ABSTRACT
Although, nowadays, the physical (light scattering) explanation of straylight and glare is almost self-evident, around WWII, a great many studies were devoted to the question whether maybe (partly) lateral neuronal interactions played a role too. The thesis work of Vos (1963) can be considered an endpoint in this discussion, in favor of the notion that light scattering is the sole determinant of straylight and glare. Later, Vos played an important role (e.g., disability glare review (Vos 1984)) in the international
Contents 23.1 Introduction 349 23.1.1 Disability glare and discomfort glare 350 23.2 Assessment 350 23.2.1 Equivalent luminance approaches 350 23.2.2 Glare testing 350 23.2.3 Optical measurements 351 23.2.4 Forward versus backward scattering 352 23.2.5 Light scattering ⇒ straylight ⇒ glare and quality of vision 352 23.3 Physics 353 23.3.1 Basic optics of scattering 353 23.3.2 Sources of light scattering in the eye 354 23.3.3 Physics of lenticular light scattering 355 23.3.4 Ciliary corona 356 23.3.5 Physics of corneal light scattering 356 23.4 Straylight in normal eyes 357 23.4.1 Normal population data collections 357 23.4.2 Wavelength dependence issue 357 23.5 Clinical straylight data 358 23.5.1 Lens aging and cataract 358 23.5.2 Pseudophakia 359 23.5.3 Corneal conditions and refractive surgery 359 23.5.4 Pigmentation insufficiencies 359 References 360
normalization committee Commission International d’Éclairage (CIE), well known among others for the definition of the spectral sensitivity function of the human eye Vλ, and the chromaticity diagram. This committee established that straylight must be used as norm for quantification of disability glare, defined using the equivalent luminance principle. Straylight is part of the functional point spread function psf of the human eye. It constitutes the periphery from say 1° to 100°, whereas the center up till say 10′ is dominated by errors of refraction and by diffraction. It is efficient to express straylight with the “straylight parameter” defined as
Straylight parameter s = θ2psf = θ2Leq/Ebl,
among others because s is relatively insensitive to differences in measurement angle because of the approximate Stiles-Holladay rule (van den Berg 1995). In normal, young eyes log(s) = 0.9, but increases with age and cataract formation. An increase of a factor of 3 (log(s) = 1.4) is considered a serious visual handicap and reason for cataract surgery. As limit factor for safe driving an increase of a factor of 4 (log(s) = 1.5) and for demanding professions such as pilots, a factor of 2 (log(s) = 1.2) is proposed (Figure 23.1).
