Either a 3-Term or 5-Term Polynomial may be chosen.  The polynomial fitting is considered a “Cache” option, because the polynomial fit data is saved in memory.  The cached data may then be used to quickly generate subsequent tolerance analyses.  For details on the other Cache option, please refer to the article, “How To Use The Tolerancing Cache.” 

Initially, for the first tolerance run, the Cache option can be set to “Recompute All.”  If for subsequent tolerances you wish to use the cached polynomial, then choose either the 3 or 5 Term Polynomial option as shown in the dialog above. 

For the initial calculation, using the polynomials is more time consuming because ZEMAX has to calculate some additional points within the minimum and maximum tolerance range.  As was described earlier, the 3-Term polynomial requires 4 evaluation points and the 5-Term polynomial requires 6 points.   This is performed for each and every tolerance in the Tolerance Data Editor.  When the fit for each individual tolerance is complete, the polynomial coefficients are saved to memory.  When tolerance analysis is complete, the polynomial terms are displayed in the output text window:

After reviewing the tolerance results, and after making adjustments to the appropriate tolerances, subsequent tolerances can use the cached polynomial data.  Just make sure to select the “Use Polynomial” Cache option for subsequent tolerances!  Especially for computationally intensive calculations, the time saved will be tremendous!