What defines the secondary power of a lens used specifically to correct astigmatism?

The secondary power of a lens used specifically to correct astigmatism is defined by the cylinder power. In the context of astigmatism, which is a refractive error caused by an irregular shape of the cornea or lens that prevents light from focusing properly, the cylinder power refers to the additional lens power needed to compensate for this irregularity.

Astigmatism typically requires a cylindrical lens, which has different optical power in different meridians. This is essential to correct the uneven curvature, allowing for proper focus of light onto the retina. The cylinder power works in conjunction with the spherical power to provide a comprehensive optical correction for vision.

Understanding this division of powers is crucial for optometrists and ophthalmic assistants, as it allows them to effectively evaluate and provide prescriptions that will give the patient the best possible vision correction for astigmatism.