Final answer:
Radial astigmatism is measured numerically as a cylindrical correction on an eyeglass prescription which compensates for the irregular focus due to corneal or lens shape irregularities.
Step-by-step explanation:
Radial astigmatism is described numerically as a cylindrical correction value in an eyeglass prescription. This value quantifies the degree of asymmetry in the focus of the eye. Astigmatism is detected using a chart, as shown in Figure 26.8, which can reveal whether lines along certain axes appear darker or clearer than others when viewed by each eye separately. The cylindrical correction in an eyeglass prescription is there to counteract the irregular shapes of the cornea or lens causing astigmatism, unlike spherical corrections that apply to short- or farsightedness, which are uniform along all axes.