Troubleshooting Power Issues

This page discusses only power issues where Rx-Universe is doing the calculations (non-digital jobs).

There are only 4 factors that affect the power of a lens:

1. front curve

2. back curve

3. index of refraction

4. thickness of the lens

All power issues are caused by one or more of the above factors.

Front Curve

Front curves are normally expressed in diopters, which means they are expressed in a particular index. In North America, the index most often used to express curves (front curves or tool curves) is “tool index”, which is 1.530. This is set in the material table, for each material, as follows:

Open

Back Curve

The back curve to be cut into a lens is calculated by Rx-Universe, base on the front curve selected, the power required, and the thickness of the resulting blank.

To illustrate, consider a minus power polycarbonate lens, where the thickness is thin and can be ignored for purposes of this illustration.

Example:

Rx -7.00 sphere
Blank: 0.45 true curve (in 1.53 index)

What curve must be cut into the back of the lens?

Recalling that the power of a lens (the back vertex power), can be calculated easily, when all measurements are in the same index as the material of the lens):

BVP = FC + BC (FC = front curve in dioptres, BC = back curve in dioptres)

BC = BVP - FC

In our example, we must first convert the 0.45 true curve, in tool (1.53) index, to poly index:

FC_1.586 = FC_1.53 x 586/530 = 0.45 x 586/530 = 0.50

then

BC_1.586 = BVP - FC_1.586 = -7.00 - 0.50 = -7.50_1.586

To obtain the back curve in tool index, we convert the -7.50 to tool index, using

BC_1.53 = BC_1.586 x 530/586 = -7.50 x 530/586 = -6.78

Thus, a 0.45 poly lens blank requires a -6.78 back curve to result in a lens of -7.00 sphere power (all curves expressed in tool index).