickness of the photoresist 16 layer at a given place is linearly dependent on the phase delay value Φ>(r, / 0) and that the difference in the thickness of the photoresist 16 layer after chemical treatment between the thickest and thinnest places is λ / (n-l) , where n is the refractive index of photoresist 16.

[0052] 6. The resulting structure is cut to a ring of 13 with an outer radius R and an inner radius R’ .

[0053] 7. Ring 13 is embedded in housing 14 with clamping thread 15. Ring 13 is attached to thread 15 by housing 14 in the form of a holding edge. When attachment 11 is applied to lens 2, the light rays 6 emitted by test strip 3 placed at the nearest point of the extended imaging range 01 or the farthest point of the extended imaging range 05 are bent so that they are focused by lens 2 on the detection array 7 into a blur spot of transverse sizes smaller than the size of a single pixel of the detection array 7. Thus, the condition of acute imaging is fulfilled. The 01 distance is the shortest distance at which this acute imaging condition is met using a depth-of-field attachment 11. The 05 distance is the greatest distance for which this acute imaging condition is met using a depth-of-field attachment 11. The range of distances between the 01 point and the 05 point is called the Extended Depth of Focus (EDoF) range. Attachment 11 is a diffraction structure with low optical power and intentionally sparse zones, causing a decrease in the diffraction angle of light, which reduces the chromatism of the structure and allows achromatic imaging in white light.

[0054] Example II

[0055] Diffractive optical attachment 11 to increase depth of field, it is screwed to thread 10 of lens 2. Its general appearance and cross-section are presented on the fig.2 and fig.3 respectively. Attachment 11 consists of a ring 13 with an outside diameter of 2*R or less than a lens diameter 2 and an inside diameter of 2*R' greater than zero but at least 5 mm smaller than the outer diameter of attachment 11. Ring 13 has the form of a glass plate covered with a layer of photoresist 16, whose profile (thickness distribution) causes diffraction of light passing through it. The profile of the photoresist 16 layer makes the board have the phase transmittance of the diffraction element. Due to the low optical power of the transmittance of photoresist 16, the phase profile in transmittance is slowly variable and thus allows achromatic broadband operation over the entire wavelength range of visible light. The photoresist 16 layer is shaped using the method shown in Example I.

[0056] The invention in question is not limited to the embodiment presented above. It is possible to modify and develop it under the attached patent claims, without departing from the essence of the invention.

Claims

Patent claims1. A method of increasing the depth of field of imaging in an imaging system containing a digital camera with an optical attachment with a diffraction structure mounted separably on the lens, consisting in transforming the visible light beam through the optical attachment, focusing this beam through the lens of the lens, and then mapping the image on the detection matrix of the digital camera, characterized by the fact that the optical attachment (11) is equipped with a photoresist layer (16), whose phase transmittance is a representation of the transmittance of the diffraction element focusing the plane wave into a focal length or a set of discrete axial foci, corresponding to a given type of objective lens (2) and is described by the formula: where:r, 0 - radial and angular coordinates in the polar coordinate system of a given transmittance point,<Φ(r, 0 ) - phase delay, k = 2π / λ, - wavenumber,X - average wavelength of light for the operating wavelength of the optical element, wherein the thickness of the photoresist layer (16) is determined in a given place depending on the phase delay value, and the difference in the thickness of the photoresist layer (16) between the thickest and thinnest place is λ / (n-1), where n is the refractive index of the photoresist.

2. The method according to claim 1, characterized by the fact that in the case of a diffraction element of the Axicon type, the phase delay is described by the formula:where: r - radial coordinate in the polar coordinate system of a given transmittance point, x1, X2- minimum desired distance of sharp imaging of the lens, minimum distance to the beginning of the depth of field zone,R, R’ - radii of the concentric circles determining the width of the photoresist layer.

3. The method according to claim 1, characterized by the fact that in the case of a diffraction element of the Light Sword type, the phase delay is described by the formula: where:r, 0 - radial and angular coordinates in the polar coordinate system of a given transmittance point, k = 2π / λ, - wavenumber,X - average wavelength of light for the operating wavelength of the optical element, x1, x2- minimum desired distance of sharp imaging of the lens, minimum distance to the beginning of the depth of field zone.

4. The method according to claim 1, characterized by the fact that in the case of a bifocal lens diffraction element, the phase delay is described by the formula: where:r - radial coordinate in the polar coordinate system of a given transmittance point, k = 2π / λ, - wavenumber,X - average wavelength of light for the operating wavelength of the optical element, x1, X2- minimum desired distance of sharp imaging of the lens, minimum distance to the beginning of the depth of field zone.

5. The method according to any of claims from 1 to 4, characterized by the fact that the photoresist layer (16) is formed in the form of a belt formed between the concentric circles, whose difference in the length of the radii is within the limits of 5 to 20%, on a round flat-parallel glass plate.

6. The method according to claim 5, characterized in that a fragment of the glass plate without a photoresist layer is removed (16).

7. The method according to any of claims from 1 to 6, characterized by the fact that the glass plate is covered on both sides with anti-reflection layers for visible light.

8. A diffractive optical attachment for increasing the depth of field of imaging containing a fixing ring inside which a round glass element equipped with a photoresist layer is set flat, characterized by the fact that the photoresist layer (16) is in the form of a strip formed between the concentric circles, whose radial length difference is between 5 and 20%, and the phase transmittance of the resistor layer (16) is a representation of the transmittance of the corresponding diffraction element to the type of objective lens (2) and is described by the formula: where:r, 0 - radial and angular coordinates in the polar coordinate system of a given transmittance point, Φ(r, 0 ) - phase delay, k = 2π / λ, - wavenumber, λ. - average wavelength of light for the operating wavelength of the optical element, where the thickness of the photoresist layer (16) is determined in a given place depending on the phase delay value, and the difference in the thickness of the photoresist layer (16) between the thickest and thinnest place is λ / (n-l), where n is the refractive index of the photoresist.

9. The attachment according to claim 8, characterized in that in the case of the diffraction element of the objective lens (2) of the Axicon type, the phase delay is described by the formula:where: r - radial coordinate in the polar coordinate system of a given transmittance point, x1, x2- minimum desired distance of sharp imaging of the lens, minimum distance to the beginning of the depth of field zone,R, R’ - radii of the concentric circles determining the width of the photoresist layer.

10. The attachment according to claim 8, characterized in that in the case of the diffraction element of the lens (2) of the Light Sword type, the phase delay is described by the formula:where: r, 0 - radial and angular coordinates in the polar coordinate system of a given transmittance point, k = 2π / λ, - wavenumber, λ - average wavelength of light for the operating wavelength of the optical element,X1, x2- minimum desired distance of sharp imaging of the lens, minimum distance to the beginning of the depth of field zone.

11. The attachment according to claim 8, characterized by the fact that in the case of the diffraction element of the objective lens (2) of the bifocal lens type, the phase delay is described by the formula: where:r - radial coordinate in the polar coordinate system of a given transmittance point, k = 2π / λ, - wavenumber, λ - average wavelength of light for the operating wavelength of the optical element,X1, x2- minimum desired distance of sharp imaging of the lens, minimum distance to the beginning of the depth of field zone.

12. The attachment according to any of claims from 8 to 11, characterized by the fact that the glass element is a flat-parallel glass plate.

13. The attachment according to any of claims from 8 to 11, characterized by the fact that the glass element is in the form of a ring 13 with a width equal to the width of the photoresist layer (16).

14. The attachment according to any of claims from 8 to 13, characterized by the fact that the glass element is covered on both sides with anti-reflection layers for visible light.

Images