*4.2.2. PoC main module*

Residual focus and astigmatism aberrations in the DCAO demonstrator that had not been compensated for by a Badal focus corrector and trial astigmatism lenses were corrected by DM1 after passing DM2, resulting in sub-optimal DM2 performance. The PoC prototype fea‐ tures a correct arrangement of the DMs where reflected light from the eye, corrected by trial lenses, first passes the pupil mirror DM1 before passing the field mirror DM2.

DM1 is a Hi-Speed DM52-15 (ALPAO S.A.S., Grenoble, France), a 52 actuator magnetic DM with a 9 mm diameter optical surface and 1.5 mm actuator separation. The magnification relative to the pupil of the eye is 1.5, thus setting the effective pupil area of the instrument to 6 mm at the eye. DM2 is a Hi-Speed DM97-15 (ALPAO S.A.S., Grenoble, France), a 97 actua‐ tor magnetic DM with a 13.5 mm diameter optical surface and 1.5 mm actuator separation. GS beam footprints on DM1 and DM2 are shown in Fig. 5. The last element of the main module is a dichroic beamsplitter (CM) that reflects collimated imaging light towards the retinal camera and transmits collimated GS light towards the WFS.

As the relay optics of the main module transmits both measurement (835 nm) and imaging (575 nm) light, custom optics were designed to assure diffraction limited performance at both wavelengths (Fig. 6). Due to the ocular chromatic aberrations the bandwidth of the flash illumination bandpass filter will induce a wavelength dependent focal shift in the in‐ strument image plane. An evaluation of the focal shift for the 575±10 nm wavelengths trans‐ mitted by the flash illumination bandpass filter using the Liou-Brennan Zemax eye model [35] yields a ±6.9 µm focal shift at the retina (Fig. 7).

**Figure 7.** Chromatic focal shift over flash illumination bandpass filter bandwidth (575±10 nm) at the retina calculated

Dual Conjugate Adaptive Optics Prototype for Wide Field High Resolution Retinal Imaging

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Transmitted GS light from the main module passes through the CLA and is reflected by a pel‐ licle beam splitter. A second polarizing filter (PFA) removes unwanted backscattered reflec‐ tions from the GS generation, and a lens brings the five GS beams to a common focus where they are spatially filtered by a single aperture (SF). A collimating lens finally relays the five beams onto a lenslet array (LA) with a focal length of 3.45 mm and a lenslet pitch of 130 µm.

The monochromatic WFS CCD camera has 1388×1038 pixels with a square pixel cell size of 6.45 µm, of which a central ROI of 964×964 pixels is used for wavefront sensing. The diameter of the diffraction limited focus spot of a lenslet is 2.44 λ *f* / d = 54 µm. Each spot will conse‐ quently be sampled by approximately 8×8 pixels, an oversampling that can be alleviated us‐ ing pixel binning. The 6 mm pupil diameter of the eye is demagnified to 1.87 mm at the WFS and each Hartmann pattern will consequently be sampled by ~13 lenslets across the diameter

**Figure 8.** Schematic drawing of the multi-reference WFS with spatial filtering.

using the Liou-Brennan eye model [35].

(Fig. 9).

**Figure 5.** GS beam footprints on DM1 (left) and DM2 (right).

**Figure 6.** RMS wavefront error of the PoC main module custom relay optics at the main module exit pupil for three retinal field positions (0, 2.5, and 3.6 deg).

#### *4.2.3. PoC WFS module*

A multi-reference WFS with spatial filtering (Fig. 8) has been implemented in both the DCAO demonstrator and the PoC prototype. The design greatly reduces system complexity by implementing a single spatial filter to reduce unwanted light from parasitic source reflec‐ tions and scattered light from the retina when imaging multiple Hartmann patterns with a single WFS camera.

strument image plane. An evaluation of the focal shift for the 575±10 nm wavelengths trans‐ mitted by the flash illumination bandpass filter using the Liou-Brennan Zemax eye model

**Figure 6.** RMS wavefront error of the PoC main module custom relay optics at the main module exit pupil for three

A multi-reference WFS with spatial filtering (Fig. 8) has been implemented in both the DCAO demonstrator and the PoC prototype. The design greatly reduces system complexity by implementing a single spatial filter to reduce unwanted light from parasitic source reflec‐ tions and scattered light from the retina when imaging multiple Hartmann patterns with a

[35] yields a ±6.9 µm focal shift at the retina (Fig. 7).

12 Adaptive Optics Progress

**Figure 5.** GS beam footprints on DM1 (left) and DM2 (right).

retinal field positions (0, 2.5, and 3.6 deg).

*4.2.3. PoC WFS module*

single WFS camera.

**Figure 7.** Chromatic focal shift over flash illumination bandpass filter bandwidth (575±10 nm) at the retina calculated using the Liou-Brennan eye model [35].

Transmitted GS light from the main module passes through the CLA and is reflected by a pel‐ licle beam splitter. A second polarizing filter (PFA) removes unwanted backscattered reflec‐ tions from the GS generation, and a lens brings the five GS beams to a common focus where they are spatially filtered by a single aperture (SF). A collimating lens finally relays the five beams onto a lenslet array (LA) with a focal length of 3.45 mm and a lenslet pitch of 130 µm.

The monochromatic WFS CCD camera has 1388×1038 pixels with a square pixel cell size of 6.45 µm, of which a central ROI of 964×964 pixels is used for wavefront sensing. The diameter of the diffraction limited focus spot of a lenslet is 2.44 λ *f* / d = 54 µm. Each spot will conse‐ quently be sampled by approximately 8×8 pixels, an oversampling that can be alleviated us‐ ing pixel binning. The 6 mm pupil diameter of the eye is demagnified to 1.87 mm at the WFS and each Hartmann pattern will consequently be sampled by ~13 lenslets across the diameter (Fig. 9).

**Figure 8.** Schematic drawing of the multi-reference WFS with spatial filtering.

**5.1. Cone photoreceptor imaging**

**5.2. Retinal capillary imaging**

Imaging of the cone photoreceptor layer (Fig. 10) is accomplished by focusing on deeper ret‐ inal layers. The variation in cone appearance from dark to bright in Fig. 10 is an effect of the directionality [40] or waveguide nature of the cones. The retinal photoreceptor mosaic pro‐ vides all information to higher visual processing stages and is many times directly or indi‐ rectly affected or disrupted by retinal disease. It is therefore of interest to study various parameters, e.g. photoreceptor spacing, density, geometry, and size, to determine the struc‐ tural integrity of the mosaic. An example of this is given in Fig. 11, where the cone density of the mosaic in Fig. 10 has been calculated. Cone spacing, where possible, was obtained from power spectra of 128×128 pixel sub-regions with a 64 pixel overlap. Spacing (*s*) was

Dual Conjugate Adaptive Optics Prototype for Wide Field High Resolution Retinal Imaging

Retinal capillaries, the smallest blood vessels in the eye, are difficult to image because of their small size (down to 5 µm), low contrast, and arrangement in multiple retinal planes. Even good-quality retinal imaging fails to capture any of the finest capillary details. The pre‐ ferred clinical imaging method is fluorescein angiography (FA), an invasive procedure in which a contrast agent is injected in the patient's bloodstream to enhance retinal vasculature contrast. The narrow depth of focus of both the DCAO demonstrator and the PoC prototype allows for imaging of retinal capillaries by focusing on the upper retinal layers. It is a noninvasive procedure with performance similar to FA [22]. An unfiltered camera raw image of the capillary network surrounding the fovea, the central region of the retina responsible for

**Figure 10.** DCAO image of cone photoreceptor layer. Variation in cone appearance from dark to bright is an effect of

the directionality or waveguide nature of cone photoreceptors.

), and the density profile was

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converted to density (*D*) using the relation *D* = sqrt(3) / (2s2

constructed by fitting a cubic spline surface to the distribution of density values.

sharp vision, is shown in Fig. 12, and a flat-fielded image is shown in Fig. 13.

**Figure 9.** Zemax simulation of Hartmann spot image (left) and actual WFS image (right).

### *4.2.4. PoC flash and imaging modules*

Retinal images are obtained by illuminating a 10×10 degree retinal field using a 4-6 ms spectral‐ ly filtered (575±10 nm) Xenon flash. A Canon EF 135mm f/2.0 L photographic lens is used to fo‐ cus reflected light from the dichroic beamsplitter onto the science camera, a 2452×2056 pixel Stingray F-504B monochromatic CCD with a square pixel cell size of 3.45 µm (Allied Vision Technologies GmbH, Stadtroda, Germany). The physical size of the full chip corresponds to a retinal FOV of 8.28×6.94 deg with a pixel resolution of 0.059 mrad (0.974 µm on the retina).
