# Mode of hologram interferometering of a flat object

FIELD: hologram interferometering of a flat object.

SUBSTANCE: the mode of hologram interferometering of a flat object is in making of a double exposure hologram of the surface of the object in counter rays. Then two images of the surface of the object reconditioned by the hologram are directed in an optical system in which one after another two direct Fournier-Frenel transform are carried out, interferograms of the images of the surface of the object are fixed. The object and the interferogram are located at such distances from the lenses that the in-focus image of the surface of the object in the surface of the interferogram is made and at the expense of variations of these distances and the focal distances of the lenses, measured sizes are divided and wishful sensitivities of measurements of travel and inclination constant along the whole surface are chosen.

EFFECT: ensuring constant surface and variable sensitivity, increased range of measuring of travel and inclination.

1 dwg

The present invention relates to experimental solid mechanics and can be used in mechanical engineering for non-contact optical detection of regions of high gradients of deformation and measurement of strain state of the flat surfaces of the parts of critical structures.

There is a method of holographic interferometry flat object (West Including Holographic interferometry. - M., 1982) to measure the microscopic movements of the elements of a deformable surface, at which the coherent radiation passes through and illuminates the photographic plate along the normal of the investigated surface. Scattered by the surface radiation falls on the photographic plate and a hologram. The exposition is repeated after deformation of the object. The recovered hologram dupexplorer the photographed image of the collimated rays and creates a focused interferogram. The local distance between the interference fringes depends on the tilt of an element of a deformable surface. To measure the displacement of the element it is necessary to illuminate the object and/or to photograph an image at an angle to the perpendicular.

However, this method does not significantly vary the sensitivity of the measurement of tilt and shift due to the lack of d is quite the number of free parameters, therefore, the measurement range is narrow.

In addition there is a method of holographic interferometry flat object using integrated optical conversion (J.T. Sheridan, R. Patten Holographic interferometry and the fractional Fourier transformation // Optics Letters. 2000. V.25, No. 7. R-450) and which is the prototype of the present invention. Allocated object along the x axis, illuminated flat coherent wave. Diffuse radiation g(x) is subjected to fractional Fourier transform. Studied as a separate area of the surface and at a distance of s=f(1-cosϕ) from sets collecting lens with a focal length f, where ϕ - parameter Fourier transform. Symmetric to the lens plane and positioned at a distance s on the other side of the lens occurs Fourier transform u=F_{u} ^{(ϕ)}{g(x)} is the fractional conversion for the initial state of the object. The photographic plate is located in the plane of u and holographic method captures the result of the conversion. Then on the photographic plate records the result of the transformation of waves from the deformed object e^{ikx}g(x+At), whereIn θ - move and the angle of the investigated area. If the conversion option isthen the Fourier images of the original and the deformed object are offset, and the phase shift between n the mi does not depend on the argument u.
The recovered hologram images of the object are next to the traditional Fourier transform. The distance between the fringes resulting interference pattern depends on. Two equations are defined In and θ. The application of this method to areas with different values In and θ gives a General picture of the deformed state of the object.

However, this method contains in addition to the focal length f one parameter ϕ, which is used for separating the contributions of displacement and tilt, which can significantly change the sensitivity of the measurement of each of the deformation parameters. The study inhomogeneously deformed surface is the sum of the measurements of many uniformly deformed plots with different values for parameter ϕ and with different sensitivity.

The task of the invention is to develop a method of holographic interferometry, providing a permanent surface and variable sensitivity and extended range of measurement of displacement and tilt.

This object is achieved in that in the method of integral optical transformation of wave fields sequentially perform two Fourier-Fresnel-based optical system of the two collecting lenses, and the object is the interferograms have at such distances from the lens system, that creates a focused image of the sample surface and due to the variation of these distances and focal lengths of lenses divide the measured value and select the desired sensitivity of the measurement of displacement and tilt, constant over the entire surface.

The circuit that implements the proposed method of holographic interferometry flat object, shown in the drawing, where: 1 - surface, 2 - hologram, 3 - flat coherent wave, 4 - translucent mirror, 5 and 6 - collecting lens 7 to the image plane of the first lens, 8 - interferogram. The method is as follows: first created in opposite rays dvuhknopochnaya hologram of the sample surface, the image is then restored the hologram is subjected to Fourier transform-Fresnel and creates a set of interferograms next interferogram is analyzed and defined the field of deformation. When creating holograms flat coherent wave 3 is reflected from the semitransparent mirror 4, crosses the plate 2 and illuminates the object 1. The scattered object wave g(x) crosses the plate 2. The exposition is repeated after deformation of the object emitting now wavewhere is the displacement of the investigated area, γ=29, θ - the angle of slope of the terrain. To obtain the interferogram is bject removed,
dvuhknopochnaya hologram is installed in its original position and restores both images the sample surface. Lens 5 with a focal length of f_{1}and the lens 6 with a focal length of f_{2}perform two consecutive Fourier-Fresnel. In the plane 8 wave g_{1}(u) andcoming from a diffuse scattering surface, coherent, if shift_{1}does not exceed the size of individual speckle. Conditionwhere d is the diameter of the aperture diaphragm of the lens. In the plane 8 is formed a focused image of the sample surface and its background system of interference fringes. The distance between the stripes depends on quantities and γand from the parameters of the optical system: f_{1}f_{2}, s_{1}, s_{2}. Changing one or more parameters, a second interference pattern. Using the remaining three free parameters are chosen, the required sensitivity of measurements and γ. Since the sensitivity is the same for the entire surface, the concentration of interference fringes detected region of high deformation gradients. Using theoretical ratio, the interferogram is determined by the field deformations In and γ.

theoretical ratio of the core is designed to transform Fourier-Fresnel implemented by the collecting lens

where u is the coordinate in the plane of the image, q^{2}=λf(sinϕ+δctgϕ), 0≤ϕ≤π. The conversion settings ϕ and δ are determined by the relationswhere f is the focal length of the lens, s and s’ is the distance between the object lens and the lens image. Conversion functionswith shifted argument and linear phase multiplier is

where

For focused valid imageWith a relatively small movement of the element in the plane of the objectfind the transformed function.

For a system of two collecting lenses with a small movement of the element in the object plane wavecoming from the deformed object, gets in the planes 7 and 8 are views respectivelyand. Using (1), we find the

Assuming s_{1}=αf_{1}, s_{2}’=(1+ε)f_{2}where options α, ε>0, and ele is they condition of focus images in the planes 7 and 8,
get thethen from (2) follows

Interference of waves g_{2}(x’) andcreates in-plane 8, the intensity distribution

where I_{0}(x’)=|g_{2}(x’)|^{2}. The distance between the interference fringes is equal towhere sensitivity follow from (3)

Whenget k_{B}=0, then the distance (Δx’)_{1}between the interference fringes is determined by the varying sensitivity of the tilt angle

where α=s_{1}/f_{1}and ε=(s_{2}’-f_{2})/f_{2}determined by the parameters s_{1}, s_{2}’shown in the drawing, and the focal distances of the lenses. Setting drawing follow from formulasandWhen a→1 we obtain from (4) small kγthen the distance (Δx’)_{2}between the interference fringes is determined by the moving

with variable sensitivity.

Thus, the advantages of the proposed method of measurement compared to the prototype status is:

in the measurement of the deformation parameters constant over the surface and variable sensitivity,

in extension of the measurement range of the displacement and tilt due to the fact that the corresponding sensitivity depend on variable parameters s_{1}, s_{2}’, f_{1}f_{2}.

The method is applicable for small displacements of the object elements and strong irised lens.

The method of holographic interferometry flat object at which to measure the displacement and the angle of the surface of the object to create in opposite rays dwuhektarowy hologram of the object surface, then the two images of the object surface, the recovered hologram, is directed to an optical system in which sequentially perform two Fourier-Fresnel, and the optical system is made on the basis of two collecting lenses, fix interferogram images of the surface of the object, and the object and the interferograms have at such distances from the lenses that create a focused image of the object surface in the plane of the interferogram and due to the variation of these distances and focal lengths of lenses, divide the measured value and select the desired sensitivity of the measurement of displacement and tilt, constant over the entire surface.

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