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.

Similarly the common path design is resistant to ambient disturbances. This is a major disadvantage of a double path systems, such as Fizeau interferometers, as shown in Figure 2. The common path design eliminates any need of having a reference optics, which are known to overlap the absolute surface form of a test object with its own surface form errors.

PDI systems are valuable tool to measure absolute surface characteristics of an optical or reflective instruments non destructively. It is very important that the reference optics(flat) be near perfect because it heavily influence the measured surface form of a test object. The device is similar in operation to phase-contrast microscopy.įigure 2: Fizeau interferometer requires a reference optics.

The transmission and the hole size are selected to balance the intensities of the test and reference beams. The zeroth order (the low frequencies in Fourier space) then passes through the hole and interferes with the rest of beam. In the centre of the mask is a hole about the size of the Airy disc, and the beam is focused onto this hole with a Fourier-transforming lens. Incident light is focused onto a semi-transparent mask (about 0.1% transmission). The device is similar to a spatial filter. The principle of a PDI is shown in Figure 1. The reference beam is created from a portion of the test beam by diffraction from a small pinhole in a semitransparent coating. This design makes the PDI extremely useful when environmental isolation is not possible or a reduction in the number of precision optics is required. In PDI systems, the test and reference beams travel the same or almost the same path. Unlike an amplitude-splitting interferometer, such as a Michelson interferometer, which separates out an unaberrated beam and interferes this with the test beam, a common-path interferometer generates its own reference beam. Figure 1: Basic layout of a PDI system, where the reference beam is generated by a pinhole etched onto a semitransparent filmĪ point diffraction interferometer (PDI) is a type of common-path interferometer.

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