Watch where you put that probe!

There are several sources of error in Goldmann applanation tonometry,1 but one which is readily minimised is misplacement of the probe on the corneal surface. Whereas the requirements for correct placement are well known, the magnitude of error thus avoided is slightly obscure in the literature. In this brief article I present results from a theoretical analysis of the two main types of misplacement error.

Displacement from centre

In use, the Goldmann probe (figure 1) slightly indents and applanates (flattens) a circular patch on a spherical cornea. The size of the applanated patch varies with the force applied by the probe. When the circle of applanation has a diameter of 3.06mm, the force applied by the probe is adjudged to be a measure of the eye’s intraocular pressure (IOP).