By a sophisticated instrument, known as an optical interferometer, any length can be measured in terms of the wavelength of laser light.

The transducer is physically designed as a pressure sensitive optical interferometer according to the Fabry-Pérot principle.

These telescopes, along with four smaller Auxiliary telescopes, are also combined to operate as an optical interferometer on certain nights of the year.

Currently operational at the site (since 2008) is a 2.4-meter fast-tracking optical telescope.; under construction is a ten-element optical interferometer.

It is an optical interferometer with two perpendicular arms 100 m long.

Although ELTs are large, they have smaller apertures than the aperture synthesis on many large optical interferometers.

A Coudé focus was planned as a later addition, to feed an optical interferometer with another telescope, but this was never built.

Despite these technical difficulties, dozens of astronomical optical interferometers are now in operation offering resolutions down to the fractional milliarcsecond range.

The current NPOI siderostat array remains the world's only long-baseline (437-meter) optical interferometer that can simultaneously co-phase six elements.

It is related to the path difference between the two arms of an optical interferometer over which the light wave is still capable to generate an interference pattern.