The curvature of the progressive front surface increases constantly below the major reference point for distance vision. Without the use of a thickness-reducing prism, this would mean that the edge thickness in the upper area of the progressive lens would be greater than that in the lower area.
To reduce the weight of the lenses, a base-up prism is ground into the back surface of the lens. A base-down prism is then effective in the finished progressive lens. The magnitude and direction of this thickness reduction prism is equal on the right and left lenses with the same addition and is not therefore effective for the spectacle wearer.
The thickness reduction prism can be measured in the prism reference point. In a prismatic prescription the value measured is the prism resulting from the powers of the thickness reduction prism and the prescribed prism.
The thickness reduction prism
The thickness reduction prism is finely
graduated as a function of the addition |  Symmetric
Horizontally symmetric
Variable inset
Thickness reduction prism
Engravings and stamp
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