Hi Julien,

For S+A, the units can be considered PTV; it’s a 50% spherical and 50% astigmatism aberration, so since there are only 2 values there is no RMS to be calculated.  The magnitude of both the spherical and the astigmatism will be random between the minimum and maximum values and the clocking angle for the astigmatism will be random between 0 and 360 deg.  This is typically used when the manufacturer will verify the optic with a Test Plate and is visually analyzing the results.  This uses the TIRR operand and converts a Standard surface to an Irregular surface.  This uses the TWAV wavelength for fringes.

For Zernike Irregularity, this uses RMS for the calculation.  All the Zernike terms up to the included term are randomized according to the either the Statistics defined in the Tolerancing window or the STAT operand, then the RSS of the Zernike terms are normalized to the max value defined in the Tolerance Data Editor.  This uses the TEZI operand and convert the surface (Standard, Even Aspheric, or Torodial) to a Zernike Standard Sag surface.  This doesn’t use fringes but rather uses lens units for the Zernike Standard Sag.  This operand should be used if your manufacturer tests optics with an interferometer (rather than test plates).  This operand replaces the TEXI operand which did use fringes and would also be controlled by the TWAV wavelength.