Straylight as measure for quality of vision and the tear film

Purpose In this presentation an overview will be given on the basic optical effects connected with the tear film. A prominent role of the tear film is to create a smooth surface for the most powerful refractive interface of the eye, i.e. the air‐cornea interface. On the other hand, the tear film itself can disturb proper imaging if light is scattered in it, or if its surface is not perfectly smooth. Methods Surface effects on optical image formation can be described with aberrometry. Only, in the case of the epithelial surface, present aberrometers do not have sufficient resolution to describe the fine irregularities. The group of Thibos designed a way to address this problem using the shape of the spot images of the Shack‐Hartmann aberrometer. Effects of light scattering can be assessed using straylight measurement (implemented in the C‐Quant from Oculus). Results Both aberrometric and straylight outcomes combine to form the ocular point‐spread‐function defining the functional problem originating from optical defects in the tear film. The aberrometric errors dominate the central part of the PSF, and thus visual acuity. Straylight corresponds to the peripheral part of the PSF, and dominates in complaints like glare and hazy vision. Literature models for both aspects of the optical problems in the eye media were used to delineate their effects on the PSF. Conclusion Two domains must be differentiated: the aberration domain and the small particle scatter domain, with corresponding parts (small angle vs large angle) to the PSF. Straylight typically originates ffrom small particles, as opposed to aberrations originating from refractile humps and bumps extending over large distances (tear film surface) or small small distances (epithelial surface).Commercial interest