New noncontact devices for measuring small microdisplacements.
The visual method suitable for longitudinal optical settings, based on the observation of equal modulations produced by two periodic tests when depth is shifted, is recalled and shown to provide sensitivity 100 times greater than conventional methods. A photoelectric adaptation of this method is described, as well as two devices that can be used to maintain focus within the same sensitivity. The first is designed to detect the difference of modulations of the image of a line or point source as measured in two planes situated on both sides of the source. The second is designed to detect the difference of encircled energies of the images of the same source as measured in two identical planes. Results from both devices are compared.
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