# Wavelength definition of the metre

Between 1960 and 1983 the metre was defined as being equal to 1 650 763.73 wavelengths of an orange emission line of krypton-86. How did they actually use that definition? Was it possible to move the mirror of an interferometer slowly over a distance of 1 metre, perhaps by a very smooth screw, and count the appearance of 1.6 million fringes without mistakes? Was the coherence length of the krypton light more than 1 metre?

There are much easier ways to measure a wavelength than by "tromboning" an interferometer. A simple two-slit grating, for example, will give you a pretty decent measurement. You don't need to measure a "meter's worth" of light, just run an experiment where the output data (such as a diffraction pattern) can be used to calculate the wavelength to high precision and accuracy.

In any case, the definition was basically "in reverse." Using existing measurement techniques, NIST or ISO or whomever took the current measurement of this narrow (no broadening, no fine-structure splitting) line and used that to define the meter.

• My question is about how the primary standard of 1960 was to be used, not about easy ways to measure a wavelength. Were they really going to count wavelengths? I am confused about your reply. I think it suggests nobody was going to count wavelengths, the new definition was just pro forma, not operational. Is that a correct summary of your reply? – jkien Feb 27 '18 at 13:46
• You are correct that nobody would try to "lay down 1.6E6 wavelengths in a row" . That's the nice thing about equations and stuff - you can get the value you need with indirect measurements. – Carl Witthoft Feb 27 '18 at 15:47
• That is amazing, and hard to believe. So between 1960 and 1983 lengths of 1 metre and longer could not be calibrated exactly anymore, not using krypton-86 light? What would have been the largest length that could still be calibrated using krypton-86 light? – jkien Feb 27 '18 at 16:36

It was actually done with a "tromboning" Michelson Interferometer

In 1892 by Michelson. He measured a distance of 10cm to an accuracy of ± 0.00004 mm using a Cadmium lamp . The 1960 re-definition with Krypton used the same data - it was just a matter of measuring the ratio of the wavelengths of the Cadmium and Krypton lines

Michelson's 1907 Noble prize speech describes it http://qsd.magnet.fsu.edu/course/exp3802/optics/AAMichelson1907.pdf