[摘自Zygo,原文和图示请参考附件]
What’s the difference between PV and RMS?
Peak-to-Valley error (PV) is a worst case error statistic. Calculation of the PV error uses
only the high and low point on the surface relative to the reference surface. Since this
only compares two points on the surface, it is possible for two very different surfaces to
have the same PV error. The PV error does have the advantage that it is very easily
estimated visually from the interferogram. It has the disadvantage that one small dig, or
other artifact in the surface, can cause the PV to be very large even though the optic may
perform quite well.
Root-mean-square error (rms) is an area weighted statistic. The rms is calculated as the
standard deviation of the height (depth) of the test surface relative to the reference at all
the data points in the interferogram. The rms has the advantage of more accurately
reflecting the optical performance of the surface being measured.
There is no specific constant of proportionality relating the PV and rms errors. This ratio
is dependent on the process used to generate the surface and the instrument used to
measure the surface. Polished optical surfaces measured with a Zygo interferometer will
typically show the PV error of the surface to be between a factor of 3 and 5 greater than
the rms error. Surface with high spatial frequencies, such as diamond-turned surfaces, will
generally show a smaller factor, in the range of 2 to 3. A very large ratio is often a sign
that there are noisy data points in the measurement. It is recommended to examine a plot
of the data to understand why the a high ratio is obtained. Cleaning and re-measuring the
part may be advisable. The problem may also be fixed by properly focusing the
interferometer on the part. |
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