counts per inch, my $0.02

If the DRO is used with a Twyman-Green interferometer using a HeNe laser at
632.8 nm wavelength, a fringe count will occur every 312.4 nm, which comes
out to 80278 counts/inch.
Other types of interferometers may have twice this sensitivity.
Shorter wavelengths (doubled YAG at 532 nm) will have even more sensitvity.

For those interested in using interferometers to measure distance, a
suitable laser source costs much less than $100. The optics required aren't
too complex. Basic precision can be 0.5 ppm, if the temperature and
humidity of the air path are compensated for.

Elliot Burke