Authored By Hui Chen
Occasionally customers have the need to bring interferometer-measured lens data, for example lens shape measured by Zygo Interferometer, into OpticStudio to check performance impact due to manufacturing tolerances. This FAQ provides a quick introduction to how to best approach this task in 4 steps.
- Data Conversion: First, when exporting data from the interferometer, one needs to pay attention to the type of data contained in the INT data file, is it an INT Grid data file or is it an INT Zernike data file? If it’s an INT Grid data file, does the grid data describe the physical sag of the lens surface in unit of length, or does it describe the surface using phase information in unit of radian? This is important because different INT Grid data will need to be imported into OpticStudio via different types of surfaces. For INT Grid file describing surface sag in length unit, one should use Grid Sag surface. For INT Grid file describing surface phase information, one should use Grid Phase surface. For INT Zernike file with phase information, one should use Zernike Fringe Phase surface, and for INT Zernike with physical sag in length unit, one should use Zernike Fringe Sag surface. Next, OpticStudio cannot directly import these INT data files so they need to be converted into a format that’s compatible with OpticStudio first. To ease the effort of data import, OpticStudio GUI offers a tool to convert interferometer INT file into OpticStudio compatible DAT file for import. These converted DAT files are in text format. One can open them using any text editor to check and make sure the converted file format appears proper. The Zernike coefficients in the converted DAT file will have the same units as the INT file. In addition, the clear semi-diameter or semi-diameter of the surface should correspond to the size of the surface or wavefront under test. If you are interested in knowing more about the INT to DAT file conversion tool, you can find it described in the manual at Convert File Formats
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Data Import into OpticStudio: Start with a new surface in the Lens Data Editor and choose a proper surface type as described above. Data file converted in step 1 can now be imported using the Lens Data Editor > Surface Properties > Import function. Grid Sag or Grid Phase surface is used for importing the converted grid DAT file, and Zernike Fringe Sag or Zernike Fringe Phase surface for the converted Zernike data file.
- Inspect data using Surface Sag or Surface Phase plot: For best practice, after importing the converted DAT file, we recommend checking the surface sag or phase information using the Surface Sag or Surface Phase plot listed under the Analyze tab\Surface dropdown. Phase surfaces are a special category in OpticStudio in the sense that they bend rays not through physical shape, but by adding phase or optical path length to a ray. For example, Grid Phase, Binary 2, Zernike Phase, etc., are all phase surfaces. Because these phase surfaces may be flat and only contain phase information, when viewing them in Surface Sag plot, the plot may appear empty. Instead, one needs to observe their phase pattern using Surface Phase plot. On the other hand, if you try to use Surface Phase plot to observe a “non-phase” surface, the phase plot may come up empty as well. When inspecting surface sag or phase plot, one needs to pay attention to two things. First is the visual checking. Does the pattern in False color plot roughly match what’s shown in the Interferometer measurement? Second, do the maximum and minimum sag or phase values reported in the color scale bar match the PV values reported in the interferometer? This step is very important. It helps make sure the data export and import process worked as expected and the surface information measured by the Interferometer has been properly implemented in OpticStudio.
- Check out results in OpticStudio: After satisfying all above checks, we are now ready to view results in OpticStudio Wavefront map. Sometimes customers ask why the shape and PV error reported in the Wavefront map do not match their expectations. If the implemented surface sag or phase information matches the interferometer results as described in step 3, then this discrepancy may have something to do with how OpticStudio calculates the Wavefront map. The Wavefront you see is the optical path length difference referenced to certain OPD reference. How OpticStudio constructs the OPD reference, spherical vs planar, at Exit Pupil vs at Image surface, will have an impact on the calculated wavefront error. If you want an “un-referenced” optical path length difference, you can adjust the Reference OPD settings in System Explorer\Advanced section. Please refer to the article How do OPD Reference settings in System Explorer\Advanced section affect wavefront calculation? to have a better understanding of how OPD Reference settings work. If you still cannot understand the difference between OpticStudio Wavefront Map and the Interferometer measurement results, please contact Ansys Zemax Support team for further discussion.