Abstract: The invention provides a method, an appliance and an installation for the termination of small displacements of points on the surface of a solid body.A catadioptric covering, such as a catadioptric paint or a piece of a sheet with catadioptric properties is applied on the point of the solid body whose displacement is to be determined, and after illuminating said covering, one determines in a viewing direction different from that of the displacement the variations in the quantity of light received by a photosensitive receiver coming from an image of the said covering in the plane of which is located a knife-edge partly covered by the said image.

Abstract: Method and apparatus for monitoring the vibratory displacements of a diffusing or reflecting solid body which in a preferred application is a turbine blade. A laser source of rectilinearly polarized monochromatic light provides a laser beam which is directed at the solid body. The beam reflected off the body is directed toward a first photocell with a photomultiplier along with part of the incident laser beam. Part of the reflected beam is directed toward a second photocell with a photomultiplier. A Kerr or Pockels cell varies the intensity of the laser beam in response to the output current strength from the second photocell to maintain the lighting of the first photocell constant regardless of the presence of speckle phenomenon. Apertures are arranged in front of the photocells and sufficiently small, e.g. one tenth the diameter of the grains of materials of the solid body, so that the lighting of the first photocell may be considered constant at all points.