Extra Questions for Module 11
          Vertex Distance Compensation
Each worksheet is different, because each one was originally written by a different person.  Use the directions at the beginning of each worksheet for that worksheet.

Vertex Power Worksheet
Round powers to two decimal places.  This power will not be recorded in the wearer's records or ordered from the lab.  We do this so you will understand that the nominal power formula does not work for powers over about 4 D, and why you may get different readings in the focimeter for plus lenses depending on how you mount the glasses in the instrument.
 
Nominal Power Front Vertex Power Back Vertex Power
1.  If a lens has a front surface power of +3.00, a back surface power of -8.50, a thickness of 2.2 mm, and is made of CR39, what are the nominal, front vertex, and back vertex powers? D1 = +3.00
D2 = -8.50

DN = +3.00 -8.50
DN = -5.50

 D1 = +3.00
D2 = -8.50
t = 0.0022
n = 1.498
Dfront = -5.40
 
 

Dback = -5.49
2.  If a lens has a front surface power of +9.50, a back surface power of -3.25, a thickness of 5.7 mm, and is made of CR39, what are the three different powers? +6.25 +6.29 +6.61
3.  The same lens in number 2 is now made of aspheric curves in a 1.66 material.  It still has an actual front surface power of +9.50,  a back surface power of -3.25, and a center thickness of 3.3 mm.  Now what are the three different powers? +6.25 +6.27 +6.43
4.  A bifocal lens made from polycarbonate has a back surface power of -4.75, a front surface power of +8.75, and the bifocal portion has a surface power of +10.50. 
  • If the center thickness of the distance portion is 4.1 mm  what are the three powers for the distance section? 
  • If the thickest part of the segment is 5.2 mm, what are the three powers for the reading portion of the lens?
  • What ADD do you get if you use the back vertex powers that you see in the focimeter?  What ADD do you get if you use the front vertex powers from the focimeter?
 Distance

+4.00

 +4.06 +4.20
Reading

+5.75

 +5.82 +6.12
ADD

+1.75

 +1.76 +1.92


 
 

 Press the BACK button at the top left of the screen to return to the assignment page.