Quantifying natural higher order aberration(s) in emmetropic human eyes and objectively evaluating retinal image quality.

Abstract

Aim: This study aimed to quantify the internal, corneal and total higher order aberrations (HOAs) in the healthy human eye using ray tracing technology in bright and dim light conditions. The study also aimed to evaluate the retinal image quality and contrast modulation at different spatial frequencies, objectively using image-quality metrics derived from wavefront aberrations.

Methodology: A cross-sectional prospective observational study was conducted on 521 emmetropic young adults. HOAs including coma, trefoil, spherical aberration and secondary astigmatism were measured separately for the cornea, internal and total eye using a ray-tracing wavefront aberrometer at mesopic and photopic pupil sizes. The Strehl ratio, derived from the point spread function (PSF), was used to evaluate the retinal image quality of the eye. Contrast modulation at spatial frequencies of 5, 10 and 15 cycles per degree was assessed through the modulation transfer function (MTF).

Results: Mean age of participants was 25.4 ± 4.8 years (54.5% male). The total HOAs of internal, corneal and total eye were 0.186 ± 0.075, 0.191 ± 0.112 and 0.222 ± 0.075 μ, respectively for a 5-mm pupil (dim light condition), and 0.069 ± 0.031, 0.061 ± 0.094 and 0.076 ± 0.031 μ, respectively, for a 3-mm pupil (bright light condition). The Strehl ratio was 0.16 ± 0.124 (5 mm pupil) and 0.601 ± 0.19 (3 mm pupil). A normative database was created and correlations drawn for the HOAs, PSF and MTF (using image quality metrics) for the cornea, internal and total eye at mesopic and photopic pupil sizes.

Conclusion: These findings provide normal population (young healthy adults) reference ranges for HOAs, retinal image quality and contrast modulation at higher spatial frequencies using ray tracing technology. The presence of these natural wavefront aberrations enhances the visual performance and may not need to be corrected. The separate assessment of aberrations and derived image quality metrics for the cornea, internal and total eye can serve as a benchmark for diagnosing and monitoring ocular conditions and allows targeted treatment planning.