What is referred to as the steepest curve on a toric surface of a lens?

The steepest curve on a toric surface of a lens is known as the cross curve. In the context of toric lenses, which are designed to correct astigmatism, the surface has two principal meridians that differ in curvature. The cross curve specifically refers to the curvature that runs perpendicular to the axis of the lens, which is where the steepest and flattest curves on the toric surface can be found.

Understanding this distinction is critical for opticians because it helps in accurately measuring and fitting lenses for individuals with astigmatism. The cross curve is essential when determining how the lens will sit and function on the wearer's eye, which in turn affects vision quality.

The base curve is typically the overall curvature of a lens providing the primary refractive power, while the axis curve defines the orientation of the toric surface. The power curve can refer to changes in power across different meridians of a toric lens but does not specifically denote the steepest curve. Recognizing that the cross curve is indeed the steepest allows opticians to make informed decisions regarding lens prescriptions and fittings.