The base curve is the curvature of the GP lens’s back surface, and measuring it requires methods that relate directly to the lens’s curvature rather than the eye’s structures. The radiuscope provides a direct readout of the lens’s back-surface radius of curvature by analyzing how the lens’ posterior surface interacts with the instrument’s optics, giving an actual numerical value for the base curve. The Drysdale principle offers a second, complementary way to determine the same curvature. It uses a calibrated, geometry-based approach—often involving how the lens sits on a reference surface or template and deriving the base curve from that fit. Together, these two methods yield a reliable measurement of the GP lens base curve. Other instruments mentioned are for different measurements: keratometry assesses the cornea’s curvature, while pachymetry, goniometry, interferometry, and tonometry measure thickness or ocular parameters, not the GP lens back surface.