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Vyas Akondi, Alfredo Dubra; Shack-Hartmann wavefront sensor bias at the pupil boundary: problem and solution. Invest. Ophthalmol. Vis. Sci. 2019;60(9):5052. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The Shack-Hartman wavefront sensor (SHWS) is widely used for refractive surgery and adaptive optics ophthalmoscopy. Here, we report on a focal shift due to diffraction as a source of SHWS bias at partially illuminated lenslets, such as those found at the pupil boundary. This bias, small when compared to typical spectacle prescription, is not reported in the context of refractive surgery. This bias is, however, known to users of adaptive optics ophthalmoscopes that use the SHWS for sensing.
Using Fresnel diffraction theory, we illustrate how the diffraction focus, defined as the point in space of maximum intensity behind a SHWS lenslet, moves towards the lenslet and away from the geometrical focus with decreasing Fresnel number. The theory is tested using a custom SHWS, with a lenslet array having a geometrical focal length of 6.5 mm and 203 μm pitch. An 850 nm light source was first collimated by translating the source while examining centroid displacements due to fully-illuminated lenslets. Applying the centroid formula derived from diffraction theory (S. Bara, J Opt Soc Am A 20(12), 2237-2245, 2003), starting from the diffraction focus, we use centroid displacements due to partially-filled lenslets for refocusing the camera at the geometrical focus of the lenslet array.
The measured focal shift is -1.5 mm, which is in close agreement with the theoretically predicted -1.44 mm (Y. Li, J. Opt. Soc. Am.72(6), 770-774, 1982. The refocusing of the camera sensor results in a reduction of the bias by 77% in partially illuminated lenslets (peak intensity within 20 and 75% of the camera’s dynamic range).
The focal shift in SHWSs due to diffraction has been identified as the reason for measurement bias at partially illuminated lenslets. Because such lenslets are present at the pupil boundary and tear film break up areas and cataracts, the proposed refocusing will be beneficial for all applications of the ophthalmic SHWS.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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