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
Multifocal terminology
- seg width
- seg depth
- seg height
- seg drop
W: longest horizontal dimension of the seg
De: longest vertical dimension of the seg
H: dist from lowest point on the lens to the top of the seg
Dr: vertical dist b/w MRP and top of the seg
Multifocal terminology
- inset (I)
- seg inset (Is)
- total inset (It)
I: distance from GC to MRP
-i.e. inset = (frame PD - distance PD) ÷ 2
SI: inset accounting for near PD
- distance from MRP to center of the seg
- i.e. SI = (distance PD - near PD) ÷ 2
TI: inset of the seg as measured from the OC of the lens
- i.e. TI = (frame PD - near PD) ÷ 2
- also total inset = seg inset + inset
Adjusting seg height
- if seg app too high
- if seg app too low
Incr panto Decr vertex dist Spread nose pads Move pads up by adjusting arms Stretch bridge
Narrow the pads Move pads down by adjusting arms Incr vertex distance Reduce panto Shrink bridge
Frame adjustments
-glasses fall down nose
Pull in temples
Bend down temple tips
Pull in nose pads
Frame adjustments
-one lens feels closer than the other
Straighten temples
Frame adjustments
-glasses touch cheek
Reduce panto
Narrow bridge/pads
Frame adjustments
-glasses too close to face
Narrow pads
Shrink bridge
Decr faceform
Frame adjustments
-frames sit too low on face
Narrow bridge
Add pads
Lower pads
Lens materials
-ophthalmic crown glass
n = 1.523
Abbe = 58.9
No longer commonly used
Lens materials
-CR-39 (plastic)
n = 1.498
Abbe = 58
Light, impact resistant, amenable to large range of optical designs
Lens materials
-polycarbonate (plastic)
n = 1.586
Abbe = 30
Excellent impact resistance, but high chromatic aberrations
Lens materials
-trivex
n = 1.53
Abbe = 44
Relatively light and impact-resistant
Relationship b/w index of refraction and abbe value
Inverse
