Two‐dimensional simulation of eccentric photorefraction images for ametropes: factors influencing the measurement

Abstract

Computational modelling demonstrates the principles and limitations of photorefraction to help users avoid potential measurement errors. Factors that could cause clinically significant errors in photorefraction estimates include high refractive error, vertex distance and magnification effects of a spectacle lens, increased higher-order monochromatic aberrations, and changes in primary spherical aberration with accommodation. The impact of these errors increases with increasing defocus.

Calibration of the PlusOptix PowerRef 3 with change in viewing distance, adult age and refractive error

“…The true baseline refractive state for each viewing distance therefore varied between individuals. The lens power plus uncorrected refractive state might also then shift the refractive state measurement outside the linear range of the instrument . Figure presents the mean measured refractive state during lens placement (mean of 100 samples collected over 2 s, pooled over four rounds) as a function of the measured refractive state pre lens placement (mean of 100 samples collected over 2 s, averaged across four rounds) plus the lens power.…”

“…Spherical trial lenses (+1, +2, +3, +4, −1 and −2 D) were placed in front of the occluded eye in sequence, and in the above order, for at least 3 s each to ensure sufficient data collection. Participants with significant refractive errors who did not have contact lenses participated without optical correction to avoid any possible interference of the optical properties of their spectacle lenses with the measurements . The angle between the photorefractor camera and the participant's gaze was approximately 4 deg, with the camera angled up towards the eye, placing the camera axis close to the axis passing perpendicularly to the cornea through the centre of the pupil.…”

“…Eccentric photorefraction in combination with Purkinje image eye tracking provides a rapid method for the objective, binocular, simultaneous assessment of refractive error, pupil size, and eye position. The double‐pass eccentric photorefraction method estimates refractive state using the slope of reflected light distributed across the pupil with a number of factors known to cause variation in the measurement . Additionally, these instruments permit open‐field measurements from a typical working distance of 1 m .…”

“…However, the slope of the relative calibration function, which we define as the calibration factor (CF) for the purposes of this study, can vary in an individual by up to 50% from this default . Individual calibration must be performed carefully as a number of optical factors will impact the results …”

“…The double-pass eccentric photorefraction method estimates refractive state using the slope of reflected light distributed across the pupil with a number of factors known to cause variation in the measurement. 1,2 Additionally, these instruments permit openfield measurements from a typical working distance of 1 m. 3 These features, with a high video sampling rate, make photorefraction a valuable tool in the assessment of typical adults and children, as well as clinical populations. [4][5][6][7] Versions of the PowerRefractor instrument 6 have been used to study the near triad of accommodation, vergence and pupil responses in normal and clinical populations.…”