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