The Optimark corneal marker designed by Prof. Luyten of Leiden University Hospital. The corneal curve normally changes when patients move from upright to horizontal position. With this corneal marker a crossmark is placed on the eye when the patient is behind the slit lamp. Simply moisture the Optimark with e.g. a sterile inkpad and place it on the arm of the applanation tonometer. When the patient is moved to horizontal position, the crossmark is exactly on the correct spot.