**1. Introduction**

The precorneal tear film is traditionally described as a structure made of three layers which make up a thickness of 2–5.5 μm [1]. The thickness of the tear film was a controversial topic, with many attempts to measure the full thickness through different imaging modalities. However, recent publications such as the DEWS II Tear Film state that ultrahigh resolution Optical Coherence Tomography (OCT) has recently resolved the debate over the tear film thickness. Furthermore, the tear film is now regarded as a complex blended two-layer structure comprising of an outer lipid layer and an inner muco-aqueous layer [1, 2]. However, to better understand the precorneal tear film, the traditional approach will be taken in this review. The three tear film layers serve to not only protect and provide nutrition to the cornea, but also act as the first refractive surface for light entering the eye. Of the three layers, the largest is the middle aqueous. (e.g. **Table 1**).
