Tear film break-up time (TBUT) is a method for determining the stability of the tear film and checking evaporative dry eye. In testing for TBUT, sodium fluorescein dye is added to the eye and the tear film is observed under the slit lamp while the patient avoids blinking until tiny dry spots develop. A short tear break-up time is a sign of a poor tear film and the longer it takes the more stable the tear film.