Lens back focus measuring device and measuring method thereof

Abstract

The present invention provides a lens back focus measuring device and a measuring method thereof, wherein the lens back focus measuring device comprises a calibration plate, a base, a light source module, a clamping and displacement module, and an imaging module. A bracket is vertically fixed on the base, and a plurality of spaced connecting rods are fixed on the bracket. The light source module is fixed to one of the connecting rods and provides parallel light. The clamping and displacement module is movably mounted on one of the connecting rods, and the lens to be measured and the calibration plate can be selectively placed on the clamping and displacement module. The imaging module is fixed to one of the connecting rods, and the clamping and displacement module is located between the light source module and the imaging module, so that after the light source module is turned on, the back focus of the lens to be measured and the calibration plate are imaged on the imaging module respectively. The lens back focus measuring device has a simple structure, high cost performance, and a simple and accurate measurement method without damaging the lens surface.

Description

Technical Field

[0001] The present invention relates to the field of lens technology, and in particular to a lens back focus measuring device and a measuring method thereof. Background Art

[0002] The back focus of a lens refers to the distance between the back surface of the lens and the back focus, that is, the working distance of the lens. It is one of the important characteristic parameters of the lens. In various optical systems using lenses, the determination of this parameter greatly affects the performance and accuracy of the system. The main methods currently used to measure the back focus of a lens include microscope and micrometer measurement, CCD (Charge-coupled Device) measurement, imaging measurement, light intensity meter measurement, interferometer measurement, etc.

[0003] The microscope and micrometer measurement method refers to the use of a micrometer and a microscope to measure the thickness and diameter of the lens, and the use of the theoretical relationship between the lens back focus and the lens size to calculate the lens back focus to complete the measurement. However, this method is generally applicable to the measurement of spherical lenses. For surface lenses such as aspherical lenses, which cannot theoretically give a back focus calculation formula, it is impossible to complete the back focus measurement. In addition, the measurement with a micrometer is a contact measurement, which is easy to cause damage to the lens surface.

[0004] The CCD measurement method directly uses CCD to image the focal spot of the lens. When the focal spot of the lens is clearly imaged on the CCD, the distance between the lens and the CCD can be measured to complete the measurement of the lens back focus. However, since the surface of the CCD camera generally has a protective cover, there is a certain distance between the protective cover surface and the CCD photosensitive surface. If the lens back focus is smaller than the distance between the protective cover surface and the CCD photosensitive surface, the measurement cannot be completed.

[0005] Imaging measurement refers to the use of an imaging module to image the rear surface and rear focus of the lens, and record the two positions where a clear image is formed. The difference between the two positions is the lens back focus. Imaging measurement is generally used to measure the back focus of a lens. When used to measure a single lens, since the lens is transparent, it is impossible to find the exact position where the rear surface of the lens forms a clear image, resulting in the inability to complete the back focus measurement.

[0006] The light intensity meter measurement method refers to the process of making parallel light pass through a semi-transparent and semi-reflective mirror and vertically incident on the lens to be measured. When a reflector is placed at the back focus of the lens to be measured, the light beam converged at the back focus will be reflected through the lens to be measured and then return along the original path, and bend to a converging lens after passing through the semi-transparent and semi-reflective mirror. After the light beam converges, it will be received by the light intensity detector through the aperture diaphragm. When the reflector deviates from the back focus, only part of the light is received. During the process of moving the reflector, the light intensity detector will only have a maximum value when the reflector is at the back focus. At this time, the distance between the reflector and the back surface of the lens is the back focus of the lens. However, the light intensity meter measurement method is limited by the size of the aperture diaphragm and the focusing ability of the converging lens. When the aperture diaphragm size is too large and the focusing ability of the lens is poor, it will result in the inability to find the maximum light intensity point and thus the inability to complete the measurement. Secondly, when this method is used to measure a short back focus lens, the maximum light intensity value will only appear when the distance between the reflector and the back surface of the lens is extremely small, but this extremely small distance is difficult to align, which will also affect the accuracy of measuring the back focus.

[0007] Interferometer measurement refers to the use of the interferometer's "cat's eye" positioning technology to determine the rear surface and rear focus position of the lens, thereby completing the measurement of its back focus. However, the measurement equipment requirements are high, the test system is expensive, the measurement process is complicated, and it is not suitable for measuring multiple lenses. Summary of the invention

[0008] In order to solve the above problems, the present invention provides a lens back focus measuring device, which includes a calibration plate, a base, a light source module, a clamping and displacement module, and an imaging module. A bracket is vertically fixed on the base, and a plurality of spaced connecting rods are fixed on the bracket. The light source module is fixed to one of the connecting rods and provides parallel light. The clamping and displacement module is movably mounted on one of the connecting rods, and the lens to be measured and the calibration plate can be selectively placed on the clamping and displacement module. The imaging module is fixed to one of the connecting rods, and the clamping and displacement module is located between the light source module and the imaging module, so that after the light source module is turned on, the back focus of the lens to be measured and the calibration plate are imaged on the imaging module respectively.

[0009] According to an embodiment of the present invention, the clamping and displacement module has a stage with a central opening, and the lens to be tested and the calibration plate can be selectively placed on the stage.

[0010] According to an embodiment of the present invention, the clamping and displacement module has a displacement stage, and the displacement stage is used to move the lens to be tested or the calibration plate.

[0011] According to an embodiment of the present invention, the translation stage is an electric translation stage or a manual translation stage with a screw micrometer.

[0012] According to an embodiment of the present invention, the light source module includes a light emitting element and a collimator that cooperate with each other to output parallel light, and the light emitting element is a halogen tungsten lamp or a laser.

[0013] According to an embodiment of the present invention, the light source module provides parallel light by a collimated LED light source.

[0014] According to an embodiment of the present invention, the light source of the light source module is one of ultraviolet light, infrared light or visible light.

[0015] According to an embodiment of the present invention, the imaging module includes a CCD camera and a microscope that cooperate with each other.

[0016] According to an embodiment of the present invention, the calibration plate has a black and white chessboard pattern or a black and white line pattern.

[0017] According to another aspect of the present invention, the present invention further provides a measurement method of the above lens back focus measurement device, comprising the following steps:

[0018] a calibration plate imaging step, placing the calibration plate on the clamping and displacement module, then turning on the light source module, moving the clamping and displacement module to adjust the distance between the calibration plate and the imaging module, so that the calibration plate is imaged on the imaging module, and recording the position of the clamping and displacement module at this time;

[0019] Lens imaging step: replacing the calibration plate placed on the clamping and displacement module with the lens to be tested, moving the clamping and displacement module to adjust the distance between the lens to be tested and the imaging module so that the light spot formed on the imaging module becomes the smallest and brightest, and recording the position of the clamping and displacement module at this time;

[0020] Lens back focus calculation step: The distance between the two positions of the clamping and displacement module in the calibration plate imaging step and the lens imaging step is calculated as the lens back focus.

[0021] Compared with the prior art, this technical solution has the following advantages:

[0022] The lens back focus measuring device provided by the present invention adopts a non-contact measuring method, which solves the defect that the lens surface is easily damaged by the contact measuring method such as the traditional microscope and micrometer measuring method.

[0023] Second, since there is a certain working distance between the imaging position of the imaging module of the lens back-focus measuring device and the front surface of the imaging module, the lens back-focus measuring device provided by the present invention can meet the measurement requirements of back-focus lenses of different specifications, and solves the defect in the CCD measurement method that the measurement cannot be completed when the lens back focus is short due to a protective cover on the surface of the CCD camera.

[0024] Third, since the lens back focus measuring device calculates the lens back focus by measuring the distance between the two positions of the clamping and displacement modules corresponding to the time when the calibration plate is imaged and the time when the lens back focus is imaged, the position and distance of the clamping and displacement modules can be measured relatively conveniently and intuitively. For example, they can be measured using a screw micrometer or an electric translation stage, thereby solving the problem of inaccurate lens back focus measurement caused by the inability to accurately measure the distance when the distance between the reflector and the lens is small in the light intensity meter measurement method.

[0025] Fourthly, since the position of the clear image of the rear surface of the lens to be measured is determined by the position of the clear image of the calibration plate when the lens back focus measuring device is used for measurement, the defect of the imaging measurement method that the position of the clear image of the rear surface of the lens cannot be found due to the transparency of the lens is solved.

[0026] Fifth, since the imaging accuracy of the lens back focus measuring device is determined by the magnification of the microscope objective of the imaging module and the displacement accuracy of moving the clamping and displacement module, there are no unknown factors that affect the measurement accuracy, which solves the many defects in the light intensity meter measurement method, such as the measurement accuracy is limited by the size of the aperture diaphragm and the unknown focusing ability of the converging lens.

[0027] Sixth, the lens back focus measuring device has a simple structure and is easy to operate. When the imaging position of the rear surface of the lens is determined by imaging the calibration plate, since the position depends on the working distance of the imaging module, the positions of the rear surfaces of different lenses where clear images are formed are the same without changing the imaging module. Therefore, after the position is determined, when measuring the back focus of different lenses, it is only necessary to image the lens to be tested to determine the imaging position of the lens back focus. This solves the defects of the interferometer measurement method, which has high equipment requirements, expensive test systems, and complicated test processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic structural diagram of the lens back focus measuring device provided in an embodiment of the present invention;

[0029] Figure 2A It is shown that the lens back focus measuring device provided by an embodiment of the present invention is in a first state, and the rear surface of the lens is located at the working distance of the imaging module;

[0030] Figure 2B It is shown that the lens back focus measuring device provided by the embodiment of the present invention is in the second state, the lens back focus is located at the working distance of the imaging module and can form a clear image on the imaging module;

[0031] Figure 3A and Figure 3BThe blurred image when the calibration plate is not at the working distance of the imaging module and the clear image when it is at the working distance are shown respectively;

[0032] Figure 4A and Figure 4B The larger and darker light spot formed when the back focus of the lens to be tested is not located at the working distance of the imaging module and the smallest and brightest light spot formed when it is located at the working distance are respectively shown. DETAILED DESCRIPTION

[0033] The following description is only used to disclose the present invention so that those skilled in the art can implement the present invention. The embodiments described below are only for example, and those skilled in the art can think of other obvious variations. The basic principles of the present invention defined in the following description can be applied to other embodiments, variations, improvements, equivalents, and other solutions that do not deviate from the spirit and scope of the present invention.

[0034] like Figure 1 As shown, the present invention provides a lens back focus measuring device for measuring the lens back focus of various lenses. Specifically, the lens back focus measuring device includes a calibration plate 10, a base 20, a light source module 30, a clamping and displacement module 40, and an imaging module 50.

[0035] The calibration plate 10 is used to assist in measuring the imaging position of the rear surface of the lens to be measured. Optionally, the calibration plate 10 has a black and white chessboard pattern or a black and white line pattern. The higher the pattern contrast on the calibration plate 10, the higher the accuracy of the entire lens back focus measurement device.

[0036] The base 20 supports the entire lens back focus imaging measurement device. A bracket 21 is vertically fixed to the base 20, and a plurality of spaced connecting rods 22 are fixed to the bracket 21, that is, the number of the connecting rods 22 is multiple. Optionally, in this embodiment, the number of the connecting rods 22 is three, and the three connecting rods 22 are parallel to each other and spaced from top to bottom along the bracket 21.

[0037] The light source module 30, the clamping and displacement module 40, and the imaging module 50 are fixed to the bracket 21 in sequence in the vertical direction, wherein the light source module 30 is located at the top of the entire lens back focus measurement device. The light source module 30 is fixed to one of the connecting rods 22 and provides parallel light. The light source of the light source module 30 is one of ultraviolet light, infrared light or visible light. The combination mode and wavelength of the light source can be freely selected to meet the lens measurement requirements of different working wavelengths.

[0038] Optionally, the light source module 30 includes a light emitting element and a collimator that cooperate with each other to output parallel light, and the light emitting element is a halogen tungsten lamp or a laser.

[0039] Optionally, in another embodiment, the light source module 30 provides parallel light by a collimated LED light source.

[0040] The clamping and displacement module 40 is movably mounted on one of the connecting rods. The clamping and displacement module 40 is used to horizontally place the lens to be tested or the calibration plate and can be moved in the vertical direction along the bracket 21. The clamping and displacement module 40 includes a stage 41 and a displacement stage 42. The stage 41 has a hole in the center, and the size of the hole depends on the size of the lens to be tested, as long as it can ensure that the lens to be tested and the calibration plate 10 can be placed horizontally. The lens to be tested and the calibration plate 10 can be selectively placed and clamped on the stage 41 of the clamping and displacement module 40. The displacement stage 42 is used to move the entire clamping and displacement module 40, thereby moving the lens to be tested or the calibration plate 10 placed on the clamping and displacement module 40. The stage 41 and the displacement stage 42 are vertically connected, that is, the moving direction of the displacement stage 42 is the vertical direction. Optionally, the displacement stage 42 is an electric displacement stage or a manual displacement stage with a screw micrometer, so that the position of the lens to be tested or the calibration plate 10 can be moved more accurately. The higher the precision of the translation stage 42 is, the better the measurement accuracy and repeatability of the entire lens back focus measurement device is.

[0041] The imaging module 50 is fixed to one of the connecting rods 22, and the clamping and displacement module 40 is located between the light source module 30 and the imaging module 50, so that after the light source module 30 is turned on, the back focus of the lens to be measured and the calibration plate 10 are imaged on the imaging module 50 respectively. Optionally, the imaging module 50 includes a camera and a microscope that cooperate with each other. The working wavelength of the camera depends on the working wavelength of the lens to be measured, and can meet the back focus measurement requirements of lenses with different working wavelengths. Optionally, the camera is a CCD camera or a CMOS (Complementary Metal Oxide Semiconductor) camera. The microscope objective lens should try to select an objective lens with a large magnification while ensuring that the back focus of the lens and the calibration plate 10 can be fully imaged. The larger the magnification, the smaller the depth of field of the microscope objective lens. The smaller the depth of field, the higher the measurement accuracy of the entire lens back focus measurement device.

[0042] like Figure 2A and Figure 2BAs shown, the principle of the lens back focus measuring device is as follows: due to the imaging principle of the lens, parallel light will converge to the back focus of the lens after being incident from the front surface of the lens. Due to the conjugate relationship between the object and the image, the imaging module 50 can only form a clear image of an object located at a fixed working distance in front of it. By moving the clamping and displacement module 40, the distance between the lens and the imaging module 50 is changed, and the rear surface and the rear focus of the lens are respectively located at the working distance of the imaging module. At this time, the imaging module 50 forms a clear image of the rear surface and the rear focus of the lens respectively, and the two positions of the clamping and displacement module 40 when the clear images are formed twice are recorded. The distance between the two positions is the back focus of the lens. However, since the lens to be tested is generally transparent, when the imaging module 50 is used to image the rear surface of the lens to be tested, a clear image cannot be obtained, and thus the position of the rear surface of the lens forming a clear image cannot be determined. To solve this problem, the present invention adopts a calibration plate 10 with a pattern, and places the calibration plate 10 on the stage 41 of the clamping and displacement module 40. Similarly, the imaging module 50 is used to form a clear image of the calibration plate. Since the imaging module 50 only forms a clear image of objects at its working distance, and the calibration plate 10 is equivalent to being placed horizontally at the same position as the imaging position of the rear surface of the lens to be tested, the position where the calibration plate 10 forms a clear image is the position where the rear surface of the lens to be tested forms a clear image. Therefore, the imaging position of the calibration plate 10 is used to replace the imaging position of the rear surface of the lens to be tested, thereby solving the problem that the imaging module 50 cannot form a clear image of the rear surface of the transparent lens.

[0043] During the measurement process, the calibration plate 10 is first placed horizontally on the clamping and displacement module 40, and the imaging module 50 is used to form a clear image of the calibration plate 10, and the position reading of the clamping and displacement module 40 at this time is recorded; then the position of the clamping and displacement module 40 is kept unchanged, the calibration plate 10 is replaced with the lens to be measured, and the light source is turned on, and then the clamping and displacement module 40 is moved, and the imaging module 50 is used to form a clear image of the back focus of the lens, and the position reading of the clamping and displacement module 40 at this time is recorded; finally, the distance between the two positions is calculated, and this distance is the back focus of the lens to be measured.

[0044] According to another aspect of the present invention, the present invention further provides a measurement method of the lens back focus measurement device, comprising a calibration plate imaging step, a lens imaging step and a lens back focus calculation step.

[0045] Calibration plate imaging step: place the calibration plate 10 on the clamping and displacement module 40, then turn on the light source module 30, move the clamping and displacement module 40 in the vertical direction to adjust the distance between the calibration plate 10 and the imaging module 50, so that the calibration plate 10 is imaged on the imaging module 50. During the adjustment process, the image will change from blurry to clear. Adjust until the imaging module 50 forms the clearest image of the calibration plate 10. At this time, the calibration plate 10 is located at the working distance of the imaging module 50. Record the position of the clamping and displacement module 40 at this time. This position is the position where the clear image is formed on the rear surface of the lens.

[0046] Lens imaging step: replace the calibration plate 10 placed on the clamping and displacement module 40 with the lens to be tested, that is, remove the calibration plate 10 from the stage 41, and then place the lens to be tested horizontally with the front surface of the lens facing upwards, turn on the light source module 30, and the imaging module 50 will now form a light spot with a larger diameter. Move the clamping and displacement module 40 in the vertical direction to adjust the distance between the lens to be tested and the imaging module 50, and the diameter of the light spot will become smaller or larger as the distance changes. Move the clamping and displacement module 40 in the direction where the light spot becomes smaller, so that the light spot formed on the imaging module 50 becomes the smallest and brightest, and record the position of the clamping and displacement module 40 at this time. After obtaining the positions of two clear images through these two measurement steps, the back focus of the lens to be tested can be calculated.

[0047] Lens back focus calculation step: The distance between the two positions of the clamping and displacement module 40 in the calibration plate imaging step and the lens imaging step is calculated as the lens back focus.

[0048] When measuring other lenses, there is no need to repeatedly measure the position where the clear image is formed on the rear surface of the lens. This position depends on the working distance of the imaging module 50. Without changing the imaging module 50, the positions where the clear images are formed on the rear surfaces of different lenses are the same. Therefore, after the position is determined, to measure the back focal length of different lenses, it is only necessary to perform the lens imaging step and the lens back focal length calculation step.

[0049] The lens back focus measuring device provided by the present invention has a simple structure, high cost performance, and a simple measurement method, and can meet the back focus measurement requirements of lenses with different working wavelengths and back focuses. Moreover, while measuring the back focus, it can also detect whether the lens has defects such as damage, defects, and poor focusing performance. The performance of the lens back focus measuring device is mainly determined by the accuracy of the translation stage 42 and the magnification of the microscope objective lens of the imaging module 50. By selecting appropriate accessories, excellent measurement performance can be achieved.

[0050] Those skilled in the art should understand that the embodiments of the present invention described above and shown in the accompanying drawings are only examples and do not limit the present invention. The purpose of the present invention has been fully and effectively achieved. The functional and structural principles of the present invention have been demonstrated and explained in the embodiments, and the embodiments of the present invention may be deformed and modified in any way without departing from the principles.

Claims

1. A lens back focus measuring device, characterized in that: include: Calibration plate; A base, on which a bracket is vertically fixed, and on which a plurality of spaced connecting rods are fixed; A light source module, fixed to one of the connecting rods and providing parallel light; A clamping and displacement module is movably mounted on one of the connecting rods, and the lens to be tested and the calibration plate can be selectively placed on the clamping and displacement module; The imaging module is fixed to one of the connecting rods, and the clamping and displacement module is located between the light source module and the imaging module, so that after the light source module is turned on, the back focus of the lens to be measured and the calibration plate are imaged on the imaging module respectively.

2. The lens back focus measuring device according to claim 1, characterized in that: The clamping and displacement module has a stage with a central opening, and the lens to be tested and the calibration plate can be selectively placed on the stage.

3. The lens back focus measuring device according to any one of claims 1 or 2, characterized in that: The clamping and displacement module has a displacement stage, and the displacement stage is used to move the lens to be tested or the calibration plate.

4. The lens back focus measuring device according to claim 3, characterized in that: The translation stage is an electric translation stage or a manual translation stage with a screw micrometer.

5. The lens back focus measuring device according to claim 1, characterized in that: The light source module comprises a light emitting element and a collimator which cooperate with each other to output parallel light, and the light emitting element is a halogen tungsten lamp or a laser.

6. The lens back focus measuring device according to claim 1, characterized in that: The light source module provides parallel light by means of a collimated LED light source.

7. The lens back focus measuring device according to any one of claims 5 or 6, characterized in that: The light source of the light source module is one of ultraviolet light, infrared light or visible light.

8. The lens back focus measuring device according to claim 1, characterized in that: The imaging module includes a CCD camera and a microscope that cooperate with each other.

9. The lens back focus measuring device according to claim 1, characterized in that: The calibration plate has a black and white chessboard pattern or a black and white line pattern.

10. The method for measuring the back focus of a lens according to any one of claims 1 to 9, comprising the following steps: a calibration plate imaging step, placing the calibration plate on the clamping and displacement module, then turning on the light source module, moving the clamping and displacement module to adjust the distance between the calibration plate and the imaging module, so that the calibration plate is imaged on the imaging module, adjusting until the imaging module forms the clearest image of the calibration plate, and recording the position of the clamping and displacement module at this time; Lens imaging step: replacing the calibration plate placed on the clamping and displacement module with the lens to be tested, moving the clamping and displacement module to adjust the distance between the lens to be tested and the imaging module so that the light spot formed on the imaging module becomes the smallest and brightest, and recording the position of the clamping and displacement module at this time; Lens back focus calculation step: The distance between the two positions of the clamping and displacement module in the calibration plate imaging step and the lens imaging step is calculated as the lens back focus.

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