THE PHYSICS OF THE EYE
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.
The values come from a Tear Film Imager study of healthy eyes. Read the study abstract.
YOUR TEST
How often do you renew the film?
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.
THE SCALE
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.
62.3× thickness difference in the TFI measurement
Picture a main layer of 3,282 units and an oil film only 52.7 units deep on the same scale. The oil is thin, but its position matters: it forms the interface with air and changes how the water beneath it moves.
THE MECHANISM
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.
5.6× descriptive ratio between the two rates in the TFI paper
The TFI authors interpret the early thinning mainly through tangential flow and redistribution of the muco-aqueous layer, with the Marangoni effect among the dominant mechanisms. Evaporation remains present, but cannot reproduce the observed speed on its own in that early window.
PLAY WITH THE MODEL
How much room is left between blinks?
Change the interval between blinks and the modelled time to film breakup. OPI is the ratio between those values. Above 1 means that, in this model, the next blink arrives before breakup.
This is an orientation model, not an eye examination. Breakup time is measured clinically with dedicated methods, and results vary with the person, attention, humidity and air movement.
WHAT REMAINS OPEN
A living film has no single number
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. These values describe that sample and those conditions, not a personal threshold.
A blink itself lasts about 200–300 milliseconds, while the interval between blinks changes with attention, humidity and air movement.
Blinking changes with attention and environment. A reader who counts fewer blinks while reading has observed a momentary behaviour. This page cannot measure film thickness or establish whether an eye problem exists.
SOURCES
Where the numbers come from
- Tear Film Imager Findings Support a New Perspective on the Mechanisms of Tear Film Thinning in Healthy Subjects, TFI measurements and the evaporation-rate synthesis.
- Tear film lipid layer thickness as a function of blinking, the study of forceful blinking and the lipid layer.
- Relation between tear breakup time and spontaneous blink rate, the 159-participant sample and its mean values.
- TFOS DEWS II, Pathophysiology, the description of the film, blinking and the lipid front.
Scientific method: measured values remain tied to their measurement method, and ratios and conversions are calculated from the published values. The studies use different populations and methods, so comparisons are presented as bounded descriptions.