Empirical variability in the calibration of slope-based eccentric photorefraction

Abstract

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.

Revisiting the impact of phenylephrine hydrochloride on static and dynamic accommodation

“…Raw photorefraction videos were analyzed using custom-designed Matlab algorithms that detected the pupil edge, first Purkinje image position and the slope of the luminance profile across the pupil in each video frame. [13] A frame was determined to contain a blink if the eyelid covered part or entire pupil of either eye and such frames were removed from the analysis. [13] Typically, a 15 s video with 900 frames contained approximately 2–3 blinks that spanned 6–10 frames each.…”

“…[13] A frame was determined to contain a blink if the eyelid covered part or entire pupil of either eye and such frames were removed from the analysis. [13] Typically, a 15 s video with 900 frames contained approximately 2–3 blinks that spanned 6–10 frames each. Reflections from the IR filter or the artificial pupil before the right eye were rare and these frames were also discarded.…”

Utility of Theoretical Hirschberg Ratio for Gaze Position Calibration

“…9Y11 Subjects monocularly fixated on an array of visible light-emitting diodes (LEDs) arranged in a rotatable mount at 95 cm viewing distance in an otherwise dark room ( Fig. 32 The top row of LEDs were 4 mm below the camera aperture whereas the other three rows were placed at 7 mm from the previous row. Each LED was separated from the adjacent one by 4 degrees, with the LEDs on the left and right extremes subtending 24 degrees of gaze angle from straight-ahead viewing.…”

Determining the relative contribution of retinal disparity and blur cues to ocular accommodation in Down syndrome

“…Refractive power and eye position calibration was attempted on all participants using spherical full aperture trial lenses and prisms of known power (+4, +3, +2, +1, −2, −4D spherical lenses and 4, 8, 12 and 16 Δ base in and base out prisms)1213. Lenses/prisms were placed in front of the non-dominant eye in combination with an infrared (IR) transmitting filter (Edmund Optics, USA) to occlude visible light.…”