Quantitative photorefraction using an off-center flash source.
ABSTRACT When an eye is refracted by "eccentric photorefraction" with a flash source off-centered from a camera lens, a crescent of light is formed in the margin of the pupil. The size of the crescent varies directly with the eye's refractive error. This photographic method has been used in vision screening studies of young children where the appearance of a crescent indicated that the refractive error was above a certain threshold. Usually quantification of the refraction could not be achieved by the photorefractor but relied upon subsequent testing using retinoscopy. My research aimed to expand eccentric photorefraction so as to enable it to provide quantification of the eye's refractive error. This was achieved by varying the eccentricity of the flash source from the camera lens and then calibrating the instrument over a large range of refractive errors. The calibration modified a previously derived optical relation which defined the eye's refractive error in terms of the eccentricity of the source for a given pupil size. Eccentric photorefraction of 26 infants and children aged 7 to 48 months showed a good correlation with retinoscopy (r = 0.82). It is concluded that this method would be complementary to other photorefractive methods (e.g., isotropic) particularly as it is able to measure a large range of refractive errors once the astigmatic meridians of the eye are known.
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ABSTRACT: The accuracy and precision of retinoscopy need to be evaluated in an objective and unambiguous manner to judge an individual's performance in this task during training or in research for consistent measurements of refractive error. This study describes and evaluates a psychophysical technique for obtaining simultaneous and unbiased estimates of accuracy and precision in retinoscopy. Subjects with zero to 12 years of experience with retinoscopy performed the psychophysical task on a model eye (75 subjects) and on a cyclopleged human eye with spherical refractive error (30 subjects). Subjects made forced choice judgments of 'with' or 'against' for lens powers within ±0.5 D of expected neutrality (in 0.12 D steps), each placed 20 times before the eye in random order. Accuracy and precision were determined from the mean and standard deviation of the resultant psychometric function. Subjects could be qualitatively divided into those with good and poor accuracy and precision based on the task outcomes. The median and inter-quartile range of accuracy (no experience: 0.16 ± 0.34 D; four or more years of experience: 0.06 ± 0.11 D) and precision (no experience: 0.30 ± 0.39 D; four or more years of experience group: 0.13 ± 0.08 D) improved with task experience (p < 0.001). Median accuracy and precision in the human eye were similar to the model eye (p > 0.8). Accuracy and precision were poorly correlated with each other for both the human eye and model eye (|ρ| ≤ 0.20; p ≥ 0.09 for all). The psychophysical retinoscopic task could differentiate subjects based on their accuracy and precision and also capture key elements of improvement in retinoscopic performance with experience. Retinoscopic performance using this task was similar to previous reports using routine clinical retinoscopy. Therefore, the psychophysical technique may be used to evaluate and monitor objectively retinoscopic performance.Clinical and Experimental Optometry 10/2013; · 1.26 Impact Factor
- Ophthalmology 12/1992; 99(12):1785-1795. · 6.17 Impact Factor
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ABSTRACT: Objective This study aimed to determine the ability of healthcare professionals and lay volunteers to grade photoscreening photographs.Ophthalmology 05/1998; 105(5):856-863. · 5.56 Impact Factor