Gentle Introduction To White Light Interferometry

Screenshot of the Zygo white light interferometry microscope software. (Credit: Huygens Optics)
Screenshot of the Zygo white light interferometry microscope software. (Credit: Huygens Optics)

White light interferometry (WLI) is a contact-free optical method for measuring surface height. It uses the phase difference between the light reflected off a reference mirror and the target sample to calculate the height profile of the sample’s surface. As complex as this sounds, it doesn’t take expensive hardware to build a WLI microscope, as [Huygen Optics] explains in a detailed introductory video on the topic. At its core you need a source of white light (e.g. a white LED), with a way to focus the light so as to get a spatially coherent light source, like aluminium foil with a pin hole and a lens.

This light source then targets a beam splitter, which splits the light into one beam that targets the sample, and one that targets the reference mirror. When both beams are reflected and return to the beam splitter, part of the reflected light from either side ends up at the camera, which captures the result of the reference and sample beams after their interference (i.e. combination of the amplitudes). This creates a Michelson interferometer, which is simple, but quite low resolution. For the demonstrated Zygo Newview 100 WLI microscope this is the first objective used, followed by a more recent innovation: the Mirau interferometer, which integrates the reference mirror in such a manner that much higher resolutions are possible, down to a few µm.

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Play Dough Simplifies Interferometer Build

An interferometer sounds like something complicated, and in a way, it is. But it is also pretty easy to build one with some common materials. [Let’s Innovate] has instructions for how to make an interferometer using a green laser pointer, some mirrors, and a CD case. one of the most mundane parts, though, might be the most important: Play Dough.

The very sensitive device needs very precise alignment of the mirrors that reflect the beam. Using Play Dough it is easy to adjust the mirrors to the spot that is just right and then have it stay there.

For the best result, the mirrors really need to be first surface mirrors and not the more common kind with the reflective part on the back. Apparently, a green laser gives better results than a red one, too. If you don’t want to hack up a CD jewel case, a DVD player may give up a beam splitter.

So what do you use it for? Well, most of us use it to see the pretty patterns. But the instrument actually has wide-ranging applications to measure very small distances in fields as diverse as astronomy, optics, and photomicroscopy. To do anything really practical, you might need to add a detector of some sort.

If you want a more robust build, this one is similar. If you have a well-stocked test bench, you might be able to get by with even less.

Hackaday Prize Entry: Radio Telescope Interferometer

Radio telescopes are one of the dark arts of science. Not only do you have to deal with RF wizardry, the photons you’re detecting are so far out of the normal human experience that you really don’t know what you’re looking at. It’s hard, but that’s the point — there’s a lot to learn with a radio telescope.

[alfazoOm]’s entry in the 2017 Hackaday Prize seeks to counteract a two-part problem: first, there is a dearth of educational radio interferometers in Latin America. Secondly, in Colombia, there’s only so much clear sky so radio astronomy is the preferred technique. Even though they’re so close to the equator, a lot of the northern stars can be seen as well. His interferometer, IMFR11GHz, answers both of those challenges.

IMFR11GHz is a Michelson interferometer, in which a light source is split into two beams, which are reflected by mirrors back to the detector. [alfazoOm] is basing his telescope off of the Stony Brook radio interferometer, though he is designing custom hardware that can position the dish in whatever direction the operator desires with an Alt-Az mount. The control system consists of an ESP32 microcontroller with an IMU and two stepper motors controlling azimuth and elevation. This is awesome citizen science, and a great entry in the Hackaday Prize.