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# Double Pass Tolerancing for Surface Tolerances (e.g. surface irregularity)

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I’m modelling a double-pass system, which can be simplified to the Knowledgebase help tutorial at this link. The tutorial only covers element tolerances, but I’m interested in modelling surface tolerances. In particular, the Surface Irregularity tolerance - I see no way to have ZEMAX “pick up” the same random perturbation onto the surface from the first pass onto the second pass of that surface. Similarly, I suspect TIND (index tolerance) is also applied separately despite double pass.

For decenter and tilt, I’ve figured out this can be approximated by “lumping together” the element and surface tolerances together. If there are two 0.1mm decenter tolerances, one on the lens surface, and on on the alignment of the lens, then the combined tolerance is 0.1*sqrt(2) mm.

What would you suggest I do for the TEZI/ TIND tolerances?

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Best answer by Sandrine Auriol 16 June 2023, 17:12

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Hi @ISHFM

For the surface irregularity, you can change the surfaces on the second pass to be Zernike Standard sag surface and a composite surface (if needed) and then set up manually the pick-up solve to the first surface. See more information about the composite surface here:
https://support.zemax.com/hc/en-us/articles/10648753483539-Introduction-to-Composite-Surface

That way when the tolerancing will change your 1st surface into a Zernike or a composite surface, you will have the solves in place ready to add the same amount of irregularity on the 2nd surface. I have shared an example using the TIRR operand.

For TIND, you can simply pick up your material. TIND will change your material to an offset solve. The pick-up solve will pick up the value with the offset: