Method for determining changes in spacing between two positions of interest

#Various means have been used in measuring displacement, such as methods used in determining strains in solids. One such system is described in the patent to Bell, U.S. Pat. No. 2,929,242 wherein a diffraction grating is used to measure strain. The patent to Pryor, U.S. Pat. No. 3,664,739 discloses a system for measuring the displacement of the edges of a slit with the use of a diffraction pattern. It is a well known fact of optics that monochromatic, coherent light passing through two narrow slits separated by a small distance will produce an interference pattern with the spacing of the interference fringes being proportional to the distance between the slits. In the measurement of displacement between positions on objects or relative movement between pairs of objects, it is sometimes difficult to provide slits to produce the interference pattern.

#According to this invention, a method is provided for determining the relative displacement between points on an object or the relative movement between pairs of objects. According to this invention, use is made of the fact that monochromatic, coherent light impinging on two narrow scattering strips, separated by a small distance will produce an identical interference pattern to that produced by spaced slits. Two scattering strips, such as wires or fibers, are attached to items for which the displacement is to be measured. A coherent, monochromatic light is then used to illuminate the strips. A measuring device is then provided to give an indication of changes in the fringe locations to provide an indication of displacement.

#FIG. 1 is a schematic illustration showing a system for determining the relative movement between two scatterers.

#FIG. 2 is a schematic drawing of one apparatus for providing an indication of relative movement between linear scatterers in an adhesive lap joint.

#FIG. 3 shows a standard specimen and test specimen prepared for an expansion measurement.

#Reference is now made to FIG. 1 of the drawing which shows radiation source 10, such as a laser, which illuminates two scatterers 12 and 14 which produce an interference pattern on screen 16 as shown schematically at 18. The scatterers may be wires or fibers which are attached to positions of interest such as two parts, on a single item or on separate items, which undergo some relative displacement with respect to each other. One of the parts might be fixed while the other is displaced or both parts could be displaced.

#To act as a scattering site, the strips must posses an index of refraction different from the surrounding media, for the wavelength of the radiation used, or must act as if opaque to the radiation. Coherent, monochromatic radiation passing over these strips interferes constructively and destructively to produce the fringe pattern indicated schematically at 18. The position of the intensity peaks is given by the expression.

#x(1-cos .alpha.) + y sin .alpha. = m.lambda.