THE EXACT SOLUTION FOR THE CENTER THICKNESS OF SPECTACLE AND CONTACT-LENSES

The available methods for the exact calculation of centre thickness pr esuppose that one knows simultaneously both surfaces of the lens and t hat the lens' power is determined afterwards. However, this approach i s of secondary importance in practical situations, The issue in daily practice is the determination of the centre thickness of lenses that a lready have explicit vertex powers. In such cases one can set only one surface. The other has to be calculated following the prescription, t he known surface and the optical effect of the centre thickness. Exact solution for this problem leads to four expressions, reflecting the n umber of ways one can treat it. Two of them are complete second-order algebraic equations and the other two, complete third-order algebraic equations. The practical value of the latter is limited by the fact th at one cannot always obtain their three roots by pure algebraic proced ures.