Answer: etalon

AnswerCrossword Clue
ETALONDistance-measuring instrument
ETALONStandard: Fr.
ETALONAn interferometer.
ETALONInterferometer
etaloninterferometer consisting of two glass plates that reflect approximately half of each ray of light incident upon them and that are separated by a small, fixed distance: used to compare wavelengths and to study atomic spectra
etalona device used to measure wave length