Posted on September 09 2015
So far we have discussed plus and minus lenses, which work in similar ways. However, there is an entirely different type of common lens called a sphero-cylinder lens (also known as a toric lens). This lens is used to treat astigmatism, a vision condition that affects a great number of people around the world. The sphero-cylinder lens is more complex than plus and minus lenses, mainly because of the complexity of astigmatism.
Both the plus lens, which is convex and treats farsightedness, and the minus lens, which helps those with nearsightedness through a concave inner surface, generally work the same way. They both compensate for a faulty focal point within the eye by bending light towards the thickest section of the lens. The real difference is a plus lens does the opposite of what a minus lens does. Both of these types of corrective lenses have spherical lens power. Neither of these lenses, however, would adequately address the problems of an eye with astigmatism. For a person with astigmatism, the lens must also contain a cylinder component. The cylinder lens addresses the issues that are caused by astigmatism.
When someone has astigmatism, there is a difference in how they see planes of space. In vision terms, these planes are called meridians. Although they have other names, let’s call them vertical and horizontal meridians. Imagine you are looking straight at the front of a house, the height of the house will be the vertical meridian (or plane) while the width of the house is the horizontal meridian. If you have astigmatism, you will see one of those meridians less well than the other. The astigmatism is actually defined by the difference in how well you see each meridian. If you see the vertical meridian 1.00 diopter better than the horizontal meridian, your astigmatism would be +1.00. The weaker meridian is classified as the “axis” and given an angle designation. To understand how the angle number is used, think of a protractor at the base of the house. In our example, the axis is the horizontal meridian, which is at 180˚ (or 0˚). If you saw the vertical meridian less well, that would be the axis with a 90˚designation.
Since astigmatism creates two focal points with varying power in the eye, lenses that treat astigmatism must have dual strength to compensate for each of these focal points. Sphero-cylinder lenses are able to account for these multiple focal points. The front surface of the lens is the spherical component while the back side of the lens, closest to the eye, is cylindrical. The prescription of the cylinder is defined by the power of a person’s astigmatism. Therefore, it measures the difference in how well the eye sees the two meridians. It is not uncommon for someone to have different vision acuity in each eye as well as different degrees of astigmatism, so their vision prescription may have different sphero-cylinder values for each eye.