A clinical test for dry eye that determines the time taken to keep the eye open without rupturing the tear film. Fluorescein is introduced into the tear film and the patient is requested to avoid blinking while the tear film undergoes observation under cobalt beam illumination. The number of seconds that elapse between the last blink and when the first dry spots appear in the tear film, are recorded (tear breakup time). Any recording below 10 seconds is considered unusual and indicative of tear instability.