CN109883340B - Method for measuring central thickness of transverse subtraction differential confocal lens - Google Patents
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Abstract
The invention discloses a method for measuring the central thickness of a transverse subtraction differential confocal lens, and belongs to the technical field of optical precision measurement. In the confocal measuring system, a large virtual pinhole detection area and a small virtual pinhole detection area are set on a Blackey spot image detected by a CCD (charge coupled device) through software, two confocal characteristic curves detected by the large virtual pinhole detection area and the small virtual pinhole detection area are subjected to subtraction processing to sharpen the confocal characteristic curves, then the sharpened confocal characteristic curves are subjected to differential subtraction processing to obtain axial highly sensitive differential confocal characteristic curves, then the high-precision focus-fixing and position-finding are carried out on the central thickness measurement vertex position of a measured lens by utilizing the characteristics of the zero point of the differential confocal characteristic curves and the precise corresponding focus of the differential confocal measuring system, and finally the central thickness of the lens is precisely obtained through light ray tracing compensation, so that the high-precision measurement of the central thickness of the lens. The invention has the advantages of high measurement precision, strong environmental interference resistance, simple structure and the like, and has wide application prospect in the technical field of optical precision measurement.
Description
Technical Field
The invention relates to a method for measuring the central thickness of a transverse subtraction differential confocal lens, in particular to a non-contact high-precision measurement method for the central thickness of the lens, and belongs to the technical field of optical precision measurement.
Background
In the field of optics, the measurement of the thickness of the center of a lens is of great significance. The central thickness of the lens is an important parameter in the optical system, and the quality of the processing of the central thickness has a great influence on the imaging quality of the optical system. Particularly, for lenses in high-performance optical systems such as an objective lens of a lithography machine and an aerospace camera, the axial gap, the radial offset and the optical axis deflection angle of the lenses need to be precisely adjusted according to the curvature radius, the refractive index and the central thickness of the lenses in the lenses. Taking the objective lens of the lithography machine as an example, the aberration of the lithography objective lens can be caused by the deviation of the central thickness of each single lens, and the imaging quality of the objective lens is affected. The precision of the central thickness of the lens is generally several micrometers, and a high-precision instrument is also required for measurement and inspection, so that the central thickness of the lens is one of the items of inspection and strict control of optical parts.
Currently, lens center thickness measurement techniques can be divided into contact measurement and non-contact measurement.
Contact measurements are typically made with a hand-held micrometer or micrometer. During measurement, the accuracy of the position of the center point of the lens directly influences the measurement precision, so an inspector needs to move the measured lens back and forth during measurement to search the highest point (convex mirror) or the lowest point (concave mirror), so the measurement speed is low, the error is large, the material is soft due to the fact that the currently used high-transmittance optical material is used, and a measuring head moves on the surface of the lens during measurement to scratch the surface of the lens easily.
The domestic scholars also carry out related research aiming at the problems existing in the contact measurement. In the document of "raster digital display type lens center thickness measuring instrument" published in practical testing technology 1999, the authors designed a lens center thickness measuring instrument using a raster sensor as a precision length measuring device, and according to the requirements of different types of optical lenses and measuring precision, the measuring instrument can adopt different types of measuring heads and measuring seats to carry out measurement, and the measuring precision is improved to 1 μm. The Chinese patent 'device for measuring the central thickness of an optical lens' (patent number: 200620125116.9) adopts a method of placing a measured lens jig on the upper part of a measuring upright post, thereby avoiding the damage to the lens caused by the movement of a measuring head back and forth on the surface of the lens when the vertex of the surface of the lens is searched.
Non-contact measurement is commonly used in photogrammetry, coplanar capacitance, white light confocal and interferometry.
In the article, "study of online measurement of assembly gap based on image measurement technology", published in sensor technology 2005, an online measurement scheme based on image measurement technology is introduced, the image of the gap in a CCD camera through an optical system is sent to image measurement software for processing and analysis, and the measurement software gives the result. The method can also be applied to the measurement of the thickness of the center of the lens, but the measurement error is within 15 mu m due to the influence of a camera imaging system, CCD resolution, image definition, calibration coefficient accuracy and the like.
In the automatic detector for central thickness of optical lens published in 1994 journal of Instrument and Meter, the central thickness of lens is measured by coplanar capacitance method. Firstly, adjusting a capacitance measuring head and a reference surface to a certain fixed distance according to requirements; then, the lens to be measured is placed on the reference surface, an air gap exists between the lens to be measured and the measuring head, and different central thicknesses of the lens correspond to different air gaps and different measuring head capacitors; finally, a voltage signal which changes corresponding to the capacitance is measured through a circuit, the relative change of the central thickness of the measured lens can be found, and the sorting precision of the method is less than 5 mu m. However, before the measurement, the relation curve between the signal voltage of the tested lens material and the air gap needs to be known, and in the engineering practice, the coplanar capacitance measuring head needs to be accurately tested to obtain reliable data as a detection basis.
In the article "Noncontact measurements of central lens thickness", 2005, Technology (GLASS SCIENCEAND Technolog), the central thickness of the lens was measured by white light confocal method. The method comprises the steps of firstly, positioning the vertex of the surface of a measured lens by using a probe formed by axial chromatic aberration after white light passes through the lens, and then calculating the thickness of the lens according to spectral information reflected by the vertexes of the upper surface and the lower surface of the measured lens. The method is characterized by realizing real-time measurement, but white light is incoherent light, the fixed focus sensitivity and the resolution are low, and the working distance is limited (30 mu m-25 mm). Particularly, it is difficult to accurately know the refractive indexes of the measured lens at different wavelengths, and the refractive indexes are generally obtained by interpolation after measuring the refractive index at a specific wavelength, and the parameter has a large influence on the measurement result, so that the method is difficult to realize high-precision measurement in practical application.
Chinese patent "optical measuring instrument for optical element thickness" (patent number: 87200715) uses a double interference system to perform non-contact measurement on the thickness of the center of a lens. The instrument consists of two Michelson interference systems, positions two surfaces of a measured lens according to white light interference fringes, and compares the measured lens with a standard block to obtain the central thickness of the measured lens. Non-contact measurement can be realized on a cemented lens, an optical element which is opaque to visible light, an optical element of unknown material and the like. However, the structure of the instrument is complex, elements need to be replaced in the measurement process, the measurement accuracy of the instrument not only depends on the positioning accuracy of a plurality of surfaces, but also depends on the accuracy of the known thickness of the standard block, and meanwhile, in order to improve the measurement accuracy, the standard block with the thickness close to the center of the measured lens needs to be selected.
Chinese patent 'a measuring device for tiny optical interval' (patent number: 93238743.8), adopts polarized light interference method to measure the thickness of sample. Two wave fronts formed by the reflection of the incident white light on the upper and lower surfaces of the sample pass through the polarizer, the birefringent prism and the analyzer to form interference fringes on the photodetector array, and the thickness of the sample can be obtained according to the distance between the interference fringes. Meanwhile, a cylindrical lens is added between the analyzer and the photoelectric detector array to amplify the interference pattern along the stripe spacing direction, so that the requirement on the photoelectric detector array is reduced, and the measurement precision is 1-5%.
The inventor applies for a Chinese patent 'differential confocal lens center measuring method and device' (patent number: 201010000555.8) in 2010, and realizes non-contact high-precision measurement of the center thickness of the lens by accurately determining the vertex positions of the front and rear surfaces of the focusing lens according to the differential confocal principle. However, two detectors are needed, and the two detectors need to be positioned at the same defocus amount, so that the system structure and the assembly and adjustment process are complex, and errors caused by inaccurate assembly and adjustment are likely to be large; after the measured lens is replaced, the defocusing amount of the two detectors may need to be adjusted again.
In order to further improve the measurement accuracy of the central thickness of the lens, the invention provides a method for measuring the central thickness of a transverse subtraction differential confocal lens, which comprises the steps of firstly setting a large virtual pinhole detection area and a small virtual pinhole detection area on a Lei spot image detected by a CCD (charge coupled device) through software, sharpening a confocal characteristic curve by subtracting two detected confocal characteristic curves, then carrying out differential subtraction on the sharpened confocal characteristic curve to obtain an axial highly sensitive differential confocal characteristic curve, then carrying out high-accuracy fixed-focus position finding on a measured lens central thickness measurement vertex position by utilizing the characteristic that the zero point of the transverse subtraction differential confocal characteristic curve accurately corresponds to the focus of a measurement system, and finally accurately obtaining the central thickness of the lens through light ray tracing compensation calculation to realize the high-accuracy measurement of the central thickness of the lens.
Disclosure of Invention
In order to solve the problem of high-precision measurement of the central thickness of the lens, the invention discloses a method for measuring the central thickness of a transverse subtraction differential confocal lens, which aims to: in the differential confocal measurement system, a confocal characteristic curve of an out-of-focus detection light path system is sharpened through transverse subtraction detection of a large virtual pinhole and a small virtual pinhole, differential confocal bipolar fixed focus measurement of a measured spherical surface is realized through differential subtraction processing of sharpening the confocal characteristic curves before and after double-light-path detection focus, and the capture precision of a focus position is improved through linear fitting of the differential confocal fixed focus curve, so that the fixed focus precision of the vertex position of a lens in the measurement of the central thickness of the lens is improved, and the high-precision measurement of the central thickness of the lens is realized.
The purpose of the invention is realized by the following technical scheme.
The invention discloses a method for measuring the central thickness of a transverse subtraction differential confocal lens, which comprises the steps of setting a large virtual pinhole detection area and a small virtual pinhole detection area on a Blackspot image detected by a CCD (charge coupled device) through software, sharpening a confocal characteristic curve by subtracting two detected confocal characteristic curves, carrying out differential subtraction on the sharpened confocal characteristic curve to obtain an axial highly sensitive differential confocal characteristic curve, carrying out high-precision focus-finding on a measured lens central thickness measurement vertex position by utilizing the characteristic that the zero point of the transverse subtraction differential confocal characteristic curve accurately corresponds to a measurement system focus, and finally accurately obtaining the central thickness of the lens through light ray tracing compensation to realize the high-precision measurement of the central thickness of the lens.
The invention discloses a method for measuring the central thickness of a transverse subtraction differential confocal lens, which comprises the following steps:
a) opening the point light source, and forming a measuring beam by light emitted by the point light source after passing through the beam splitter, the collimating lens and the measuring objective lens to irradiate on the measured lens;
b) adjusting the measured lens to enable the measured lens to be coaxial with the measuring objective lens and the collimating lens, enabling parallel light beams emitted by the collimating lens to be converged into a focused measuring light beam through the measuring objective lens to be focused on a vertex A of the measured lens, enabling the focused measuring light beam (5) reflected by the vertex of the measured lens to be reflected by the beam splitter after passing through the measuring objective lens and the collimating lens to enter a transverse subtraction differential confocal detection system, and detecting a formed in-focus measuring Airy spot by a in-focus CCD detector;
c) moving in the direction of the optical axisThe measuring objective lens enables the focus of the focused measuring beam to coincide with the position of the vertex A of the measured lens; scanning and measuring the objective lens in the axial direction near the vertex A of the lens, and respectively detecting a large virtual pinhole detection area in front of a focus and a small virtual pinhole detection area in front of the focus in a transverse subtraction differential confocal detection system to obtain a confocal characteristic curve IB1(z,-uM) Confocal characteristic curve I of small virtual pinhole detection before focusingS1(z,-uM) Carrying out subtraction processing to obtain a semi-width compressed confocal characteristic curve I with transverse subtraction sharpening in front of focus1(z,-uM)＝IS1(z,-uM)-γIB1(z,-uM) A confocal characteristic curve I of the large virtual pinhole detection after focus detected by the large virtual pinhole detection area after focus and the small virtual pinhole detection area after focus in the transverse subtraction differential confocal detection systemB2(z,+uM) Confocal characteristic curve I of small virtual pinhole detection after focusingS2(z,+uM) Carrying out subtraction processing to obtain a semi-height-width compressed confocal characteristic curve I of transverse subtraction sharpening after focusing2(z,+uM)＝IS2(z,+uM)-γIB2(z,+uM) Where z is the axial coordinate, γ is the adjustment factor, uMThe normalized distance is the normalized distance of the distance M between the focal plane of the microscope objective before the deflection of the pre-focus CCD detector and the focal plane of the microscope objective after the deflection of the post-focus CCD detector; confocal characteristic curve I of post-focus transverse subtraction sharpening2(z,+uM) And-transverse-to-focus subtraction sharpening confocal characteristic curve I1(z,-uM) Carrying out differential subtraction to obtain axial highly sensitive transverse subtraction differential confocal characteristic curve ID(z)：
ID(z)＝I2(z,+uM)-I1(z,-uM) (1)
Differential confocal characteristic curve I by transverse subtractionD(Z) determining the vertex A of the front surface of the lens (6) to be measured by fitting the zero point (28) of the straight line, and then accurately determining the position Z of the vertex A of the front surface of the lens (6) to be measured1；
d) The measuring objective (4) is moved further in the direction of the optical axis, so that the focal point of the focused measuring beam (5) and the measured transmission are brought into contactThe rear surface vertexes B of the mirrors (6) are overlapped; the measuring objective lens (4) is axially scanned near the vertex B of the lens, and at the moment, a focused measuring beam (5) is reflected by the vertex B of the rear surface of the measured lens (6) in a primary path and enters a transverse subtraction differential confocal detection system (7) to be detected; scanning the measuring objective (4) near the position, measuring a discrete transverse subtraction differential confocal characteristic curve (26) by a transverse subtraction differential confocal detection system (7), accurately determining the focal position B of the measured lens (6) by a main control computer (30) through a fitting straight line zero point (28) of a differential confocal linear fitting straight line (27), and recording the focal position Z of the focused measuring beam (5) at the moment2；
e) According to the established ray tracing and the compensation model thereof, the calculation formula of the central thickness d of the lens is obtained as follows:
substituting the known parameter, the numerical aperture angle α of the measuring beam1Radius of curvature r of front surface of lens to be measured1Refractive index n of airairThe central thickness t of the measured lens and the distance d between the two fixed focus positions1＝|Z2-Z1And obtaining the refractive index n of the measured lens.
The method for measuring the central thickness of the transverse subtraction differential confocal lens organically integrates a laser differential confocal technology and a light tracing technology, establishes a light tracing and a compensation model thereof, eliminates the mutual influence among parameters of each analytic fixed-focus surface, and further obtains a calculation formula of the central thickness of the lens. As shown in formula (2), rNIs the Nth surface SNRadius of curvature of, nNIs the Nth surface SNAnd the (N + 1) th surface SN+1Refractive index of material in between, dN-1Is the (N-1) th surface SN-1And the Nth surface SNAxial clearance between lNIs' SNVertex to SNDistance of intersection of ray and optical axis, uNIs' SNThe angle between the emergent ray and the optical axis.
The formula (1) for calculating the central thickness of the lens can be derived according to the formula, and the accurate measurement of the central thickness of the lens is further realized.
According to the method for measuring the central thickness of the transverse subtraction differential confocal lens, the annular pupil is added in the light path to modulate the measuring light beam to form the annular light beam, so that the influence of wave phase difference on the measuring light beam when the parameters of the measuring element are measured is reduced, and the measuring error is reduced.
Has the advantages that:
1) the invention discloses a method for measuring the central thickness of a transverse subtraction differential confocal lens, which utilizes large and small virtual pinholes to detect a transverse subtraction sharpened confocal characteristic curve, utilizes differential subtraction processing of the sharpened confocal response characteristic curve to realize differential confocal bipolar fixed focus measurement of a measured surface, further obviously improves the fixed focus sensitivity and signal-to-noise ratio of the differential confocal fixed focus curve, obviously improves the fixed focus precision of the vertex positions of the front surface and the rear surface of the measured lens in the measurement of the central thickness of the lens, and can obviously improve the measurement precision of the central thickness of the lens.
2) Compared with a differential confocal measurement system, the method for measuring the central thickness of the transverse subtraction differential confocal lens disclosed by the invention has the advantage that the measurement precision is improved under the condition that the hardware cost is not increased.
3) According to the method for measuring the central thickness of the transverse subtraction differential confocal lens, disclosed by the invention, the transverse subtraction processing detection is carried out on the large virtual light spot detection area and the small virtual light spot detection area, so that the common-mode noise is effectively eliminated, and the environmental interference resistance of a measurement system is improved.
4) The invention discloses a method for measuring the central thickness of a transverse subtraction differential confocal lens, which introduces an annular pupil in a light path to shield paraxial rays to form a hollow measuring light cone and reduce the influence of aberration on a measurement result.
5) Compared with a classical high-precision interference lens center thickness measuring method, the transverse subtraction differential confocal lens center thickness measuring method disclosed by the invention can overcome the defect that the existing interference fixed focus method is extremely sensitive to system aberration, environmental vibration, air flow interference and sample surface roughness due to the adoption of a non-interference airy disk center intensity point detection mode, greatly improves the capability of resisting system aberration, environmental interference and surface scattering, and obviously improves the lens center thickness measuring precision.
Drawings
FIG. 1 is a schematic diagram of the method for measuring the central thickness of a transverse subtraction differential confocal lens of the invention
FIG. 2 is a schematic diagram of horizontal subtraction sharpening of confocal characteristic curves of large and small virtual pinholes
FIG. 3 is a schematic diagram of the bilateral dislocation differential subtraction of the sharpened confocal characteristic curve according to the present invention
FIG. 4 is a schematic diagram of the linear fitting trigger focusing of the transverse subtraction differential confocal curve of the present invention
FIG. 5 is a schematic diagram of a ray trace and its compensation model according to the present invention;
FIG. 6 is a schematic diagram of a method for measuring the central thickness of a lateral subtraction differential confocal lens according to an embodiment of the present invention;
fig. 7 is a graph showing the measurement results of the central thickness of the lateral subtraction differential confocal lens according to the embodiment of the present invention.
Wherein: 1-point light source, 2-beam splitter, 3-collimating lens, 4-measuring objective lens, 5-measuring light beam, 6-measured lens, 7-transverse subtraction differential confocal detection system, 8-beam splitter, 9-pre-focus microscope lens, 10-pre-focus CCD detector, 11-post-focus microscope lens, 12-post-focus CCD detector, 13-pre-focus measurement Airy spot, 14-pre-focus large virtual pinhole detection domain, 15-pre-focus small virtual pinhole detection domain, 16-post-focus measurement Airy spot, 17-post-focus large virtual pinhole detection domain, 18-post-focus small virtual pinhole detection domain, 19-pre-focus large virtual pinhole detection confocal characteristic curve, 20-pre-focus small virtual pinhole detection characteristic curve, 21-pre-focus transverse subtraction sharpening characteristic curve, 22-large virtual pinhole detection confocal characteristic curve after focusing, 23-small virtual pinhole detection confocal characteristic curve after focusing, 24-transverse subtraction sharpening confocal characteristic curve after focusing, 25-transverse subtraction differential confocal characteristic curve I, 26-transverse subtraction differential confocal characteristic curve II, 27-differential confocal linear fitting straight line, 28-fitting straight line zero point, 29-image acquisition system, 30-main control computer, 31-multi-path motor driving system, 32-axial measurement motion system, 33-five-dimensional adjustment system, 34-laser, 35-microscope objective, 36-pinhole and 37-annular pupil.
Detailed Description
The invention is further illustrated by the following figures and examples.
The invention uses a method for measuring the central thickness of a transverse subtraction differential confocal lens to realize high-precision measurement of the central thickness of the lens, and the core idea is as follows: in the differential confocal measurement system, a confocal characteristic curve is sharpened through transverse subtraction detection of a large virtual pinhole and a small virtual pinhole, and the focusing precision of the vertex position of the lens in the measurement of the central thickness of the lens is realized through differential subtraction processing detection of the sharpened confocal response characteristic curve, so that the aim of improving the high precision of the measurement of the central thickness of the lens is fulfilled.
Example (b):
as shown in fig. 1 to 6, the method for measuring the central thickness of a transverse subtraction differential confocal lens disclosed in this embodiment includes the following specific steps:
a) the measurement software of the main control computer 30 is started, the laser 34 is turned on, and light emitted by the laser 34 forms a point light source 1 after passing through the microscope objective lens 29 and the pinhole 36. The light emitted by the point light source 1 passes through the beam splitter 2, the collimating lens 3 and the measuring objective 4 to form a focused measuring beam 5 and irradiates on the measured lens 6.
b) The measured lens 6 is adjusted to be coaxial with the measuring objective lens 4 and the collimating lens 3, so that parallel light beams emitted by the collimating lens 3 are converged into a focused measuring light beam 5 through the measuring objective lens 4 and focused on the vertex A of the measured lens 6, the focused measuring light beam 5 reflected by the vertex of the measured lens 6 is reflected by the beam splitter 2 after passing through the measuring objective lens 4 and the collimating lens 3 and enters a transverse subtraction differential confocal detection system 7, and measurement software in a main control computer 30 obtains a pre-focus measurement Airy spot 13 collected by a pre-focus CCD detector 10 through an image collection system 23.
c) Moving the measured lens 6 along the optical axis direction to make the focus of the focused measuring beam 5 coincide with the vertex A position of the measured lens 6; relatively axially scanning the measurement objective 4 near the vertex A of the lens, and focusing the transverse subtraction differential confocal detection system 7Confocal characteristic curve 19I of large virtual pinhole detection before focus detected by large virtual pinhole detection area 14 and small virtual pinhole detection area before focus 15 respectivelyB1(z,-uM) Confocal characteristic 20I of small virtual pinhole detection before focusingS1(z,-uM) The pre-focal transverse subtraction sharpening confocal characteristic curve 21I with half-width compression is obtained by subtraction processing1(z,-uM)＝IS1(z,-uM)-γIB1(z,-uM) Wherein z is an axial coordinate and gamma is an adjustment factor;
the method for detecting the confocal characteristic curve based on the large/small virtual pinhole detection domain comprises the following steps: selecting a concentric circle region with a preset size on each frame of image of the pre-focus measurement Airy spots 13 detected by the pre-focus CCD detector 10, and integrating the light intensity of each pixel in the large circle region to obtain a confocal intensity response curve IB(z) integrating the light intensity of each pixel in the small circle to obtain a confocal intensity response curve IS(z) then adding IB(z) and IS(z) carrying out subtraction processing to obtain a transverse subtraction confocal response curve I (z, u)M)＝IS(z,uM)-γIB(z,uM) And changing the adjusting factor gamma to realize the optimization of the confocal characteristic curve.
In this embodiment, the diameter of the large circle is 11 pixels, the diameter of the small circle is 5 pixels, and γ is 0.5, uM＝2.98。
d) The measurement software of the main control computer 30 respectively detects the confocal characteristic curve 22I of the large after-focus virtual pinhole detection in the large after-focus virtual pinhole detection domain 17 and the small after-focus virtual pinhole detection domain 18 in the transverse subtraction differential confocal detection system 7B2(z,+uM) Confocal characteristic curve 23I of small virtual pinhole detection after focusingS2(z,+uM) Carrying out subtraction processing to obtain a semi-height-width compressed confocal characteristic curve 24I of transverse subtraction sharpening after focusing2(z,+uM)＝IS2(z,+uM)-γIB2(z,+uM)；
e) Confocal characteristic 21I by post-focus transverse subtraction sharpening2(z,+uM) And-front transverse subtraction sharpening confocal characteristic 24I1(z,-uM) Using equation (1)The horizontal subtraction differential confocal characteristic curve I25I with high axial sensitivity can be obtained by row differential subtractionD(z)ID(z)＝I2(z,+uM)-I1(z,-uM)。
f) The measurement software of the main control computer 30 is paired with the discrete transverse subtraction differential confocal characteristic curve I25IDThe zero point 28 of the fitted straight line of (Z) determines the vertex A of the front surface of the measured lens 6, and thus precisely determines the position Z of the vertex A of the front surface of the measured lens 61The lens vertex position Z of the lens 6 to be measured at this time is recorded1＝0.0018mm。
g) The main control computer 30 controls the five-dimensional adjusting system 33 to continuously move the measuring objective 4 along the optical axis direction of the measuring objective 4 in the opposite direction through the multi-path motor driving system 31 and the axial measuring moving system 32, so that the focus of the focused measuring beam (5) is superposed with the vertex B of the rear surface of the measured lens 6; the measuring objective 4 is axially scanned near the vertex B of the lens, and the focused measuring beam 5 is reflected by the vertex B of the rear surface of the measured lens 6 and enters the transverse subtraction differential confocal detection system 7 to be detected. Scanning the measuring objective 4 near the position, measuring a discrete transverse subtraction differential confocal characteristic curve two 26 by a transverse subtraction differential confocal detection system 7, accurately determining the focal position B of the measured lens 6 by a main control computer 30 through a fitting straight line zero point 28 of a differential confocal linear fitting straight line 27, and recording the Z of the vertex position B of the rear surface of the measured lens 6 at the moment2The result of focusing the optical path on the front and rear surfaces of the test mirror 6 is shown in fig. 7, which is-2.6745 mm.
h) The main control computer 30 obtains a calculation formula of the lens center thickness d according to the established ray tracing and compensation model formula (2) as follows:
substituting the known parameter, the numerical aperture angle α of the measuring beam0Radius of curvature r of front surface of lens to be measured1Refractive index n of air0The distance between the refractive index n of the measured lens and the two fixed focus positions is equal to | Z2-Z1I, then the center of the measured lens 6 is obtainedThe thickness d is 4.0068 mm.
While the invention has been described in connection with specific embodiments thereof, it will be understood that it is not intended to limit the scope of the invention, which is defined by the appended claims, but rather should be construed in breadth and scope in accordance with any variation of the following claims.
Claims (3)
1. The method for measuring the central thickness of the transverse subtraction differential confocal lens is characterized by comprising the following steps of: comprises the following steps of (a) carrying out,
a) opening the point light source (1), and forming a focused measuring beam (5) through light emitted by the point light source (1) after passing through the beam splitter (2), the collimating lens (3) and the measuring objective lens (4) to irradiate on the measured lens (6);
b) adjusting a measured lens (6) to be coaxial with a measuring objective lens (4) and a collimating lens (3), so that parallel light beams emitted by the collimating lens (3) are converged into a focused measuring light beam (5) through the measuring objective lens (4) and focused on a vertex A of the measured lens (6), the focused measuring light beam (5) reflected by the vertex of the measured lens (6) is reflected by a beam splitter (2) to enter a transverse subtraction differential confocal detection system (7) after passing through the measuring objective lens (4) and the collimating lens (3), and a formed in-focus measuring Airy spot (13) is detected by a in-focus CCD detector (10);
c) moving the measuring objective lens (4) along the optical axis direction to enable the focus of the focused measuring beam (5) to coincide with the vertex A position of the measured lens (6); the measuring objective lens (4) is relatively axially scanned near the vertex A of the lens, and a confocal characteristic curve (19) I of the large virtual pinhole detection before the focus (14) and a confocal small virtual pinhole detection before the focus (15) in the transverse subtraction differential confocal detection system (7) are respectively detectedB1(z,-uM) Confocal characteristic curve (20) I of small virtual pinhole detection before focusingS1(z,-uM) The pre-focal transverse subtraction sharpening confocal characteristic curve (21) I with half-width compression is obtained by subtraction processing1(z,-uM)＝IS1(z,-uM)-γIB1(z,-uM) A large virtual pinhole detection area (17) behind the middle focus and a large virtual pinhole detection area (18) behind the middle focus of the transverse subtraction differential confocal detection system (7) are respectively used for detecting the confocal large virtual pinhole after the middle focusCharacteristic curve (22) IB2(z,+uM) Confocal characteristic curve (23) I of small virtual pinhole detection after focusingS2(z,+uM) Carrying out subtraction processing to obtain a semi-width-at-half compressed confocal characteristic curve (24) I of transverse subtraction sharpening after focusing2(z,+uM)＝IS2(z,+uM)-γIB2(z,+uM) Where z is the axial coordinate, γ is the adjustment factor, uMThe normalized distance is the normalized distance of the focal plane distance M of the off-focus front microscope objective (8) of the pre-focus CCD detector (9) and the normalized distance of the focal plane distance M of the off-focus back microscope objective (10) of the post-focus CCD detector (11); post-focus transverse subtractive sharpening confocal characteristic (24) I2(z,+uM) And the confocal characteristic (21) I of the transverse subtraction sharpening1(z,-uM) Carrying out differential subtraction to obtain a transverse subtraction differential confocal characteristic curve I (25) I with high axial sensitivityD(z)：
ID(z)＝I2(z,+uM)-I1(z,-uM) (1)
Differential confocal characteristic line I (25) by transverse subtractionD(Z) determining the vertex A of the front surface of the lens (6) to be measured by the zero point (28) of the fitted straight line, and further accurately determining the position Z of the vertex A of the front surface of the lens (6) to be measured1；
d) Continuously moving the measuring objective lens (4) along the optical axis direction to ensure that the focus of the focused measuring beam (5) is superposed with the vertex B of the rear surface lens of the measured lens (6); the measuring objective lens (4) is axially scanned near the vertex B of the lens, and at the moment, a focused measuring beam (5) is reflected by the vertex B of the rear surface lens of the measured lens (6) in a primary path and enters a transverse subtraction differential confocal detection system (7) to be detected; scanning the measurement objective lens (4) near the position, measuring a transverse subtraction differential confocal characteristic curve II (26) by a transverse subtraction differential confocal detection system (7), accurately determining the lens vertex B of the measured lens (6) by a main control computer (30) through a fitting straight line zero point (28) of a differential confocal linear fitting straight line (27), and recording the focus position Z of the focused measurement light beam (5) at the moment2；
e) According to the established ray tracing and the compensation model thereof, the calculation formula of the central thickness d of the lens is obtained as follows:
substituting the known parameter, the numerical aperture angle α of the measuring beam0Radius of curvature r of front surface of lens to be measured1Refractive index n of air0The distance between the refractive index n of the measured lens and the two fixed focus positions is equal to | Z2-Z1And | obtaining the central thickness d of the measured lens (6).
2. The method of measuring the central thickness of a lateral subtractive differential confocal lens of claim 1 comprising: organically integrating a laser differential confocal technology and a light tracing technology, establishing a light tracing and a compensation model thereof, eliminating the mutual influence among parameters of each analytic fixed-focus surface, and further obtaining a calculation formula of the central thickness of the lens, wherein r is shown in formula (2)NIs the Nth surface SNRadius of curvature of, nNIs the Nth surface SNAnd the (N + 1) th surface SN+1Refractive index of material in between, dN-1Is the (N-1) th surface SN-1And the Nth surface SNAxial clearance between lNIs' SNVertex to SNDistance of intersection of ray and optical axis, uNIs' SNThe angle between the emergent ray and the optical axis,
and (3) deriving a formula (1) for calculating the central thickness of the lens according to the formula, and further realizing accurate measurement of the central thickness of the lens.
3. The method of measuring the central thickness of a lateral subtractive differential confocal lens of claim 1 comprising: an annular pupil (37) is added in the light path to modulate the measuring light beam to form an annular light beam, so that the influence of wave phase difference on the measuring light beam when the parameters of the measuring element are measured is reduced, and the measuring error is reduced.
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