A diffuse reflectance based dual optical path measurement system and method

Abstract

The application provides a double optical path measurement system and method based on diffuse reflection, a Wollaston prism divides linearly polarized light into orthogonal polarized reference light and measurement light; the reference light returns through a reference mirror, the measurement light is focused to a rough surface of a sample to be measured through a second lens, and diffuse reflection echoes are collected and collimated back through the second lens. After the two-way return light is superimposed, polarized interference light is formed through a second polarizer. The transmission axis angle of the first and second polarizers is adjusted, the projection power ratio of the two-way return light is changed, and the working angle combination is determined according to the measured fringe contrast. A phase modulation mechanism drives the sample to be measured to generate a modulation displacement, a signal processing module carries out phase demodulation, phase unfolding and displacement conversion on the output of a photoelectric detector, and the axial relative displacement change between the reference reflection surface and the illuminated diffuse reflection effective area is obtained. The application can improve the problem of reduced interference fringe contrast caused by low diffuse reflection echo power and power mismatch of the two-way return light.

Description

Technical Field

[0001] This invention relates to the fields of precision optical measurement and interferometric measurement technology, and more specifically, to a dual-optical-path measurement system and method based on diffuse reflection. Background Technology

[0002] Suspended optical systems can be used for the precise measurement of physical quantities such as displacement, force, and acceleration in micro / nano oscillators. In such systems, non-contact readout of displacement changes in the measured object can be achieved using optical interferometry. Taking diamagnetic levitation pyrolytic graphite sheets as an example, existing interferometric readout schemes typically require the addition of miniature mirrors to the sample surface to obtain sufficiently strong specular reflection echoes. The additional mass and adhesive layer introduced by the mirrors alter the mass distribution and dynamic characteristics of the micro / nano oscillator, which is particularly detrimental to levitation oscillators with very small mass.

[0003] If a reflective mirror is not placed on the surface of the sample under test, and interferometry is performed directly using the diffuse reflection echo generated by its microscopic rough surface, the echo component that can be received and form an effective cross-interference term with the reference light will be very weak, easily causing power mismatch between the two return beams and a reduction in interference fringe contrast. Traditional intensity balancing methods can reduce the return beam power by placing an attenuation element in the stronger beam, thus making the power of the two return beams more similar; however, this method requires additional intensity modulation elements in the optical path, and balance is achieved by reducing the effective power of the stronger beam, thus limiting the overall effective power that can participate in the interference. For measurement scenarios with low diffuse reflection echo power, it is desirable to achieve power balancing of the two return beams while fully utilizing the diffuse reflection echo from the measurement arm.

[0004] Furthermore, in existing free-space two-beam interferometry schemes for targets facing mirror reflection, the propagation segments not shared between the reference arm and the measurement arm may still be affected by mechanical vibration, air disturbance, and temperature changes, introducing differential phase perturbations.

[0005] Therefore, there is a need for a dual-optical-path measurement scheme that can directly collect diffuse reflection echoes generated by rough surfaces without setting up reflective mirrors on the surface of the sample to be tested, adjust the projection power ratio of the two return beams in the polarization direction without setting up a dedicated light intensity attenuation element for power balancing in the dual optical path, and help reduce the influence of environmental disturbances on the relative phase of the two arms. Summary of the Invention

[0006] To address the issues of weakened diffuse reflection echoes from rough surfaces of the test sample, power mismatch between the two return beams, and differential phase disturbances that may be introduced by non-shared propagation sections in existing technologies, this invention proposes a dual-optical-path measurement system based on diffuse reflection, along with a corresponding measurement method. The aim is to improve the power mismatch between the two return beams in diffuse reflection scenarios and reduce the impact of shared optical elements and common disturbances in adjacent propagation sections on the relative phase of the two arms.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A dual-optical-path measurement system based on diffuse reflection, comprising: The light source and polarization adjustment module are used to emit monochromatic linearly polarized beams and adjust the polarization direction of the beam incident on the Wollaston prism to adjust the ratio of polarization components in the two orthogonal polarization eigenaxis directions. The polarization beam splitting and dual-path propagation module is used to decompose the incident beam into a reference beam and a measurement beam with mutually orthogonal polarization directions, and propagate back and forth along the reference arm and the measurement arm respectively. A phase modulation mechanism is used to generate a relative phase change between the reference arm and the measurement arm to form a polarization interference light intensity signal that varies with the relative phase change; The optical path combining and polarization projection module is used to make the returned reference light and the diffuse reflection echo light coincide in space, and to project the two return lights with mutually orthogonal polarization directions onto the same polarization direction to form polarization interference light; by adjusting the transmission axis angle of the first polarizer and the second polarizer, the projection power ratio of the two return lights in the transmission direction of the second polarizer is adjusted, which helps to improve the contrast of polarization interference fringes. The photoelectric detection module is used to convert the polarization interference light intensity signal formed by polarization projection into a response electrical signal; The signal processing module, connected to the photoelectric detection module, is used to demodulate the phase of the response electrical signal, expand the phase of the interference phase information according to the sampling time sequence, and calculate the axial relative displacement change based on the continuous phase change after expansion.

[0008] The structural features of the dual-optical-path measurement system based on diffuse reflection proposed in this invention also lie in: The light source and polarization adjustment module includes a laser and a first polarizer; the laser is used to emit a monochromatic coherent beam, and the first polarizer is disposed in the output optical path of the laser to form linearly polarized light along the transmission axis of the first polarizer; the angle between the transmission axis of the first polarizer and the first polarization eigenaxis of the Wollaston prism. The first polarizer can be adjusted by rotating it, and the included angle can be adjusted. This is used to change the ratio of polarization components of the incident beam along the two orthogonal polarization eigenaxis directions of the Wollaston prism, and to adjust the initial power distribution between the reference arm and the measuring arm. When the laser outputs linearly polarized light with a fixed polarization direction, the actual transmitted light power of the first polarizer can vary with the included angle. As changes occur, the present invention does not require the total transmitted power to remain constant during the adjustment of the first polarizer as a necessary condition for implementing the polarization balancing adjustment.

[0009] The polarization beam splitter and dual-path propagation module includes: The beam splitter is a non-polarized beam splitter, which is set in the output light path of the first polarizer. It is used to transmit forward-incident monochromatic linearly polarized light and reflect the combined return light to the second polarizer. A Wollaston prism, acting as a polarization beam splitter, is placed in the output optical path of the beam splitter to decompose the incident beam into a first linearly polarized beam and a second linearly polarized beam that are orthogonal to each other. The first linearly polarized beam propagates along the reference arm as a reference beam, and the second linearly polarized beam propagates along the measurement arm as a measurement beam. A first lens, used to make the reference beam and the measurement beam form a parallel and spatially separated double beam, is set in the outgoing light path direction of the Wollaston prism, and the axial distance between the first lens and the Wollaston prism is maintained at the focal length of the first lens. A reference mirror, located at the end of the reference arm, is used to reflect the reference beam after it has been aligned. In a preferred embodiment, the system further includes a displacement adjustment mechanism connected to the reference mirror, used to adjust the position of the reference reflecting surface of the reference mirror along the optical axis of the reference arm, so as to adjust the relative phase between the reference arm and the measuring arm.

[0010] The second lens, located in the measuring arm, focuses the measuring light onto the irradiated rough surface of the sample under test. It collects the diffuse reflection echo generated by the irradiated rough surface within the collection angle range corresponding to the numerical aperture of the second lens. The collected diffuse reflection echo is collimated and returned along the measuring arm. The reference light returns via a reference mirror, and the returned reference light and the diffuse reflection echo spatially coincide via the first lens and the Wollaston prism. The actual numerical aperture of the second lens is determined by its focal length and effective aperture, and its specific value can be selected based on the angular distribution of the diffuse reflection echo of the sample under test, the predetermined axial measurement range, and the effective receiving range of the photodetector. The field component in the collected diffuse reflection echo that can form an effective cross-interference term with the reference light within the effective receiving area of ​​the photodetector is determined by the spatial field overlap and phase correlation of the two returning beams on the detection surface. Its coherent coupling degree can be determined by the effective spatial coherent field amplitude coupling coefficient. Characterization. The collected light component that does not form an effective cross-interference term is not included in the AC coherent modulation term, but its stable average component is included in the DC light intensity component. The effective spatial coherent field amplitude coupling coefficient... For the characterization of the interference light intensity model, this invention does not require determining its specific value as a necessary condition for implementing the measurement system and method.

[0011] The initial irradiated position of the sample under test is located within the focal plane of the second lens or its adjacent effective measurement area. The effective measurement area refers to the region where the second lens can still collect sufficient diffuse reflection echoes to form demodulate interference signals when the sample under test moves within a predetermined axial displacement range. The finite numerical aperture of the second lens is used to define the range of collection angles for the diffuse reflection echoes and is not equivalent to the selection of a single spatial mode.

[0012] The effective region of diffuse reflection, as referred to in this invention, is the region on the irradiated rough surface of the sample under test where the diffuse reflection echo, after being collected by the second lens, can form the effective cross-interference term. During the measurement process, the axial displacement of the sample under test, as well as any possible lateral drift or tilting changes, should be within a range that ensures the effective region of diffuse reflection remains within the effective collection range of the second lens, and that the interference signal output by the photodetector can be continuously demodulated.

[0013] like Figure 4 As shown, The beam-splitting angle is the angle between the two orthogonally polarized beams separated by the Wollaston prism. According to the focal length of the first lens and effective light transmission aperture The selection is made such that the axial distance between the Wollaston prism and the first lens is equal to the focal length of the first lens. To ensure that the two beams of light split by the Wollaston prism can completely pass through the first lens without edge obstruction, the center-to-center distance between the two beams at the first lens is equal to the diameter of the incident ray-polarized beam. The sum must be less than or equal to the effective aperture of the first lens. That is, it satisfies the following constraint formula: (1) In the formula, The beam-splitting angle of the Wollaston prism is... The focal length of the first lens. The effective aperture of the first lens, The diameter of the incident ray-polarized beam is given.

[0014] The phase modulation mechanism is connected to the sample under test and is used to drive the sample under test to generate a modulation displacement along the optical axis of the measuring arm, so as to change the round-trip optical path of the measuring arm, so as to generate a relative phase change between the reference arm and the measuring arm, thereby causing the photodetector to output a response electrical signal covering at least one interference period for fringe contrast calculation and phase demodulation.

[0015] The first lens, the Wollaston prism, the beam splitter, and the second polarizer together constitute the optical path combining and polarization projection module. In the optical path combining and polarization projection module: The first lens is used to propagate the returned reference light and the diffuse reflection echo light collected and collimated by the second lens to the Wollaston prism; The Wollaston prism is used to make the returning reference light spatially coincide with the diffused echo light, forming a combined return light containing two orthogonal polarization components; The beam splitter is used to guide the combined return light to the second polarizer; The second polarizer is disposed in the reflected optical path of the beam splitter to project the reference light and the diffuse reflection echo light in the combined return light onto the same polarization direction, thereby forming polarized interference light; the angle between the transmission axis of the second polarizer and the first polarization eigenaxis of the Wollaston prism. Adjustable, by adjusting the transmission axis angle of the first polarizer and the second polarizer, the projection power ratio of the returned reference light and the diffuse reflection echo light on the transmission direction of the second polarizer can be adjusted, which is beneficial to improving the contrast of polarization interference fringes.

[0016] The photoelectric detection module includes a photodetector.

[0017] In a preferred embodiment, the diffuse reflection-based dual-optical-path measurement system further includes a displacement adjustment mechanism connected to the reference mirror, used to adjust the position of the reference reflecting surface along the optical axis of the reference arm, so that the relative phase between the reference arm and the measurement arm is at the orthogonal working point of the polarization interference light; after the adjustment is completed, the position of the reference mirror remains unchanged, and the signal processing module obtains the relative displacement change based on the approximately linear relationship between the change of the response electrical signal near the orthogonal working point and the change of the axial relative displacement.

[0018] This invention also proposes a dual-optical-path measurement method based on diffuse reflection, implemented using the aforementioned dual-optical-path measurement system based on diffuse reflection, comprising the following steps: Step 1: Emit a monochromatic coherent beam, and then direct the monochromatic coherent beam through the first polarizer and the unpolarized beam splitter before it enters the Wollaston prism. Step 2: Using the Wollaston prism, the incident beam is decomposed into a reference beam propagating along the reference arm and a measurement beam propagating along the measurement arm in orthogonal polarization dimension, and the reference beam and the measurement beam are made to form a parallel and spatially separated double beam after passing through the first lens. Step 3: Focus the measurement light onto the micro-rough surface of the sample under test using the second lens to generate a diffuse reflection echo, and collect the diffuse reflection echo within the collection angle range corresponding to the numerical aperture of the second lens. Collimate the collected diffuse reflection echo back to the measurement arm; use the reference mirror to reflect the reference light propagating along the reference arm. Step 4: Using the phase modulation mechanism, drive the sample under test to generate a modulation displacement along the optical axis of the measuring arm to change the round-trip optical path of the measuring arm, so as to generate a relative phase change between the reference arm and the measuring arm; so that the returned reference light and the diffuse reflection echo light collected and collimated by the second lens are propagated through the first lens to the Wollaston prism, and the Wollaston prism spatially coincides to form a combined return light containing two orthogonal polarization components, which is then guided by the unpolarized beam splitter to the second polarizer; Step 5: Using a second polarizer, project the two orthogonally polarized beams after beam combining onto the same polarization direction to form polarized interference light. Then, use the photodetector to convert the polarized interference light into a response electrical signal; based on the effective round-trip power response coefficient of the reference arm... Effective round-trip power response coefficient of the measuring arm The ratio between them determines the initial angle combination of the first polarizer and the second polarizer; the relative phase change of the two arms covers at least one interference period, the response electrical signal is collected and the measured fringe contrast is calculated; using the measured fringe contrast as an evaluation parameter, at least one of the transmission axis angles of the first polarizer and the second polarizer is adjusted to determine the working angle combination for displacement measurement. Step 6: Under the working angle combination, the response electrical signal output by the photodetector is acquired according to the predetermined sampling sequence. The response electrical signal is processed by the signal processing module to obtain the relative displacement change along the optical axis between the reference reflecting surface of the reference mirror and the effective area of ​​diffuse reflection of the sample under test.

[0019] In a preferred embodiment, in step six, the signal processing module is used to demodulate the response electrical signal acquired according to the predetermined sampling sequence to obtain interference phase information; the interference phase information is phase-expanded according to the sampling sequence to correct the phase jump caused by phase periodicity; and the axial relative displacement change is calculated based on the continuous phase change after expansion.

[0020] The aforementioned dual-optical-path measurement method based on diffuse reflection also has the following characteristics: Step five involves adjusting the transmission axis angles of the first and second polarizers, including staged polarization balancing adjustment. The staged polarization balancing adjustment includes: Initial trimming phase: based on the effective round-trip power response coefficient of the reference arm. Effective round-trip power response coefficient of the measuring arm The ratio between them determines the polarization angle of the first polarizer. and the polarization angle of the second polarizer The theoretically symmetrical initial angle combination is determined, and the first polarizer and the second polarizer are adjusted to the theoretically symmetrical initial angle combination. Actual adjustment phase: Starting from the theoretically symmetrical initial angle combination, and using the measured fringe contrast obtained from the response electrical signal output by the photodetector as the evaluation parameter, the polarization angle is adjusted according to a preset angle step size. and At least one of the following: after each adjustment, acquire the response electrical signal within at least one interference cycle, calculate the corresponding measured fringe contrast, and determine the working angle combination for subsequent measurements based on the comparison results of the measured fringe contrast before and after adjustment.

[0021] Furthermore, during the initial balancing stage, the polarization angle of the first polarizer... and the polarization angle of the second polarizer The theoretically symmetrical initial angle combination satisfies: (2) In the formula, The effective round-trip power response coefficient is the value of the reference light as it propagates back and forth through the reference arm and returns to the incident side of the second polarizer. The effective round-trip power response coefficient is used to measure the light's return to the incident side of the second polarizer after diffuse reflection from the sample under test, collection by the second lens, and round-trip propagation through the measuring arm. The theoretical symmetrical initial angle combination is used as the starting angle combination for the actual adjustment stage. In the measured adjustment phase, the theoretically symmetrical initial angle combination is used as the initial reference angle combination, and the polarization angle is changed according to a preset angle step size. and At least one of the following: After each change, the phase modulation mechanism is controlled to make the relative phase change between the reference arm and the measuring arm cover at least one interference cycle, and the measured fringe contrast is calculated based on the response electrical signal output by the photodetector; when the measured fringe contrast under the candidate angle combination improves, the candidate angle combination is used as the new reference angle combination; otherwise, the reference angle combination before the change is restored, and the adjustment direction is changed or another polarizer is adjusted; the above process is repeated until the polarization angle is changed by the preset angle step size. and Neither of these methods could improve the measured fringe contrast, nor could they bring the measured fringe contrast to the preset threshold. The final retained reference angle combination was then determined as the working angle combination for subsequent measurements.

[0022] Compared with existing technologies, the beneficial effects of this invention are reflected in: 1. The present invention sets a second lens in the measuring arm to collect diffuse reflection echoes generated by the rough surface of the sample under test, so that diffuse reflection echoes for forming polarization interference signals can be obtained without setting a reflecting mirror on the surface of the sample under test. This helps to avoid the influence of additional reflecting mirrors and their connecting structures on the original mass distribution and dynamic characteristics of the object under test.

[0023] 2. When there is a mismatch between the return power of the diffuse reflection echo light and the reference light, the present invention adjusts the transmission axis angles of the first polarizer and the second polarizer to adjust the polarization component ratio of the incident beam in the two orthogonal polarization eigenaxis directions of the Wollaston prism and the projection ratio of the two return lights in the transmission direction of the second polarizer, thereby adjusting the projection power ratio of the two return lights, which is beneficial to improving the contrast of the polarization interference fringes.

[0024] 3. The present invention uses the theoretically symmetrical initial angle combination determined by the ratio of the effective round-trip power response coefficients of the reference arm and the measuring arm as the starting point for actual adjustment, and further determines the working angle combination based on the actual fringe contrast output by the photodetector. This allows the determination of the working angle to take into account both the ideal optical model and the actual detection conditions, which is beneficial to narrowing the actual angle adjustment range.

[0025] 4. The reference arm and measuring arm of the present invention share optical elements such as the Wollaston prism and the first lens, and are arranged adjacent to each other in part of the propagation section. For mechanical vibration, air disturbance and temperature change introduced by the shared optical elements and adjacent propagation sections, the two light paths may produce phase changes in the same or similar directions, which is beneficial to reduce the influence of the disturbance on the relative phase of the two arms. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure and axial relative displacement of a dual-optical-path measurement system based on diffuse reflection; Figure 2 This is an interference curve of the response electrical signal collected under the unbalanced control angle combination; Figure 3 This is a comparison chart of the electrical signal interference curves of the first and second stage responses; Figure 4 This is a schematic diagram of the beam splitting principle of the Wollaston prism.

[0027] In the figure: 1. Laser; 2. First polarizer; 3. Beam splitter; 4. Wollaston prism; 5. First lens; 6. Reference mirror; 7. Displacement adjustment mechanism; 8. Second lens; 9. Sample under test; 10. Phase modulation mechanism; 11. Second polarizer; 12. Photodetector; 13. Signal processing module. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] This embodiment proposes a dual-optical-path measurement scheme capable of collecting diffuse reflection echoes generated by rough surfaces, adjusting the projection power ratio of the two returning beams in the polarization analysis direction, and reducing the impact of common disturbances on shared optical elements and adjacent propagation sections. For example... Figure 1 As shown, the dual-optical-path measurement system based on diffuse reflection in this embodiment includes: a laser, a first polarizer, a beam splitter, a Wollaston prism, a first lens, a reference mirror, a displacement adjustment mechanism connected to the reference mirror, a second lens, a pyrolytic graphite sheet (with a microscopically rough surface that generates diffuse reflection) serving as the sample to be measured, a phase modulation mechanism, a second polarizer, a photodetector, and a signal processing module connected to the photodetector signal. Figure 1 middle This represents the initial axial distance between the reference reflecting surface of the reference mirror and the effective area of ​​diffuse reflection of the sample under test. + rel This indicates the axial spacing in the current state. rel This indicates the change in the current axial spacing relative to the initial axial spacing.

[0030] The system parameters are set as follows: The laser has a center wavelength of 635 nm and outputs single-mode linearly polarized light with a beam diameter d of approximately 1.1 mm. The round-trip optical path difference between the reference arm and the measuring arm is set within the coherence length range of the laser. Both the first and second polarizers are mounted on a manually rotating adjustment frame. A 50:50 unpolarized beam splitter is used. The focal length of the first lens is... The effective light-transmitting aperture is 45 mm. The effective aperture of the second lens is 25 mm, and the beam splitting angle of the Wollaston prism satisfies geometric constraint equation (1); the focal length of the second lens is 25 mm. The aperture is 15 mm, the effective light-transmitting aperture is about 6 mm, and the corresponding effective numerical aperture is about 0.20. The pyrolytic graphite sheet used as the test sample is not mirror-polished and has a micro-rough surface that can produce diffuse reflection. Its initial irradiation position is set at the focal plane of the second lens or its adjacent effective measurement area.

[0031] Within a predetermined axial measurement range, the second lens continuously collects diffuse reflection echoes used to form a demodulated interference signal. This embodiment employs a piezoelectric actuator as a phase modulation mechanism to drive the sample under test to generate a modulated displacement along the optical axis of the measuring arm, causing a relative phase change between the reference arm and the measuring arm, and enabling the photodetector to obtain a response electrical signal covering at least one interference cycle. Figure 2 and Figure 3 The response curves shown are obtained by varying the driving voltage of the phase modulation mechanism within the range of 0 V to 60 V. Within this driving range, the response electrical signal output by the photodetector covers multiple interference cycles. The driving voltage serves only as a control quantity for generating the relative phase change between the reference arm and the measuring arm, and is not used as a basis for calculating the axial relative displacement change. The piezoelectric actuator is only one implementation of the phase modulation mechanism in this embodiment. The axial relative displacement change between the effective area of ​​the diffuse reflection of the sample under test and the reference reflecting surface of the reference mirror is obtained based on the interference phase change, and is not directly determined by the driving voltage or nominal displacement of the piezoelectric actuator.

[0032] The displacement adjustment mechanism is used to adjust the position of the reference reflecting surface of the reference mirror along the optical axis of the reference arm. In this embodiment, a piezoelectric displacement stage is used. The photodetector's response band covers 635 nm. Its output is amplified and acquired by a data acquisition circuit that matches the amplitude of the response signal, and is acquired at a sampling rate of 30 kHz for fringe contrast calculation and phase demodulation. The photodetector and its amplification and acquisition parameters are set to ensure that the response electrical signal operates within a linear and unsaturated range.

[0033] The above laser parameters, lens focal length, and beam splitting angle Numerical aperture Parameters such as sample roughness and sampling rate can be adjusted according to the specific object, and the collected diffuse reflection echo should contain a field component that can form a stable interference with the reference light on the detection surface, and the output signal of the photodetector should not be saturated.

[0034] Under the condition that the axial relative displacement of the sample under test is within the predetermined axial measurement range and the interference signal corresponding to the effective area of ​​diffuse reflection can be continuously demodulated, the measurement target demodulated by the system is the relative displacement change along the optical axis between the reference reflecting surface of the reference mirror and the effective area of ​​diffuse reflection of the sample under test. .

[0035] (3) It represents the equivalent positional change of the effective area of ​​diffuse reflection of the sample under test relative to the predetermined initial state along the optical axis of the measuring arm; This represents the change in position of the reference reflecting surface of the reference mirror relative to the predetermined initial state along the optical axis of the reference arm. When performing axial relative displacement measurement using a phase demodulation method that keeps the reference mirror position constant, the position of the reference mirror remains fixed; therefore, the change in position of its reference reflecting surface relative to the predetermined initial state is... =0, therefore we have At this time, the axial relative displacement change is equal to the displacement change of the effective area of ​​the sample under test under diffuse reflection relative to its initial position along the optical axis of the measuring arm.

[0036] When the phase change between adjacent sampling times satisfies a predetermined continuity condition, the demodulated interference phase is phase-expanded according to the sampling sequence to correct for jumps caused by phase periodicity, and the axial relative displacement change is calculated based on the continuous phase change after expansion. The predetermined continuity condition is that the actual phase change between adjacent sampling times is within the range that the phase expansion algorithm can correctly identify.

[0037] To quantitatively describe the intensity tuning behavior of the two-arm echoes during polarization interference, this embodiment models the interference light intensity received by the photodetector. The total interference light intensity output by the system... With the angle of the first polarizer Angle of the second polarizer The evolutionary relationship is expressed as: (4) In the formula, The total relative phase of the two arms at the sampling time; the phase used for displacement conversion is its phase relative to a predetermined initial state. The continuous phase change of the state; The DC light intensity component is represented as follows: (5) The amplitude of the coherent modulation is expressed as: (6) In the formula, The transmission axis of the first polarizer is at the polarization angle. The actual intensity of the light emitted and incident on the Wollaston prism. When the laser outputs linearly polarized light with a fixed polarization direction and the polarization angle is adjusted by rotating the first polarizer. At that time, the actual light intensity It can vary with polarization angle change.

[0038] and These are relative to the actual light intensity. Normalized reference arm effective round-trip power response coefficient and measurement arm effective round-trip power response coefficient. Used to characterize the power response of the reference light after it propagates through the reference arm and is reflected by the reference mirror back to the incident side of the second polarizer; It is used to characterize the power response of the measurement light after it propagates through the measurement arm, is diffusely reflected by the sample under test, and is collected by the second lens before returning to the incident side of the second polarizer.

[0039] and Excluding the polarization angle of the first polarizer The resulting orthogonal polarization component distribution effect does not include the polarization angle caused by the second polarizer. The resulting polarization projection effect.

[0040] The effective spatial coherent field amplitude coupling coefficient represents the coefficient of coupling in the same spatial coherent field. , Under the given polarization angle conditions, the ratio of the actual AC coherent modulation amplitude to the theoretical AC coherent modulation amplitude under ideal, perfectly coherent conditions with completely overlapping spatial fields, and satisfying the following conditions: The coherent power weight corresponding to the effective spatial coherent field amplitude coupling coefficient is determined by... Characterization.

[0041] Under the same combination of polarization angles, This is a common factor for the DC light intensity component and the AC coherent modulation amplitude. Therefore, under ideal conditions where the equivalent DC bias voltage of the photodetector is neglected, the common factors cancel each other out in the calculation of the ideal optical fringe contrast, without changing the... , and The theoretical balancing relationship is established. The actual working angle combination is determined based on the measured fringe contrast output by the photodetector.

[0042] To establish the correspondence between the system's output light intensity and the photodetector's response electrical signal, the response voltage output by the photodetector is... Represented as: (7) In the formula, This is the comprehensive conversion factor, which includes the photodetector responsivity and signal amplification gain. The equivalent DC bias voltage is approximately constant during a single signal acquisition, including the dark output of the photodetector and the DC output component corresponding to unmodulated stray light. Time-varying electronic noise is not included in the equivalent DC bias voltage, and its impact on peak, valley, and phase extraction results can be reduced through multi-cycle acquisition, curve fitting, or averaging.

[0043] In this embodiment, unless otherwise stated, the measured fringe contrast refers to the value before deducting the equivalent DC bias voltage. The stripe contrast is calculated based on the peak and valley values ​​of the photodetector output voltage: (8) In the formula, and These represent the peak and valley values ​​of the response voltage obtained under the same polarization angle combination and the same signal acquisition conditions. The measured fringe contrast is used to compare the changes in the interference signal under different polarization adjustment states.

[0044] When the actual transmitted light power of the first polarizer varies with the polarization angle When the transmitted light power changes, it simultaneously affects both the DC optical component and the AC coherent modulation component in the response electrical signal. Furthermore, the measured fringe contrast is also affected by the equivalent DC bias voltage. Due to the influence of the ideal balancing formula, the actual working angle combination is not determined separately, but is further determined based on the measured fringe contrast comparison results output by the photodetector.

[0045] The following describes how the working angle combination is determined in this embodiment, in conjunction with the polarization balancing adjustment process.

[0046] First, the response electrical signal was acquired under an unbalanced control angle combination where the polarization angles of the first and second polarizers were both 15°. The acquired interference response electrical signal is as follows: Figure 2 As shown. The unbalanced reference angle combination deviates from the theoretical balancing curve, resulting in a mismatch between the projected power of the returned reference light and the diffuse reflection echo light in the transmission direction of the second polarizer. Under this angle combination, the maximum value of the response voltage collected by the photodetector is... Approximately 1.12663 V, minimum value The value is approximately 1.11558 V. According to equation (8), the value is calculated without deducting the equivalent DC bias voltage. Measured stripe contrast It is approximately 0.49%.

[0047] The first stage is the initial balancing stage, used to determine the initial angle combination for subsequent measured adjustments based on the ideal optical model. According to equations (5) and (6) and the definition of ideal optical fringe contrast, under ideal conditions where the equivalent DC bias voltage and random noise are negligible, the polarization angles of the first and second polarizers satisfy: (9) At this time, the electric field amplitudes of the reference light component and the diffuse reflection measurement light component in the transmission direction of the second polarizer satisfy the theoretical balancing relationship. The combination of angles that satisfy this relationship constitutes the theoretical balancing curve, rather than a single angle point.

[0048] To obtain an initial angle combination that is easy to adjust in practice, this embodiment selects a combination of angles from the theoretical balancing curve that satisfies the desired effect. The symmetrical operating point, from which we obtain: (10) This embodiment, through pre-performed polarization projection response tests, obtains data on the change of the DC component in the photodetector output response signal with the polarization angle of the second polarizer while keeping the polarization angle of the first polarizer constant. This data is then fitted using the polarization projection relationship of the two returning beams. Based on the fitting results and the fixed polarization angle of the first polarizer, the effective round-trip power response coefficient ratio between the reference arm and the measuring arm is estimated. ≈4.58, therefore the theoretically symmetrical initial angle combination is: ≈55.6°. The ratio and corresponding angle combination are only used to determine the starting position of the measured adjustment. The actual working angle combination is further determined based on the measured fringe contrast output by the photodetector. Theoretically symmetrical initial angle combination is adopted to facilitate the synchronous setting of the first and second polarizers and to maintain a relatively balanced angle adjustment range for the two polarization angles during subsequent measured adjustments.

[0049] Both the first and second polarizers are mounted on a manually rotating frame. During operation, the polarization angles of the first and second polarizers are adjusted to the theoretically symmetrical initial angle combination, i.e. ≈55.6°. Under the action of the voltage scanning signal, the response electrical signal acquired by the photodetector during the initial balancing stage is as follows: Figure 3As shown. After data processing, the measured fringe contrast without subtracting the equivalent DC bias voltage under this polarization adjustment state was calculated according to equation (8). Approximately 6.39%, of which the peak response voltage Approximately 0.30439 V, trough value It is approximately 0.26780 V.

[0050] It should be noted that the theoretically symmetrical initial angle combination is based on neglecting the equivalent DC bias voltage. The measured fringe contrast is determined under ideal conditions based on the ideal optical model and the ratio of the effective round-trip power response coefficients used. In actual testing, the dark output of the photodetector and the DC output component formed by unmodulated stray light will change the denominator of the measured fringe contrast, which may cause the measured fringe contrast to vary with the polarization angle compared to the ideal model. Therefore, the theoretically symmetrical initial angle combination is only used as the starting angle combination for actual adjustment, and the polarization angle is further adjusted based on the measured fringe contrast output by the photodetector.

[0051] The second stage is the actual measurement and adjustment stage. Using the theoretically symmetrical initial angle combination as the starting angle combination, the polarization angle is changed according to a preset angle step size. and At least one of the following. After each adjustment, the phase modulation mechanism is used to make the relative phase change between the reference arm and the measuring arm cover at least one interference cycle, the response electrical signal is collected, and the measured fringe contrast under the current angle combination is calculated according to equation (8). If the measured fringe contrast is improved after adjustment, the adjusted angle combination is retained; otherwise, the angle combination before adjustment is restored, and the adjustment direction is changed or another polarizer is adjusted. The above process is repeated until the polarization angle of the first polarizer or the polarization angle of the second polarizer cannot be improved by changing the polarization angle of the first polarizer or the second polarizer respectively with the preset angle step size, or the measured fringe contrast reaches the preset threshold, so as to determine the working angle combination for subsequent measurement. The preset angle step size can be determined according to the angular resolution of the rotating adjustment frame and the change of the measured fringe contrast.

[0052] In this embodiment, the actual adjustment was performed using (55.6°, 55.6°) as the starting angle combination. The measured response curve after adjustment is shown below. Figure 3 As shown. In this test, the maximum response voltage was... Approximately 0.30619 V, minimum value The value is approximately 0.26683 V. According to equation (8), the value of the equivalent DC bias voltage before deduction is calculated. Measured stripe contrast The actual fringe contrast ratio is approximately 6.87%. Compared to the measured fringe contrast ratio of approximately 6.39% under the theoretically symmetrical initial angle combination, the measured fringe contrast ratio in this test increased by 0.48 percentage points.

[0053] The above test results are used to illustrate that further adjusting the polarization angle based on the comparison results of the measured fringe contrast can be used to determine the working angle combination suitable for the current optical path, the sample under test, and the detection conditions. The test results do not indicate that the same contrast change amplitude is obtained under different measurement conditions, nor do they indicate that the determined working angle combination is the only optimal angle combination.

[0054] Neglecting the equivalent DC bias voltage Under ideal conditions, ignoring the effects of random noise, the ideal optical fringe contrast of the system Defined as the ratio of the AC coherent modulation amplitude to the DC light intensity component, i.e.: (11) Ideal optical fringe contrast is used to characterize the effects of polarization angle, effective round-trip power response characteristics of both arms, and effective spatial coherent field amplitude coupling coefficient on the degree of optical interference modulation.

[0055] As can be seen from equation (11), under ideal conditions where the effects of equivalent DC bias voltage and random noise are neglected, the ideal optical fringe contrast does not exceed the effective spatial coherent field amplitude coupling coefficient. When the projected power of the reference light component and the diffuse reflection measurement light component in the transmission direction of the second polarizer satisfies the theoretical balancing relationship, the ideal optical fringe contrast reaches its upper limit.

[0056] (12) The above relationship is used to illustrate the limiting effect of the effective spatial coherence coupling degree of diffuse reflection echoes on the ideal optical fringe contrast. This embodiment does not determine this. The specific numerical value is a necessary condition for implementing the measurement method.

[0057] After completing the polarization balancing adjustment and determining the working angle combination, this embodiment uses the linear response near the orthogonal working point to implement displacement readout. Specifically, the position of the reference reflecting surface of the reference mirror along the optical axis of the reference arm is adjusted by the displacement adjustment mechanism so that the relative phase between the reference arm and the measuring arm is at the working point where the absolute value of the slope of the polarization interference light fringe is the largest, i.e., the orthogonal working point; after completing the working point adjustment, the position of the reference mirror is kept unchanged. At this time, when the axial relative displacement change of the sample under test is within the preset linear measurement range near the orthogonal working point, the change in response voltage and the change in axial relative displacement approximately satisfy a linear relationship: (13) In the formula, The axial relative displacement change ΔV between the reference reflecting surface of the reference mirror and the effective area of ​​diffuse reflection of the sample under test, taking the state at which the orthogonal operating point adjustment is completed as the local initial state, is given by the following formula: PD I represents the change in response voltage relative to the operating point voltage when the relative phase between the reference arm and the measuring arm is near the orthogonal operating point. ac For the AC coherent modulation amplitude determined according to equation (6), K d To integrate the conversion coefficients, the preset linear measurement range is determined based on the allowable nonlinear error. Therefore, the change in axial relative displacement can be obtained from the change in the response electrical signal. When the change in axial relative displacement exceeds the preset linear measurement range, the position of the reference mirror can be readjusted using a displacement adjustment mechanism so that the relative phase between the reference arm and the measuring arm is back at the orthogonal working point, and the state at the time of readjustment of the orthogonal working point is used as the new local initial state; or the demodulated interference phase can be phase-expanded according to the sampling sequence to correct the jump caused by phase periodicity, and then the axial relative displacement change can be calculated according to equation (3) based on the continuous phase change after expansion; phase expansion requires that the true phase change between adjacent sampling times is within the range that the phase expansion algorithm can correctly identify. The displacement direction is determined based on the slope sign of the selected orthogonal working point and the direction of the response voltage change.

[0058] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. In practical applications, based on the measured fringe contrast output by the photodetector, the first and second polarizers can be further adjusted using smaller angle adjustment steps to determine a suitable working angle combination for the current measurement conditions. The data in this embodiment is used to illustrate the implementation of the above measurement system and method under the above test conditions. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A dual-optical-path measurement system based on diffuse reflection, characterized in that, It includes a light source and polarization adjustment module, a polarization beam splitting and dual-path propagation module, a phase modulation mechanism, a beam combining and polarization projection module, a photoelectric detection module, and a signal processing module; The light source and polarization adjustment module includes a laser and a first polarizer; the laser is used to emit a monochromatic coherent beam, and the first polarizer is disposed in the output optical path of the laser to make the transmitted light linearly polarized; the angle between the transmission axis of the first polarizer and the first polarization eigenaxis of the Wollaston prism. Adjustable to adjust the ratio of polarization components of the incident beam along the two orthogonal polarization eigenaxis directions of the Wollaston prism; The polarization beam splitting and dual-path propagation module includes a non-polarizing beam splitter, the Wollaston prism, a first lens, a reference mirror, and a second lens. The non-polarizing beam splitter is disposed in the outgoing optical path of the first polarizer, and the Wollaston prism is disposed in the transmission optical path of the non-polarizing beam splitter, for splitting the incident beam into a reference beam and a measurement beam with mutually orthogonal polarization directions. The first lens is disposed in the outgoing optical path direction of the Wollaston prism, for making the reference beam and the measurement beam form a parallel and spatially separated dual beam, with the reference beam propagating along the reference arm and the measurement beam propagating along the measurement arm. The reference mirror is disposed at the end of the reference arm and is used to reflect the reference beam; The second lens is disposed in the measuring arm and is used to focus the measuring beam onto the irradiated rough surface of the sample to be tested, and collect the diffuse reflection echo generated by the irradiated rough surface within the collection angle range corresponding to the numerical aperture of the second lens, and collimate the collected diffuse reflection echo and return it along the measuring arm. The optical path beam combining and polarization projection module includes a first lens, a Wollaston prism, a non-polarizing beam splitter, and a second polarizer. The returning reference light and the diffuse reflection echo light pass through the first lens and return to the Wollaston prism. The Wollaston prism is used to make the returning reference light and the diffuse reflection echo light coincide in space to form a beam combining return light containing two orthogonal polarization components. The non-polarizing beam splitter is used to guide the beam combining return light to the second polarizer. The second polarizer projects the reference light and diffuse reflection echo light in the combined return light onto the same polarization direction to form polarized interference light; the angle between the transmission axis of the second polarizer and the first polarization eigenaxis of the Wollaston prism. Adjustable, by adjusting the transmission axis angle of the first polarizer and the second polarizer, the projection power ratio of the returned reference light and the diffuse reflection echo light on the transmission direction of the second polarizer can be adjusted; The phase modulation mechanism is connected to the sample under test and is used to drive the sample under test to generate a modulation displacement along the optical axis of the measuring arm, so as to change the round-trip optical path of the measuring arm and generate a relative phase change between the reference arm and the measuring arm. The photoelectric detection module includes a photodetector for converting the polarization interference light into a response electrical signal; The signal processing module is connected to the photodetector and is used to demodulate the phase of the response electrical signal to obtain interference phase information. The interference phase information is then phase-expanded according to the sampling sequence of the response electrical signal. Based on the continuous phase change after expansion, the relative displacement change along the optical axis between the reference reflecting surface of the reference mirror and the effective area of ​​diffuse reflection of the sample under test is obtained. The working angle combination of the first polarizer and the second polarizer for displacement measurement is determined by using the measured fringe contrast calculated from the response electrical signal as the evaluation parameter when the relative phase change covers at least one interference period under the drive of the phase modulation mechanism.

2. The dual-optical-path measurement system based on diffuse reflection according to claim 1, characterized in that: The axial distance between the Wollaston prism and the first lens is equal to the focal length of the first lens.

3. The dual-optical-path measurement system based on diffuse reflection according to claim 2, characterized in that: The beam splitting angle of the Wollaston prism, the focal length and effective aperture of the first lens, and the diameter of the incident ray-polarized beam satisfy the following: (1) In the formula, The beam-splitting angle between the two orthogonally polarized beams split by the Wollaston prism. The focal length of the first lens. The effective aperture of the first lens, The diameter of the incident ray-polarized beam is given.

4. The dual-optical-path measurement system based on diffuse reflection according to claim 1, characterized in that: The initial irradiated position of the sample under test is located at the focal plane of the second lens.

5. The dual-optical-path measurement system based on diffuse reflection according to claim 1, characterized in that: The determination of the working angle combination starts with the initial angle combination, which is determined based on the ratio between the effective round-trip power response coefficient of the reference arm and the effective round-trip power response coefficient of the measuring arm.

6. The dual-optical-path measurement system based on diffuse reflection according to claim 5, characterized in that: The initial angle combination is a theoretically symmetrical initial angle combination that satisfies: (2) In the formula, The angle between the transmission axis of the first polarizer and the first polarization eigenaxis of the Wollaston prism. Rr is the angle between the transmission axis of the second polarizer and the first polarization intrinsic axis of the Wollaston prism; Rr is the effective round-trip power response coefficient when the reference light travels back and forth through the reference arm and returns to the incident side of the second polarizer; and Rm is the effective round-trip power response coefficient when the measurement light is diffusely reflected by the sample under test, collected by the second lens, and travels back and forth through the measurement arm before returning to the incident side of the second polarizer.

7. The dual-optical-path measurement system based on diffuse reflection according to claim 1, characterized in that: It also includes a displacement adjustment mechanism connected to the reference mirror, used to adjust the position of the reference reflecting surface along the optical axis of the reference arm, so that the relative phase between the reference arm and the measuring arm is at the orthogonal working point where the absolute value of the slope of the interference fringes of the polarized interference light is the largest; after the working point adjustment is completed, the position of the reference mirror remains unchanged, and the signal processing module is used to obtain the relative displacement change based on the approximately linear relationship between the change of the response electrical signal near the orthogonal working point and the change of the axial relative displacement.

8. The dual-optical-path measurement system based on diffuse reflection according to claim 7, characterized in that: The relative displacement change is within a preset linear measurement range near the orthogonal working point, and the preset linear measurement range is determined based on the allowable nonlinear error.

9. The dual-optical-path measurement system based on diffuse reflection according to claim 1, characterized in that: The phase modulation mechanism is a piezoelectric actuator.

10. A dual-optical-path measurement method based on diffuse reflection, employing a system including a laser, a first polarizer, a second polarizer, a non-polarizing beam splitter, a Wollaston prism, a first lens, a second lens, a reference mirror, a phase modulation mechanism, a photodetector, and a signal processing module to measure a sample under test; the phase modulation mechanism is connected to the sample under test, and the signal processing module is connected to the photodetector signal, characterized in that... Includes the following steps: Step 1: Emit a monochromatic coherent beam, and then direct the monochromatic coherent beam through the first polarizer and the unpolarized beam splitter before it enters the Wollaston prism. Step 2: Using the Wollaston prism, the incident beam is decomposed into a reference beam propagating along the reference arm and a measurement beam propagating along the measurement arm in orthogonal polarization dimension, and the reference beam and the measurement beam are made to form a parallel and spatially separated double beam after passing through the first lens. Step 3: Focus the measurement light onto the micro-rough surface of the sample under test using the second lens to generate a diffuse reflection echo, and collect the diffuse reflection echo within the collection angle range corresponding to the numerical aperture of the second lens. Collimate the collected diffuse reflection echo back to the measurement arm; use the reference mirror to reflect the reference light propagating along the reference arm. Step 4: Using the phase modulation mechanism, drive the sample under test to generate a modulation displacement along the optical axis of the measuring arm to change the round-trip optical path of the measuring arm, so as to generate a relative phase change between the reference arm and the measuring arm; so that the returned reference light and the diffuse reflection echo light collected and collimated by the second lens are propagated through the first lens to the Wollaston prism, and the Wollaston prism spatially coincides to form a combined return light containing two orthogonal polarization components, which is then guided by the unpolarized beam splitter to the second polarizer; Step 5: Using the second polarizer, project the two orthogonally polarized beams after beam combining onto the same polarization direction to form polarized interference light, and use the photodetector to convert the polarized interference light into a response electrical signal; Based on the effective round-trip power response coefficient of the reference arm Effective round-trip power response coefficient of the measuring arm The ratio between them determines the initial angle combination of the first and second polarizers; the relative phase change of the two arms covers at least one interference cycle, the response electrical signal is collected and the measured fringe contrast is calculated; Using the measured fringe contrast as an evaluation parameter, at least one of the transmission axis angles of the first polarizer and the second polarizer is adjusted to determine the working angle combination for displacement measurement. Step 6: Under the working angle combination, the response electrical signal output by the photodetector is acquired according to the predetermined sampling sequence. The response electrical signal is processed by the signal processing module to obtain the relative displacement change along the optical axis between the reference reflecting surface of the reference mirror and the effective area of ​​diffuse reflection of the sample under test.

11. The dual-optical-path measurement method based on diffuse reflection according to claim 10, characterized in that: Step six includes: using the signal processing module to perform phase demodulation on the response electrical signal acquired according to the predetermined sampling sequence to obtain interference phase information; performing phase unrolling on the interference phase information according to the sampling sequence to correct the phase jump caused by phase periodicity; and calculating the axial relative displacement change based on the continuous phase change after unrolling to obtain the relative displacement change along the optical axis between the reference reflecting surface of the reference mirror and the effective area of ​​diffuse reflection of the sample under test.

12. The dual-optical-path measurement method based on diffuse reflection according to claim 10, characterized in that: The measured fringe contrast is calculated based on the peak and valley values ​​of the response electrical signal within a complete interference cycle under the same angle combination and the same signal acquisition conditions.

13. The dual-optical-path measurement method based on diffuse reflection according to claim 10, characterized in that, Step five involves adjusting the transmission axis angles of the first and second polarizers, including staged polarization balancing adjustment. The staged polarization balancing adjustment includes: Initial trimming phase: based on the effective round-trip power response coefficient of the reference arm. Effective round-trip power response coefficient of the measuring arm The ratio between them determines the polarization angle of the transmission axis of the first polarizer relative to the first polarization eigenaxis of the Wollaston prism. and the polarization angle of the transmission axis of the second polarizer relative to the first polarization eigenaxis of the Wollaston prism. The theoretically symmetrical initial angle combination is determined, and the first polarizer and the second polarizer are adjusted to the theoretically symmetrical initial angle combination. Actual adjustment phase: Starting from the theoretically symmetrical initial angle combination, and using the measured fringe contrast obtained from the response electrical signal output by the photodetector as the evaluation parameter, the polarization angle is adjusted according to a preset angle step size. and At least one of the following: after each adjustment, acquire the response electrical signal within at least one interference cycle, calculate the corresponding measured fringe contrast, and determine the working angle combination for subsequent measurements based on the comparison results of the measured fringe contrast before and after adjustment.

14. The dual-optical-path measurement method based on diffuse reflection according to claim 13, characterized in that: In the initial balancing stage, the polarization angle of the first polarizer and the polarization angle of the second polarizer The theoretically symmetrical initial angle combination satisfies: (2) In the formula, The effective round-trip power response coefficient is the value of the reference light as it propagates back and forth through the reference arm and returns to the incident side of the second polarizer. The effective round-trip power response coefficient is used to measure the light's return to the incident side of the second polarizer after diffuse reflection from the sample under test, collection by the second lens, and round-trip propagation through the measuring arm. The theoretical symmetrical initial angle combination is used as the starting angle combination for the actual adjustment stage.

15. The dual-optical-path measurement method based on diffuse reflection according to claim 13, characterized in that, In the measured adjustment phase, the theoretically symmetrical initial angle combination is used as the initial reference angle combination, and the polarization angle is changed according to a preset angle step size. and At least one of the following: After each change, the phase modulation mechanism is controlled to make the relative phase change between the reference arm and the measuring arm cover at least one interference cycle, and the changed measured fringe contrast is calculated based on the response electrical signal output by the photodetector; when the changed measured fringe contrast increases, the changed angle combination is retained as the new reference angle combination; otherwise, the reference angle combination before the change is restored, and the adjustment direction is changed or another polarizer is adjusted; the angle change and measured fringe contrast comparison are repeated until the polarization angle is changed by the preset angle step size. and Neither of these methods could improve the measured fringe contrast, nor could they bring the measured fringe contrast to the preset threshold. The final retained reference angle combination was then determined as the working angle combination for subsequent measurements.

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