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Consider the case of a Michelson 'Interferometer', from what I have read:

  • If you measure the output as a function of mirror separation that's interferometry.
  • If you measure the output as a function of wavelength (some how) that's spectroscopy.

But what is it called if I am measuring the intensity distribution as a function of position on the focal plane of a lens whilst keeping everything else constant?

My guess would be that it could be etalon (or a word related) due to the similarity with a Fabry-Perot 'interferometer'.

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    $\begingroup$ You can also do interferometric experiments without moving the mirrors if, for instance, you put a gas cell on one leg and vary the pressure or composition of the contents. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Apr 7, 2016 at 15:09

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You're close. In some very general way we could start with definitions:

  • Interferometry You use a constant and well understood light and vary some parameter of the light's paths through the machine (either by changing the length of a leg, changing the material on one leg, or changing the motion of the material on one leg.

    To repeat: hold the light steady and change something about the optical path.

  • Spectroscopy You use a well understood and unvarying machine to analyze the spectral composition of the light (that is, how bright it is at different frequencies.

    To repeat: keep the optical path constant1 and examine different lights (or different frequencies in a single mixed light source).

And see where that takes us.

However, in the case of Fourier transform spectroscopy mentioned in the comments by WhatRoughBeast, we perform what this rule would describe as a interferometric measurement using a sinusoidal variation of one path length and then transform the data to get arrive at a spectral measurement.

This isn't an accident because the two ideas are intimately linked. both involve measuring differences in (distance traveled/wavelength). You can use this to investigate either of these quantities.

1 I'm anticipating a little confusion over what I mean by "keep the optical path constant" because some spectrometers have swinging telescope. I'm going to define the out-going "optical path" for this purpose as the space between the analyzer and the objective of the telescope--the scope is just a magnifier. Similarly for machines that use a telescope to focus the incident light: we treat that step as separate from the spectroscopic measurement.

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  • $\begingroup$ Actually, you need to rethink this. Fourier transform spectroscopy uses a variable path to analyze the wavelengths present in the light beam, which may or may not be constant. $\endgroup$
    – WhatRoughBeast
    Commented Apr 7, 2016 at 15:55
  • $\begingroup$ And here I thought I was being slick. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Apr 7, 2016 at 15:55
  • $\begingroup$ Hi, I am still a bit confused of what we would call it when simply looking at the pattern on the focal plane, as this does not fall into your (very helpful) definitions of interferometry or spectroscopy? (unless it does and I just can't see it) $\endgroup$
    – Quantum spaghettification
    Commented Apr 19, 2016 at 16:48
  • $\begingroup$ The simple version is "if you a measuring the composition of the light (or other source) it's spectroscopy, otherwise it's interferometry". $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Apr 19, 2016 at 22:44

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