Wongs midterm study guide Flashcards
(25 cards)
what are 5 responsibilities in designing the most appropriate spectacle design
- optically precise ansi tolerance
- comfortable and proper fit
- safe and highly impact resistant material-FDA approved
- aesthetically pleasing/cosmetically correcy
- 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?
- 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
- the beginning curve
- always in the front surface
- 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
- duplication
- multiple pair of same rx
- single lens replacement
- 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
- change lens index
- 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
CR39-1.498
glass-1.523
poly-1.586
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
FPD = GCD = DBC
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
deviation
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
- 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
