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Gongpu Lan, Jason Porter; First-order optical design to accommodate large uncorrected refractive errors in adaptive optics ophthalmoscopes. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5196. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
With the development of high-stroke correcting elements and the drawbacks of trial lens corrections, it is advantageous to design adaptive optics (AO) systems to operate over a wide range of uncorrected refractive errors. Here we propose a set of generalized formulae to be considered when optimizing an AO system to accommodate for the high vergence demands presented when trying to image patients with moderate to large uncorrected refractive errors.
We derived generalized first-order equations to predict the amount of vergence present at different pupil planes in a general AO system consisting of afocal, lens-based 4f telescope pairs. These equations were used to calculate vergence-related beam diameters on each optical element in the system and generate equations to predict the minimum clear aperture (MCA) required of each optical element to avoid vignetting over a specified vergence range (i.e., desired range of uncorrected refractive errors to be accommodated). The optical performance of a design example, an out-of-plane, mirror-based adaptive optics scanning laser ophthalmoscope (AOSLO), was evaluated following this vergence-optimization procedure.
The vergence of light at a system pupil plane is given by the vergence of light exiting the eye’s pupil divided by the product of the square of the magnifications of all intervening telescope pairs. The MCA of each optical element can then be calculated by adding the vergence-related spot size on the element to the product of the element’s focal length and the field angle made by the chief ray between the element and its nearest pupil plane. The change in an element’s MCA due to a change in vergence is proportional to the element’s focal length and the diameter of the pupil directly preceding (or following) the element. Application of these principles to the design of an AOSLO (1.5° field, 7.6-mm pupil, 840nm) using a large-stroke deformable mirror to compensate for uncorrected spherocylindrical errors resulted in diffraction-limited performance with no vignetting over vergences from -9D to +7D.
The proposed formulae provide a method to minimize vignetting due to large amounts of vergence created by uncorrected refractive error. The technique can be applied to any AO imaging modality, as well as to lens, mirror and non-planar based systems with small angles (e.g., ≤ ~5-10°).
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