In tonometry, the intraocular pressure (IOP) of an eye is measured. In Goldmann applanation tonometry specifically, performed with an attachment to the slit lamp, IOP is inferred from the amount of force required to flatten an area of 3.06 mm in diameter on the cornea.
Applanation tonometry is based on Newton’s third law of motion, in particular that the pressure (ie, force per surface area) inside the eyeball equals the force applied to its surface divided by the area of contact. This assumes that the eye is infinitely thin-walled, perfectly elastic, and dry, none of which hold true, producing 2 confounding forces: (1) a force generated by the eye’s corneal rigidity (because the eye is not infinitely thin-walled or perfectly elastic), which is directed away from the globe; and (2) a force generated by the surface tension of the tear film (because the eye is not dry), which is directed toward the globe. Hans Goldmann determined empirically that if enough force is applied to produce a circular area of flattening 3.06 mm in diameter, the opposing forces caused by scleral rigidity and surface tension cancel each other out, allowing the pressure in the eye to be inferred from the force applied.
The head of the applanation tonometer, which is placed against the patient’s cornea, creating a tear film meniscus, contains split-field prisms that split the magnified image of tear film meniscus into 2 (like the image doubling associated with keratometry) separated by exactly 3.06 mm. The tear film is often stained with fluorescein dye and viewed under a cobalt blue light to enhance the visibility of the resultant yellow/green circle of tears. The examiner adjusts the applanation pressure until the half circles are aligned so that their inner margins just touch one another (Fig 8-12). At this point, the circle is exactly 3.06 mm in diameter, and the reading on the tonometer (multiplied by a factor of 10, as it is measured in dynes of force) represents the IOP in millimeters of mercury.
Several factors, especially central corneal thickness, can substantially affect the accuracy of applanation tonometry (see BCSC Section 10, Glaucoma).
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.