Handheld ESPI-speckle interferometer

A "handheld" speckle interferometer for measuring out-of-plane displacements on reflective as well as diffusely scattering object surfaces is presented. The interferometer is a nearly path length compensated set-up which uses diffuse illumination of the object in combination with a speckled reference. The reference wave is established by reflecting a part of the diffuse object illumination from a glass plate located just in front of the object. The glass plate is mounted on a piezoelectric translator in order to control the phase of the reference wave when using phase stepping algorithms. The coherent light source is a laser diode. A web camera with a Universal Serial Bus (USB) interface is employed as the image-capturing device. Likewise, is the piezoelectric translator controlled through the USB interface. The necessary size of the optical set-up depends on the size of the object. The interferometer presented here is a compact version of the set-up, Which is capable of measuring displacements of small objects, having either a specularly reflecting-or a diffusely scattering surface. The small optical set-up together with the use of the popular USB-communication for acquiring the images and controlling the phase of the reference wave constitutes a compact "handheld" instrument and eliminates the need for installing extra hardware, such as frame grabber and Digital to Analog converter, in the host computer.