CN109990983B - Bilateral dislocation differential confocal super-long focal length measuring method - Google Patents
Bilateral dislocation differential confocal super-long focal length measuring method Download PDFInfo
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Abstract
The invention discloses a bilateral dislocation differential confocal super-long focal length measuring method, 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 firstly 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 bilateral dislocation differential subtraction processing to obtain axial highly sensitive differential confocal characteristic curves, and then the zero point of the bilateral dislocation differential confocal characteristic curves and the focus of the confocal measuring system are utilized to accurately correspond to the characteristics to carry out high-precision focus locating on each position point of a mobile measuring plane mirror in the ultralong focus measurement, so that the high-precision measurement of the ultralong focus is realized. Compared with the existing ultra-long focal length measuring method, the method has the advantages of high measuring 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 bilateral dislocation differential confocal ultra-long focal length measuring method, belonging to the technical field of precision measurement of optical element parameters.
Background
The long-focus optical element is widely applied to the research fields of large optical systems such as laser nuclear fusion, space optical systems, high-energy lasers and the like. However, the high-precision measurement of the focal length value of the long-focus lens is a big problem which is not solved in the field of optical testing, and the measurement precision directly affects the imaging quality and the service performance of a large-scale optical system. Therefore, finding a long-focus high-precision measurement method has important application value, and is a technical bottleneck to be solved urgently in research and adjustment of national important special projects and national important engineering projects such as laser nuclear fusion, space optical instruments, high-energy lasers and the like.
At present, for high-precision measurement of long focal length, researchers at home and abroad have proposed many different measurement methods, which can be generally classified into two types.
The first type is a measurement method based on the conventional geometric imaging principle. For example, in "Measuring the focal length of long-distance Optical systems" published in "Journal of Optical Technology" in 1999, the authors proposed a method of Measuring the measured focal length value by inserting a small-angle Optical wedge in a collimated light path and Measuring the change in position of an image with/without the wedge on the focal plane of an Optical system, and the relative measurement accuracy of this method to a focal length of 25000mm was 0.1%. As in the "Determination of the focal length of the non-polar lenses by lens deflection" published in "Applied optics" in 1987, the authors propose to use the molar effect to achieve the measurement of the focal length of the measured mirror by measuring the rotation angle of the molar fringes, with a theoretical relative measurement error of less than 0.1%. These conventional measurement methods based on the geometric imaging principle are limited by diffraction limit, and it is difficult to further improve the measurement accuracy.
The second category is the measurement method based on the taber effect, which is also the most commonly used method in the field of long focus measurement research at present. According to the Talbot effect, when a grating is irradiated by spherical light waves, a generated periodic Talbot image has a corresponding relation with the wave front curvature radius, moire fringes can be generated by placing another grating at the position of the Talbot image surface, and the focal length measurement is realized according to the corresponding relation between the moire fringe deflection angle and the curvature radius. Based on the principle, in the document of "Measurement of the focal length of a collimating lens using the Talbot effect and the Moire technique" published in "Applied optics" in 1991, the author replaces a complex collimating system with a collimating lens, and the Measurement accuracy of the focal length value of 200mm is only 2% limited by the Moire angle discrimination accuracy. In order to improve the judgment precision of the angle of the fringe, in the article of "Measurement of focal length with phase-shifting Talbot interferometry" published in "Applied optics" in 2005, an author adopts a Fourier analysis technology to filter image noise caused by grating fringes so as to improve the judgment precision of the angle of the moire fringe, and under the condition that the focal length is 240mm, the Measurement error is less than 0.3%. The method is also deeply researched by Zhejiang university In China, And In 2005, "Optics And Lasers In Engineering" a Novel method for testing the local length of the lens of large aperture is published, authors combine the Taber effect And the scanning measurement technology to scan different positions of the measured lens In real time to measure the measured focal length, And the relative measurement accuracy is better than 0.13% under the lengths of aperture 150mm And focal length 18000 mm. To further improve the measurement accuracy, in the context of "Long focal-length measurement using divergent beams and two gratings of differential properties" published in "Optics express" 2014, the authors propose a measurement method using divergent beams and an unequal period grating instead of the conventional collimated beams and equal period grating, which has a relative error of less than 0.0018% at a focal length of 13500 mm. Compared with the first type of measurement method, the measurement method based on the Talbot effect achieves higher measurement accuracy, but the fringe change information is used as an evaluation scale, and interference fringes are easily influenced by environmental factors such as air flow, temperature, jitter and the like in actual measurement, so that the popularization and application of the method in engineering are restricted, and the measurement accuracy is further improved.
In summary, high-precision measurement of long focal length is still a big problem in the field of optical testing, and the main difficulties are:
1) the depth of focus is long, and is affected by diffraction effect, so that the focus is difficult to be accurately fixed;
2) the focal length is long, the measuring light path is long, and the precise length measurement is difficult under the influence of measuring environment interference and system drift;
3) the measurement light path is long, harsh requirements are provided for the construction and measurement environment of a measurement system, and the difficult problem of realizing long-focus high-precision measurement through small-size measurement is urgently needed to be solved, so that the volume of an instrument is reduced, and the environment interference resistance is improved.
Aiming at the difficult problem of high-precision measurement of ultra-long focal length, the inventor has conducted principle level innovation on the confocal microscopic imaging principle used in the microscopic measurement field, and has successfully used the confocal microscopic technology in the microscopic imaging test field for the first time internationally in the large-size optical element measurement field, the related papers are published on the international optical field famous journals such as Optics Express, v17, n22,2009, Optics Express, v18, n3,2010, Optics Express, v21, n19,2013, and also applied and issued multiple invention patents such as "confocal combined ultra-long focal length measurement method and device" (ZL 200810226967.6), "differential confocal combined ultra-long focal length measurement method and device" (ZL 200810226966.1) and "differential confocal internal focusing method and device" (ZL 201010121848.1), but the inventor still has fixed focus sensitivity and not high fixed focus sensitivity in the ultra-long focal length measurement in the above-mentioned papers and patents, Insufficient anti-environment interference capability, complex fixed focus system and the like.
Based on the above, the present invention provides a bilateral dislocation differential confocal super-long focal length measuring method, in the confocal measuring system, firstly, a large and a small virtual pinhole detection area (image area) are set on a alice spot image detected by a CCD through software, two confocal characteristic curves detected by the large and the small virtual pinhole detection area are subtracted to sharpen the confocal characteristic curve, then the sharpened confocal characteristic curve is subjected to bilateral dislocation differential subtraction to obtain an axial highly sensitive differential confocal characteristic curve, and finally, the characteristic that the zero point and the focus of the bilateral dislocation differential confocal characteristic curve accurately correspond is used to realize high-precision focusing on each characteristic point in super-long focal length measurement, so as to realize the high-precision measurement of the super-long focal length. The method for measuring the ultra-long focal length provides a brand new technical approach for high-precision measurement of the ultra-long focal length.
Disclosure of Invention
In order to solve the problem of high-precision measurement of the ultra-long focal length, the invention discloses a bilateral dislocation differential confocal ultra-long focal length measuring method, which aims to improve the fixed focal precision of each position in the ultra-long focal length measurement so as to realize the high-precision measurement of the ultra-long focal length.
The differential confocal super-long focal length refers to an element with a focal length not less than 5 m.
The purpose of the invention is realized by the following technical scheme.
In a confocal measurement light path system, a confocal response characteristic curve is sharpened through transverse subtraction processing of confocal characteristic curves of a large virtual pinhole and a small virtual pinhole, differential confocal bipolar fixed-focus measurement of a measured surface is realized through bilateral dislocation differential subtraction processing of the sharpened confocal response characteristic curve, 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 each position in the super-long focus measurement is improved, and the high-precision measurement of the super-long focus is realized. The invention discloses a bilateral dislocation differential confocal super-long focal length measuring method, which comprises the following steps:
a) and opening the point light source, and irradiating the light emitted by the point light source on the plane reflecting mirror after the light penetrates through the beam splitter, the collimating lens and the reference lens.
b) Adjusting a plane reflector (6) to be coaxial with a reference lens (4) and a collimating lens (3), converging parallel light beams emitted by the collimating lens (3) into measuring light beams (5) through the reference lens (4) and focusing on a point A of the plane reflector (6), reflecting the focusing measuring light beams (5) reflected by the plane reflector (6) through the reference lens (4) and the collimating lens (3) and then reflecting the light beams by a beam splitter (2) to enter a transverse subtraction confocal detection system (7), and obtaining a measuring Airy spot (10) collected by a CCD detector (9) through image collection software in a main control computer (24) through an image collection system (23); the transverse subtraction confocal detection system (7) is composed of a microscope objective (8) and a detector (9); the light beam reflected by the beam splitter (2) passes through the microscope objective (8) and is collected by the detector (9);
c) moving the plane reflector along the optical axis direction to make the focus of the measuring beam coincide with the position A of the plane reflector, scanning the plane reflector along the axial direction near the position A, and detecting the large virtual pinhole detection confocal characteristic curve I respectively detected by the large virtual pinhole detection domain and the small virtual pinhole detection domain in the transverse subtraction confocal detection systemB(z) confocal characteristic line I for detection of small virtual pinholeS(z) obtaining a half-width compressed sharpened confocal characteristic curve I (z) IS(z)-γIB(z), wherein z is an axial coordinate and γ is an adjustment factor;
the method for acquiring and optimizing the detection confocal characteristic curves of the large virtual pinhole detection domain and the small virtual pinhole detection domain (12) comprises the following steps: selecting a concentric circle domain with a specific size on each frame of image of the Airy spots (10) detected and measured by the CCD detector (9), and integrating the light intensity of each pixel in the large circle domain 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) obtaining a half-width compressed sharpened confocal characteristic curve (15) by subtraction (I), (z) IS(z)-γIB(z), changing the adjustment factor gamma to optimize the confocal characteristic curve.
d) Translating the sharpened confocal characteristic curve (15) along a transverse coordinate S to obtain a translational sharpened confocal characteristic curve (16), converging the sharpened confocal characteristic curve (15) and the side edge of the translational sharpened confocal characteristic curve (16), respectively carrying out same-transverse coordinate point interpolation processing on the sharpened confocal characteristic curve (15) and the translational sharpened confocal characteristic curve (16), and then carrying out point-by-point subtraction processing to obtain a first dislocation subtraction differential confocal characteristic curve (17) ID(Z) I (Z) -I (Z-S), linear fitting the linear segment data of the first misalignment subtraction differential confocal characteristic curve (17) by using a differential confocal linear fitting line (18), precisely determining the coincidence position of the focal point of the measuring beam (5) and the vertex of the plane mirror (6) by reversely shifting the shift fitting straight line zero point (21) of the shift differential confocal fitting line (20) of the S/2 position of the differential confocal linear fitting line (18), and further obtaining the position Z of the plane mirror (6)1；
e) And inserting the measured lens between the collimating lens and the reference lens, and adjusting the measured lens to be coaxial with the collimating lens and the reference lens, so that the focal position of the measuring beam is changed from A to B.
f) Moving the plane reflecting mirror oppositely along the direction of the optical axis to ensure that the focus of the measuring beam is superposed with the surface of the plane reflecting mirror; axially scanning the plane reflector near the coincidence position of the focus and the surface, processing the measured airy disc by a transverse subtraction confocal detection system to obtain a sharpened confocal characteristic curve, then performing bilateral dislocation subtraction to obtain a dislocation subtraction differential confocal characteristic curve corresponding to a lens surface B point of the plane reflector 6, and accurately determining the position B of the plane reflector by performing linear fitting, linear fitting and linear retracement on the dislocation subtraction differential confocal characteristic curve and determining a retracement fitting straight line zero point according to the step of e) by a main control computer, and recording the position Z of the plane reflector at the moment2While measuring the distance d between the measured lens and the reference lens0Calculating the distance between the positions A and B of the plane mirror as Z2-Z1；
g) Calculating the principal plane spacing d of the measured lens from the reference lens by:
wherein, the parameters of the lens (31) to be measured are as follows: thickness b1Refractive index n1Radius of curvature r11、r12(ii) a The reference lens parameters are: focal length f2', thickness b2Refractive index n2Radius of curvature r21、r22。
h) Calculating the focal length value of the lens (31) to be measured by the following formula:
preferably, the bilateral dislocation differential confocal super-long focal length measuring method disclosed by the invention is implemented by processing the measured Airy spots by a transverse subtraction confocal detection system to obtain a sharpened confocal characteristic curve, and comprises the following steps: in the scanning process of the plane reflector, detecting and measuring Airy spots through a CCD (charge coupled device) detector, selecting a large virtual pinhole detection domain with a specific size on each frame detection image of the CCD detector by taking the gravity center of the measured Airy spots as a center, and integrating the intensity of each pixel in the large virtual pinhole detection domain to obtain a large virtual pinhole detection confocal characteristic curve;
step two, simultaneously taking the center of gravity of the measured Airy spots detected by the CCD detector as a center, selecting a small virtual pinhole detection domain of another smaller area, and integrating the intensity of the small virtual pinhole detection domain to obtain another small virtual pinhole detection confocal characteristic curve, wherein the full width at half maximum and the peak intensity of the small virtual pinhole detection confocal characteristic curve are lower than those of a large virtual pinhole detection confocal characteristic curve;
multiplying the large virtual pinhole detection confocal characteristic curve by a coefficient gamma, so that the light intensity of the large virtual pinhole detection confocal characteristic curve is 1/2 times that of the small virtual pinhole detection confocal characteristic curve;
and step four, subtracting the large virtual pinhole detection confocal characteristic curve obtained by multiplying the coefficient gamma from the small virtual pinhole detection confocal characteristic curve to obtain a sharpened confocal characteristic curve.
Preferably, according to the bilateral dislocation differential confocal ultra-long focal length measuring method, 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 bilateral dislocation differential confocal super-long focal length measuring method, which utilizes a large virtual pinhole and a small virtual pinhole to detect a transverse subtraction sharpened confocal characteristic curve in a confocal measuring system, utilizes bilateral dislocation 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 the signal-to-noise ratio of the differential confocal fixed-focus curve, obviously improves the fixed-focus precision of each fixed-focus position in super-long focal length measurement, and obviously improves the measuring precision of the super-long focal length.
2) Compared with a differential confocal measurement system, the bilateral dislocation differential confocal ultra-long focal length measurement method disclosed by the invention can improve the ultra-long focal length measurement precision under the condition of not increasing the hardware cost.
3) According to the bilateral dislocation differential confocal super-long focal length measuring method disclosed by the invention, the horizontal 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 super-long focal length measuring system is improved.
4) Compared with a classical high-precision interference focusing method, the bilateral dislocation differential confocal super-long focal length measuring method disclosed by the invention can overcome the defects of extreme sensitivity of the existing interference focusing method to system aberration, environmental vibration, air flow interference and sample surface roughness by adopting a non-interference airy disk central intensity point detection mode, greatly improves the capability of resisting system aberration, environmental interference and surface scattering, and obviously improves the super-long focal length measuring precision.
Drawings
FIG. 1 is a schematic diagram of a bilateral dislocation differential confocal ultra-long focal length measurement method of the present invention;
FIG. 2 is a schematic diagram of horizontal subtraction sharpening of confocal characteristic curves of large and small virtual pinholes according to the present invention;
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 bilateral dislocation differential confocal curve linear fitting trigger focusing of the present invention;
FIG. 5 is a schematic diagram of a bilateral dislocation differential confocal ultra-long focal length measurement method according to an embodiment of the present invention;
FIG. 6 is a data diagram of an embodiment of the present invention;
wherein: 1-point light source, 2-beam splitter, 3-collimating lens, 4-reference lens, 5-measuring light beam, 6-plane reflector, 7-transverse subtraction confocal detection system, 8-first microscope, 9-CCD detector, 10-measuring Airy spot, 11-large virtual pinhole detection domain, 12-small virtual pinhole detection domain, 13-large virtual pinhole confocal characteristic curve, 14-small virtual pinhole confocal characteristic curve, 15-sharpening confocal characteristic curve, 16-translation sharpening confocal characteristic curve, 17-first dislocation differential characteristic curve, 18-differential confocal linear fitting straight line, 19-fitting straight line confocal zero point, 20-retracement differential fitting straight line, 21-shift fitting straight line zero point, zero point, 22-a second dislocation subtraction differential confocal characteristic curve, 23-an image acquisition system, 24-a main control computer, 25-a multi-path motor driving system, 26-an axial measurement movement system, 27-a five-dimensional adjustment system, 28-a laser, 29-a second microscope objective, 30-a pinhole and 31-a measured lens.
Detailed Description
The invention is further illustrated by the following figures and examples.
In the embodiment, a bilateral dislocation differential confocal ultra-long focal length measurement method is used for realizing high-precision measurement of the lens group gap, and the core idea is as follows: in the differential confocal measurement system, a large virtual pinhole and a small virtual pinhole are transversely subtracted and detected to sharpen a confocal characteristic curve, and bilateral dislocation differential subtraction processing and detection of the sharpened confocal response characteristic curve are used to realize accurate focusing of the top point position of a lens group in the ultralong focal length measurement, so that the aim of improving the ultralong focal length measurement precision is fulfilled.
Example (b):
as shown in fig. 5, the measuring steps of the bilateral dislocation differential confocal ultra-long focal length measuring method are as follows:
a) and starting measurement software of the main control computer 24, turning on the laser 28, and forming the point light source 1 after light emitted by the laser 28 passes through the second microscope objective 29 and the pinhole 30. The light emitted from the point light source 1 passes through the beam splitter 2, the collimator lens 3 and the reference lens 4 and then irradiates the plane mirror 6.
b) The plane reflector 6 is adjusted to be coaxial with the reference lens 4 and the collimating lens 3, so that parallel light beams emitted by the collimating lens 3 are converged into measuring light beams 5 through the reference lens 4 and focused on the plane reflector 6, the focused measuring light beams 5 reflected by the plane reflector 6 are reflected by the beam splitter 2 after passing through the reference lens 4 and the collimating lens 3 and enter a transverse subtraction confocal detection system 7, and measuring software in a main control computer 24 obtains the measured Airy spots 10 collected by the CCD detector 9 through an image collection system 23.
c) Moving the plane mirror 6 along the optical axis direction to make the focus of the measuring beam 5 coincide with the position of the A point of the plane mirror 6; a large virtual pinhole confocal characteristic curve 13I detected by a large virtual pinhole detection region 11 in a transverse subtraction confocal detection system 7 relative to an axial scanning plane mirror 6 near the vertex positionB(z) detected Small virtual pinhole confocal Curve 14IS(z) the half-width compressed sharpened confocal characteristic 15I (z) IS(z)-γIB(z), wherein z is an axial coordinate and γ is an adjustment factor;
as shown in fig. 2, the method for acquiring and optimizing the detection confocal characteristic curves of the large virtual pinhole detection area 11 and the small virtual pinhole detection area 12 includes: selecting a concentric circle domain with a specific size on each frame of image of the measured Airy spots 10 before the CCD detector 9 detects the focus, and integrating the light intensity of each pixel in the large circle domain to obtain the large virtual pinhole confocal characteristicSexual curve 13IB(z) integrating the intensity of each pixel in the small circle to obtain a small virtual pinhole confocal characteristic 14IS(z) then adding IB(z) and IS(z) the subtraction processing is performed to obtain sharpened confocal characteristic 15I (z) ═ IS(z)-γIB(z), changing the adjustment factor gamma to optimize the confocal characteristic curve.
d) As shown in fig. 3, the measurement software of the main control computer 24 translates the sharpened confocal characteristic curve 15 obtained by subtraction along the horizontal coordinate S to obtain a translational sharpened confocal characteristic curve 16, joins the side edges of the sharpened confocal characteristic curve 15 and the translational sharpened confocal characteristic curve 16, performs interpolation processing on the sharpened confocal characteristic curve 15 and the translational sharpened confocal characteristic curve 16 at the same horizontal coordinate point, and performs subtraction processing point by point to obtain a first staggered subtraction differential confocal characteristic curve 17ID(z)＝I(z)-I(z,-S)。
e) As shown in fig. 4, the measurement software of the main control computer 24 performs a line fitting on the linear segment data of the first dislocation subtraction differential confocal characteristic curve 17 to obtain a differential confocal linear fitting line 18, and then precisely determines the coincidence position of the focal point of the converged measurement beam 5 and the vertex of the plane mirror 6 by moving back the shifted fitting line zero point 21 of the differential confocal fitting line 20 at the position of the reverse backward shifted differential confocal linear fitting line 18S/2, and further determines the position Z of the plane mirror 61The position Z of the plane mirror 6 at this time is recorded1＝-0.0002mm。
f) Inserting a measured lens 31 between the collimating lens 3 and the reference lens 4 for changing the focal position A of the measuring beam 5, aligning the measured lens 31 with the collimating lens 3 and the reference lens 4, and measuring the distance d between the measured lens 31 and the reference lens 40。
g) The main control computer 24 controls the five-dimensional adjusting system 27 to continuously move the plane mirror 6 in the opposite direction along the optical axis direction of the reference lens 4 through the multi-path motor driving system 25 and the axial measuring moving system 26, so that the focus of the measuring beam 5 is superposed with the surface of the plane mirror 6; scanning the plane mirror 6 axially near the position where the focus coincides with the surface, and processing the measured measurement beam by a transverse subtraction confocal detection system 7The inner spot 10 obtains a sharpened confocal characteristic curve 15, then bilateral dislocation subtraction processing is carried out to obtain a second dislocation subtraction differential confocal characteristic curve 22 corresponding to a lens surface B point of the plane reflector 6, the main control computer 24 accurately determines the position B of the plane reflector 6 by carrying out linear fitting, fitting linear retracement, determining a retracement fitting linear zero point and the like on the second dislocation subtraction differential confocal characteristic curve 22 according to the step of e), and as shown in figure 6, the position Z of the plane reflector 6 at the moment is recorded2＝286.4262mm。
h) Let Z be the moving distance Δ between the position A and the position B of the plane mirror 62-Z1286.4264mm, and the distance d between the measured lens 31 and the reference lens 4 is measured0＝418.29mm；
i) The principal in-plane distance d of the measured lens (31) from the reference lens 4 is calculated by:
wherein, the parameters of the lens (31) to be measured are as follows: thickness b146.5mm, refractive index n11.5067, radius of curvature r11＝9377mm、r1228133 mm; the reference lens (4) parameters are: focal length f2' 2797.5220mm, refractive index n21.5067, radius of curvature r21、r22＝∞。
j) The focal length of the lens (31) to be measured is 31218.34mm as calculated by the following formula:
while the invention has been described in connection with specific embodiments thereof, it will be understood that these should not be construed as limiting the scope of the invention, which is defined in the following claims, and any variations which fall within the scope of the claims are intended to be embraced thereby.
Claims (2)
1. The bilateral dislocation differential confocal super-long focal length measuring method is characterized in that: comprises the following steps of (a) carrying out,
a) opening the point light source (1), and irradiating light emitted by the point light source (1) on the plane reflector (6) after penetrating through the beam splitter (2), the collimating lens (3) and the reference lens (4);
b) adjusting a plane reflector (6) to be coaxial with a reference lens (4) and a collimating lens (3), converging parallel light beams emitted by the collimating lens (3) into measuring light beams (5) through the reference lens (4) and focusing on a point A of the plane reflector (6), reflecting the focusing measuring light beams (5) reflected by the plane reflector (6) through the reference lens (4) and the collimating lens (3) and then reflecting the light beams by a beam splitter (2) to enter a transverse subtraction confocal detection system (7), and obtaining a measuring Airy spot (10) collected by a CCD detector (9) through image collection software in a main control computer (24) through an image collection system (23); the transverse subtraction confocal detection system (7) is composed of a microscope objective (8) and a detector (9); the light beam reflected by the beam splitter (2) passes through the microscope objective (8) and is collected by the detector (9);
c) moving the plane reflector (6) along the optical axis direction to make the focus of the measuring beam (5) coincide with the position A of the plane reflector (6), axially scanning the plane reflector (6) near the position A, and detecting a large virtual pinhole confocal characteristic curve (13) I detected by a large virtual pinhole detection domain (11) in a transverse subtraction confocal detection system (7)B(z) small virtual pinhole confocal characteristic (14) I detected by small virtual pinhole detection field (12)S(z) obtaining a half-width compressed sharpened confocal characteristic curve (15) by subtraction (I), (z) IS(z)-γIB(z), wherein z is an axial coordinate and γ is an adjustment factor;
the method for acquiring and optimizing the detection confocal characteristic curves of the large virtual pinhole detection domain (11) and the small virtual pinhole detection domain (12) comprises the following steps: selecting a concentric circle domain with a specific size on each frame of image of the Airy spots (10) detected and measured by the CCD detector (9), and integrating the light intensity of each pixel in the large circle domain 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) sharpening by subtraction to half-width compressionConfocal characteristic curve (15) I (z) ═ IS(z)-γIB(z), changing the adjusting factor gamma to realize the optimization of the confocal characteristic curve;
the transverse subtraction confocal detection system (7) processes the measured measurement airy disk (10) to obtain a sharpened confocal characteristic curve (15) by the following method:
in the scanning process of the plane reflector (6), detecting and measuring an Airy spot (10) through a CCD (charge coupled device) detector (9), selecting a large virtual pinhole detection domain (11) with a preset size on each frame detection image of the CCD detector (9) by taking the gravity center of the measured Airy spot (10) as a center, and integrating the intensity of each pixel in the large virtual pinhole detection domain (11) to obtain a large virtual pinhole confocal characteristic curve (13);
step two, simultaneously, taking the gravity center of a measurement Airy spot (10) detected by a CCD detector (9) as a center, selecting another small virtual pinhole detection domain (12), wherein the size of the small virtual pinhole detection domain (12) is smaller than that of a large virtual pinhole detection domain (11), integrating the intensity of the small virtual pinhole detection domain (12) to obtain a small virtual pinhole confocal characteristic curve (14), and the full width at half maximum and the peak intensity of the small virtual pinhole confocal characteristic curve (14) are both lower than those of the large virtual pinhole confocal characteristic curve (13);
multiplying the large virtual pinhole confocal characteristic curve (13) by an adjusting factor gamma to enable the light intensity of the large virtual pinhole confocal characteristic curve (13) to be 1/2 times that of the small virtual pinhole confocal characteristic curve (14);
step four, subtracting the large virtual pinhole confocal characteristic curve (13) multiplied by the adjusting factor gamma from the small virtual pinhole confocal characteristic curve (14) to obtain a sharpened confocal characteristic curve (15);
d) translating the sharpened confocal characteristic curve (15) along a transverse coordinate S to obtain a translational sharpened confocal characteristic curve (16), converging the sharpened confocal characteristic curve (15) and the side edge of the translational sharpened confocal characteristic curve (16), respectively carrying out same-transverse coordinate point interpolation processing on the sharpened confocal characteristic curve (15) and the translational sharpened confocal characteristic curve (16), and then carrying out point-by-point subtraction processing to obtain a first dislocation subtraction differential confocal characteristic curve (17) ID(z) I (z) -I (z-S) by subtracting the differential confocal line (18) from the first offset lineLinear fitting is carried out on linear segment data of the characteristic curve (17), the coincidence position of the focal point of the measuring beam (5) and the vertex of the plane reflector (6) is accurately determined through a displacement fitting straight line zero point (21) of a retransferred differential confocal fitting straight line (20) at the S/2 position of a reverse retransferred differential confocal linear fitting straight line (18), and then the position Z of the plane reflector (6) is obtained1；
e) Inserting a measured lens (31) between the collimating lens (3) and the reference lens (4), and adjusting the measured lens (31) to have the same optical axis as the collimating lens (3) and the reference lens (4), so that the focal position of the measuring beam (5) is changed from A to B;
f) the plane mirror (6) is moved in the direction of the optical axis in the opposite direction to make the focus of the measuring beam (5) coincide with the surface of the plane mirror (6), axially scanning the plane reflector (6) near the coincidence position of the focus and the surface, processing the measured Airy spots (10) by a transverse subtraction confocal detection system (7) to obtain a sharpened confocal characteristic curve (15), and then carrying out bilateral dislocation subtraction processing to obtain a second dislocation subtraction differential confocal characteristic curve (22) corresponding to a lens surface B point of the plane reflector (6), and then carrying out linear fitting, fitting linear retracement and determining a retracement fitting linear zero point on the second dislocation subtraction differential confocal characteristic curve (22) by the main control computer (24) according to the step e) to accurately determine the position B of the plane reflector (6) and record the position Z of the plane reflector (6).2Simultaneously measuring the distance d between the measured lens (31) and the reference lens (4)0Calculating the distance delta between the position A and the position B of the plane mirror (6) as Z2-Z1；
g) Calculating the principal plane separation d of the measured lens (31) and the reference lens (4) by:
wherein, the parameters of the lens (31) to be measured are as follows: thickness b1Refractive index n1Radius of curvature r11、r12(ii) a The reference lens (4) parameters are: focal length f2', thickness b2Refractive index n2Curvature of the surfaceRadius r21、r22；
h) Calculating the focal length value of the lens (31) to be measured by the following formula:
2. the bilateral dislocation differential confocal ultra-long focal length measuring method according to claim 1, characterized in that: an annular pupil is added in the light path to modulate the measuring light beam (5) to form an annular light beam, so that the influence of wave aberration on the measuring light beam (5) when the parameters of the element are measured is reduced, and the measuring error is reduced.
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