3 - Lens Measurements & Design Flashcards
Where do you measure the OC height from?
Lowest part of the lens to the pupil centre
Where do you measure seg height from?
Lower limbus or lower iris to lowest part of the lens
What does MRP stand for?
Major Reference Point
The MRP and OC are at the same point in the lenses. (when the px looks through their is no prescribed prism) T/F
True
If prism is prescribed what happens with the MRP and the OC
MRP stays the same in the lens.
OC is shifted away in the necessary direction to create the prismatic effect
What is the MRP?
This is the point in the lens where the prism is equal to that called for by the prescription.
Recite + Look at steps for Bifocal and Trifocal heights
What is the formula to find the segment inset?
Distance PD - Near PD / 2
What are 7 bifocal designs?
Round seg
Flat top
Curved top
B seg (ribbon)
R seg (ribbon)
Panoptik
Executive
4 characteristics of Stock Lenses?
Uncut lenses with power already ground in
Found in retail stores + manufacturing labs
Mostly single vision
Turn around time can be same day/ next day
4 characteristics of Surfaced Lenses?
The is made when ordered
Custom made to px meas + rx
Turn around 7 - 10 days
For speciality lenses such as ground in prism, progressives, bifocals or very large lens.
What does MBS stand for?
Minimum blank size
What is MBS used for? 2
To see if stock lens will fit or we need a custom made surfaced one
determine if single vision lens will be able to cut out properly
What are the formulas for calculating MBS? 2
MBS = ED + 2(decentration per lens) + 2
MBS = ED + total decentration + 2
What are the formulas for calculating decentration?
Decentration per lens = (A+DBL) - PD/2
total decentration = (A+DBL) - PD
