Refraction estimates from eccentric infrared (IR) photorefraction depend critically on the calibration of luminance slopes in the pupil. While the intersubject variability of this calibration has been estimated, there is no systematic evaluation of its intrasubject variability. This study determined the within subject inter- and intra-session repeatability of this calibration factor and the optimum range of lenses needed to derive this value. Relative calibrations for the MCS PowerRefractor and a customized photorefractor were estimated twice within one session or across two sessions by placing trial lenses before one eye covered with an IR transmitting filter. The data were subsequently resampled with various lens combinations to determine the impact of lens power range on the calibration estimates. Mean (±1.96 SD) calibration slopes were 0.99 ± 0.39 for North Americans with the MCS PowerRefractor (relative to its built-in value) and 0.65 ± 0.25 Ls/D and 0.40 ± 0.09 Ls/D for Indians and North Americans with the custom photorefractor, respectively. The ±95% limits of agreement of intrasubject variability ranged from ±0.39 to ±0.56 for the MCS PowerRefractor and ±0.03 Ls/D to ±0.04 Ls/D for the custom photorefractor. The mean differences within and across sessions were not significantly different from zero (p > 0.38 for all). The combined intersubject and intrasubject variability of calibration is therefore about ±40% of the mean value, implying that significant errors in individual refraction/accommodation estimates may arise if a group-average calibration is used. Protocols containing both plus and minus lenses had calibration slopes closest to the gold-standard protocol, suggesting that they may provide the best estimate of the calibration factor compared to those containing either plus or minus lenses.

Purpose Studying the full shape crystalline lens geometry is important to understand the changes undergone by the crystalline lens leading to presbyopia, cataract, or failure of emmetropization, and to aid in the design and selection of intraocular lenses and new strategies for correction. We used custom-developed three-dimensional (3-D) quantitative optical coherence tomography (OCT) to study age-related changes in the full shape of the isolated human crystalline lens. Methods A total of 103 ex vivo human isolated lenses from 87 subjects (age range, 0–56 years) were imaged using a 3-D spectral-domain OCT system. Lens models, constructed after segmentation of the surfaces and distortion correction, were used to automatically quantify central geometric parameters (lens thickness, radii of curvatures, and asphericities of anterior and posterior surfaces) and full shape parameters (lens volume, surface area, diameter, and equatorial plane position). Age-dependencies of these parameters were studied. Results Most of the measured parameters showed a biphasic behavior, statistically significantly increasing (radii of curvature, lens volume, surface area, diameter) or decreasing (asphericities, lens thickness) very fast in the first two decades of life, followed by a slow but significant increase after age 20 years (for all the parameters except for the posterior surface asphericity and the equatorial plane position, that remained constant). Conclusions Three-dimensional quantitative OCT allowed us to study the age-dependency of geometric parameters of the full isolated human crystalline lens. We found that most of the lens geometric parameters showed a biphasic behavior, changing rapidly before age 20 years and with a slower linear growth thereafter.

Eccentric infrared photorefraction is an attractive tool for measuring refractive errors of young children and uncooperative subjects, for it allows quick and non-invasive acquisition of data from both eyes simultaneously over a reasonably large dioptric range. Accuracy of refraction in this technique depends on calibration of luminance slope formed across the pupil into diopters (defocus calibration factor). Commercial photorefractors, like the PowerRef 3™ used in this study, employ an universal defocus calibration factor from one population (Caucasian) to convert raw data of all populations. This study reports significantly larger defocus calibration factors of PowerRef 3™ in 132 East Asian, African and Indian eyes, relative to the machine's default calibration (p < 0.001). The calibration slope of 50 Indian eyes was over-estimated by 64 ± 11% (mean ± 95%CI), vis-à-vis, retinoscopy (p < 0.001). The error reduced to ~6–7% upon rescaling the data using a calibration factor specific for Indian eyes or to that individual (p > 0.9, relative to no over-estimation). Our results therefore strongly suggest the use of an ethnicity- or individual-specific defocus calibration factor for accurate estimation of refraction using photorefraction. Inaccurate refraction estimates due to calibration errors will otherwise severely undermine the advantages of this technique.

We present a new in vitro instrument for measuring shape and wavefront aberrations of the primate crystalline lens, both on-and off-axis, while simulating accommodation with a motorized lens stretching system. The instrument merges spectral domain optical coherence tomography (SD-OCT) imaging and ray tracing aberrometry using an approach that senses wavefront aberrations of the lens with the OCT probing beam. Accuracy and repeatability of aberration measurements were quantified. Preliminary experiments on two human and four cynomolgus monkey lenses demonstrate the ability of the system to measure the lens shape, spherical aberration and peripheral defocus, and their changes during simulated accommodation.

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