Rigid Gas Permeable (RGP) lenses are hard lenses that can also be used to treat many vision problems. Prescriptions for RGP lenses are similar to soft lenses. Many of the same measurements, such as power, base curve and diameter, are used in both RGP and soft lenses. Soft lenses are malleable and can be manufactured to fit comfortably over the typical cornea in a roughly uniform manner. RGP lenses, on the other hand, must be specifically measured and designed to fit the eye precisely. Due the precise measurements that must accompany the production of RGP lenses, there are generally more detailed numbers assigned to RGP lens prescriptions.
The brand in an RGP lens prescription generally doesn’t indicate an actual brand but rather the material used to manufacture the lens. While soft lenses are mass produced, RGP lenses are custom made in a lab specifically for each patient. Companies that make the plastic polymers for RGP lenses do so in bulk and ship plastic polymer rods to production labs that in turn produce the individual lenses. To make the RGP lenses, the manufacturer will shave a small circle of plastic off the end of a rod and grind it to meet the parameters noted in a particular prescription.
Some measurements, like power, are similar between soft and RGP lenses but there are factors that differ between the two. For example, the base curve of a rigid lens is more specific than in a soft lens and thus is measured in hundredths of millimeters rather than simply millimeters. The average base curve for a rigid gas permeable lens is between 7.00 and 8.5 millimeters, although this varies from person to person. Base curve, diameter, and other measurements are required for RGP lenses.
Previous: The Soft Contact Lens Prescription