Tear film break-up time (BUT)

Tear film break-up time, often abbreviated to BUT or TBUT, is a measurement that indicates how long the tear film remains intact on your cornea after you blink. After each blink, the tear film is redistributed across the surface of the eye. In a healthy tear film, this layer remains even and stable, but in some people the film breaks up more quickly, leading to dry patches.

The BUT is measured in seconds and is calculated from the moment you last blinked until the moment the first dry spot or break in the tear film becomes visible. An optician, optometrist or ophthalmologist carries out this measurement using a slit lamp and a dye, usually fluorescein. The dye makes the tear film visible under blue light, allowing the moment it breaks to be clearly seen.

A BUT of 10 seconds or longer is generally considered normal. A shorter BUT, for example under 5 to 10 seconds, may indicate a less stable tear film. This is common in people with dry eyes or in contact lens wearers who experience symptoms of dryness. The measurement provides insight into the quality of the tear film and helps in assessing eye comfort and lens suitability.

Why is BUT measured?

Tear film break-up time is measured to assess the stability of your tear film. This is particularly useful for symptoms such as dry, burning or irritated eyes, or for contact lens wearers experiencing discomfort. A short BUT may indicate that the tear film is not functioning properly, which affects the comfort of your eyes and your ability to wear contact lenses.

What factors influence BUT?

Various factors can influence tear film break-up time. Dry eyes, reduced tear production, an altered composition of the tear film (for example, too few lipids or mucin), environmental factors such as air conditioning or wind, and wearing contact lenses can all lead to a shorter BUT. Age, hormonal changes and certain medicines also play a role.

BUT and contact lenses

For contact lens wearers, the stability of the tear film is particularly important. A short BUT can lead to symptoms of dryness, blurred vision and reduced comfort whilst wearing lenses. If you suffer from dry eyes whilst wearing lenses, your optician or optometrist can measure your BUT to assess whether your tear film is stable enough. Sometimes it helps to switch to a different type of lens or to use moisturising eye drops. At SMEYEL , you’ll find moisturising eye drops designed to support comfort whilst wearing contact lenses.

When should you see a specialist?

If you have symptoms such as dry, burning or irritated eyes, or if you are experiencing discomfort with your contact lenses, have your eyes assessed by your optician, optometrist or ophthalmologist. They can measure your BUT and advise you on possible adjustments to your lens type, lens care or eye care.