Certified Paraoptometric (CPO) Practice Exam 2025

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What is the primary curve on the backside of a lens called?

Base Curve Radius
Secondary Curve
Optical Curve
Surface Curve

The correct answer is: Base Curve Radius

The primary curve on the backside of a lens is known as the base curve radius. This term refers to the main curvature that shapes the lens' back surface, primarily determining how the lens aligns with the eye and how it refracts light. The base curve is crucial for achieving an appropriate fit and optimal vision correction for the wearer. Other options refer to different aspects of lens design. For instance, the secondary curve provides additional adjustments to the lens profile, while the optical curve, though related to the lens’s overall design, does not specifically pertain to the primary curve on the back surface. The term surface curve is more generic and can apply to any curve on the lens rather than the specific primary backing curve. Therefore, recognizing the base curve radius as the primary curve helps clarify its importance in lens fitting and optics.