Abstract
Purpose: :
Retinal straylight is the cause of important visual disturbances such as glare hindrance and contrast loss. It assesses the functional spreading of light (as e.g. seen around bright lights at night) over angles of 1 to 90 degrees. It addresses part of visual function not accessible with other means, such as visual acuity, contrast sensitivity and optical techniques, addressing light spreading over much smaller angles. How independent are those two domains?
Methods: :
A forced choice psychophysical assessment technique was developed for objective assessment of straylight, including a reliability estimate ("Compensation Comparison", implemented in a commercial product, C-Quant from Oculus). The outcomes were compared to classical visual function measures (visual acuity and contrast sensitivity) in 2400 subjects showing a high level of independence. Literature models for different aspects of the optical irregularities in the eye media, in particular Thibos et al. JOSA A 2002 for the aberration structure and Van den Berg et al. VR 1999 for small particle scattering, were used to study potential overlap between the two domains. The Thibos et al. model was extrapolated to allow inclusion of the high orders of aberration not normally included in aberrometry, but potentially visible with double pass.
Results: :
As a reference the full scale psf standard model of Vos et al. CIE 1999 was used. The modeled aberration structures of the eye proved to predict the central part of the psf up till about 0.3 degrees if extreme high orders were included. For angles >1 degree predicted values were far below the actual values of the psf. Small particle scattering was essential to predict the psf for angles above 1 degree.
Conclusions: :
In the assessment of disturbances to the optical media two domains must be discriminated: the aberration domain and the small particle domain, with corresponding parts (small angle vs large angle) to the psf. Straylight typically originates from irregularities in the optical media of small characteristic size (collections of particles sized 10 micrometer and below), as opposed to disturbances to the central part of the psf (<0.3 degrees), originating from large scale irregularities (refractile humps and bumps extending over 100 micrometer and more). From this the independence of straylight as compared to VA and CS can be understood. Such complementary aspects of media disturbances must be considered in aging of the lens, corneal haze, PCO, multifocal IOL artifacts, corneal scars, vitreous turbidity, etc. Straylight assesses a domain of visual function not accessible with classical or optical means.
Keywords: aberrations • optical properties • cataract