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Applanation tonometry is a method used primarily for measuring intraocular pressure (IOP), which assesses the pressure inside the eye. The technique involves flattening a small area of the cornea and measuring the amount of force required to achieve that flattening. This process is crucial because elevated IOP can be a risk factor for glaucoma and other ocular conditions.

In applanation tonometry, mercury is typically used in the form of a calibrated device that provides an accurate measurement of the pressure based on the amount of force applied to the cornea. This precise measurement is essential for diagnosing and monitoring patients at risk for various eye diseases.

The other testing methods mentioned do not utilize mercury nor are they related to the measurement of intraocular pressure. For example, corneal thickness measurement is often performed using pachymetry, color discrimination tests assess the ability to distinguish colors without applying pressure to the eye, and visual field testing evaluates peripheral vision rather than intraocular pressure. Each of these tests serves different purposes in ophthalmology and employs unique techniques and instruments.