# Three Micrometres of Water Between Air and Sight

A film thinner than a speck of dust rests on the cornea. When you blink, the lid spreads it, mixes its layers and brings fresh oil back to the surface.

In a recent measurement, the muco-aqueous layer was **3,282 nanometres** thick, while the surface lipid layer was only **52.7 nanometres**. The thickness difference is **62.3 times**.

## Measure your blink rate

Press the button once for every blink during 30 seconds. The result turns your count into blinks per minute and an average interval between blinks. The timer uses no camera and saves no count.

## One thick layer and one almost invisible layer

The tear film is not a uniform sheet of water. Its muco-aqueous core carries salts, proteins and mucins. At the surface, lipids form a sheet measured in tens of nanometres and spread as the lid rises.

## A blink pushes a surface wave

A moving lid does more than close the eye. On the upstroke, surface tension spreads the lipids and sends a front across the muco-aqueous film. The TFOS DEWS II report describes a speed of about **10 millimetres per second** and stabilisation after **1 second or more**.

In the TFI study, the reported muco-aqueous thinning rate was **63 nanometres per second**, while the weighted mean evaporation rate from the literature synthesised in the paper was **11.3 nanometres per second**. The descriptive ratio between them is **5.6 times**.

## How much room is left between blinks?

The page model uses OPI, the modelled time to breakup divided by the interval between blinks. Above 1 means that the next blink arrives before breakup in this model.

In a study of 159 healthy young women, the mean observed rate was **18.6 blinks per minute**, and the mean time to film breakup was **18.9 seconds**. The mean rate implies an interval of **3.23 seconds**.

Those values describe that sample and those conditions. The page test is a personal observation, not an eye examination, and cannot establish whether an eye problem exists.

## Sources

- [Tear Film Imager Findings Support a New Perspective on the Mechanisms of Tear Film Thinning in Healthy Subjects](https://pubmed.ncbi.nlm.nih.gov/42112682/)
- [Tear film lipid layer thickness as a function of blinking](https://pubmed.ncbi.nlm.nih.gov/7924337/)
- [Relation between tear breakup time and spontaneous blink rate](https://www.sciencedirect.com/science/article/pii/S089289670000033X)
- [TFOS DEWS II, Pathophysiology](https://www.tfosdewsreport.org/report-pathophysiology/106_36/en/)