2 - Lenses and Frames Flashcards Preview

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Flashcards in 2 - Lenses and Frames Deck (28)
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1

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

2

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)

3

Base curves
-single vision lenses

BC is always on the front surface for SV lenses

4

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

5

Base curves:
-contact lenses

Typically on the back surface

6

Lens thickness
-when doing a problem for

DRAW A PICTURE and use geometry (don’t memorize the formula)

7

Describe chord length

Chord = where curve starts
-where we measure sag from is curve to chord

h = half chord length

8

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

9

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

10

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)

11

Decentration per lens (d) equation

GCD equation

d = (frame PD - wearer’s PD)/2

GCD = A + DBL

12

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

13

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

14

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

15

Trifocal
-intermediate add

One half power of near add

16

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

17

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

18

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

19

Frame adjustments
-glasses fall down nose

Pull in temples
Bend down temple tips
Pull in nose pads

20

Frame adjustments
-one lens feels closer than the other

Straighten temples

21

Frame adjustments
-glasses touch cheek

Reduce panto
Narrow bridge/pads

22

Frame adjustments
-glasses too close to face

Narrow pads
Shrink bridge
Decr faceform

23

Frame adjustments
-frames sit too low on face

Narrow bridge
Add pads
Lower pads

24

Lens materials
-ophthalmic crown glass

n = 1.523
Abbe = 58.9
No longer commonly used

25

Lens materials
-CR-39 (plastic)

n = 1.498
Abbe = 58
Light, impact resistant, amenable to large range of optical designs

26

Lens materials
-polycarbonate (plastic)

n = 1.586
Abbe = 30
Excellent impact resistance, but high chromatic aberrations

27

Lens materials
-trivex

n = 1.53
Abbe = 44
Relatively light and impact-resistant

28

Relationship b/w index of refraction and abbe value

Inverse