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Thomas J T P Van Den Berg, Ivanka J.E. van der Meulen; Understanding the (lack of) relationship between Straylight and Visual Acuity with clinical examples. Invest. Ophthalmol. Vis. Sci. 2016;57(12):209.
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© ARVO (1962-2015); The Authors (2016-present)
Straylight is the visual effect of light scattering in the eye. It corresponds to the peripheral part of the (functional) PSF. In ophthalmology, visual acuity is the dominant visual function assumed to explain patient complaints. In case a patient suffers from straylight increase, it may be ill understood if visual acuity is not deteriorated. The present study aims to show how the (lack of) relation between straylight and visual acuity can be understood.
The CIE standard for the PSF was used to derive theoretical relations between visual acuity and straylight (Vos et al. 1999 CIE Coll. 135/1:1-9), showing complete independence (no effect on straylight) for pure refractive type blur (defocus and higher order aberrations) to be expected. This result was verified experimentally using defocusing lenses and straylight measurement with the C-Quant from Oculus. Six subjects participated. Using derived MTFs complete independence (no effect on visual acuity) is also found for the reverse situation, of increased light scattering. Several clinical conditions were checked for these effects, including cornea conditions, and cataracts.
The MTF shows little effects in case of customary values for straylight in clinical examples. Correspondingly, visual acuity predictions are not affected. Clinical cases were found with this kind of behavior. The most extreme cases in the sense of high straylight elevation (1 log or 10x and more) and low visual acuity effects (less than 3 lines, or 2x) included: (1) for defects to the crystalline lens, polar cataracts, and pulverent congenital cataract; (2) for defects to the cornea, crystalline corneal dystrophy, and fish eye disease.Experimental testing of the potential effect of refractive blur on straylight showed virtually no such effect (less than 0.1 log) to exist for all dioptric errors tested, corresponding to visual acuities worse than 1 logMAR . The typical clinical condition corresponding with this situation is deformation of the cornea without haze. Corneal scarring can show this, but also the reverse. For the mentioned conditions between 2 and 23 patients were included.
Straylight and visual acuity are quite independent aspects of quality of vision. The independence can be understood on the basis of simple optical considerations. Clinical conditions exist of extreme straylight hindrance and mild acuity problems.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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