Optical Ed Articles » Back Vertex Power

1. If a polycarbonate lens (n = 1.586) is made with a front surface power of +10.25, a back surface power of -3.00, and a center thickness of 4 mm, what power will the lensmeter show? Give the power rounded to two decimal places, not to 1/8 diopter steps.

Remember when using the lensmeter we will check the back vertex power, so we use the back vertex power formula

$B_v = \frac{D_1}{1 - (\frac{t}{n})(D_1)} + D_2\\$

$B_v=\frac{10.25}{1- (\frac{0.004}{1.586})(10.25)} + (-3.00)$

$\ B_v=\frac{10.25}{1 -(0.002522068095839)(10.25)} + (-3.00)$

$\ B_v=\frac{10.25}{1 -(0.02585119798234975)} + (-3.00)$

$\ B_v= \frac{10.25}{0.97414880201765025} + (-3.00)$

$\ B_v =10.52 + (-3.00)$

$\ B_v =+7.52$

2. For a crown glass lens with a front surface power of +15.00, a back surface power of -6.00, and a thickness of 6.5 mm, what power will the wearer see? For crown glass, n = 1.523. Enter just the sign and power.

The wearer will always look through the back vertex power, so we use the back vertex power formula.
$\ B_v=\frac{15.00}{1- (\frac{0.0065}{1.523})(15.00)} + (-6.00)$

$\ B_v=\frac{15.00}{1- (\frac{0.0065}{1.523})(15.00)} + (-6.00)$

$\ B_v = \frac{15.00}{1 - 0.064018384767} + (-6.00)$

$\ B_v = \frac{15.00}{0.935981615233} + (-6.00)$

$\ B_v = 16.03 + (-6.00)$

$\ B_v = +10.03$