For measurements made by Laser Fizeau Interferometers PV is frequently increased erroneously by outliers and artefacts of the measurement which the metrologist is confident are not associated with the surface under test. Methods for removing those outliers from the measurement result are not standardized. Introducing PVr   PVr is a newly defined parameter pioneered by C. Evans of Zygo Corporation that is related to imaging and is robust over a range of instruments. Briefly, PVr for circular apertures is defined as: PVr = PV36 Zernikes + 3 x δ36 Zernike Resid
What Makes PVr Robust? PVr is detector independent and reasonably insensitive to noise, specifically from small defects. By describing the surface via its Power Spectral Density function and then applying a 36 term Zernike fit, the underlying, low order, optical surface figure is captured. This represents the bulk of the surface amplitude. The table shows the relationship of detector resolution to PV and PVr for a 300 mm flat optic. It shows the effect of reducing pixel resolution computationally on PV and PVr. Note the stability of the PVr value as the detector resolution is changed.
Detector Resolution PV, nm PVr, nm
1024x1024 53.81 39.46
512x512 38.27 39.41
256x256 36.88 39.33
128x128 33.9 39.19
To summarise, PVr is a Robust Amplitude Parameter with the following attributes
  • It is a repeatable method for specifying optical surfaces
  • It has low sensitivity to noise and detector resolution
  • The PVr application is available in MetroPro™ 8.3 or higher
  • It reduces instrument to instrument variability