Potential vision tester using Maxwellian view, small pupil, and different levels of wavefront correction with adaptive optics

Abstract

We demonstrate a potential vision tester (PVT) designed to study and improve the accuracy of visual acuity (VA) measurements in the aging eye. Key features include a high-resolution visual display presented in Maxwellian view, a 3 mm pupil to limit wavefront (WF) aberrations, a Hartmann Shack wavefront sensor to quantify the ocular aberrations, and a deformable mirror to correct optical aberrations. VA was measured using four alternative forced choices for a single black-on-white E stimulus in each trial, with three different levels of correction for the ocular aberrations. For normally sighted subjects, VA was not significantly better when higher-order aberrations beyond second order were corrected.

Fig. 1.

A) Schematic of the optical layout of the PVT2 system. In the visual stimulus channel (orange-shaded), a high-resolution OLED microdisplay is placed at a plane conjugate to the retina to present the stimulus, thus serving as both the source and the retinal target plane of a Maxwellian view system. Subsequent pupil and retinal planes are carefully separated. Light from the microdisplay is collimated by an achromatic lens (L1). There is a pupil plane between the two achromatic lenses (L1,L2). Next the beam passes through the second achromatic lens (L2) and a short pass dichroic mirror (SPD), forming a retinal plane prior to a fold mirror M1. Together L2 and a reflective spherical mirror (SM1) image the pupil plane of the visual stimulus channel onto the deformable mirror (DM). Subsequently, light emerging from the DM is relayed to the eye via reflective spherical mirror SM2, fold mirrors M2 and M3, spherical mirror SM3, and fold mirror M4. Focusing the retinal plane of the stimulus channel on to the retina is performed by the refractive optics of the eye and the DM, which is in a pupil plane conjugate to the pupil of the eye. The unwanted astigmatism introduced by the horizontal off axis folding of SM1 is compensated by vertical folding at SM2 and SM3. The dashed blue line indicates the HS-WF channel. An 854 nm SLD is introduced into the visual path using a beamsplitter (BS) to act as a wavefront beacon. Light returning from the eye is split from the visual channel by the SPD beamsplitter and relayed to the HS-WF sensor. To reduce corneal specular reflections, a pair of cross-polarizers (P1, P2) is placed in front of the SLD (P1) and the HS-WF (P2). Two pupil cameras, angled such that their optical axes converge at a point 80 mm along the Z-axis from their centers, monitored the eye’s pupil for alignment. B) Photograph of the PVT2 optical benchtop setup, with the OLED microdisplay at the top left and the pupil of the eye at the far right. To minimize captions, only the main components are labelled.

Fig. 2.

Spot diagrams from the Zemax model, for the stimulus channel across +/- 2 deg on the retina (3.8 mm on the display). These spots represent geometric point spread function across visible wavelengths (blue spots: 450 nm, green spots: 550 nm, and red spots: 650 nm), and the black circle represents the Airy disk.

Fig. 3.

The VA data of a 32-yr old female subject (F1037) for the Z₂ (left), Z₂-Z₄ (center), and Z₂-Z₆ (right) correction conditions. The orange dots represent the proportion of correct responses across the presented letter sizes (MAR). Z₂: 50% threshold VA (0.93 MAR), SD (0.22 MAR), Z₂-Z₄: 50% threshold VA (0.72 MAR), SD (0.43 MAR), and Z₂-Z₆: 50% threshold VA (0.76 MAR), SD (0.25 MAR). The blue line represents the cumulative Gaussian fit to the VA data.

Fig. 4.

Association between VA measurements and residual RMS error for the first trial across different wavefront correction conditions. The top row shows the 50% threshold VA (MAR) as a function of residual RMS error (microns) for Z₂ (left column), Z₂-Z₄ (center column), and Z₂-Z₆ (right column) correction conditions. The center row shows the SD (MAR) as a function of residual RMS error (microns), and the bottom row shows the CV as a function of residual RMS error (microns) across all the wavefront correction conditions. Each colored dot represents an individual subject, and the dotted line represents the trend line (statistically not significant). For Z₂-Z₄ correction condition (top center), the 50% threshold VA had a significant positive correlation with residual RMS error shown as solid trend line (VA = 5.01 x Residual RMS + 0.243, r² = 0.729, p = 0.019).

Fig. 5.

Association between VA measurements and residual RMS error for the time 2 measurements across different wavefront correction conditions. The top row shows the 50% threshold VA (MAR) as a function of residual RMS error (microns) for Z₂ (left column), Z₂-Z₄ (center column), and Z₂-Z₆ (right column) correction conditions. The center row shows the SD (MAR) as a function of residual RMS error (microns) and the bottom row shows the CV as a function of residual RMS error (microns) across all the wavefront correction conditions. Individual subject data are denoted by colored dots, with each subject's color code consistent with the representation used in Fig. 4 for ease of comparison. The dotted line represents the trend lines (statistically not significant).

Fig. 6.

The 50% threshold visual acuity (VA) plotted as a function of residual RMS error in microns across different correction conditions. The black, blue, and red dots correspond to VA for the Z_2, Z₂- Z₄, and Z₂-Z₆ correction conditions, respectively.

Fig. 7.

Higher order Zernike coefficients of the ocular wavefront error of three subjects (M850, F007, F1037). M850 is the 33-yr old male subject with the most inconsistent performance across the correction conditions. The 50% threshold +/- SD of VA for Time 1 were Z₂ : 0.68 +/- 0.14; Z₂-Z₄: 1.1 +/- 0.34; Z₂-Z₆: 0.79 +/- 0.29, for Time 2 were Z₂ : 1.3 +/- 0.39; Z₂-Z₄: 1.5 +/- 0.69; Z₂-Z₆: 1.1 +/- 0.41. F007 is a 46-yr old female subject with largest RMS wavefront error among the 10 ten subjects, but with better performance on VA than subject M850. Her 50% threshold +/- SD of VA for Time 1 were Z₂ : 1.3 +/- 0.22; Z₂-Z₄: 0.76 +/- 0.41; Z₂-Z₆: 0.70 +/- 0.31, for Time 2 were Z₂ : 0.91 +/- 0.21; Z₂-Z₄: 0.57 +/- 0.48; Z₂-Z₆: 0.73 +/- 0.31. In comparison a typical subject F1037 is the 32-yr old female subject shown in Fig. 3. The 50% thresholds +/- SD of VA for Time 1 were Z₂ : 0.52 +/- 0.33; Z₂-Z₄: 0.79 +/- 0.29; Z₂-Z₆: 0.66 +/- 0.36, for Time 2 were Z₂ : 0.94 +/- 0.23; Z₂-Z₄: 0.72 +/- 0.43; Z₂-Z₆: 0.77 +/- 0.26.