Contact lenses are medical devices that require precise measurements to ensure they function correctly and maintain eye health. Two fundamental measurements found on every contact lens prescription are the Base Curve (BC) and the Diameter (DIA). These figures, typically expressed in millimeters, dictate the physical fit of the lens on the eye’s surface. Understanding these measurements is important because they directly influence comfort, vision clarity, and the overall well-being of the eye.
What Base Curve (BC) and Diameter (DIA) Mean
The Base Curve (BC) defines the curvature of the back surface of the contact lens, which rests directly on the cornea. This measurement is given in millimeters and typically ranges from 8.0 mm to 10.0 mm for soft contact lenses. The BC is designed to match the dome shape of the cornea. A lower BC number (e.g., 8.2 mm) indicates a steeper, more curved lens suitable for a steeper cornea, while a higher BC number (e.g., 9.0 mm) signifies a flatter lens chosen for a flatter corneal shape.
The Diameter (DIA) measures the total width of the contact lens from one edge to the opposite edge, also expressed in millimeters. Soft contact lens diameters commonly range from 13.0 mm to 14.5 mm. The DIA ensures the lens adequately covers the entire cornea and rests appropriately on the surrounding white part of the eye, known as the sclera. This width ensures stability and proper positioning on the eye’s surface.
The Role of Fit and Comfort
The precise matching of the Base Curve and Diameter to the eye’s anatomy is necessary for both comfort and health. A correctly fitted lens remains centered on the cornea, allowing for stable and clear vision. Proper positioning supports the natural movement of the lens with each blink, which is necessary for tear exchange beneath the lens. Tear exchange washes away debris and ensures a continuous supply of oxygen reaches the cornea, a process called oxygen permeability.
If the Base Curve is too flat, the lens will move excessively, causing irritation, blurred vision, or potentially dislodging easily. Conversely, a BC that is too steep will suction tightly to the cornea, restricting movement and hindering the flow of tears and oxygen. This tight fit can lead to discomfort, redness, and compromise corneal health due to oxygen deprivation. Similarly, an incorrect Diameter causes issues; a lens that is too small may not fully cover the cornea, while one that is too large can irritate the edges or fail to move correctly.
How BC and DIA are Determined and Documented
The correct Base Curve and Diameter are determined by an eye care professional during a comprehensive contact lens examination. The process begins by measuring the curvature of the cornea using instruments like a keratometer or a corneal topographer. This non-invasive test, known as keratometry, provides a radius of curvature that serves as the starting point for selecting the appropriate BC. The professional also measures the horizontal visible iris diameter (HVID) to determine the most suitable lens Diameter, often using trial lenses before finalizing the prescription. These fixed measurements are documented on the contact lens prescription and are distinct from the Power (PWR) values, which correct vision.