Method and device for interferometric measuring of shape deviation of optical surfaces

FIELD: measuring engineering.

SUBSTANCE: method comprises directing a coherent light beam at the surface to be tested, producing and recording interferogram of the light path difference, and processing the interferogram. The tested and reference surfaces are exposed to the second coherent light beam, and the second interferogram of the light path difference is created. The second interferogram is provided with the additional light path difference with respect to that of the first interferogram, which is equal to the one fourth of the beam wavelength. The light path difference of the first interferogram is determined at specific points of the surface to be tested from the signal of illumination in one of two interferograms. The device comprises source of coherent light, first filter-condenser, first and second light-splitting units, interferometer composed of tested and reference surfaces, unit for measuring optical length of the beam, first projecting unit, recording unit, observing unit, and unit for processing the interferogram. The device also has two light-splitting units between which two pairs of transparent diffraction lattices are interposed. The filter-condenser, the second light-splitting unit, and λ/4 lattice are arranged in series in the direction of the beam.

EFFECT: enhanced precision.

4 cl, 8 dwg

 

The invention relates to the control and measuring equipment, and in particular to methods and devices for measurement of form deviations polished surfaces from nominal, and can be used, for example, when controlling the shape of the optical parts.

Known interferometric method of measuring the deflection shapes of the surfaces implemented in the interferometer [1]. The method is based on determining the coordinates of the centers of interference fringes on the interferogram amplitude method, that is, minimum illumination, further calculating the shape of the wave front reflected the controlled surface and, accordingly, the deviation of the shape of this surface.

It is known device [1], intended for the measurement of form deviations of polished surfaces of nominal. Device [1] contains a source of coherent radiation (gas laser), a condenser with a small aperture, low pass parasitic rays, lens, creating a parallel beam, the interferometer, the device used is a flat Fizeau interferometer, the projection system interferogram camera and pre-configuration of the interferometer.

Also known amplitude method of measurement of form deviations of the surfaces described in the literature [2].

Device [2] to implement the content is tons of helium-neon laser, special filter that converts the laser beam into a divergent spherical wave front, splitters, lens, actually the Fizeau interferometer, the projection system interferogram camera and projection system camera autocollimating images of the iris.

Both the known method [1] and [2], as well as devices for their implementation are the amplitude and have the disadvantages of amplitude measurement technique. These disadvantages are that all the defects of an interference pattern resulting from defects of the illuminator and the optical system of the interferometer, are perceived by the registration system interferogram as deformation interferogram caused by the deviation of the shape of the surface of the inspected items from nominal.

In addition, the use of amplitude methods for determining the deformation of the wave front by the coordinates of the points of interference fringes leads to significant errors and does not provide sufficient accuracy of the calculation of the shape of the test surface.

In addition to amplitude and known phase methods of measuring the deflection shape of optical surfaces and devices implement, based on the implementation phase modulation of the interference pattern. So in a patent [3], the phase shift of the interference pattern is methodology which changes the wavelength of the radiation source, as implemented in a known way by changing the optical length of the resonator of the laser.

In one embodiment of the device [3], performs the method described in literature [3], as the radiation source used is a gas laser. Changing the optical length of the resonator of the laser is achieved by either moving one mirror, in devices with an external mirror or pulling tubes, devices with an internal mirror. In another embodiment of the known device [3], is used as the radiation source, a semiconductor laser, the change of the wavelength is achieved by changing the excitation current, which in turn changes the temperature of the laser and, therefore, its optical length.

Thus, in the known method [3] implementation phase modulation, as well as all proposed devices requires the use of special lasers that is a definite drawback, limiting the wide application of the method and implement it devices for control of optical products in their mass production.

The closest in technical essence to the proposed method is phase method for measurement of form deviations polished surfaces, described in the source [4].

Schematic diagram of the prototype [4] is shown in Fig..

The prototype method is based on the fact that the reference surface makes oscillatory motion along the optical axis. In the end, the entire interference pattern is shifted to one lane while moving the reference surface to 1/2 the wavelength of the radiation used in the interferometer source. By measuring three or more intensity values of the interference pattern in the points field, corresponding to the position of the pixel matrix, during one cycle of movement of the reference surface, it is possible to calculate the phase of the interferogram in these field points, which corresponds to the phase of the wave front.

The known device [4] contains a source of coherent radiation, located behind the filter condenser mounted in the focal plane of the lens and consisting of a condenser lens and aperture of small diameter, the beam splitting unit, the first and second beam-splitting elements, the lens and the interferometer. The interferometer consists of the controlled and reference surfaces. Both surfaces perpendicular to the optical axis. The reference surface oscillates along the optical axis of the interferometer, so the length of the beam reflected from this surface is continuously changing. The device [4] also contains a projection system, which together with the lens projecting an interference pattern on the T-cell and photodiode matrix, and projection autocollimating images intended for pre-setting of the interferometer. With photodiode matrix connected computer, in which the processing of measurement results.

However, the known method and device [4] have the disadvantage inherent phase methods and implementing their devices, namely the intensity of the interferogram at each point of the field is changing not only because of the displacement of the reference surface, but also because of the inevitable wavefront distortions caused by vibration as the reference and the controlled surfaces, fluctuation of air between the reference and the controlled surfaces and other factors. In the phase of the interferogram is determined with substantial errors.

In addition, the known device [4] can only be used for inspection of parts with a diameter up to 100 mm, so as to provide high frequency oscillatory movement of the parts with large dimensions, it is almost impossible.

When using the known method and device for controlling spherical surfaces are to be paid additional error caused by the shift of the center of curvature of the reference surface relative to the center of curvature of the test surface, which also affects the accuracy of the measurement.

The objective of the proposed image is eteni is to improve the accuracy and reliability of measurements of the deviation of the shape of optical surfaces on the phase of the interferometer due to the exclusion of the influence of vibrations on the measurement results of the wavefront distortions on phase interferometer, as well as extending the scope of application of the method and device.

To achieve this, the technical result of the proposed method interferometric measuring the deflection shape of optical surfaces and systems for its implementation.

The interferometric method of measuring the deflection shape of the optical surfaces based on the fact that controlled surface directed coherent beam focused near the center of curvature of the test surface placed in the path of beams exemplary surface with a center of curvature located near the center of curvature of the test surface, form and record the interferogram difference of the rays reflected from the controlled and the model surface, and process it to obtain the value of this difference of turn.

The method differs from the known fact that controlled and exemplary guide surface of the second coherent beam and form a second interferogram difference of the rays reflected from the controlled and the model surface, introducing a path difference of beams of the second interferogram additional difference of the rays in comparison with the difference of the first interferogram, is equal to a quarter wavelength, and the set points of the controlled surface ODA is really the difference of the first interferogram signal light in one of the two interferograms for which fair condition:

,

where I registered the signal light at a given point of the interferogram when the measured difference of the rays;

I0is known in advance direct component of illuminance at a given point of the interferogram when changes in the difference of the rays;

ΔI known in advance amplitude signal changes at a given point of the interferogram when changes in the difference of the rays

The proposed method can be implemented in the inventive system interferometric measurement of the shape of optical surfaces.

System for interferometric measuring the deflection shape of the optical surface includes a source of coherent radiation, emitting two plane-polarized beam, the beam splitting unit that separates the light source into two rays, the polarization planes which are mutually perpendicular, two pairs of transparent diffraction gratings, directing the radiation in the appropriate order, i.e. at right angles, the connecting unit, the directional radiation with different polarization direction in one direction, two filter-condenser located in the focal plane of the lens and consisting of a condenser lens in the focal plane where the aperture is small diameter, led lights-Senso the calculating unit, separating the radiation with different direction of the plane of polarization. In addition, the system includes first and second beam-splitting elements, the interferometer comprising a reference and controlled surfaces, a projection system for projecting an interferogram on a CCD sensor and associated processing system interference pattern and the projection system autocollimating images.

Feature of the proposed system is that 4 formed of a light source of the interferometer of which two are valid diaphragm condenser and two imaginary image. These 4 source when reflected radiation from the reference and the controlled surfaces form two interferentsionnye paintings, which are projected onto a single CCD. When this alignment of the interferometer is performed so that the second interference pattern was shifted in phase by π/2 with respect to the first, and the first or second diffraction grating can oscillate perpendicular to the lines of the grating to change the phases of the interfering beams. While the first and the second interference pattern is shifted on to 1 page when you change the phase difference of 2π.

The proposed system for interferometric measurement of the shape of optical surfaces that implements the claimed method allows to use it for high-precision control of the plane and spherical parts of any size, and in the presence of vibration testing part and the device itself, which has become possible due to the following.

In the proposed system intensity, which is calculated phase interferogram recorded simultaneously at all points of the picture; in the proposed system by using a beam-splitting devices are formed four measuring channel, unlike the one in the prototype. Thanks to the additional channels it was possible to obtain two interferograms shifted in phase relative to each other on π/2. In addition, the device changes the optical length of the beam situated in the prototype directly in the interferometer, installed in the proposed device in the lighting system and executed in the form of two diffraction gratings, one of which can move perpendicular to the lines of the grating. This arrangement of the device for changing the optical length of the beam, as well as its implementation allows the measurement of large parts with high accuracy. The introduction to the second projection system, the separation and coupling prism provides a projection of the two interferograms at the registration unit. Blocks splitting a laser beam into two, with different direction of polarization planes, and then mixing in each of the two capacitors filters on Volat get two identical interference pattern, dephased π/2. The slope of the reference and the controlled surfaces relative to the normal to the axis of the device is determined by the formula

α=s/4f′about,

where s is the distance between the apertures of the first and second filter-condenser;

f′about- focal distance of the lens, provides the formation of interference patterns.

Thus, the combination of the above features allows you to solve tasks.

Obtain two interferograms shifted relative to each other on π/2 and simultaneously removing the information from these interferograms allows you to get the correct results even when the displacement of the interferogram caused by random factors, such as vibration. The shift of the interference pattern due to a change in length of the rays allows to apply this method interferometers for control plane details of any size, provided the dimensions of the parts of the interferometer, because the node that creates the phase shift, is located in the lighting part of the interferometer and is not dependent on the dimensions of the testing details.

Method and implements his system is also suitable for control of spherical components, so as not disturbed alignment of parts in the scanning process.

The proposed method and system for measuring the deflection shape of the optical surface is TEI illustrated by drawings.

In Fig.6 and Fig.7 shows the diagram of the rays forming the interference pattern, explaining the inventive method.

In Fig.6 and Fig.7 shows:

And the reference surface;

B - controlled surface;

α - the angle of the reference and the test surface;

O - a plane passing through the principal point of the lens;

D and E - diaphragm filter-condenser;

D1and E1- imaginary image of the apertures of the filter condensers;

About1center of the interference pattern resulting from interference of the beams I and II;

About2accordingly rays III and IV.

In figure 2, 3, 4 shows a schematic diagram of one specific example of a complete system for interferometric measurement of the deviation of the shape of optical surfaces in accordance with the invention.

The system includes a source 1 of coherent radiation, such as laser emitter of LGN-303, installed behind the mirror 2 and the beam splitting unit containing the beam splitting element 3 which divides the laser light into two rays with mutually perpendicular planes of polarization, each of the beams passes a transparent diffraction grating 4, razlagaemoi beam so that the maximum energy to send in ±I order. Then the beams pass through the second transparent diffraction p is the brush 5 and become parallel to each other. For diffraction gratings installed svetosilinyh element unit 6, which beams with mutually perpendicular directions of polarization plane sends in one of the two filter condensers 7 and 8, consisting of a condenser lens in the focal plane where the aperture 9 and 10. For filter condenser has a second beam splitting unit 11 (figure 4 and figure 5), consisting of two beam-splitting plates 12 and 13, reflecting the rays in one direction of polarization and transmits light perpendicular to the polarization direction. Mirrors 14 change the radiation direction. Plate 15 installed between the beam-splitting plate 12 and the mirror 14, shifting the virtual image of the aperture of the condenser so that the distance between the center aperture of the condenser and its virtual image was ≈1.6 mm (the size of the diameter of the lens of the condenser). After the beam splitting unit installed vinyl λ/4, which converts the plane-polarized light in the light with circular polarization, the first 17 and second 18 of the beam-splitting elements, the lens 19 and a flat interferometer 20, made by the scheme Fizeau and consisting of reference 21 and 22 controlled surfaces.

When measuring the variation of the spherical surfaces is used nozzle that converts a planar wave front in the spherical converging in General, the required curvature of the last surface of the nozzle, which is a reference. When combining the centers of curvature of the reference and the controlled surfaces formed Fizeau interferometer, the equivalent flat interferometer. Controlled 22 and the reference surface 21 is inclined to the normal to the optical axis of the interferometer at an angle

α=s/4f′about,

where s is the distance between the apertures of the filter condensers;

f′about- focal length lenses.

The system includes a separator lens 23 mounted behind the first beam-splitting element 17 in the reverse course of the rays, the first projection system 24 that projects together with the lens first interference pattern in the intermediate image plane. The second projection system 25 projects a second interference pattern is also in an intermediate plane in which is located a coupling prism 26, which is the third projection 27 projecting an intermediate image of the interference patterns on the CCD camera 28 recording unit associated with the processing system interference pattern. The third projection system consists of two lenses 29 and 30, and the lens 29 has a variable focal length to maintain a constant image size of the interference pattern on the CCD matrix when resizing conrol what has been created details.

To maintain a constant distance between the interference paintings on the CCD sensor, the prism 26, together with the lens 30 is moved along the axis is proportional to the change in focal length. For pre-setting of the interferometer has a projection system autocollimating images 31.

System (1, 2, 3, 4) is as follows.

The laser light beam splitting unit is divided into two components with mutually perpendicular planes of polarization. Each of the beams passes through the two transparent gratings and dirigeret on them. Using ±I order diffraction get to the exit grating two beams parallel to each other. When moving one of the gratings in the direction perpendicular to the stroke direction, the phase of one beam is changed relative to another by the value of 2π when moving grating on the value of the

l=d/2,

where d is the grating pitch.

Then beams the beam splitting unit and get on the lenses filter-condenser. Through each condenser are beams having mutually perpendicular polarization planes. Of the diaphragms of the respective filter-condenser released 4 spherical wavefront, which after lens 19 is flat.

When the contact plate 12 beamsplitter block the beam with one plane field is Itachi is reflected from it, and with perpendicular passes. Mirrors change the direction of the rays and they fall on the beam splitter plate, which also reflects the rays reflected by the plate 12 and transmits the rays passing through the plate 12.

Plane-parallel plate 15 is shifted imaginary image of the diaphragms 9 and 10 on their own by 1.6 mm (the diameter of the lens of the condenser).

All four beam I, II, III and IV pass through the plate λ/4, which turns the plane-polarized light in the light with circular polarization.

Rays are semi-transparent elements 17 and 18 and out of the lens 14 in the form of a flat light wave. The reference surface 21 which is located at an angle α relative to the normal to the optical axis, reflects the incident rays and the focal plane of the lens 19 is formed the image of the diaphragm 9, which can be seen using the system 31 projecting autocollimating images. The tilt of the test surface 11 is superimposed upon the image of the diaphragm 9 with the image of the diaphragm 10, reflected from the surface 22. This autocollimating imaginary image of the diaphragms 8 and 10 will also be combined. The angle between the reference 21 and 22 controlled surfaces provides the parallel reflected rays and receiving interference patterns that lens 19 through the beam splitting element is s 17 and 18, the separator 23 and the connection 26 of the prism, a projection system 24 or 25 and projection system 27, are projected onto the CCD 28. Because the optical length of the rays I and II when the displacement of the diffraction grating 4 is changed, the interference pattern will shift to one lane when changing the stroke length of one wavelength. The displacement of the interference pattern measured maximum and minimum value of luminance for each pixel of the CCD array, which are used in subsequent calculations of the phase of the wave front. Moving one of the diffraction gratings 5 during alignment of the interferometer, making that the second interference pattern was shifted relative to the first π/2. After the offset of the interference patterns to one lane by shifting the grating 5 moving grating 5 is stopped and the measured intensity at each pixel of the CCD array for both interferograms that the calculated phase of the wave front. Knowing the phase of the wave front reflected from the test surface, it is possible to calculate all parameters of this surface, namely the radius, the standard deviation from the sphere at each point.

Literature

1. The interferometer IKD-110. Technical description and operating instructions. Part I. Yu - 30.60.025 THE 1991

2. U.S. patent No. 4201473, CL G 01 In 9/02, op.

3. EB is opaski patent No. 0144510, CL G 01 B 9/02, op. 19.06.85.

4. The interferometer. Model Mark III, technical description of the prototype.

1. The interferometric method of measuring the deflection shape of the optical surfaces, namely, that on controlled surface directed coherent beam focused near the center of curvature of the test surface placed in the path of beams exemplary surface with a center of curvature located near the center of curvature of the test surface, form and record the interferogram difference of the rays reflected from the controlled and exemplary surfaces, and process it to obtain the value of this difference stroke, characterized in that the controlled and reference guide surface of the second coherent beam and form a second interferogram difference of the rays reflected from the controlled and exemplary surfaces, introducing a path difference of beams of the second interferogram additional difference of the rays in comparison with the difference of the rays of the first interferogram, is equal to a quarter wavelength, and in specific points of the surface determine the path difference of the first interferogram signal light in one of the two interferograms for which fair condition

|I-I0|≤ΔI/√2,

where I registered the signal light at a given point of the interferogram when the measured difference of the rays;

I0is known in advance direct component of illuminance at a given point of the interferogram when changes in the difference of the rays;

ΔI known in advance the amplitude of the signal changes at a given point of the interferogram when the signal change at a given point of the interferogram when changes in the difference of the rays.

2. System for interferometric measurement of the deviation of the shape of optical surfaces containing a source of coherent radiation, the first filter condenser located in the focal plane of the lens and consisting of a condenser lens in the focal plane where the aperture is of small diameter, the first and second beam-splitting elements, interferometer, consisting of the controlled and reference surfaces, installed perpendicular to the optical axis, and a device for changing the optical length of the beam, the first projection system, which together with the lens projects a first interference pattern on the recording unit, and a monitoring device associated with the registering unit system processing the interference pattern and the projection system autocollimating images, characterized in that the system introduced two beam-splitting unit, the first beam splitting unit is a source of coherent radiation and consists of a beamsplitter and svetovalno elements, between which there are two pairs of transparent diffraction gratings, the first beam splitting unit are two filter condenser, the second beam splitting unit, separating the beams in each channel into two, having mutually perpendicular polarization planes and offset from each other so that the distance between the imaginary image of the diaphragm of the condenser is equal to the distance between the diaphragms is located behind the filter condensers, for the second beam splitting unit installed vinyl λ/4, converting linearly polarized light into light with circular polarization while the system additionally introduced separation prism installed for the first beam-splitting element in the reverse course of the rays, the second projection system projecting an image of the second interferogram in the intermediate plane, and a coupling prism installed in the intermediate image plane of the interferogram, and the third projection system projecting an image of two interference patterns on the CCD recording unit, and the reference and controlled surface interferometer is tilted to the optical axis in which terferometer 90± αwhere α=s1/4 f'aboutwhere s1- the distance between the apertures of the filter condensers, f'about- the focal length of the lens.

3. The system according to claim 2, characterized in that the second beam splitting unit, located behind the filter condensers made in the form of two beam-splitting plates coated permeable to one direction of the plane of polarization and reflecting mutually perpendicular, the two mirrors located between the two plates and two inclined plane-parallel glass plates, bias rays and provides the desired offset of the beams relative to each other.

4. The system according to claim 2, characterized in that the third projection system projecting an intermediate image plane of the interferogram on the CCD recording unit consists of two lenses, one of which has a variable focal length, and the other is connected with the coupling lens can be moved in proportion to the focal length of the first lens.



 

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The invention relates to information-measuring technique and can be used for contactless measurement of geometrical parameters of the compressor, turbine blades, molds, moulds and tooling in the manufacture of gas turbine engines (GTE), templates, membranes, machining tool, etc

The invention relates to a geodetic instrument and can be used to control the straightness of suspensions of fuel assemblies for nuclear power plants with RBMK-type reactors

The invention relates to measuring technique and can be used for precise non-contact monitoring form concave surfaces (uncoated and mirror) of the second order in laboratory and industrial conditions optical instrumentation

FIELD: railway transport; instrument technology.

SUBSTANCE: proposed wear checking system contains optical receiving projection system and converting-and-calculating unit. It includes also car position pickup and car counter whose outputs are connected to inputs to inputs of converting-ands-calculated unit. Optical receiving projection system consists of sets of stereo modules. Rigid structure of each module includes two CCD television cameras and lighting unit. Outputs of stereomodules are connected to corresponding inputs of converting-and-calculating unit. Stereomodules are rigidly installed relative to each other.

EFFECT: enlarged operating capabilities.

3 cl, 2 dwg

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