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what are 5 responsibilities in designing the most appropriate spectacle design

1. optically precise ansi tolerance
2. comfortable and proper fit
3. safe and highly impact resistant material-FDA approved
4. aesthetically pleasing/cosmetically correcy
5. vision and functionality


what is decentration

the physical separation of the OC of the lens from the GC of the frame


why decenter a lens?

1. eliminate unwanted prism
2. create prescribed prism


What is the importance of the base curve of a lens?

chnages the thickness and magnification
-always on the front, if there are 2 surfaces it is the flattest one
-foundation and determination of all other curves
-changes the depth perception of patient so you want to keep it consistent


Which base curves are most appropriate for which power?

-always a positive convex lens EXCEPT when the rx is > 10.00D then it is concave surface on both sides, only time the base curve is NEGATIVE


what is a slab off lens and in what situations would it offer a benefit to the pt

BUMM glass
-takes prism imbalance and places prism in one lens/segment to cancel it out

presbyopic and vert anisometropia (2 diopter diff in 90 meridan)


why is vertical placement of the OC so importnat in clinical care and why is it so often ignored

vert tolerate less but PD can vary more


whta is the 3 part definition of a base curve

1. the beginning curve
2. always in the front surface
3. if there are 2 curves on the front-the BC is the FLATTEST curve (closest to plano)


when is it recommended to MATCH an existing base curve

1. duplication
2. multiple pair of same rx
3. single lens replacement
4. match to dd BC, if new rx is w/in 1.5-2D of change

-CAREFUL at 4D or greater, sensitive to BC changes


when is it recommended to NOT MATCH a base curve

1. change lens index
2. if rx is over 2D of change

-sports/wrap goggles (esp negative rx)
-negative lens = flat base curve


what are the indexes for CR 39, glass, and poly



what is the diff btwn the gross height of a bifocal segment compared to the net height and why is it an important fitting concept

gross height: bottom of the frame to the LLM (lower limbal edge)

net height: any adjustment made to gross = orderded height
-changes that could be made by head tiltins, depends what they do most, lower height i pt nervous about bifocal


how are the DBC, spectacle PD, and patient PD related

pt pd is the pd w/o gl


what is the diff btwn slab off and slab off CR39

slab off: BU most minus
slab off CR39 = BD most plus


what is a lens

a transparent medium "band by" 2 highly polished surfaces


rx diopter is based on...
prism diopter is based on...

focal length


what is the def of prism diopter

deviation in cm at distance of 1 meter


what is the formula for prism power

prism power = dev (cm) / distance (m)


what is the purpose of a prism

1. to make eye turn in certain direction
2. to redirect light to eye that is turned


what is human prism tolerance for horiz and vert

H: 8-10 prism diopters
V: 1.5-2 prism diopters


what is ANSI tolerance

H: OD-0.33, OS 0.33 total= 0.66
V: 0.33 per eye and 0.33 as a whole


what are the rules for combing prism for horizontal in the same base direction and opp base direction
what about for vertical

H: same = add, opp = subtract
V: same = subtract, opp = add


what is the gross height for bifocal, trifocal, and PAL

bifocal: bottom of frame to LLM
trifocal: to lower pupil edge
PAL: to pupil center


what is the best shape for power in 180? 45?

180: aviator, circle
45: oval/round w/ no corners
CMA WR: oval or rectangle but oval is best


what does it mean when A+DBL > PD?
A + DBL < PD?