Goldmann applanation tonometry. Top image shows two semicircles that were converted from a single circular meniscus through prisms inside the biprism applanation tip. The semicircles of the Goldmann-type applanation tonometer in the top image depict proper width, position, vertical and horizontal alignment. When proper position and alignment is reached, the intraocular pressure can be recorded. The principle used in this type of measurement of intraocular pressure is the Imbert-Fick Principle, best summarized as Pressure=Force/Area The lower photo shows the Goldmann applanation tonometer in contact with he patient's cornea.