It&#x27;s pretty much as Velox and bbrazil said.<p>You can use &quot;low-coherence interferometry&quot; to measure tiny signals that would be undetectable otherwise. Combine a &quot;reference&quot; beam with a &quot;signal&quot; beam and you get a measurable interference pattern, even when the magnitude of the signal beam is miniscule.<p>This is what a real-life interference pattern looks like (I just acquired this from an actual interferometer illuminating a painted metal surface):<p><a href="https:&#x2F;&#x2F;dl.dropboxusercontent.com&#x2F;u&#x2F;30682604&#x2F;fringe_signal.png" rel="nofollow">https:&#x2F;&#x2F;dl.dropboxusercontent.com&#x2F;u&#x2F;30682604&#x2F;fringe_signal.p...</a><p>This is now an established medical imaging method (Optical Coherence Tomography) to create 3d scans of biological tissue. It can also be used to measure distance or elevation changes on a surface of anything from a micrometer-level scale to a planetary surface. All you need is to use light with the right wavelengths and two measurements &quot;arms&quot; of roughly the same length.