Final answer:
The diameter of a contact lens is the width across the lens at its widest point. It is vital for fitting purposes as the cornea is often not a perfect sphere, and its curvature, measured when fitting lenses, impacts the lens's efficiency and comfort.
Step-by-step explanation:
The diameter (or chord diameter) of a contact lens refers to the distance across the lens at its widest point. In fitting contact lenses, it is crucial to match the diameter of the lens properly to the size of the cornea to ensure a comfortable and effective fit. Complications in fitting lenses arise because many corneas are not perfectly spherical, and the radius of curvature of the cornea can vary significantly, affecting how well a lens fits and functions. Optometrists often use instruments like a keratometer to measure the curvature of the cornea for contact lens fitting. The curvature of the cornea is related to the focusing power of the eye, which is measured in diopters (D), and provides the ability to focus light accurately on the retina, located approximately 2.0 cm from the cornea in an adult eye.