A holographic process for testing, utilizing the coherent light of the laser, in which the specimen is not damaged. Holographic nondestructive testing systems usually incorporate holography (lens-less, three- dimensional laser photography), interferometry, and an appropriate means of stressing the test specimen. Mild stressing is sometimes accomplished with a hot air gun, a quartz heater, or even amplified sound.
Inspection consists of optical comparison of a test specimen in two or more (unstressed and stressed) states. Performance characteristics of the test object can then be evaluated directly by observing the resulting holographic interferometric fringe pattern. Lines in the hologram that show abrupt bends indicate defects. Where a defect exists, the surface under stress will move more. Lines with sharp angular bends are one indication of a defect; rings or spread lines are other possible indications. Besides being relatively fast, the process permits scan-testing of fairly sizeable weldments.