Immediately following the instillation of fluorescein the B.U.T. should be measured. The room lights should be out and the filters in place on the biomicroscope. A wide light beam should be used. The patient is asked to blink 3 or 4 times and then to look straight ahead and not blink. The time (in seconds) from the last blink until a dark spot or streak develops in the tear film is the B.U.T. Do not hold the lids open.
