A new electronic speckle pattern interferometry technique (ESPI) has been developed and is used for non-contact examination of the head-tape interface in a revolving head configuration of a video tape recorder (VTR). Dynamic tape motion is measured by illuminating the surface of the tape with a pulsed Nd-YAG laser (wavelength of 532 nm). A television camera (NTSC standard), combined with the pulsed laser, makes it possible to measure the transient motion of the tape. The pulse repetition rate of the laser is 30 pps, such that the speckle pattern of the tape's surface can be observed and recorded at intervals of 1/30 of a second and stored in frame memories. Isothetic fringes for in-plane displacement are obtained by computing the difference between the initial and succesive images. These isothetic fringes are processed automatically to obtain the values of the displacement by using digital techniques. By this method, the transient behavior of the tape can be analyzed. An example of the results is presented to show the dynamic motion of a tape traced by a revolving head at a speed of 5.77 m/s in a VTR.
"Dynamic Motion Measurement Using Digital TV Speckle Interferometry", Proc. SPIE 0549, Image Quality: An Overview, (2 July 1985); doi: 10.1117/12.948809; https://doi.org/10.1117/12.948809