2 - Lenses and Frames Flashcards
ANSI standards for impact
- high mass impact
- high velocity impact
Mass = drop ball test
-pointed projectile, 500g, dropped from 50 inches
Velocity:
-steel ball, 0.25 inches in diameter, fired at 150 ft/sec
Lens form
- equiconvex/cave
- meniscus
- plano cylinder
- toric
EQ: half of the total power is due to the front surf, half the back
M: convex front, concave back
PC: one flat surf, one cyl surf
T: one toric surface and spherical surface
*most lenses used in the USA are minus cyl lenses (toric surface on the back)
Base curves
-single vision lenses
BC is always on the front surface for SV lenses
Base curves: spectacles
- spherical lens
- plus cyl
- minus cyl
Sph: front sphere curve
Plus cyl: BC is the flatter of the front (toric) surface curves
- the other front curve is called the cross curve
- the back curve is called the sphere curve
Minus cyl: BC is the front sphere curve
- the back flatter curve is called the toric base curve
- the other back curve is called the cross curve
Base curves:
-contact lenses
Typically on the back surface
Lens thickness
-when doing a problem for
DRAW A PICTURE and use geometry (don’t memorize the formula)
Describe chord length
Chord = where curve starts
-where we measure sag from is curve to chord
h = half chord length
Conceptually describe equations for sag, lens power, and thickness
te, tc ⟷ s1, s2 ⟷ r1, r2 ⟷ F1, F2
1) sags can be related to tc/te (center/edge thickness) by drawing a picture of the lens
2) use s = (h^2)/(2r) to relate sag to roc
- h is semi-diameter or chord length in meters
3) use F = (n2-n1)/r to relate power to roc
Describe isothickness curves
Curves drawn on a power cross to show the curves on which thickness is the same
-any 2 regions lying on the same curve have the same thickness
-if lines are close together, the thickness is changing quickly (abs value)
—similar to elevation contour maps
Frame boxing system
- geometrical center
- eye/lens size (A)
- B distance
- bridge size (DBL)
- GCD or frame PD
- effective diameter
- major reference point
GC: point on the datum line halfway b/w the 2 vertical lines which are tangent to the edges
A: horizontal length
B: vertical length
DBL: shortest horizontal dist b/w lenses
GCD: horizontal distance b/w the geometrical centers of the 2 lenses
ED: longest diameter of the lens
MRP: point on the lens thru which the line of sight/visual axis passes (would correspond to optic axis if no prism power were needed)
Decentration per lens (d) equation
GCD equation
d = (frame PD - wearer’s PD)/2
GCD = A + DBL
Minimum blank size equation
M = ED + 2(d) + 2mm
Min blank size = effective diameter (mm) + 2*decentration per lens (mm) + 2mm
THIS EQUATION IS IN MILIMETERS
Multifocal: distance b/w optical center (OC) and edge of bifocal segment
- flat top 28 or less
- flat top 35
- flat top >35
- franklin/executive
- round/kryptok
- curve top/panoptic/ribbon-b
- ribbon r
FT28: 5mm FT35: 4.5mm FT>35: 0mm Frank/Exec: 0mm Round/Kryp: r (radius of seg) Curve/panop/rib-b: 4.5mm Ribbon-r: 7mm
Progressives
-hard vs soft designs
Hard = short corridor and/or high add power
Soft = long corridor and/or low add power
*refers to transition from D to N
Trifocal
-intermediate add
One half power of near add
