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Zygo's PVr – A Robust Amplitude Parameter Specifying Optical Surfaces

Zygo’s PVr – A Robust Amplitude Parameter Specifying Optical Surfaces

Zygo’s PVr as a robust amplitude parameter unravels the complexity of optical surface specification. 


It is a well-established practice in optics manufacturing to specify optical surfaces based on the peak-to-valley (PV) departure from nominal surface shape. PV is defined as height difference between the highest and lowest points on the surface after removal of piston and tilt for flats, after a best-fit sphere for spherical surfaces, and after removing the nominal asphere in aspheric metrology.  

The increase in spatial resolution in interferometers has resulted in more artifacts being visible in a measurement. In addition, the difference in resolution from system-to-system results in a wide range of noise characteristics across metrology equipment.  

In an effort to generate a “realistic result,” the metrologist will often attempt to manually remove pixels of data that incorrectly drive the PV. This usually results in the potential for a large variation in the reported PV of a surface across operators and instruments.

To counter these issues, PVr – A Robust Amplitude Parameter, has been developed. This application note provides an overview of PVr and describes the benefits in using PVr to specify optical surfaces.

The Problem With PV And Modern Optical Manufacturing

The PV result is driven by the highest and lowest pixels and is biased by noise, often producing a result that is not representative of actual surface form. When selectable filtering is used to modify artifacts or noise, it is not easy to ensure measurement reproducibility.  

In addition to the above conditions, dramatic changes in optical fabrication and metrology methods over the past two decades have made PV a troublesome parameter to use when buying and selling optics. Small tool fabrication processes such as MRF, computer-controlled polishers, diamond turning, etc., do not necessarily produce sur-faces that approximate the 5:1 rule for PV/rms.

Small artifacts in the measured surface have little or no impact on the imaging performance of an optical system and can be resolved with interferometers that utilise high resolution cameras. The lack of standards for frequency cut-offs, filters, and outlier rejection, techniques often used to remove questionable pixels of data, make it nearly impossible to achieve acceptable measurement reproducibility.  

Differences in instrument resolution and the frequency content of the test surface can also contribute to measurement variations from one interferometer to another.

 

Read more here at page 42

 

 

 

 

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