What defines a cross curve in lens design?

In lens design, a cross curve specifically refers to the stronger curvature found on a toric lens surface. Toric lenses are designed to correct for astigmatism and have two different radii of curvature, which correspond to two different meridians. The cross curve represents the curvature that provides the most significant optical power in the meridian where astigmatism is present. This unique design allows the lens to provide clear vision across different axes by compensating for the varying refractive error.

Understanding that a cross curve is related to toric lenses is essential for those involved in ophthalmic dispensing, as accurate measurement and fitting of these lenses are crucial for optimal vision correction. The other terms and definitions mentioned—like base curve, peripheral curve, or a single curve—do not capture the dual curvature aspect that characterizes a toric lens and, therefore, are not correct in this context.