Goldmann applanation tonometry measures intraocular pressure by applying a flat surface to the cornea. According to Imbert-Fick principle, IOP is correctly calculated when the applanation area has a diameter of:

• A 2.06 mm (area ~3.14 mm²)
• B 4.00 mm (area ~12.57 mm²)
• C 1.00 mm (area ~0.79 mm²)
• D 3.06 mm (area ~7.35 mm²) ✓

Explanation

At an applanation diameter of 3.06 mm, the surface tension of the tear film (pulling the tonometer toward the eye) exactly counterbalances the rigidity of the cornea (pushing the tonometer away), so the measured force directly equals IOP × area by the Imbert-Fick principle. This diameter was chosen because it produces cancellation of these opposing errors, yielding an accurate IOP reading in the physiological range.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.