Quantitative Photorefraction Using an Off-Center Flash Source
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.
Available from: Manuel Costa
- "Photorefraction can be implemented in three different approaches: orthogonal; isotropic; and eccentric photorefraction       . The differences arise basically from the type and arrangement of the optical components. "
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ABSTRACT: The importance of an early evaluation of infants’ visual system condition is long time recognized. Non-corrected visual disorders may lead to major vision and developmental non-reversible limitations in the future. Among the objective methods of refraction, photorefractive techniques are specifically designed for screening young children. Over the years a number of photorefraction systems with different grades of complexity and automation were developed. A critical problem that one needs to deal with in any approach to these systems is the interpretation and classification of the photorefraction images. In digital photorefraction conventional image processing operators and Fourier techniques were currently used. In this communication we will report on the use of Neural Networks for automated classification of digital photorefraction images.
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ABSTRACT: Early screening for refractive errors is highly desirable. Techniques for doing this must be usable with noninstructable subjects (infants), noninvasive, and relatively easy to use. Photorefraction has been used to examine infants' refractive status. However, unless cycloplegia is used, the results can be difficult to evaluate, inasmuch as the subject's plane of focus is not known. This paper describes a photorefraction system that is being used for routine screening of young infants, without cycloplegia and without using highly skilled personnel. The major innovation is the systematic presentation of attractive targets at distances that present a range of demands to accommodation. Changes in the fundal reflections, seen in the photographs taken as the infant views the different targets, can be interpreted unequivocally to identify severe myopia and hyperopia, anisometropia, heterotropia, and anisocoria. The results can also be quantified and are compared with retinoscopic refractions.
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