Optical metrology and holography

What is NDT? 

Laser light, with appropriate optical equipment, can produce accurate, non-contact methods of measuring size, position, spacing, etc, or measure very small displacements of a surface.

Laser interferometry (optical holography) measures displacements to the highest accuracy and can be used to measure vibration amplitudes in machinery etc. The method is used to measure small amounts of displacement of a surface due to stress or to compare specimens with a standard. A surface or near-subsurface flaw in a stressed specimen is likely to modify the stress pattern and so may be detected by the surface displacement pattern.

The basic method of optical holography is to use a beam of coherent light from a laser and split this into two beams. One part illuminates the specimen surface and the other part is used as a reference beam. When these two beams are recombined the result is a hologram which is normally recorded on a special (holographic emulsion) film. When this hologram is in turn illuminated with laser light an image of the specimen can be seen, with interference fringes superimposed when there is any change in position of the specimen surface. The image is seen three-dimensionally. The frequency and width of these fringes is a measure of surface displacement and so of local stress. The image can also be recorded on a television camera.

If the surface of a specimen is illuminated with laser light, the surface appears speckled and this speckle pattern is related to the detailed surface structure. Speckle can therefore also be used to detect displacement of a surface. The speckle image is usually photographed and used as a negative mask, but again, television systems can be used for speckle pattern interferometry.

Optical laser systems are suitable for large-scale surface inspection by using powerful lasers and optical beam spreaders.