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.
Optik - International Journal for Light and Electron Optics 03/2011; 122(6-122):553-556. DOI:10.1016/j.ijleo.2010.04.010 · 0.68 Impact Factor
Ophthalmology 12/1992; 99(12):1785-1795. DOI:10.1016/S0161-6420(92)31722-1 · 6.14 Impact Factor
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ABSTRACT: The authors describe a new photoscreening camera designed to detect amblyogenic factors, including strabismus, asymmetric and abnormal refractive errors, and media opacities. The photoscreener uses eccentric photorefraction principles and provides two meridian photographs of the retinal reflex.
Pass/fail screening data from the photographs of 202 nondilated children as determined by two masked observers were compared with data from complete ophthalmologic examinations.
The photoscreener had a sensitivity rate of 87%, a specificity rate of 89%, a positive predictive value of 93%, a negative predictive value of 80%, and an overall agreement rate of 88%. The prescreening probability for amblyogenic factors was 63%. Constant strabismus was detected in all cases. Five intermittent strabismus cases were missed. Two patients with myopia, three patients with hyperopia, and five patients with astigmatism were missed. All media opacities were detected.
This new two-flash photoscreening camera, which uses high-speed Polaroid film, is an accurate, reliable method of detecting amblyogenic factors in undilated children. The camera offers promise as a useful mass screening tool.
Ophthalmology 01/1993; 99(12):1785-95. · 6.14 Impact Factor
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