CN111238781A - Lens back focus measuring device and measuring method thereof - Google Patents

Abstract

The invention provides a lens back focus measuring device and a measuring method thereof. The base is vertically fixed with a support, and the support is fixed with a plurality of connecting rods distributed at intervals. The light source module is fixed on one of the connecting rods and provides parallel light. The clamping and displacement module is movably arranged on one of the connecting rods, and the lens to be measured and the calibration plate are selectively placed on the clamping and displacement module. The imaging module is fixed on one of the connecting rods, the clamping and displacement module is located between the light source module and the imaging module, so that after the light source module is opened, the imaging module images the back focus of the lens to be measured and the calibration plate respectively. The lens back focus measuring device is simple in structure, high in cost performance, simple and accurate in measuring method and free of damage to the surface of the lens.

Description

Lens back focus measuring device and measuring method thereof

Technical Field

The invention relates to the technical field of lenses, in particular to a lens back focus measuring device and a measuring method thereof.

Background

The back focus of the lens, which is the distance from the back surface of the lens to the back focal point, i.e., the working distance of the lens, is one of the important characteristic parameters of the lens, and in various optical systems using the lens, the determination of the parameter greatly affects the performance and accuracy of the system. The currently adopted lens back focus measuring method mainly includes a microscope and micrometer measuring method, a CCD (Charge-coupled Device) measuring method, an imaging measuring method, a light intensity meter measuring method, an interferometer measuring method, and the like.

The microscope and the micrometer measurement method are used for measuring the thickness and the diameter of a lens by adopting a micrometer and the microscope, and the lens back focus is calculated by utilizing a theoretical relation formula of the lens back focus and the lens size so as to finish measurement. However, this method is generally suitable for measuring spherical lenses, and for surface lenses such as aspheric lenses, which cannot theoretically provide a back focus calculation formula, the back focus measurement cannot be completed, and the surface of the lens is easily damaged because the contact measurement is performed when a micrometer is used for measurement.

The CCD measurement method is to directly utilize CCD to image the focal spot of the lens, and when the CCD forms clear image to the focal spot of the lens, the distance between the lens and the CCD is measured to complete the measurement of the back focal of the lens. However, because the surface of the CCD camera is generally provided with a protective cover, the surface of the protective cover has a certain distance to the CCD photosurface, and the measurement cannot be completed if the back focal length of the lens is smaller than the distance from the surface of the protective cover to the CCD photosurface.

The imaging measurement method is that an imaging module is used for imaging the rear surface and the back focus of a lens, two positions of clear images are recorded respectively, and the difference between the two positions is the back focus of the lens. The imaging measurement method is generally used for measuring the back focus of a lens, and when the imaging measurement method is used for measuring a single lens, the lens is transparent, so that the accurate position of the back surface of the lens for forming a clear image cannot be found, and the back focus measurement cannot be completed.

The measuring method of the light intensity meter is that parallel light vertically enters a lens to be measured after passing through a semi-transparent semi-reflecting mirror, when a reflecting mirror is placed at a rear focal position of the lens to be measured, light beams converged at the rear focal point are reflected to return along the original path after passing through the lens to be measured, and are bent to a converging lens after passing through the semi-transparent semi-reflecting mirror, and the light beams are received by a light intensity detector after being converged through an aperture diaphragm. When the reflector deviates from the back focus, only part of light is received, and in the process of moving the reflector, the maximum value can appear only when the reflector is at the back focus, and the distance between the reflector and the rear surface of the lens is the back focus of the lens. However, the measurement method of the light intensity meter is limited by the size of the aperture diaphragm and the light-gathering capability of the converging lens, and when the size of the aperture diaphragm is too large and the light-gathering capability of the lens is not good, the maximum point of light intensity cannot be found, so that the measurement cannot be completed. When the method is used for measuring the short back focus lens, the maximum value of the light intensity can be generated when the distance between the reflector and the back surface of the lens is extremely small, but the extremely small distance is difficult to be laterally aligned, and the accuracy of measuring the back focus can be influenced.

Interferometric measurement refers to the determination of the back surface and back focal position of a lens using the "cat-eye" positioning technique of an interferometer to complete the measurement of its back focus. However, the requirement for measuring equipment is high, the test system is expensive, the measuring process is complicated, and the method is not suitable for measuring a plurality of lenses.

Disclosure of Invention

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. The base is vertically fixed with a support, and the support is fixed with a plurality of connecting rods distributed at intervals. The light source module is fixed on one of the connecting rods and provides parallel light. The clamping and displacement module is movably arranged on one of the connecting rods, and the lens to be measured and the calibration plate are selectively placed on the clamping and displacement module. The imaging module is fixed on one of the connecting rods, the clamping and displacement module is located between the light source module and the imaging module, so that after the light source module is opened, the imaging module images the back focus of the lens to be measured and the calibration plate respectively.

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 measured and the calibration plate are both selectively placed on the stage.

According to an embodiment of the invention, the clamping and displacement module has a displacement stage for moving the lens to be measured or the calibration plate.

According to an embodiment of the invention, the displacement table is an electric displacement table or a manual displacement table with a micrometer screw.

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

According to an embodiment of the invention, the light source module provides parallel light from a collimated LED light source.

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.

According to an embodiment of the invention, the imaging module comprises a CCD camera and a microscope cooperating with each other.

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.

According to another aspect of the present invention, the present invention further provides a measurement method of the above lens back focus measurement apparatus, comprising the steps of:

a calibration plate imaging step, namely placing a 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 the moment;

a lens imaging step: replacing the calibration plate placed on the clamping and displacement module with a lens to be measured, moving the clamping and displacement module to adjust the distance between the lens to be measured and the imaging module, enabling the light spot formed on the imaging module to become minimum and minimum, and recording the position of the clamping and displacement module at the moment;

calculating the back focus of the lens: and calculating the distance between two positions of the clamping and displacement module in the steps of imaging the calibration plate and imaging the lens to obtain the back focal length of the lens.

Compared with the prior art, the technical scheme has the following advantages:

the non-contact measuring method is adopted by the lens back focus measuring device provided by the invention, and the defect that the surface of the lens is easily damaged due to the contact measuring method of the traditional microscope and micrometer measuring method is overcome.

Secondly, because the imaging position of the imaging module of the lens back focus measuring device has a certain working distance with the front surface of the imaging module, the lens back focus measuring device provided by the invention can meet the measurement requirements of back focus lenses with different specifications, and the defect that the measurement cannot be finished when the back focus of the lens is short due to a layer of protective cover on the surface of a CCD camera in a CCD measuring method is overcome.

Thirdly, because burnt measuring device behind the lens corresponds when passing through the formation of image of measurement calibration board and burnt formation of image behind the lens the distance between two positions that centre gripping and displacement module locate to it is burnt behind the lens to calculate, consequently the position and the distance of centre gripping and displacement module can be more convenient and measure out directly perceivedly, for example can utilize spiral micrometer or electronic displacement platform to go to measure, thereby solved in the light intensity meter measuring method because can't accurate measurement distance lead to burnt measuring inaccurate problem behind the lens when less to speculum and lens distance.

Fourthly, when the lens back focus measuring device is used for measuring, the position of the back surface imaging clear image of the lens to be measured is determined through the position of the imaging clear image of the calibration plate, so that the defect that the imaging clear position of the back surface of the lens cannot be found due to the fact that the lens is transparent in an imaging measuring method is overcome.

Fifthly, because the imaging precision of the lens back focus measuring device is determined by the magnification of the microscope objective of the imaging module and the displacement precision of the movable clamping and displacement module, unknown factors influencing the measuring precision do not exist, and the problems that the measuring precision in a light intensity meter measuring method is limited by the size of the aperture diaphragm and the light condensing capacity of the convergent lens is unknown and the like are solved.

Sixthly, burnt measuring device behind lens simple structure, the simple operation, thereby when confirming lens rear surface imaging position through calibration board formation of image, because this position depends on imaging module's working distance, the position that different lens rear surface imaging is clear under the condition that does not change imaging module is the same, therefore this position is confirmed the back, when carrying out burnt measurement behind the lens to different lens, only need to await measuring lens form images with confirming lens back focus imaging position can. Therefore, the defects of high equipment requirement, expensive test system and complex test process of the interferometer measurement method are overcome.

Drawings

Fig. 1 is a schematic structural diagram of the lens back focus measuring device provided by the embodiment of the invention;

FIG. 2A shows the lens back focus measuring device provided by the embodiment of the invention in a first state, where the lens back surface is located at the working distance of the imaging module;

fig. 2B shows the lens back focus measuring device provided in the embodiment of the present invention in a second state, where the lens back focus is located at the working distance of the imaging module and can form a clear image on the imaging module;

FIGS. 3A and 3B show a blurred image when the calibration plate is not located at the working distance of the imaging module and a sharp image when the calibration plate is located at the working distance, respectively;

fig. 4A and 4B show a relatively large dark spot formed when the rear focus of the lens to be measured is not located at the working distance of the imaging module and a minimum bright spot formed when the rear focus of the lens to be measured is located at the working distance, respectively.

Detailed Description

The following description is only for the purpose of disclosing the invention so as to enable a person skilled in the art to practice the invention. The embodiments in the following description are given by way of example only, and other obvious variations will occur to those skilled in the art. The principles of the invention, as defined in the following description, may be applied to other embodiments, variations, modifications, equivalents, and other arrangements without departing from the spirit and scope of the invention.

As shown in fig. 1, 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.

The calibration plate 10 is used for assisting 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, and the higher the contrast of the pattern on the calibration plate 10 is, the higher the accuracy of the whole lens back focus measuring device is.

The base 20 plays a supporting role for the whole lens back focus imaging measuring device. The base 20 is vertically fixed with a support 21, and a plurality of connecting rods 22 which are distributed at intervals are fixed on the support 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 sequentially spaced from top to bottom along the bracket 21.

The light source module 30, the clamping and displacement module 40 and the imaging module 50 are sequentially fixed on the bracket 21 at intervals in the vertical direction, wherein the light source module 30 is positioned at the top of the whole lens back focus measuring device. The light source module 30 is fixed to one of the connection bars 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 and wavelength of the light sources can be freely selected to meet the lens measurement requirements of different operating wavelengths.

Alternatively, the light source module 30 includes a light emitting member and a collimator that cooperate with each other to output parallel light, and the light emitting member is a tungsten halogen lamp or a laser.

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

The clamping and displacing module 40 is movably installed at one of the connection bars, and the clamping and displacing module 40 is used to horizontally place a lens to be measured or a calibration board and is movable in a vertical direction along the support 21. The clamping and displacement module 40 comprises an object stage 41 and a displacement stage 42. The center of the objective table 41 is provided with a hole, and the size of the hole is determined according to the size of the lens to be measured, so long as the lens to be measured and the calibration board 10 can be horizontally placed. Both the lens to be measured and the calibration plate 10 are optionally placed and held on the stage 41 of the holding and displacing module 40. The displacement stage 42 is used to move the entire clamping and displacement module 40, and thus the lens to be measured or the calibration plate 10 placed on the clamping and displacement module 40. The stage 41 and the displacement stage 42 are vertically connected, i.e., the moving direction of the displacement stage 42 is a vertical direction. Alternatively, the displacement stage 42 is a motorized displacement stage or a manual displacement stage with a micrometer screw, so that the position of the lens to be measured or the calibration plate 10 can be moved more precisely. The higher the precision of the displacement table 42, the better the measurement accuracy and repeatability of the whole lens back focus measuring device.

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 50 is opened, the back focus of the lens to be measured and the calibration board 10 are imaged on the imaging module 50 respectively. Optionally, the imaging module 50 includes a cooperating camera and microscope. The working wavelength of the camera is determined according to the working wavelength of the lens to be measured, and the requirement of measuring the back focal length of the lens with different working wavelengths can be met. Optionally, the camera is a CCD camera or a CMOS (Complementary Metal Oxide Semiconductor) camera. The microscope objective should select the objective with a large magnification factor as much as possible on the premise of ensuring that the back focus of the lens and the calibration plate 10 can be imaged completely, wherein the larger the magnification factor is, the smaller the depth of field of the microscope objective is, the smaller the depth of field is, and the higher the measurement precision of the whole lens back focus measurement device is.

As shown in fig. 2A and 2B, the principle of the lens back focus measuring device is as follows: due to the imaging principle of the lens, parallel light is incident from the front surface of the lens and then is converged to the back focal point of the lens. Due to the conjugate relationship of the object image, the imaging module 50 can only image the object located in front of the imaging module at a fixed working distance. By moving the clamping and displacement module 40, the distance between the lens and the imaging module 50 is changed, the rear surface and the back focus of the lens are respectively positioned at the working distance of the imaging module, at the moment, the imaging module 50 respectively forms clear images on the rear surface and the back focus of the lens, two positions of the clamping and displacement module 40 are recorded when two clear images are formed, and the distance between the two positions is the rear focus of the lens. However, since the lens to be measured is generally transparent, when the imaging module 50 is used to image the rear surface of the lens to be measured, a clear image cannot be obtained, and thus the position of the clear image formed on the rear surface of the lens cannot be determined. In order to solve the problem, the invention adopts the calibration plate 10 with patterns, the calibration plate 10 is placed on the objective table 41 of the clamping and displacement module 40, the imaging module 50 is also used for clearly imaging the calibration plate, because the imaging module 50 only clearly images an object at the working distance, and the calibration plate 10 is horizontally placed at the position which is the same as the imaging position of the rear surface of the lens to be measured, the position where the calibration plate 10 clearly images is the position where the rear surface of the lens to be measured clearly images, thereby replacing the imaging position of the rear surface of the lens to be measured by the imaging position of the calibration plate 10, and solving the problem that the imaging module 50 cannot clearly image the rear surface of the transparent lens.

In the measuring process, firstly, the calibration plate 10 is horizontally placed on the clamping and displacement module 40, the imaging module 50 is used for clearly imaging the calibration plate 10, and the position reading of the clamping and displacement module 40 at the moment is recorded; then, keeping the position of the clamping and displacement module 40 unchanged, replacing the calibration plate 10 with a lens to be measured, turning on a light source, moving the clamping and displacement module 40, forming a clear image of a rear focus of the lens by using the imaging module 50, and recording the position reading of the clamping and displacement module 40 at the moment; and finally, calculating the distance between the two positions, wherein the distance is the back focus of the lens to be measured.

According to another aspect of the present invention, the present invention further provides a measurement method of the lens back focus measurement apparatus, including a calibration plate imaging step, a lens imaging step, and a lens back focus calculation step.

A calibration plate imaging step: the calibration board 10 is placed in the clamping and displacement module 40, the light source module 30 is turned on, the clamping and displacement module 40 is moved in the vertical direction to adjust the distance between the calibration board 10 and the imaging module 50, so that the calibration board 10 images on the imaging module 50, the image changes from fuzzy to clear in the adjusting process until the imaging module 50 forms the clearest image on the calibration board 10, the calibration board 10 is located at the working distance of the imaging module 50 at the moment, the position of the clamping and displacement module 40 at the moment is recorded, and the position is the clear imaging position of the rear surface of the lens.

A lens imaging step: will place calibration plate 10 on centre gripping and displacement module 40 is replaced into the lens that awaits measuring, will be about to calibrate plate 10 and take away from objective table 41, and the lens that awaits measuring is placed to level again, and the lens front surface is up, opens light source module 30, and imaging module 50 can become the great facula of diameter this moment, moves in vertical direction centre gripping and displacement module 40 are with the adjustment lens that awaits measuring and the distance between imaging module 50, and the facula diameter can be along with the change of distance and diminish or grow. The clamping and displacement module 40 is moved in a direction in which the light spot becomes smaller so that the light spot formed on the imaging module 50 becomes minimum brightest, and the position of the clamping and displacement module 40 at this time is recorded. After the positions of two clear images are obtained through the two measurement steps, the back focus of the lens to be measured can be calculated.

Calculating the back focus of the lens: and calculating the distance between two positions of the clamping and displacement module 40 in the calibration plate imaging step and the lens imaging step to obtain the rear lens focus.

When other lenses are measured, the imaging position of the rear surface of the lens does not need to be repeatedly measured, the position depends on the working distance of the imaging module 50, the imaging positions of the rear surfaces of different lenses are the same under the condition that the imaging module 50 is not changed, and therefore after the position is determined, the measurement of the rear focuses of different lenses is only needed to carry out the lens imaging step and the lens rear focus calculation step.

The device for measuring the rear focal length of the lens has the advantages of simple structure, high cost performance and simple and convenient measuring method, can meet the requirements of measuring the rear focal length of the lens with different working wavelengths and rear focal lengths, and can detect whether the lens has the defects of damage, defects, poor light-gathering performance and the like while measuring the rear focal length. The performance of the lens back focus measuring device is mainly determined by the precision of the displacement table 42 and the magnification of the microscope objective of the imaging module 50, and excellent measuring performance can be achieved by selecting proper accessories.

It will be appreciated by persons skilled in the art that the embodiments of the invention described above and shown in the drawings are given by way of example only and are not limiting of the invention. The objects of the invention have been fully and effectively accomplished. The functional and structural principles of the present invention have been shown and described in the examples, and the embodiments of the present invention may be subject to any changes or modifications without departing from the principles.

Claims (10)

1. A lens back focus measurement apparatus, comprising:

calibrating the plate;

the device comprises a base, a support and a plurality of connecting rods, wherein the support is vertically fixed on the base;

the light source module is fixed on one of the connecting rods and provides parallel light;

the clamping and displacement module is movably arranged 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 on one of the connecting rods, the clamping and displacement module is located between the light source module and the imaging module, so that after the light source module is opened, the imaging module images the rear focus of the lens to be measured and the calibration plate respectively.

2. The lens back focus measuring device of claim 1, wherein the clamping and displacement module has a centrally apertured stage on which both the lens under test and the calibration plate are selectively placed.

3. A lens back focus measuring device according to any one of claims 1 or 2, wherein the clamping and displacement module has a displacement stage for moving the lens to be measured or the calibration plate.

4. A lens back focus measuring device according to claim 3, wherein the displacement stage is a motorized displacement stage or a manual displacement stage with a micrometer screw.

5. The apparatus of claim 1, wherein the light source module comprises a light emitting element and a collimator, the light emitting element is a halogen tungsten lamp or a laser, and the light emitting element and the collimator cooperate with each other to output parallel light.

6. The device of claim 1, wherein the light source module provides collimated light from a collimated LED light source.

7. The device for measuring back focus of a lens according to any one of claims 5 or 6, wherein the light source of the light source module is one of ultraviolet light, infrared light or visible light.

8. The device of claim 1, wherein the imaging module comprises a CCD camera and a microscope in cooperation.

9. The device of claim 1, wherein the calibration plate has a black and white checkerboard pattern or a black and white line pattern.

10. The measuring method of the device for measuring back focus of a lens according to any one of claims 1 to 9, comprising the steps of:

a calibration plate imaging step, namely placing a 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 the moment;

a lens imaging step: replacing the calibration plate placed on the clamping and displacement module with a lens to be measured, moving the clamping and displacement module to adjust the distance between the lens to be measured and the imaging module, enabling the light spot formed on the imaging module to become minimum and minimum, and recording the position of the clamping and displacement module at the moment;

calculating the back focus of the lens: and calculating the distance between two positions of the clamping and displacement module in the steps of imaging the calibration plate and imaging the lens to obtain the back focal length of the lens.

Images