Eye prescriptions often contain letters and numbers that seem like a secret code. Contact lens prescriptions include specific measurements beyond standard vision correction values. The measurement labeled “BC” (Base Curve) pertains entirely to the physical fit of the lens on the eye’s surface. Understanding this notation is necessary for comfortable wear and long-term eye health.
Understanding the Base Curve (BC) Measurement
The abbreviation BC stands for Base Curve, measuring the curvature on the inside, or back, surface of a contact lens. This surface sits directly against the cornea, the clear, dome-shaped front part of the eye. The Base Curve describes the lens’s curvature radius, designed to match the unique shape of the user’s cornea.
This measurement is expressed in millimeters (mm) and typically ranges between 8.0 and 9.5 mm for soft contact lenses. A lower BC number, such as 8.4 mm, indicates a steeper curve, suited for steeply curved corneas. Conversely, a higher BC number, such as 8.9 mm, signifies a flatter curve, intended for corneas with a less pronounced curvature. Even slight variations influence how the lens interacts with the eye.
How BC Relates to Other Contact Lens Parameters
The Base Curve is a fitting parameter, distinct from the vision correction values on a prescription. Measurements like sphere (SPH), cylinder (CYL), and axis (AXIS) determine the optical power needed to sharpen sight. The BC is a physical specification that ensures the lens rests properly on the eye.
The Base Curve works in conjunction with the Diameter (DIA). The Diameter is the width of the lens from edge to edge, measured in millimeters, usually ranging between 13 and 15 mm for soft lenses. Both BC and DIA are essential for a comfortable and stable fit, but they measure different aspects of the lens’s size and shape.
The BC measurement is found almost exclusively on contact lens prescriptions. Eyeglass prescriptions also contain a base curve value, but it refers to the curvature of the lens’s front surface and is measured in diopters. This eyeglass measurement affects optical performance and aesthetics, unlike the contact lens BC which relates to physical fit.
The Importance of a Precise Base Curve Fit
The precision of the Base Curve is paramount for comfort and the physiological health of the eye. An eye care professional determines the appropriate BC using specialized equipment, such as a keratometer, to accurately measure the corneal curvature. This measurement guides the selection of a diagnostic lens, which is assessed for proper movement and centering.
An incorrect BC can lead to two types of poor fit, both compromising eye health.
Too Flat (BC Number Too High)
If the lens is too “flat,” it will move excessively with each blink. This movement can cause discomfort, irritation, blurry or fluctuating vision, and may increase the risk of corneal abrasion.
Too Steep (BC Number Too Low)
If the lens is too “steep,” it will fit too tightly, suctioning onto the eye. A tight fit restricts the flow of tears and oxygen beneath the lens, which is vital for corneal health. This oxygen deprivation can lead to corneal swelling, redness, and corneal edema, potentially causing long-term complications.