Abstract
Mechanical scanning devices are commonly based on a rotating drum or a moveable opaque screen, with slits, knife edges or pinholes mounted on it and a single-element detector positioned behind. The Hartmann principle is based on an orthogonal or hexagonal array of lenses or pinholes, which divides the incoming beam into a large number of sub-rays. In principle, there are many interferometer designs which are capable of laser beam phase determination. The point-diffraction interferometer (PDI) is a common path interferometer exhibiting a simple design. A laser beam is directed onto a neutral density filter containing a small pinhole. Diffraction at this pinhole creates a virtually undisturbed spherical reference wave which interferes with the beam. A possible way of determining the coherence properties of laser beams, which is still frequently employed in practice, is Young’s experiment employing pairs of slits or pinholes in an opaque screen.
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