Abstract
Purpose:
The visual phenomenon of radiation of light around a bright light source (“ciliary corona”, glare, straylight) is known as result of forward light scattering in the eye. The scattered light corresponds to the peripheral part of the ocular point-spread-function. The slitlamp image (or Scheimpflug image) of the human eye lens, results from backward light scattering. Light scattering can be predicted from the characteristics of scattering elements. Reversely from scattering characteristics predictions for the scattering elements can be derived. Optical bench study of human eye lenses will be presented to (1) detail the light scattering characteristics, (2) the (lack of) relationship between forward and backward scattering, and (3) the comparison to the known in vivo psf including age effects.
Methods:
Data on 15 human eye lenses, isolated from donor eyes, from an earlier study, with LOCSIII values ranging from NO=1 to 5, with median 2.3 are used. The angular and wavelength characteristics are compared to light scattering theory, in particular the strong blue dominant Rayleigh pattern, with 1/λ4 dependence, valid for small particles (molecules), and the general Mie theory.
Results:
Scatter plots testing Rayleigh behaviour for different angles show that small particles dominate light scattering for angles larger than 30 degrees, including backward directions. Surprisingly, this holds for all lenses, including advanced cataractous states. At backward directions appraoching 180 degrees, light sctattering intensifies, indicative for the process of rough surface reflection. At forward directions, as relevant for the psf, light scattering has essentially different characteristics, typical for particles with sizes of the order of wavelength. This scattering corresponds very closely to what is known for the peripheral part of the in vivo psf according the CIE standard functions (CIE Collection 1999;135/1:1-9).
Conclusions:
Different processes govern forward and backward scatter in the human eye lens. Correspondingly forward and backward scatter are not strongly related, explaining why the slit image is not an accurate predictor of forward scatter or straylight. Forward light scatter characteristics correspond very closely to the periphery of the in vivo psf, indicating that particles of about wavelength size are responsible for in vivo light scattering, irrespective of cataractous status.