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Why does OpticStudio give me smaller spot radius than the airy disk?

  • January 29, 2021
  • 3 replies
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Hello,

I set up a telecentric optical system consisting of multiple lenses. Now after optimizing the system my spot radius appears smaller than the airy disk radius. I would assume the airy disk is the smallest possible spot my system would have when free of any aberrations. How is it then possible for the system to have an image quality even better than the airy radius? Is that an error in my system or does Zemax have a different definition of spot radius/airy radius? Iif you need any more details I will try to provide them. Here is an image of the spot diagram:

Any help would be greatly appreciated. Thank you!

Best answer by Alissa Wilczynski

The spot radius being displayed here is based only on the geometric rays and doesn't account for the diffraction limited behavior of your system. Since your spot is smaller than the Airy disk, that's one indication that your system is diffraction limited and you will likely instead want to use other analyses such as the PSF to get a more realistic understanding of your system performance.

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Alissa Wilczynski
Zemax Staff
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The spot radius being displayed here is based only on the geometric rays and doesn't account for the diffraction limited behavior of your system. Since your spot is smaller than the Airy disk, that's one indication that your system is diffraction limited and you will likely instead want to use other analyses such as the PSF to get a more realistic understanding of your system performance.

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2 people like this
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    Asuku
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    Lauritz.Hartmann
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Thank you very much for your answer!

Alissa Wilczynski
Zemax Staff
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No problem! FYI, the reason that we have both a geometric spot radius analysis as well as other analyses that consider diffraction (like PSF) is because the geometric calculation is MUCH faster and it's accurate for cases with sufficient aberration. But, as you've illustrated, there are also times when the geometric analysis isn't an accurate reflection of the system because it predicts overly optimistic performance. So if you're not sure whether your system needs to account for diffraction behavior, a general recommendation is to compare the results of the two similar analyses (Spot v. PSF, for example) and if they differ significantly, go forward with the diffraction calculation. We have more examples and guidance on this in the Optical System Design Using OpticStudio course available in OpticsAcademy.

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    Naif.Alsalem

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