SU658402A2 - Object dimensions and shape checking method - Google Patents

Description

one

The invention relates to optical measurements based on holography and spectrum-interferometry, and is intended for non-contact monitoring of the size and shape of objects, for example, mass-produced items by comparing with a reference object and obtaining differential interference contour maps of the surface topography. The invention can also be used to control the size and shape of objects and in the absence of a reference object by obtaining interference contour maps of the surface relief .;

There is a method of controlling the size and shape of objects, which consists in obtaining differential interference contour maps of the surface relief, which characterize the difference in size and shape between the control and reference objects l.

In order to increase the accuracy of the control, in the process of recording the hologram on hee, the combined images of the reference and controlled objects are focused onto the high-intensity beams, reflected from these objects, are used as reference to each other and as object to them.

The disadvantage of the known method lies in its reduced sensitivity, which is more than an order of magnitude less than theoretically possible.

The aim of the invention is to increase the sensitivity.

The goal is achieved by the fact that the reflected wave field of radiation is modulated after the first exposure according to the law5

AcicosoL

ii. ,

dr

COBfi

where OS is the angle of illumination of the object;

B is the angle of reflection of radiation from the object.

According to the invention, the compared objects are illuminated with coherent radiation, after which the radiation reflected from the compared objects is optically combined and a hologram is recorded by a single exposure (real time scale method) or double exposure (double exposure method). Further, as in the basic method, the operation of changing the optical path in the part of the holographic scheme containing the illuminating source is performed, comparing -5th objects to the hologram. This operation is performed after the first eksposidii and can be performed in various.; options: by shifting the illuminating source, by changing its wavelength or by changing the refractive index of the fluid in which the compared objects are placed.

Consider, as an example, a variant of this operation, based on the displacement of the illuminating source. As is known, information on the surface microscope of an object, if the latter is longer than the wavelength of the illuminating radiation, is transferred to the reflected radiation. The microstructure of the surface is diffusely reflective: objects are a random function of spatial coordinates, so the reflected wave field also becomes a random function of the above argument. As a result of the displacement of the illuminating source, the radiation pattern of the reflected radiation is rotated and the wave field in the plane of the hologram (or in the plane of the optical system, if the hologram of the focused image was recorded) is shifted relative to its original position from

In view of the fact that the diffusely reflected wave fields randomly depend on the spatial coordinates, the correlated contribution on the reconstructed image to the formation of difference interference contour maps of the surface relief is made only by those parts of the wave fields (existing at the first exposure time or and second exposures) that spatially overlap within the aperture of the hologram or the aperture of the optical system in the case of a hologram focusing nnogo image.

Thus, an increase in the displacement of the illuminating source, which is necessary to increase the sensitivity, leads to an increase in the decorrellation of the wave fields, which form differential interference contour relief surface maps on the reconstructed image, which determines the maximum possible sensitivity achieved in the basic prototype method.

- According to the invention, between the objects being compared and the hologram, a diaphragm is installed, i.e. an opaque screen with one or several openings, which after the first exposure is moved so as to compensate for the change in the bottom of the radiation pattern reflected from the objects being compared. If a hologram of the rfocused image is recorded, then the specified aperture is set to

between the objects being compared and the optical system focusing them, the combined images in the hologram plane. For example, let the position of the illuminating source be such that the surface of the object being monitored is illuminated at an angle d- and the wave fields diffusely reflected from the object surface under mid angle

P), that is, the surface diffracted on a microstructure, having. mean period P. As is known, 5 for the first order of diffraction, the diffraction grating formula has the form:

p (s-indL-sinp) - A, (L

where D is the wavelength. Differentiator (1), we get:

AdCOSdL Dr, (2-

Cosfb

Thus, when the illumination is rotated by an angle dL wave field in the plane of the diaphragm, located at a distance of 6 from the object, is shifted by the value of g | 32.

After the first exposure, the aperture is moved so as to compensate for the change in the radiation pattern reflected from the objects being compared, i.e., move by the amount of others. If the hologram was recorded using Fresnel or Fourier-Fraunhofer holographic schemes, the wave field is shifted not only in the aperture plane, but also in the plane of the hologram. Therefore, the wave fields forming the interference pattern in the reconstructed image will be reconstructed from different parts of the photographic plate or other recording medium used to record holograms, which can distort the interference pattern in the reconstructed image, for example, due to the different thickness of the emulsion on different parts of the photo plate. . The foregoing considerations make it preferable to prefer the holographic version of the focused image, which is also chosen in the prototype method. In this case, the interference pattern can be distorted only because of changes in aberrations in the lens of the optical system, which introduces a much smaller, and also not random, but systematic error.

After performing the described operations using the real-time method or the double-exposure method, differential interference contour maps of the surface relief are obtained, carrying information about the difference between the compared objects in size and shape.

Claims (1)

The proposed method is also applicable to methods for obtaining interference contour maps of the surface topography of the object under test B, the absence of a reference object. The specific increase in sensitivity resulting from the application of the invention depends on the quality of the optical system used to record the hologram of the focused image. Thus, the lens of the type Industar-61, used in amateur cameras, with experimental verification of the proposed method made it possible to obtain an increase in sensitivity of 5 times. Obviously, the use of higher-quality lenses will make it possible to obtain an increase in sensitivity to the maximum possible theoretically, which is determined by the mean-square height of the surface microstructure and gain in sensitivity by about an order of magnitude. 026 The invention The method of controlling the size and shape of objects according to the USSR author's certificate No. 470699, characterized in that in order to increase the sensitivity, the reflected wave radiation field is modulated after the first exposure according to the law. where cos is the angle of illumination of the object j P is the angle of reflection of radiation from the object. Sources of information taken into account during the examination 1. USSR Author's Certificate No. 470699, cl. G 01 B 9/021, 3.5.73.