An under-screen fingerprint collection optical module and an under-screen fingerprint collection method

Abstract

This application belongs to the field of image acquisition technology and discloses an under-screen fingerprint acquisition optical module and an under-screen fingerprint acquisition method, which comprises a touch screen, a display screen pixel layer, a polygonal gradient refractive index lens array, a micro-hole array plate, and an image sensor in order; the upper surface of the touch screen is arranged on the object-side principal plane of the polygonal gradient refractive index lens array, and the upper surface of the image sensor is arranged on the image-side principal plane of the polygonal gradient refractive index lens array; the fingerprint to be measured is diffusely reflected under the luminous illumination of the display screen pixel layer, and the diffusely reflected light of the fingerprint to be measured passes through the display screen pixel layer and then enters the polygonal gradient refractive index lens array. The light emitted by each sub-lens passes through the micro-hole array plate and is comprehensively imaged on the image sensor. The conjugate images formed by all sub-lenses on the fingerprint to be measured are superimposed and enhanced on the image sensor, effectively improving the imaging light flux, resolution, and image contrast, thereby improving the fingerprint imaging quality.

Description

Technical Field

[0001] The present application belongs to the field of image acquisition technology, and specifically relates to a fingerprint acquisition optical module and a fingerprint acquisition method. Background Art

[0002] With the development of the demand for full-screen smart devices such as smartphones and pads, under-screen optical fingerprint recognition technology, which can avoid the appearance of notch screens, is favored by smart device manufacturers because of its advantages such as high image acquisition resolution, large-area fingerprint image acquisition, good system stability, strong anti-static ability and long life.

[0003] To ensure that the visual and touch functions of smart device screens are not affected, the under-screen fingerprint acquisition optical module is placed beneath the pixel layer of the smart device's screen. It detects the depth contrast and distribution of valleys and ridges of a fingerprint pressed against the screen glass surface to obtain a fingerprint feature image. Acquiring high-quality fingerprint images is difficult due to factors such as the fingerprint's ridge-valley depth contrast, the uniformity of diffuse illumination, the amount of oil and moisture between the skin and the screen glass, dust on the finger and screen surface, and fingerprint residue from multiple fingerprint acquisitions. Especially when the fingerprint image acquisition area is large, to avoid distortion of the edge shape of the captured image within a large field of view, the focal length of traditional optical acquisition systems increases, resulting in larger acquisition equipment, which hinders the application of portable smart devices. To facilitate smart device applications, fingerprint acquisition technologies based on micro-optical devices have emerged, including microlens arrays, micro-hole arrays, and optical fiber panel arrays. These micro-optical device arrays have drawbacks such as high processing difficulty, high cost, and difficult alignment with the pixel layer of the screen during assembly, making them difficult to implement. Summary of the Invention

[0004] In order to solve the technical problems pointed out in the background technology, the following technical solutions are adopted:

[0005] An under-screen fingerprint collection optical module, which comprises, from top to bottom, a touch screen, a display pixel layer, a polygonal gradient refractive index lens array, a micropore array plate, and an image sensor; the upper surface of the touch screen is arranged on the object-side principal plane of the polygonal gradient refractive index lens array, and the upper surface of the image sensor is arranged on the image-side principal plane of the polygonal gradient refractive index lens array.

[0006] In the under-screen fingerprint collection method of the under-screen fingerprint collection optical module of the present application, the fingerprint to be tested is diffusely reflected under the luminous illumination of the pixel layer of the display screen. The diffusely reflected light of the fingerprint to be tested passes through the pixel layer of the display screen and is incident on the polygonal gradient refractive index lens array. The light emitted by all sub-lenses of the polygonal gradient refractive index lens array passes through the microporous array plate and is comprehensively imaged on the image sensor.

[0007] The polygonal gradient refractive index lens array is composed of a plurality of polygonal gradient refractive index lenses arranged in a polygonal cross-section. Each polygonal gradient refractive index lens is a sub-lens of the polygonal gradient refractive index lens array. The polygon preferably has 6n sides, where n is a natural number. The refractive index of the sub-lens cross-section is symmetrically distributed about the center.

[0008] Preferably, the length of the polygonal gradient refractive index lens array is 0.5P to 0.75P, where P is the period length of light transmission in the polygonal gradient refractive index lens array.

[0009] The magnification of each sub-lens in the polygonal gradient refractive index lens array is 1 and each sub-lens forms an erect image.

[0010] The external dimensions of the end face of the microhole array plate are the same as those of the polygonal gradient refractive index lens array, and the number of microholes is the same as the number of polygonal gradient refractive index lenses in the polygonal gradient refractive index lens array.

[0011] The micropore array plate is made of black glass plate, on which a micropore array is arranged; the thickness of the black glass plate is 0.5-1 mm, and the diameter of the micropore is 0.3-0.6 times the diameter of the inscribed circle of the sub-lens of the gradient refractive index lens array.

[0012] The working principle of this application is as follows: when a finger is pressed on the surface of the touch screen, the fingerprint of the finger is diffusely reflected under the luminous illumination of the pixel layer, and the ridge and valley image of the fingerprint on the surface of the touch screen is successively imaged on the image sensor through the micropores of the pixel layer of the display screen, the polygonal gradient refractive index lens array, and the micropore array plate. The imaging effect of each sub-lens is a 1:1 upright image. Each sub-lens in the polygonal gradient refractive index lens array forms a conjugate image of the fingerprint to be measured. The conjugate images of the fingerprint to be measured formed by all sub-lenses are superimposed and enhanced on the image sensor, effectively improving the luminous flux of a single sub-lens imaging; the micropores in the micropore array plate effectively isolate the stray light passing through the polygonal gradient refractive index lens array, thereby improving the contrast of the fingerprint image; through the above technical means, the imaging quality of the collected fingerprint is greatly improved.

[0013] The method for preparing the polygonal gradient refractive index lens array described in this application:

[0014] 1) Melting the cesium-containing glass in a platinum crucible at a temperature of 1380-1420 degrees and forming it into a cylindrical glass;

[0015] 2) Processing the cylindrical glass into a polygonal columnar glass;

[0016] 3) The polygonal columnar glass is placed on a drawing machine for the first drawing to form continuous polygonal glass fibers at a drawing temperature of 700-800°C; the inscribed circle diameter of the polygonal glass fibers is 1.5-3 mm;

[0017] 4) Place the polygonal glass fiber in 500-600°C potassium nitrate molten salt for Cs + -K + Ion exchange, thus forming a polygonal gradient refractive index fiber with a gradually decreasing refractive index from the center to the edge;

[0018] 5) placing the polygonal gradient refractive index fiber into a drawing machine for a second drawing, drawing the polygonal gradient refractive index fiber with an inscribed circle diameter of 0.1-0.5 mm at a drawing temperature of 700-800° C.;

[0019] 6) Arranging the polygonal gradient refractive index fibers formed after the second drawing into a fiber lens array rod in a polygonal cross-section;

[0020] 7) placing the fiber lens array rod into a melting furnace at 750-850° C. for melting and pressing to form a polygonal gradient refractive index fiber lens array mother rod;

[0021] 8) The polygonal gradient refractive index fiber lens array mother rod is cut into a length designed for optical conjugate imaging with a linear magnification of 1, and both end surfaces are polished to obtain a polygonal gradient refractive index lens array.

[0022] The polygonal gradient refractive index lens array is made of cesium-containing glass because it has a large numerical aperture gradient refractive index and thus has high resolution.

[0023] The polygonal gradient refractive index lens array of the present application has the following characteristics: 1) Each sub-lens is polygonal, which reduces the triangular gaps that exist when the pixels of the fingerprint image are stacked; 2) The sub-lenses are made of gradient refractive index material, each sub-lens can be imaged independently, and multiple sub-lenses are combined together for imaging superposition to achieve comprehensive imaging; 3) The sub-lenses are made of large numerical aperture gradient refractive index material, which has high resolution when imaging; 4) The inscribed circle diameter of the sub-lenses is 0.1-0.5mm, which is easy to process and manufacture.

[0024] The cylindrical glass preferably has a diameter of 50-70 mm, the polygonal columnar glass preferably has a cross-sectional side length of 15-35 mm and a length of 400-600 mm. The polygonal gradient refractive index fiber preferably has a length of 400-600 mm and a cross-sectional side length of 1-2 mm. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 : Schematic diagram of the optical module structure for under-screen fingerprint collection;

[0026] Figure 2 :Schematic diagram of the principle of comprehensive conjugate imaging of polygonal gradient refractive index lens array;

[0027] Figure 3 : Schematic diagram of the hexagonal gradient refractive index lens array structure and cross-sectional refractive index distribution;

[0028] Among them: 1. Touch screen; 2. Display screen pixel layer; 3. Polygonal gradient refractive index lens array; 4. Microhole array plate; 5. Image sensor. DETAILED DESCRIPTION

[0029] The present application will be further described below with reference to the accompanying drawings and embodiments:

[0030] Example 1

[0031] An optical module for collecting fingerprints under the screen, such as Figure 1 As shown, from top to bottom are a touch screen 1, a display pixel layer 2, a polygonal gradient refractive index lens array 3, a micro-hole array plate 4, and an image sensor 5; the upper surface of the touch screen is arranged on the object-side cylindrical plane of the polygonal gradient refractive index lens array, and the upper surface of the image sensor is arranged on the image-side principal plane of the polygonal gradient refractive index lens array;

[0032] The fingerprint to be tested is diffusely reflected under the luminous illumination of the pixel layer of the display screen. The diffusely reflected light of the fingerprint to be tested passes through the pixel layer of the display screen and enters the polygonal gradient refractive index lens array. The microhole array effectively isolates the stray light in the optical system. The light emitted from the polygonal gradient refractive index lens array passes through the microhole array plate and is integrated into the image sensor.

[0033] The polygonal gradient refractive index lens array has a simple structure, short focal length, equal object distance and image distance, and forms a positive real image with a magnification of 1. Taking the under-screen fingerprint acquisition optical module used in portable smart devices as an example, its imaging principle is as follows: Figure 2 As shown, both the object distance and the image distance are denoted as l0. The object distance is the distance between the top surface of the touch screen and the upper end face of the polygonal gradient index lens array. To achieve thinness and lightness, the technical requirement is l0 ≤ 2mm. The length of the polygonal gradient index lens array is denoted as Z0, and the conjugate distance between the object and the image is denoted as TC. TC = 2l0 + Z0. This conjugate distance is required to be less than the thickness of the portable smart device. Table 1 provides five example parameters of the polygonal gradient index lens array.

[0034] Table 1 Polygonal gradient refractive index lens array parameters

[0035]

[0036] Take the hexagonal gradient refractive index lens array as an example, Figure 3The figure shows a schematic diagram of the hexagonal gradient refractive index lens array structure and cross-sectional refractive index distribution; the hexagonal gradient refractive index lens array consists of hexagonal gradient refractive index lenses as sub-lenses, and all sub-lenses are closely arranged to form a hexagonal array structure with a cross section.

[0037] Example 2

[0038] A method for preparing a micropore array plate in an under-screen fingerprint collection optical module includes the following steps: coating a layer of photoresist on the surface of a black glass plate and forming a circular micropore pattern through holographic lithography; then removing the photoresist in the micropore pattern area through development, hot melting, and etching steps in sequence; and finally etching a micropore array on the black glass plate in the micropore pattern area through hydrofluoric acid etching.

[0039] The thickness of the black glass plate is preferably 0.5-1 mm; the diameter of the microhole is 0.3-0.6 times the diameter of the inscribed circle of the sub-lens of the gradient refractive index lens array.

[0040] Example 3

[0041] The polygonal gradient refractive index lens array is made of cesium-containing glass, which is made of the following compounds in percentage by weight: 40%-60% Cs2O; 25%-40% SiO2; 5%-10% B2O3; 3%-8% Al2O3; 3%-9% ZnO; 5%-10% Na2O+K2O; 0.5%-2% ZrO2; and 3%-5% InF3.

[0042] Cs2O is the main component of glass, which is used to realize the reaction with K in high temperature molten salt. + Ions are replaced by ions to achieve Cs from the center to the edge of the glass fiber + The ion concentration gradually decreases and K + The ion concentration gradually increases. + The unit refractive index of ions in glass is 1.76, K + The unit refractive index of ions in glass is 1.57. + Ions and K in high temperature molten salts + The change in ion concentration gradient caused by ion replacement reduces the refractive index, which in turn causes the refractive index of the glass fiber to gradually decrease from the center to the edge.

[0043] SiO2 is a glass former and the main component of the glass. To ensure that the glass production temperature does not exceed 1450 degrees, the content of this component generally does not exceed 65%. Selecting a weight composition of 25-40% can ensure that the glass melting temperature is between 1380-1420 degrees.

[0044] B2O3 is a flux, ensuring that SiO2 and other glass components can be fully melted into glass;

[0045] Al2O3 and ZnO are glass intermediates, which are used as connecting materials of Si in structure. 4+ ions and Cs + ions, Na + ions, K + Ions, In 3+ ions such as monovalent and trivalent ions, while ensuring that Cs + Ions and K in molten salts + The stability of the overall structure of the glass during the ion exchange process is improved, and the high temperature corrosion resistance of the glass is improved;

[0046] Na2O+K2O monovalent oxides are glass structure network modifiers used to reduce the glass forming temperature. The mixing ratio of these components can regulate the ion exchange temperature and speed of cesium glass;

[0047] ZrO2 is used to improve the anti-crystallization performance during glass drawing or multiple optical fiber drawing processes.

[0048] InF3 for Cs + ions, Na + ions, K + The bridging of monovalent network modifier ions such as ions ensures the stability of the glass network structure and improves the chemical stability of the glass during high-temperature ion exchange. At the same time, the mixing ratio with Cs2O is conducive to regulating the deviation between the refractive index distribution index produced by ion exchange and theoretical calculation.

[0049] As a preferred embodiment: the weight percentage of ingredients is: 42-55% Cs2O; 25-33% SiO2; 6%-8% B2O3; 3-5% Al2O3; 4-6% ZnO; 5-8% Na2O+K2O; 1-2% ZrO2; 3-5% InF3.

[0050] The advantages of the cesium-containing glass are: high cesium content, through the Cs + The difference in ion concentration gradient caused by the replacement of ions and K+ ions in high-temperature molten salt is large, which makes the refractive index of the glass fiber gradually decrease from the center to the edge, with a large numerical aperture and small chromatic aberration.

Claims

1. An optical module for collecting fingerprints under the screen, characterized by From top to bottom: touch screen, display pixel layer, polygonal gradient refractive index lens array, micro-hole array plate, image sensor; the upper surface of the touch screen is arranged on the object-side principal plane of the polygonal gradient refractive index lens array, and the upper surface of the image sensor is arranged on the image-side principal plane of the polygonal gradient refractive index lens array; The polygonal gradient refractive index lens array is composed of a plurality of polygonal gradient refractive index lenses arranged in a polygonal cross-section, each polygonal gradient refractive index lens being a sub-lens of the polygonal gradient refractive index lens array; the number of sides of the polygon is 6n, where n is a natural number, and the refractive index of the sub-lens cross-section is symmetrically distributed about the center; The length of the polygonal gradient refractive index lens array is 0.5P~0.75P, where P is the period length of light transmission in the polygonal gradient refractive index lens.

2. The under-screen fingerprint acquisition optical module according to claim 1, characterized in that: The magnification of each sub-lens in the polygonal gradient refractive index lens array is 1 and each sub-lens forms an erect image.

3. The under-screen fingerprint acquisition optical module according to claim 1, characterized in that: The external dimensions of the end face of the microhole array plate are the same as those of the polygonal gradient refractive index lens array, and the number of microholes is the same as the number of polygonal gradient refractive index lenses in the polygonal gradient refractive index lens array.

4. The under-screen fingerprint acquisition optical module according to claim 3, characterized in that: The micropore array plate is made of black glass plate, on which a micropore array is arranged; the thickness of the black glass plate is 0.5-1 mm, and the diameter of the micropore is 0.3-0.6 times the diameter of the inscribed circle of the sub-lens of the gradient refractive index lens array.

5. The under-screen fingerprint acquisition optical module according to claim 1, characterized in that The method for preparing the polygonal gradient refractive index lens array is as follows: 1) Melt the cesium-containing glass in a platinum crucible at a temperature of 1380-1420 degrees and shape it into cylindrical glass; 2) Processing cylindrical glass into a polygonal columnar glass cross section; 3) Place the polygonal columnar glass on the drawing machine for the first drawing, drawing it into continuous polygonal glass fiber at a drawing temperature of 700-800°C; The diameter of the inscribed circle of polygonal glass fiber is 1.5-3mm; 4) Place the polygonal glass fiber in 500~600°C potassium nitrate molten salt for Cs + -K + Ion exchange, thus forming a polygonal gradient refractive index fiber with a gradually decreasing refractive index from the center to the edge; 5) placing the polygonal gradient refractive index fiber into a drawing machine for a second drawing, drawing the polygonal gradient refractive index fiber with an inscribed circle diameter of 0.1-0.5 mm at a drawing temperature of 700-800°C; 6) Arranging the polygonal gradient refractive index fibers formed after the second drawing into a fiber lens array rod in a polygonal cross-section; 7) placing the fiber lens array rod into a melting furnace at 750-850°C for melting and pressing to form a polygonal gradient refractive index fiber lens array mother rod; 8) Cutting the polygonal gradient refractive index fiber lens array mother rod into a length designed for optical conjugate imaging at a linear magnification of 1, and polishing both end faces to obtain a polygonal gradient refractive index lens array.

6. The under-screen fingerprint acquisition optical module according to claim 5, characterized in that The weight percentages of the ingredients of the cesium-containing glass are: 40%-60% Cs2O; 25%-40% SiO2; 5%-10% B2O3; 3%-8% Al2O3; 3%-9% ZnO; 5%-10% Na2O+K2O; 0.5%-2% ZrO2; and 3%-5% InF3.

7. The under-screen fingerprint acquisition optical module according to claim 4, characterized in that The micropore array plate preparation method comprises the following steps: coating a layer of photoresist on the surface of a black glass plate and forming a circular micropore pattern through holographic lithography; then removing the photoresist in the micropore pattern area through development, heat melting, and etching in sequence; and finally etching the micropore array on the black glass plate in the micropore pattern area through hydrofluoric acid etching.

8. A method for collecting fingerprints under a display, characterized by: Fingerprint collection is performed using the under-screen fingerprint collection optical module described in any one of claims 1 to 7.

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