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A point diffraction interferometer is a type of common path interferometer. Unlike an amplitude splitting interferometer, such as a Michelson, which separates out an unaberrated beam and interferes this with the test beam, a common path interferometer generates its own reference beam. The device is similar to a spatial filter. Incident light is focused onto a semi-transparent mask (about 0.1% transmission). In the centre of the mask there is a hole about the size of the Airy disc and the beam is focused onto this hole with a Fourier transforming lens. The zeroth order (the low frequencies in Fourier space) then passes through the hole and interferes with the rest of beam. The transmission and the hole size are selected to balance the intensities of the test and reference beams. The device is similar in operation to Zernike phase contrast microscopy. Since the device is self-referencing it can be used in environments with a lot of vibrations or when no reference beam is available such as in many adaptive optics and short-wavelength scenarios. Phase-shifting (Interferometry ) versions have been created to increase measurement resolution and efficiency. These include a diffraction grating interferometer by Kwon and the Phase-Shifting Point Diffraction Interferometer. The main criticisms of the original design are (1) that the required low-transmission reduces the efficiency, and (2) when the beam becomes too aberrated, the intensity on-axis is reduced, and less light is available for the reference beam, leading to a loss of fringe contrast. These problems are largely overcome in the phase-shifting point diffraction interferometer design, in which a grating or beamsplitter creates multiple, identical copies of the beam that is incident on an opaque mask. The test beam passes through a somewhat large hole or aperture in the membrane, without losses due to absorption; the reference beam is focused onto the pinhole for highest transmission. In the grating-based instance, phase-shifting is accomplished by translating the grating perpendicular to the rulings, while multiple images are recorded. ==References== 〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Point diffraction interferometer」の詳細全文を読む スポンサード リンク
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