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News » LENSES FOR GLASSES - LENSES FOR ASTIGMATISM
LENSES FOR GLASSES - LENSES FOR ASTIGMATISM
Spectacle lenses on the market today are trying to meet the need to provide perfect image quality even in their eccentric portions.
This is achieved, at least from a practical point of view and for the powers that range from -25D to + 7,5D, by employing the meniscus lenses constituted by a suitable combination of positive and negative surfaces punctually said lenses.
In practice, in order to simplify the manufacturing processes, instead of using "bases" different for every power it is used for a base of the same contiguous power lenses.
In the medium and weak power lens it is widely used the "base" + - 6D. As the power of the lens increases the curvature of the base is gradually reduced until you get, the higher powers, the piano-convex or plano-concave.
They are now manufactured in the form of a meniscus lens in which one surface is toric, that is equivalent to that of a cylinder of which has been curved axis. A toric surface has the maximum curvature and the minimum of two perpendicular meridians; their difference determines the astigmatic effect desired.
Toric lenses in the positive and negative customarily used, the toric surface is the front one.
In the astigmatic lenses in which the cylindrical component has a value exceeding 3D and in the positive lenses of high power, the front toric surface produces the drawbacks presented particularly by an increase in distortion.
Therefore the high astigmatism and thick lenses it is recommended that the toric surface and the rear; this, however, entails a change of the current process of manufacture which makes it the most expensive lens.
The lenses assosimmetriche and astigmatic commonly employed, are today built with CR39 synthetic material that has a refractive index of 1.52 and a specific gravity of about 1.3 which is about half compared to the weight of the old glass lenses.
They are usually supplied by the manufacturers, to optical, in the form of discs that have the diameter of about 50-52mm, and in which the optical center corresponds to the center of the disc.
The operator, by means of sophisticated computerized machines, cuts the lenses shaping them according to the shape of the frame in which it will be inserted and will slightly chamfer the perimeter.