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I'm reading about an experiment done with this piece of equipment. The aim is to measure the thickness of a piece of plastic. They use white light, so the central fringe in the interference pattern, corresponding to equal path lengths in the two beams can be used as a reference. Why does that work?? I don't understand why it can be used as a reference.

The central fringe corresponds to both beams having travelled equal distances... The displacement of one fringe has to be tracked when the plastic is placed. That's about as much as I understand. Why is the central fringe a reference? If it's the only one that doesn't move, why is that?

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  • $\begingroup$ Interesting idea... there is generally nothing wrong with using incoherent light in an interferometer, the resulting patterns will simply be harder to analyze. By recording the intensity of multiple spectral channels one could even use an interferometer without tracking individual fringes... but I don't know how well that works in practice. As for the details of this setup, can you give a citation? $\endgroup$ – CuriousOne Jan 10 '16 at 23:20
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I'm not sure that I understand your set up, but it appears to refer to Optical Coherence Tomography. There are various ways to do it, but in the simplest incarnation, light with a short coherence length is used, and one "mirror" of a Michelson is a semi-transparent interface (such as the surface of a piece of plastic). As you move the other mirror, fringes are observed when the optical path length of the first, and later the second, surface match the distance to the mirror. The optical distance between the surfaces is equal to the distance that the mirror was moved. The technique is used to produce 3D images of semitransparent objects.

This is now widely used in opthalmology, but has other medical applications. In non-medical fields it's sometimes referred to as Optical Low Coherence Reflectometry, but given the success of the medical devices, one doesn't see that name too much anymore.

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