| 摘要 |
<p>The following characteristics are combined in a contact lens prepared from a hard material, esp. form polymethyl-methyacrylate:- a thickness at the centre of =0.09mm; a central optical zone of 7mm, with a spherical basic curve; and two concentric curves in the zoneof the outer rim, both of which have a larger radius than the basic curve with the outer curve having a larger radius than the inner curve. The dia. is pref. standardised at 8mm. The radius of the second curve is 1.5mm greater than the basic curve and the radius of the outermost, third curve is 12.5mm. Used to overcome the disadvantages of the soft material which are widely used at the present time for contact lenses. Whilst these can be made very thin, they are porous, so that optical fluids quickly cause deposits on them, and they age rapidly. On the other hand PMMA is highly transparent and resistant to ageing.</p> |
