How does a radiuscope determine the base curve radius?

• A. It measures the lens diameter
• B. It computes curvature from the focal length
• C. Determines BC by the distance between real and aerial images observed
• D. Reads the front curve radius directly

Answer: (C)

Radiuscope uses imaging geometry to determine the base curve radius. When you view a reference target through the instrument, the curved back surface of the contact lens creates two images: a real image where light actually converges, and an aerial (virtual) image formed by the refractive effect of the curved surface. The separation between these two images changes with the curvature of the lens’ back surface. The instrument is calibrated so that this image separation corresponds to a specific base curve radius, allowing you to read off the BC from the scale.

Measuring lens diameter wouldn’t reveal curvature, and calculating curvature from focal length isn’t how this device operates in practice. Reading the front curve radius directly would imply a direct surface measurement, whereas the radiuscope derives BC from the observed image pair separation.