Though the TEZI operand is a direct tolerance for the exact RMS error of the surface in lens units, we can approximate the inhomogeneity spec as sag error.  Most conservatively, we can estimate the RMS error by multiplying the center thickness (assuming this is your largest thickness over the aperture of the lens) of your element by the inhomogeneity tolerance.

In the example given on the previous page, we determined that the maximum ΔOPL was 250nm for a 25mm PPP of H2 grade.  If we use the TEZI tolerance operand in the Tolerance Data Editor (TDE), the maximum tolerance can be set to 2.5E-4 (provided that the Lens Units are in millimeters).  The minimum tolerance for the TEZI operand is automatically set to the negative of the max value.





The number of Zernike terms used for the analysis may be between 0 and 231.  Generally speaking, if fewer terms are used, the irregularity will be of low frequency, with fewer “bumps” across the surface.  The maximum number of terms should be chosen accordingly. 

In a number of Monte Carlo Runs, we can gather a significant amount of statistical data relating to the change in RMS Wavefront Error due to the inhomogeneity of the glass.  The more Monte Carlo tolerance runs that are performed, the better the statistical average of performance degradation (change in criteria) will be.