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Why is the Nominal Criteria Different From the Value Reported Elsewhere?

This article will talk about what the nominal criteria is and how we arrive at that value.
Dan Hill
Frequently Asked Questions

Why is the nominal criteria different from the value reported elsewhere?

Question:  Why is the nominal criteria in the tolerance output different from the value which is reported elsewhere in Zemax, such as the merit function or an analysis feature? 

There are a few reasons why the reported nominal criteria from the tolerancer may be different from that of the value reported elsewhere in Zemax: 

  1. When computing the nominal criteria in the tolerance analysis, Zemax considers any compensators which are defined.  This means that Zemax optimizes for the smallest criteria value using the defined compensators as variables.  The merit function or any analysis feature in Zemax on the other hand, is evaluated without any compensator adjustment.
  2. Sampling methods and sampling sizes may be different.  In constructing physically significant merit functions for a selected criterion, Zemax uses Gaussian Quadrature pupil sampling.  Some analysis features in Zemax use Rectangular Array sampling, which may result in a slightly different value than that determined using the GQ method.  Theoretically, as the sampling size of both methods increases, the calculated results will approach equality. 
    Even if the same sampling method is used, sampling size can have an effect on the outcome of a calculated value.  Make sure that the sampling size is sufficient in either calculation before making a comparison of the results.
  3. By default, Zemax reports a single criteria value which is the root-mean-square (RMS) across multiple wavelengths, fields and/or configurations.  The results may be separated by field and configuration, but make sure that when comparing a value from a specific analysis feature vs. that of the nominal criteria, the data corresponds to a single field or the RMS across all defined fields and/or configurations.