Electronic speckle pattern interferometry

Electronic speckle pattern interferometry (ESPI),[1] also known as TV holography, is a technique that uses laser light, together with video detection, recording and processing, to visualise static and dynamic displacements of components with optically rough surfaces.

ESPI can be used for stress and strain measurement, vibration mode analysis and nondestructive testing.

[2] ESPI is similar to holographic interferometry in many ways, but there are also significant differences[3] between the two techniques.

The component under investigation must have an optically rough surface so that when it is illuminated by an expanded laser beam, the image formed is a subjective speckle pattern.

To visualize this effect, the image and reference beams are combined on a video camera and recorded.

The resulting image is a speckle pattern with black 'fringes' representing contours of constant 2nπ.

The plate has been rotated about a vertical axis and the fringes represent contours of constant displacement.

That means a rigid body motion of a half wavelength (0.3μm) towards the camera does not change the fringe pattern.

ESPI fringes – flat plate rotated about a vertical axis – the fringes represents displacement in the viewing direction; the difference in displacement between fringes is about 0.3 μm.
Optical arrangement to obtain out-of-plane ESPI fringes
ESPI fringes showing one of the vibration modes of a clamped square plate
Optical arrangement for viewing in-plane displacement sensitive fringes