Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
• The standard deviation each of the measurements must be taken into account. Since
each spot is measured 3 times we divide it by the square root of 3, giving the
standard uncertainty of the mean (or standard deviation of the mean). Where N
measurements are made, the standard deviation is divided by √N.
So the combined standard uncertainty for each of the measurements, by summing in
quadrature, is ±√ 0.079+ (SD/√3) 2 , where SD is the standard deviation of the
measurements. This gives us a standard uncertainty of ±0.28 µm for the measurements
and the expanded uncertainty is ±0.56 µm, for a 95% confidence level.
By comparing results, it can be seen that the measurement uncertainty when using the
system as a comparator is seven times smaller than when using it in a direct measurement
mode. For highest accuracy measurements vision systems should therefore be used as
comparators whenever possible. However, it is also worth noting that the accuracy of the
results from the direct measurement mode example is well within the measurement
uncertainty. It is, of course, up to the end user to decide what level of accuracy and
uncertainty is acceptable for his particular measurements. Further information regarding
the calculation of uncertainty may be found in the NPL Measurement Best Practice Guide
No. 11, ‘A Beginner’s Guide to Uncertainty of Measurement.’
8 WORDS OF WARNING
Making accurate measurements is as much about being confident of the measuring
system’s 1 performance history as it is about its day-to-day repeatability. Whilst modern
vision systems generally exhibit excellent measurement repeatability and reproducibility,
measurement errors can still be produced. This can be due for example to the use of
incorrect or inappropriate measurement practices, technical problems with the equipment,
calibration errors, changes in environmental conditions or operator errors. In order to
minimize such problems and ensure that accurate measurements are made, users are
recommended to take steps to regularly monitor the performance of their measuring
systems by acquiring and analysing statistics on repeatability and reproducibility. This will
enable departures from ‘the normal’ to be spotted at an early stage and allow action to be
taken to identify and correct the source of the errors in order to return the measuring system
to its desired operating state.
1 The measuring system here includes the vision system, any associated equipment or
accessories required for the measurement and the operator.
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