Apr 30 2014
A toric lens is a special type of lens. It has two different focal lengths, and optical power in orientations that are perpendicular to each other. Like other spherical contact lenses, a toric lens is made of either soft gel-like plastic, for soft lenses, or rigid plastic, for rigid gas permeable lenses.
Toric lenses are manufactured based on the torus, which is a spatial body resulting from the rotation of a circle, with radius r, around an axis at a distance, R, from the circle center. A ring torus is produced if R > r; a horn torus is produced if R = r; and a spindle torus is produced if R < r. One of the surfaces of the toric lens is cap-shaped, while the other surface is spherical in shape.
Design of Toric Lenses
Unlike spherical lenses that have same power throughout the lens, toric lenses feature two distinct powers and a special design, which prevents them from freely rotating on the eye. Toric lenses float on the thin fluid layer in the eye. Since these lenses are weighted at the bottom, they can be maintained at the right place on the eye.
Toric lenses are manufactured in multi-focal, colored, daily disposable, and non-disposable forms. Several manufacturers also produced silicone hydrogel toric lenses with prolonged service life. Silicone lenses also ensure that the eye gets more oxygen than other, more conventional, lenses.
Working Principle
Light rays passing through the axis of revolution of the torus are refracted with respect to the greatest radius of curvature, and the rays passing via the axis of revolution of the torus are refracted with respect to the smallest radius of curvature. This results in two different refractive powers at perpendicular orientations. The refractive power in the intermediate orientation changes from the greatest to the smallest value, or vice-versa. This helps in compensating the astigmatic irregularity of the eye.
Applications
Toric lenses are primarily used in the following to correct astigmatism:
- Intraocular lenses
- Contact lenses
- Eyeglasses
References