The difference between using a conventional toric lens and the Purevision 2 HD for Astigmatism is like watching your grandparent’s old black and white TV compared to your crisp, clear 1080p high definition television. The design features a concept called asphericity which simply means “not spherical.” Asphericity is a relatively simple concept, right? Well, not exactly. Let’s examine this concept for a moment.
Consider this: all lenses are prisms which refract light. A lens used for nearsightedness is akin to having two prisms stacked apex to apex. A lens used for nearsightedness is like having two prisms stacked base to base. The former lens design allows light to converge whereas the latter design causes light to diverge. The curvature of the lens will ultimately determine its power with each lens possessing a center at which there is the least amount of deviation from the prescription. This is known as the optical center. If we were to read the power of the lens, especially in lenses with a significant amount of correction, we would see that the farther we get from the optical center the more distorted the lens appears from a visual and optical standpoint. This is due to the radius of curvature of the lens and how it steeply bends light back toward the optical center. Lenses which are termed “aspheric” bend light at a more gradual pace which results in the following benefits to the wearer: a wider field of view, crisper vision and less blur along the peripheral edges of the lens. This is only one reason why Purevision 2 HD for Astigmatism lenses provide the toric contact lens patient with better vision than traditional toric lenses. The other design improvement can be summed up in one word: stability.
Stability is exceptionally important to the toric lens wearer. Astigmatism is caused by the inability of light as it passes through the visual mediums of the cornea and lens to come to a single focal point on the surface of the retina. Instead, light will focus at two different focal lengths causing images to blur. To correct this error, lenses are designed with power which runs in a horizontal and vertical meridian with different powers angled at 90⁰ to one another. This aligns light so that it presents itself at a single point of focus on the retina, therefore providing clear vision. Toric lenses used to treat astigmatic patients differ from simple spherical lenses in that they must remain stable in order for the patient to see clearly. Spherical lenses will rotate constantly as the patient blinks or rubs their eye. This presents no visual challenge to the patient since the powers are equal in each meridian of the lens. It goes completely unobserved by the patient. Toric lenses must remain stable. If it loses its proper orientation, it will blur the patient’s vision since the correct will reside in the wrong meridian. The patented Auto Align Design™ possesses a hybrid ballasting system which not only orients the lens in the correct position, but also provides the patient with stable vision throughout the course of wear.