Bicentric Grind Worksheet

Vertical imbalance. Methods for correcting vertical imbalance. Bicentric grind. 

 
  1. Slab-off is performed

    2. on the front of a glass lens.
    on the back of a glass lens.
    slab-off is not done on glass lenses. 
     
  2. Slab-off is performed

    3. on the front of a plastic lens.
    on the back of a plastic lens.
    slab-off is not done on plastic lenses. 
     
  3. Reverse slab-off is molded

    4. on the front of a glass lens.
    on the back of a glass lens.
    reverse slab-off is not done on glass lenses. 
     
  4. Reverse slab-off is molded

    5. on the front of a plastic lens.
    on the back of a plastic lens.
    reverse slab-off is not done on plastic lenses. 
     
  5. Rx:   +8.50 sph              ADD +1.50        20/20

    6.        +8.50 -2.50 x180   ADD +1.50        20/40
    What is the vertical imbalance at a reading level of 8 mm below OC?
    [Show answers in the form "0.5 BO" for 1/2 diopter of base out prism. {This is not the correct answer here.}]
  6. If the Rx in problem 5 is the wearer's first pair of bifocals, is she likely to experience a problem reading?

    7. yes no 
     
  7. A person comes to you after wearing his first pair of bifocals for one week and indicates that he is having problems reading. You check over all of the attributes of the glasses and find that the Rx is correct and the vertex distance, base curve, pantoscopic tilt, and optical centers are all equivalent to his old pair. You check to make sure he is actually looking down through the bifocals to read, and then mark about where in the bifocal he is looking when he is reading. On measuring you determine that the reading level is 9 mm and then you compute that the vertical imbalance is 1.2. What options will you present to him for correcting the imbalance? List at least three possible ways to correct this not including slaboff or reverse slaboff , and tell me why you would or would not recommend each method.


    8.     .
  8. Bicentric grind on a glass lens produces ___ in the reading area.

    9. base up prism.
    base down prism.
    bicentric grind is not done on glass lenses. 
     
  9. Bicentric grind on a plastic lens produces ___ in the reading area.

    10. base up prism.
    base down prism.
    bicentric grind is not done on plastic lenses. 
     
  10. Reverse slab-off on a glass lens produces ___ in the reading area.

    11. base up prism.
    base down prism.
    Reverse slab-off is not done on glass lenses. 
     
  11. Reverse slab-off on a plastic lens produces ___ in the reading area.

    12. base up prism.
    base down prism.
    Reverse slab-off is not done on plastic lenses. 
     
  12. Rx:   -8.50 sph              ADD +2.50        20/40

    13.        -9.50 -2.50 x180   ADD +2.50       20/35
  13. Rx:   -0.50 -2.25 x130   ADD +2.25        20/20

    14.         +1.50 -0.50 x080   ADD +2.25        20/20
  14. If the wearer is anisometropic, but the vision in each eye is not correctable to or better, then correcting for vertical imbalance will not be useful/necessary.

    15.    .
  15. Plastic bicentric grind or reverse slab-off lenses do not have to be drop-ball tested.

    16. true  false
    [I did not tell you that one, did I? Make an educated guess and find out if you are correct when you press 'submit'.]
     
  16. Glass bicentric grind lenses do not have to be drop-ball tested.

    17. true  false
    [Ditto for this question.]
     
  17. Lenses that are exempted from drop-ball testing are exempt from safety requirements.

    18. true  false 
    [Ditto for this question.]
     
  18. The lowest amount of bicentric grind that is practical is .

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Textbooks:
Brooks & Borish, Systems for Ophthalmic Dispensing, 2nd Ed, Butterworth-Heinemann, 1996
Stoner, Optical Formulas Tutorial, Butterworth-Heinemann, 1997

Copyright 1998 - 2001, Ellen Stoner, MALS, ABOM, NCLC
Registered students may print one copy for study purposes.