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September 9, 2014Measurement Repeatability: How much is enough?

How many times have you looked at an interferometer specification and to see a number under Simple Repeatability like λ/10,000 and wondered, “Who cares?”  Add to this the long footnotes that tell you how the number was obtained, usually through significant averaging, idealized short interferometer cavities (think 1 mm) and statistics from around 30 measurements.  Then the RMS is reported which further averages the data.  When you consider how an interferometer is used, a 1 mm cavity coupled with averaged data seems irrelevant.

The escalation of specifications like Simple Repeatability has occurred for a couple reasons:  First, some purchasing departments buy purely on specification, and if one system’s Repeatability is lower than another system’s…guess who wins.  And second, there is potentially a bit of pride in saying our numbers are the best.  Notice the reason is not to tell the user what is important, or what is “good enough”.

Repeatability considerations:

  • Repeatability only applies to each particular measurement situation.  Environment, part interactions with the interferometer ( for example stray light), and the interferometer data acquisition play a role.
  • Another variable not seen on specification sheets is REPRODUCIBILITY.  It does not appear because it can only be measured for a particular measurement in the use environment.  Reproducibility is the variation due to operator to instrument interactions (tilt fringes for example), long term environmental variations, fixturing induced errors and other unforeseen complications.
  • In many industries a GR&R test is used to quantify and separate the contributions of repeatability and reproducibility. The typical target for a GR&R test is ≤10% (combining both contributions) of the measurement tolerance.  For a λ/10 P-V surface that means a λ/100 P-V, or ~λ/500 RMS GR&R.  A far cry from λ/10,000 reported.
  • This does not mean that ~λ/500 RMS GR&R is easy to achieve.  When considering real life conditions ~λ/500 RMS GR&R can be very hard to achieve.  The important issue is knowing what your GR&R is compared to the desired tolerances to see if the measurement is meaningful.

From a metrology viewpoint repeatability as reported on specification sheets is meaningless.  GR&R is the meaningful parameter to use for your measurement set up and tells the true story.

What other specification really matter and why?

Let us know what you would like discussed.

Thank you for reading.


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