CN111536896B - A kind of laser interference surface shape detection automatic detection device and method - Google Patents

Abstract

The invention relates to a laser interference surface shape detection automatic detection device and a method, comprising the following steps: the device comprises a laser interferometer, a binocular camera, a diffraction light spot receiving camera, a CGH, a light spot receiving screen with a central light through hole, a measured mirror and a projection cross-hair detector, wherein the laser interferometer is fixedly connected with the binocular camera, the diffraction light spot receiving camera is fixedly connected with the CGH, and the measured mirror is fixedly connected with the projection cross-hair detector; the CGH is arranged in front of the focus of the standard mirror of the laser interferometer and far away from the direction of the laser interferometer; the spot receiving screen with the central light through hole can be removed and replaced at the focus of the standard mirror of the laser interferometer. Based on a binocular camera, a projection cross-hair centroid detector, a multi-stage diffraction light spot identification and positioning sensor and an interference fringe aberration decoupling laser interference surface shape detection light path multi-stage pose automatic adjustment method, the optical positioning from mm-stage mechanical positioning to sub-mum-stage optical positioning is achieved, and therefore automatic surface shape detection of the nm-stage optical element is completed.

Description

A kind of laser interference surface shape detection automatic detection device and method

technical field

The invention belongs to the field of automatic detection of surface shapes of optical elements, in particular to an automatic detection device and method for detection of laser interference surface shapes.

Background technique

Due to the extremely high precision requirements of optical component surface inspection, the inspection process often requires the participation of experienced professionals, and the degree of automation is very low. Classical high-precision optical component surface inspection technologies such as contour scanning method, Shack-Hartmann inspection method, phase deflection method, laser interference inspection method, etc. Currently, only the probe directly measures the contour of the sag of each point on the mirror surface along the preset path The scanning method can realize automatic detection within the travel range of the scanning equipment. However, the point-by-point detection of this method requires a long time, and the sampling density is limited, which cannot reflect the medium and high frequency errors of the mirror surface, and the detection accuracy is also limited by the performance of the scanning mechanism and the environmental stability and other factors.

The laser interference surface detection method is the current standard method for high-precision optical detection. The plane/spherical interferometer developed by Zygo has become the benchmark in the optical detection industry, and its detection accuracy can be better than 3nm. However, due to the small dynamic range of interferometry, various types of aspheric surfaces and free-form surfaces need to be equipped with corresponding optical compensation components for detection, such as Offner compensator, Dall compensator, CGH (Computer Generated Hologram) compensator, etc. However, the addition of optical compensation components greatly increases the complexity of the pose adjustment of each component in the laser interference optical path, posing challenges to automatic detection, and generally requires professional optical engineers to participate in the adjustment.

At present, there is no fully automatic laser interference surface shape detection automatic detection device and method.

SUMMARY OF THE INVENTION

Based on the above technical problems, the present invention provides an automatic detection device and method for laser interference surface shape detection, which is used to realize fully automatic laser interference detection.

The present invention adopts the following technical scheme to realize: the method for automatic detection of laser interference surface shape detection includes the following steps: (1) fixing the laser interferometer with the binocular camera, fixing the diffraction spot receiving camera with the CGH, and fixing the projection cross-wire detector with the CGH. The tested mirror is fixed; (2) A spot receiving screen with a central light hole is placed near the focus of the standard mirror of the laser interferometer, and the screen can exit the detection optical path along the plane perpendicular to the optical axis of the laser interferometer; (3) Adjust the CGH Four-dimensional positioning; (4) Adjust the six-dimensional pose of the CGH and the laser interferometer; (5) Adjust the four-dimensional coarse positioning of the mirror under test; (6) Finely adjust the four-dimensional pose of the tested mirror; (7) Adjust the spot Move it to the central light hole of the receiving screen; (8) Complete the six-dimensional pose adjustment between the measured mirror and the laser interferometer; (9) Analyze the interference fringes of the laser interferometer at this time, and output the measured mirror surface shape.

Further, in the step (1), the optical axis of the laser interferometer and the binocular camera are approximately parallel, and the pose calibration is performed by a standard plane with a cross-wire mark; the field of view of the diffraction light spot receiving camera can cover the center pass light. The full frame of the light spot receiving screen of the hole; use the position measuring equipment such as a three-coordinate instrument to calibrate the relative position of the projection fork wire detector and the measured mirror.

Further, adjusting the four-dimensional positioning of the CGH in the step (3) includes the following steps: placing a CGH compensator in front of the focal point of the standard mirror of the laser interferometer, that is, in a direction away from the laser interferometer; withdrawing the light spot with the central light hole to receive screen; use the binocular camera to collect the cross-wire mark on the CGH, calculate the position of the CGH relative to the laser interferometer; adjust the position of the CGH in the X/Y direction translation, the X/Y plane rotation and the Z direction distance.

Further, the six-dimensional pose adjustment of the CGH and the laser interferometer in the step (4) includes the following steps: moving the spot receiving screen with the central light hole back to the focus of the standard mirror of the laser interferometer, and the screen will A series of diffraction light spots reflected by the CGH alignment area 12 appear; the diffraction light spot receiving camera collects the light spot image and finds the diffraction light spot of the preset order through calculation and analysis, and adjusts the CGH pitch and yaw to move the light spot to the center of the receiving screen. At the hole; the reflected wavefront of the CGH alignment area 12 will enter the laser interferometer 1, and the pitch and yaw of the CGH are slightly adjusted to make the interference fringes the most sparse.

Further, in the step (5), the four-dimensional coarse positioning adjustment of the mirror under test includes the following steps: placing the mirror under test according to the design value of the apex curvature radius of the mirror under test; withdrawing the light spot receiving screen with the central light hole; using The binocular camera collects the projected scribe mark near the measured mirror, and calculates the position of the measured mirror relative to the laser interferometer, including X/Y translation, X/Y in-plane rotation, and Z distance.

Further, the fine adjustment of the four-dimensional pose of the mirror under test in the step (6) includes the following steps: judging the deviation of the mirror under test relative to the ideal position according to the three groups of projection fork wires: if the three groups of fork wires are all far away from the mirror body , and the fork wire is slightly larger than the design reference pattern, the mirror under test needs to move along the Z axis of the laser interferometer; if the three groups of fork wires are all close to the mirror body, and the fork wire is slightly smaller than the design reference pattern, the measured The mirror needs to move away from the laser interferometer along the Z axis; the relative positions of the three sets of fork wires are close to the design value, but there is a relative rotation or translation relationship with the mirror under test, then adjust the X and Y directions of the mirror under test. Rotate and pan to preset positions.

Further, the step (7) of adjusting the light spot to move the light spot to the central light hole of the receiving screen includes the following steps: moving the light spot receiving screen with the central light hole back to the focal point of the standard mirror of the laser interferometer, and the screen There will be a series of diffraction spots transmitted by the light reflected by the mirror under test and transmitted through the CGH wavefront compensation area 14; the diffraction spot receiving camera collects the spot image and finds the diffraction spot of the preset order through calculation and analysis, and adjusts the pitch and yaw of the mirror under test. .

Further, the judging criteria for completing the six-dimensional pose adjustment of the mirror under test and the laser interferometer in the step (8) are: the reflected wavefront of the mirror under test enters the laser interferometer, and the pitch and torsion of the mirror under test are slightly adjusted to interfere with each other. Stripes are the most sparse.

In addition, the present invention also provides an automatic detection device for laser interference surface shape detection, including: a laser interferometer, a binocular camera, a diffraction spot receiving camera, a CGH, a spot receiving screen with a central aperture, a mirror to be measured, and a projection Fork wire detector 2, in which the laser interferometer is fixedly connected to the binocular camera, the diffraction spot receiving camera is fixedly connected to the CGH, the measured mirror is fixedly connected to the projection fork wire detector; the CGH is placed in front of the focal point of the standard mirror of the laser interferometer away from the laser interferometer; the spot receiving screen with the central aperture can be removed and replaced at the focal point of the standard mirror of the laser interferometer.

Further, the binocular camera, whose optical axis is approximately parallel to the optical axis of the laser interferometer, is used to complete the interferometer standard mirror and the CGH reference plane, X/Y translation with the measured mirror, rotation in the X/Y plane, Z Coarse positioning of the distance in the direction; the projection cross-wire detector is located near the mirror under test, and is used to complete the X/Y translation, rotation in the X/Y plane, and precise positioning of the distance in the Z direction; the diffraction spot receiving camera is located near the CHG It can collect the diffracted light spot image on the receiving screen with the central light hole, which is used to complete the adjustment of the X/Y plane pitch and yaw of the tested mirror until the light spot returns to the field of view of the interferometer; the pose analyzed according to the interference fringes Fine adjustment, complete the fine adjustment of the position and orientation of the mirror under test, until the interference pattern is close to zero fringes.

The technical scheme of the present invention has the following beneficial effects:

The invention proposes a multi-level position and attitude automatic adjustment method of laser interference surface shape detection optical path based on binocular camera, projection cross-wire centroid detector, multi-level diffraction spot identification and positioning sensor, and interference fringe aberration decoupling. Positioning to sub-μm-level optical positioning, so as to complete the automatic detection of nm-level optical element surface shape. On the one hand, this method helps to reduce the manual participation in the inspection process of optical components in the laboratory/factory, improve the inspection efficiency, and improve the quality control ability of the production line; Scientific exploration such as optical manufacturing/maintenance in the working environment reduces the need for astronauts' professional background in optics and improves the technical feasibility of on-orbit applications.

Description of drawings

Figure 1 is a schematic diagram of the distribution of sensors for automatic position and orientation adjustment of the optical path of the optical element surface shape detection;

Fig. 2 is a schematic diagram of a CGH pose adjustment reference mark;

3 is a schematic diagram of the shape and position distribution of the multi-order diffraction spot;

Figure 4(a) is a schematic diagram of the positioning reference projection of the mirror under test;

Figure 4(b) is a schematic diagram of the measured mirror positioning reference projection mismatch

specific embodiment

The automatic detection method of laser interference surface shape includes the following steps: (1) fixing the laser interferometer with the binocular camera, fixing the diffraction spot receiving camera with the CGH, and fixing the projection cross-wire detector with the measured mirror; A light spot receiving screen with a central light hole is placed near the focus of the standard mirror of the laser interferometer, which can exit the detection optical path along the plane of the optical axis of the vertical laser interferometer; (3) Adjust the four-dimensional positioning of the CGH; (4) Adjust the CGH and the laser Six-dimensional pose of the interferometer; (5) Coarse four-dimensional positioning adjustment for the mirror under test; (6) Fine-tune four-dimensional pose for the mirror under test; (7) Adjust the light spot to move to the center of the receiving screen to pass light (8) Complete the six-dimensional pose adjustment of the mirror under test and the laser interferometer; (9) Analyze the interference fringes of the laser interferometer at this time, and output the surface shape of the mirror under test.

The specific basic workflow of the present invention is as follows:

(1) The laser interferometer 4 is fixedly connected with the binocular camera 1, and the optical axes of the two are approximately parallel, and a standard plane with a cross-wire mark is used to calibrate the pose of the two;

(2) The diffraction spot receiving camera 3 is fixedly connected to the CGH, and the camera's field of view can cover the entire frame of the spot receiving screen with the central aperture;

(3) The projection fork wire detector 2 is fixedly connected with the mirror under test, and the relative position of the two is calibrated using position measuring equipment such as a three-coordinate instrument;

(4) A spot receiving screen with a central light hole is placed near the focal point of the standard mirror of the laser interferometer, and the screen can exit the detection optical path along the plane of the optical axis perpendicular to the laser interferometer;

(5) Place the CGH compensator in front of the focal point of the standard mirror of the laser interferometer (away from the direction of the laser interferometer), withdraw the spot receiving screen with the central aperture, and use the binocular camera to collect the cross-wire mark 11 ( For example, four crosshairs of different sizes and shapes shown in Figure 2), calculate the position of CGH relative to the laser interferometer, including X/Y translation, X/Y in-plane rotation, Z distance, and complete the four-dimensional positioning of CGH Adjustment;

(6) Move the spot receiving screen with the central light hole back to the focal point of the standard mirror of the laser interferometer, a series of diffraction spots reflected by the CGH alignment area 12 will appear on the screen; the diffraction spot receiving camera 3 collects the spot image And through calculation and analysis, find the diffraction spot of the preset order (for example, the circular spot with the highest energy concentration on the far right in Figure 3), adjust the pitch and yaw of the CGH to move the spot to the center light hole of the receiving screen; this At this time, the reflected wavefront of the CGH alignment area 12 will enter the laser interferometer 1, and the pitch and yaw of the CGH are slightly adjusted to make the interference fringes the most sparse; so far, the six-dimensional pose adjustment of the CGH and the laser interferometer is completed;

(7) Place the mirror under test according to the design value of the apex curvature radius of the mirror under test, withdraw the spot receiving screen with the central light hole, and use the binocular camera to collect the projected cross-hatched marks located near the mirror under test, and calculate the measured The position of the mirror relative to the laser interferometer, including X/Y translation, X/Y in-plane rotation, and Z distance; since the measured mirror is generally farther from the laser interferometer than the CGH, the calculation accuracy is relatively low1-2 orders of magnitude, at this time only the four-dimensional coarse positioning adjustment of the mirror under test has been completed;

(8) After the rough positioning adjustment, the projection fork wire mark has entered the projection fork wire detector 2 fixedly connected with the mirror under test. Taking Figure 4 as an example, the three groups (the shape of the visible mirror body change the shape of the projection fork wire, The number and position) of the projected fork wires can reflect the deviation of the mirror under test relative to the ideal position: if the three groups of fork wires are far away from the mirror body, and the fork wires are slightly larger than the design reference pattern, the mirror under test needs to be along the Z axis. The laser interferometer moves; if the three groups of fork wires are all close to the mirror body (even some of them enter the mirror area to form a fork wire pattern that is partially missing), and the fork wire is slightly smaller than the design reference pattern, the mirror under test needs to be along the Z axis. Move away from the laser interferometer; the relative positions of the three sets of fork wires are close to the design value, but there is a relative rotation or translation relationship with the mirror under test, then adjust the X, Y direction rotation and translation of the mirror under test to the preset value position, at this time the mirror under test has completed the fine adjustment of the four-dimensional pose;

(9) Move the spot receiving screen with the central light hole back to the focal point of the standard mirror of the laser interferometer, on the screen there will be a series of diffraction spots transmitted by the light reflected by the measured mirror through the CGH wavefront compensation area 14; diffraction; The spot receiving camera 3 collects the spot image and finds the diffraction spot of the preset order through calculation and analysis (similar to Fig. 3, the shape and energy distribution are more complicated), adjust the pitch and yaw of the measured mirror to move the spot to the center of the receiving screen at the aperture;

(10) At this time, the reflected wavefront of the measured mirror will enter the laser interferometer 1, and the pitch and yaw of the measured mirror will be slightly adjusted to make the interference fringes the most sparse; so far, the six-dimensional pose adjustment of the measured mirror and the laser interferometer is completed;

(11) Analyze according to the interference fringes of the laser interferometer at this time, and output the mirror surface shape under test.

In addition, the present invention also provides an automatic detection device for laser interference surface shape detection, including: a laser interferometer, a binocular camera, a diffraction spot receiving camera, a CGH, a spot receiving screen with a central aperture, a mirror to be measured, and a projection Fork wire detector 2, in which the laser interferometer is fixedly connected to the binocular camera, the diffraction spot receiving camera is fixedly connected to the CGH, and the measured mirror is fixedly connected to the projection fork wire detector; the CGH compensator is placed on the laser interferometer standard mirror Away from the laser interferometer in front of the focal point; the spot receiving screen with the central aperture can be removed and replaced at the focal point of the standard mirror of the laser interferometer.

The present invention realizes the automatic adjustment of the position and attitude of the interference detection optical path through the joint adjustment of the multi-level position and attitude, and the position and attitude adjustment sensor includes:

1) Binocular camera that is approximately parallel to the optical axis of the laser interferometer: completes the rough positioning of the interferometer standard mirror and the CGH reference plane, X/Y translation with the measured mirror, rotation in the X/Y plane, and Z distance;

2) Projection fork wire detector located near the mirror under test: complete the precise positioning of the X/Y direction translation, X/Y plane rotation, and Z direction distance of the measured mirror;

3) Diffraction spot receiving camera located near the CGH (collecting the diffraction spot image on the receiving screen with the central aperture): complete the adjustment of the X/Y plane pitch and yaw of the CHG and the mirror under test, until the spot returns to the view of the interferometer. in the field;

4) Fine-tune the pose according to the analysis of the interference fringes: complete the fine-tune of the pose of the mirror under test until the interference pattern is close to zero fringes.

The above only describes the preferred specific embodiments of the present invention in detail. It should be understood that those skilled in the art can make many modifications and changes according to the concept of the present invention without creative work. On the basis of the existing technology, the concept can be obtained through logical analysis, reasoning or limited experiments, and the equivalent structure or equivalent process transformation made by the description and accompanying drawings of the present invention is used, or it is directly or indirectly used in other All relevant technical fields should fall within the protection scope determined by the claims of this application.

Claims (10)

1. A laser interference surface shape detection automatic detection method is characterized in that: the method comprises the following steps: (1) the laser interferometer is approximately parallel to the optical axis of the binocular camera, the view field of the diffraction light spot receiving camera can cover all the frames of the light spot receiving screen with the central light through hole, and a calculation holographic compensator is arranged in front of the focus of the standard mirror of the laser interferometer, namely in the direction far away from the laser interferometer; (2) a facula receiving screen with a central light through hole is arranged near the focus of the standard mirror of the laser interferometer, and the screen can exit from a detection optical path along the plane vertical to the optical axis of the laser interferometer; (3) adjusting the four-dimensional positioning of the computational holographic compensator; (4) adjusting and calculating six-dimensional poses of the holographic compensator and the laser interferometer; (5) carrying out four-dimensional coarse positioning adjustment on the measured lens; (6) carrying out four-dimensional pose fine adjustment on the measured mirror; (7) adjusting the light spot to move to the central light through hole of the receiving screen; (8) completing the six-dimensional pose adjustment of the measured mirror and the laser interferometer; (9) analyzing according to the interference fringes of the laser interferometer at the moment, and outputting the surface shape of the measured mirror;

the laser interferometer, the facula receiving screen, the holographic compensator and the measured mirror are sequentially arranged along the optical axis of the laser interferometer;

the light spot receiving screen is positioned at the focus of the laser interferometer; the distance from the measured mirror to the light spot receiving screen is the focal length of the measured mirror;

The calculation holographic compensator is positioned between the measured mirror and the light spot receiving screen; the binocular camera is arranged on the laser interferometer, and the view field of the binocular camera covers the measured mirror; the diffraction light spot receiving camera is arranged on the calculation holographic compensator, and the view field of the diffraction light spot receiving camera covers all the frames of the light spot receiving screen with the central light through hole; the projection cross wire detector is arranged on the side end surface of the measured mirror.

2. The automatic detection method for laser interference surface shape detection according to claim 1, characterized in that: the laser interferometer and the binocular camera in the step (1) are subjected to pose calibration by a standard plane with cross hair identification; and calibrating the relative position of the projection cross wire detector and the measured mirror by using position measuring equipment comprising a three-coordinate instrument.

3. The automatic detection method for laser interference surface shape detection according to claim 1, characterized in that: the step (3) of adjusting the four-dimensional positioning of the computed holographic compensator comprises the following steps: withdrawing the light spot receiving screen with the central light through hole; utilizing a binocular camera to collect and calculate a cross hair mark engraved on the holographic compensator, and calculating the position of the holographic compensator relative to the laser interferometer; and adjusting and calculating the position of the holographic compensator in X/Y direction translation, X/Y plane rotation and Z direction distance.

4. The laser interference surface shape detection automatic detection method according to claim 1, characterized in that: the step (4) of calculating the six-dimensional pose adjustment of the holographic compensator and the laser interferometer comprises the following steps: moving a light spot receiving screen with a central light through hole back to the focus of the standard mirror of the laser interferometer, wherein a series of diffraction light spots reflected by an alignment area (12) of the computer holographic compensator appear on the screen; collecting a light spot image by a diffraction light spot receiving camera, finding out diffraction light spots of a preset order through calculation and analysis, and adjusting the pitching and twisting of the calculation holographic compensator to enable the light spots to move to a central light through hole of the receiving screen; the reflected wave front of the alignment area (12) of the calculation holographic compensator enters a laser interferometer, and the pitching and the torsion of the calculation holographic compensator are adjusted in a micro-scale mode so that interference fringes are sparsest.

5. The automatic detection method for laser interference surface shape detection according to claim 1, characterized in that: the four-dimensional coarse positioning adjustment of the measured mirror in the step (5) comprises the following steps: placing the measured mirror according to the design value of the vertex curvature radius of the measured mirror; withdrawing the light spot receiving screen with the central light through hole; and acquiring a projection cross wire mark positioned near the measured mirror by using a binocular camera, and calculating the position of the measured mirror relative to the laser interferometer, wherein the position comprises X/Y direction translation, X/Y plane rotation and Z direction distance.

6. The laser interference surface shape detection automatic detection method according to claim 1, characterized in that: the fine adjustment of the four-dimensional pose of the measured mirror in the step (6) comprises the following steps: and (3) judging the deviation condition of the measured mirror relative to the ideal position according to the three groups of projection cross wires: if the three groups of fork wires are far away from the mirror body and the fork wires are slightly larger than the design reference pattern, the measured mirror needs to move along the Z-axis laser interferometer; if the three groups of fork wires are close to the mirror body and the fork wires are slightly smaller than the design reference pattern, the measured mirror needs to move along the Z-axis direction away from the laser interferometer direction; the relative positions of the three sets of cross hairs are close to the design value, but have relative rotation or translation relation with the measured mirror, and then the measured mirror X, Y is adjusted to rotate and translate to the preset position.

7. The automatic detection method for laser interference surface shape detection according to claim 1, characterized in that: the step (7) of adjusting the light spot to move to the central light through hole of the receiving screen comprises the following steps: moving the light spot receiving screen with the central light through hole back to the focus of the standard mirror of the laser interferometer, wherein a series of diffraction light spots transmitted by reflected light of the measured mirror through the wave front compensation area (14) of the computer holographic compensator appear on the screen; the diffraction light spot receiving camera collects light spot images, finds out diffraction light spots of a preset order through calculation and analysis, and adjusts the pitching and twisting of the measured mirror.

8. The automatic detection method for laser interference surface shape detection according to claim 1, characterized in that: the judgment standard for completing the six-dimensional pose adjustment of the measured mirror and the laser interferometer in the step (8) is as follows: before the reflected wave of the measured mirror enters the laser interferometer, the pitching and the torsion of the measured mirror are adjusted slightly to make the interference fringes sparsest.

9. The utility model provides a laser interference profile of face detects automatic checkout device which characterized in that: the automatic detection device includes: the system comprises a laser interferometer, a binocular camera, a diffraction light spot receiving camera, a calculation holographic compensator, a light spot receiving screen with a central light through hole, a measured mirror and a projection cross-hair detector, wherein the laser interferometer is approximately parallel to the optical axis of the binocular camera, and the view field of the diffraction light spot receiving camera can cover all frames of the light spot receiving screen with the central light through hole; the calculation holographic compensator is arranged in front of the focus of the standard mirror of the laser interferometer and far away from the direction of the laser interferometer; the light spot receiving screen with the central light through hole can be removed and replaced at the focus of the standard mirror of the laser interferometer;

the laser interferometer, the facula receiving screen, the calculation holographic compensator and the measured mirror are sequentially arranged along the optical axis of the laser interferometer;

The light spot receiving screen is positioned at the focus of the laser interferometer; the distance from the measured mirror to the light spot receiving screen is the focal length of the measured mirror; the calculation holographic compensator is positioned between the measured mirror and the light spot receiving screen; the binocular camera is arranged on the laser interferometer, and the view field of the binocular camera covers the measured mirror; the diffraction light spot receiving camera is arranged on the calculation holographic compensator; the projection cross wire detector is arranged on the side end surface of the measured mirror.

10. The automatic detection device for detecting the laser interference surface shape according to claim 9, wherein: the optical axis of the binocular camera and the laser interferometer are used for completing coarse positioning of the interferometer standard mirror, the calculation holographic compensator reference plane, the measured mirror in X/Y direction translation, X/Y plane rotation and Z direction distance; the projection cross wire detector is positioned near the measured mirror and is used for finishing X/Y direction translation, X/Y plane rotation and Z direction distance fine positioning of the measured mirror; the diffraction light spot receiving camera is positioned near the computer holographic compensator and can collect diffraction light spot images on a receiving screen with a central light through hole, and the diffraction light spot receiving camera is used for completing the adjustment of the pitching and the torsion of the X/Y plane of the measured mirror until the light spots return to the field of view of the interferometer; and finishing the fine adjustment of the pose of the measured mirror according to the fine adjustment of the pose analyzed by the interference fringes until the interference pattern is close to zero fringes.

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