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Thomas J T P Van Den Berg, Luuk Franssen, Joris Coppens; Straylight angular dependency. Invest. Ophthalmol. Vis. Sci. 2017;58(8):4225.
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© ARVO (1962-2015); The Authors (2016-present)
Ocular straylight is defined as the visual effect of light spreading around bright point light sources. It is the result of light scattering in the eye. It is the outer part of the PSF. Its intensity declines with angle approximately according angle-2 between 1 and 30 degrees. Straylight is specified by means of the straylight parameter, defined as PSF x angle2, which is more or less constant as function of angle for normal eyes. The angular dependency is important for quality of vision, and because it follows the physical characteristics of the significant scatterers in the eye.
Straylight measurements were performed with the C-Quant instrument from Oculus. This instrument measures at 7 degrees. We made adaptations to measure also at 2.5 and 3.5 degrees, by changing the length of the shaft of this instrument. Moreover a previous version of straylight meter was used, enabling measurements at 3.5, 10 and 28 degrees. Isolated clinical cases of media turbidity were studied and compared to large groups of normal controls. Straylight will be reported with the straylight parameter s, given logarithmically.
On average log(s) was higher by 0.100±0.023 (s.e.) for 3.5 degrees as compared to 7 degrees in controls (33 eyes, 30 subjects). Log(s) was higher by 0.194±0.014 (s.e.) for 2.5 degrees as compared to 7 degrees in another control study (118 eyes, 84 subjects). As these groups also included older individuals, and individuals with cataracts, the results were analysed with age and the average straylight value as independent variables. No significant difference was found as function of either. The 28 degree control value is larger by 0.18 log as compared to 7 degrees, according a study by De Waard et al. (n=34). Isolated clinical cases included patients with excessive amounts of straylight, yet (near) normal visual acuity, viz. 3 patients with pulverent congenital cataract and 2 patients with fish eye disease, which is a cornea condition. The angular dependencies were clearly different: For the congenital cataract straylight was relatively weak at small angles, with a relative difference of 0.16 to 0.40 at 2.5 degrees. For the fish eye patients straylight was relatively weak at large angles, with a relative difference of around 0.50.
In case of normal aging and early cataract formation the angular dependence of straylight is relatively stable. In case of special scatterers, one must be aware that the angular dependency may change.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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