Consultant's Corner
Checking the Center Thickness of a GP with a Lens Clock
Did you know you can check the approximate center thickness of a GP contact lens with a standard base curve lens clock? Most ophthalmic practices have a lens clock in their office for checking the base curve of spectacle lenses. This same instrument can be used to check the approximate center thickness of a rigid contact lens. While we do not suggest that this replace the standard center thickness gauge, it comes in handy when checking lenses that have other than standard thickness (i.e., extra thin, aphakic, high hyperopia, prism ballasted, etc.).
First, make sure the lens clock has protective tips so that you do not scratch or otherwise mar the contact lens. Place the contact lens, concave side up, on a clean, flat, hard surface, such as a desk or countertop. Place the center prong of the lens clock in the absolute center of the contact lens, this will leave the other two prongs on the flat surface on the outside of the contact lens. Take the reading off the dial of the lens clock and divide by two. This will approximate the thickness of the GP contact lens.
Example: Lens clock reads +7.00 diopters, divide by 2 = 3.50, move the decimal point one place to the left = .35, this will equal the approximate center thickness of the contact lens.
We suggest that if you frequently check the center thickness of GP lenses, you are better off investing in a thickness gauge intended for this purpose. However, if you are occasionally problem solving or duplicating a GP lens when center thickness is an issue, this method is quite accurate (within .03mm).
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