What color is the light filter used in applanation tonometry?

The light filter used in applanation tonometry is blue. This is because the blue light filter helps to enhance the visibility of the fluorescent dye (often fluorescein) that is applied to the surface of the cornea during the procedure. The blue light makes it easier for the clinician to observe the alignment of the tonometer's measuring device with the corneal surface, which is essential for accurate pressure readings. The contrast provided by the blue light combined with the fluorescent dye allows for precise observations and measurements, which is crucial for diagnosing conditions like glaucoma.

The other colors mentioned in the options do not serve the same purpose; they do not provide the necessary contrast needed for effective visualization in applanation tonometry.