Tonometry and Tonography

The Goldmann applanation tonometer (Haag-Streit) remains a standard of reference in clinical tonometry (Figure 6-1). The principle of applanation tonometry is old. It is based on the physical relationship that applies to the flat end of a piston at rest: The pressure against the piston is calculated from the force applied to the piston divided by the area of the face of the piston. This principle has been used in the Russian Maklakov applanation tonometer since the 19th century, but it was never applied with great precision until the advent of the Goldmann applanation tonometer. In the Goldmann instrument, the force is measured with a sensitive spring or counterpoise balance, and the area is precisely established by an accurate split-field device.