When ordering contact lenses, particularly online, patients encounter abbreviations and numbers that can be confusing. The measurement abbreviated as “BC” (Base Curve) is often misunderstood. This specification is important for ensuring the safe and comfortable wear of lenses, as it dictates the physical interaction between the lens and the eye’s surface.
Defining Base Curve (BC)
The abbreviation BC stands for Base Curve, and it is a precise measurement of the curvature on the back, or internal, surface of the contact lens. This curve is designed to match the dome-like shape of your cornea, the clear front surface of the eye. The measurement is expressed in millimeters, typically appearing as a number between 8.0 and 9.5. A lower BC number indicates a steeper curve, while a higher number corresponds to a flatter curve. This value is determined by your eye care professional using a specialized instrument called a keratometer.
The Role of BC in Proper Contact Lens Fit
Matching the lens’s Base Curve to the cornea’s natural shape is important for both comfort and maintaining eye health. An improperly fitted lens can lead to significant problems because the lens must allow for tear exchange and oxygen flow beneath it. If the lens is too flat (BC number is too high), it will fit too loosely, causing it to slide excessively with each blink. This constant movement can lead to irritation, discomfort, blurred vision, or mechanical abrasion on the corneal surface.
Conversely, if the lens is too steep (BC number is too low), it will fit too tightly against the eye. A tight fit restricts the natural circulation of tears and oxygen to the cornea, which can lead to oxygen deprivation (hypoxia). This restriction increases the risk of debris being trapped and can cause corneal swelling, redness, or inflammation. The Base Curve ensures the lens remains centered and stable while allowing necessary physiological exchange for good eye health.
Locating and Understanding Your BC Measurement
The Base Curve measurement is found either on your contact lens prescription or on the packaging of your current contact lens boxes. On a written prescription, the value is clearly labeled with “BC.” On the side or end of a contact lens box, you will see the same abbreviation, sometimes next to the diameter (DIA) and power (PWR). The BC value for a specific lens is determined during a professional contact lens fitting and is a required part of a valid prescription.
This measurement is brand and lens-specific, meaning that even if you have the same BC number, switching to a different brand or type of lens may require a new fitting to ensure the new lens fits correctly. A valid, unexpired prescription is required to purchase them because contact lenses are regulated medical devices. While your vision correction power may change, the Base Curve often remains consistent unless your eye care professional notes a change in your corneal shape or you switch lens types.