Shifted reference holographic interferometry.

Conventional holographic interferometry of vibrating objects yields reconstructed images in which the radiance of each point is proportional to the square of the zero-order Bessel function of an argument proportional to the vibration amplitude. Therefore, the vibration pattern appears as a dark outline on the bright background of the nonvibrating points. By appropriately shifting the frequency of the reference radiation used in the recording of the hologram, the radiance of each image point can be made proportional to the square of the first-order Bessel function of the above argument. Nonvibrating points are therefore black and vibrating points appear bright on a black background. A quantitative analysis of the technique is reviewed and extended and shows the shifted-reference method to be almost one order of magnitude more sensitive than the conventional method. Experimental verification of the theory is presented and some possible future applications are indicated.