The precorneal tear film (ie, tear film) is the first ocular structure that light encounters. Because of the lower refractive index of air relative to that of the tear film, the air–tear film interface at the surface of the cornea constitutes a major refractive element of the eye, directing light toward the cornea.
Evidence supports a 2-phase model of the tear film, in which a lipid layer overlies a mucoaqueous layer.
Elevated tear film osmolarity is diagnostic of dry eye syndrome.
There is mounting evidence that ocular surface inflammation is integral to the pathology of dry eye syndrome.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.