Low-cost electronic speckle pattern interferometry: Obtaining reliable signals

Designs for low cost electronic speckle pattern interferometry (ESPI) systems allow broad use of this technique to study the vibration or deformation of surfaces. For example, the technique allows direct imaging of the normal mode shapes of musical instruments, which can be used to assess design changes or optimize instrument fabrication. While the typical surface measurement system is quite simple in design, obtaining reliable signals is often challenging. Random phase changes due to object motion or reference signal light path can lead to decorrelations resulting in an unstable final image that washes out with averaging. Depolarization of the scattered light also diminishes the interference pattern intensity. We will be discussing the particulars of surface preparation, optical alignment, and phase control that help lead to successful ESPI image contrast and stability. We will also discuss the differences between using a stationary reference image for ESPI subtraction and a frame-by-frame difference method to obtain higher contrast interference patterns.