What measurement does a tonometer primarily assess?

A tonometer is a device specifically designed to measure intraocular pressure (IOP), which is the fluid pressure within the eye. This measurement is crucial in the diagnosis and management of glaucoma, a condition characterized by increased eye pressure that can lead to vision loss. By assessing the IOP, a tonometer helps healthcare providers determine whether the pressure within the eye falls within a normal range or if it indicates a risk for glaucoma.

In contrast, visual acuity refers to the clarity or sharpness of vision and is typically assessed with an eye chart, while corneal thickness measurements are often taken with a pachymeter to evaluate the risk of glaucoma and other ocular conditions. Pupil size, a measurement related to the response of the pupils to light, is not relevant to the tonometry process. Therefore, the primary function of a tonometer is to evaluate intraocular pressure, making this the correct choice for the question.