Characterization of the thickness of the tear film lipid layer using high resolution microscopy

Interferometry is an optical technique that have been used to quantify the lipid layer of the precorneal tear film, and to investigate the relationship between lipid layer thickness and tear film evaporation. However, the relationship between lipid layer thickness and the rate of evaporation is far from consistent. One possible reason is the inherent limit of contemporary interferometric systems, which employ objectives with relatively long depth of focus (DOF) (>15 μm or more), which tend to collect excessive extra-planar noise. This limitation may negatively affect the accuracy of the characterization and thickness measurement of the lipid layer. The current system incorporated an objective with limited DOF (∼1.5 μm) into a custom-built optical microscope to image the tear film lipid layer in humans. An algorithm was also developed to process these images. One major outcome of this system is that thick lipid layers exhibit higher variation in thickness values than thin or normal-thickness lipid layers. The variations may reflect the structural differences of the lipid layer, which may offer a novel dimension to explain the missing correlation between lipid layer thickness and evaporation. In summary, the development of the high resolution microscopy system and associated data processing algorithm may provide new insights into the lipid layer structure, topography and their relation to the tear film evaporation rate.