Flashcards in 5 - Sizes, Absorption Deck (23)

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1

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Spectacle magnification

-equation/what are we comparing

###
SM = Ig ÷ I0

Spec mag = retinal image size with glasses/correction ÷ RIS without/uncorrected

Comparing retinal image size, corrected and uncorrected with specs

2

##
Spectacle magnification for thick lenses

-2 contributors

-equations (3)

###
Shape and power

SM = (shape factor)*(power factor)

Where:

Shape factor (Ms) = 1 ÷(1-(t/n)(F1))

Power factor (Mp) = 1 ÷ (1-(h*Fv))

“h” is the distance b/w back surface of the lens and entrance pupil of the eye in meters = VERTEX + 3mm!

“Fv” is back vertex power

NOTE: thickness in METERS

3

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Spectacle magnification trends

-for PLUS lenses

—incr h (vertex distance)

—incr t

—incr BC

—incr n

###
SM:

Incr (only one diff)

Incr

Incr

Decr

4

##
Spectacle magnification trends

-for MINUS lenses

—incr h (vertex distance)

—incr t

—incr BC

—incr n

###
SM:

Decr (only one diff)

Incr

Incr

Decr

5

##
Relative spectacle magnification

-equation/what are we comparing

###
RSM = Ia ÷ Is

Relative spec mag = retinal image size in corrected ametropic eye ÷ RIS in standard eye

Comparing retinal image size, corrected ametrope and standard eye

6

##
Relative spectacle magnification

-axial ammetropes

###
Best corrected with spectacles (Knapp’s Law)

7

##
Relative spectacle magnification

-refractive ametropes

### Best corrected with contact lenses

8

## Knapp’s Law

### Essentially: if someone is an AXIAL ametrope, you want to correct them in SPECS to minimize spectacle magnification

9

##
Comparative retinal image sizes

-uncorrected axial ametrope

###
Myope image size > emmetrope > hyperope

Think projector and screen

10

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Comparative retinal image sizes

-uncorrected refractive ametrope

### All retinal image sizes are the same/no change in distance

11

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Comparative retinal image sizes

-corrected refractive ametrope

### With specs: larger image for hyperopes, smaller for myopes

12

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Aniseikonia

-describe anatomical aniseikonia

###
Due to anatomical asymmetry, such as discrepancy in density of PRs

-e.g. wet AMD, mac edema/Irvine-Gass, ERM

13

##
Aniseikonia

-describe induced aniseikonia

###
Due to optics of corrected eye

-esp difference in spectacle mag

14

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Aniseikonia

-describe meridonial aniseikonia

###
Due to differences in cyl power

-effect is prominent in one meridian

-vertical obect may appear to be tilted

15

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Aniseikonia

-how many diopters power diff per percent aniseikonia

-when does this become problematic

###
1D = 1% aniseikonia

Problems start ~3%

16

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Anisometropia

-describe

-major concern

###
Refractive state of OD vs OS differs, usually by more than 1D

Ambylopia - esp hyperopes

-less hyperopic eye will always be in focus

-with myopia, each eye will see clearer at a certain distance

17

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Anisometropia

-describe antimetropia

### One eye is hyperopic, the other myopic

18

## Loss of light passing thru a lens

### Due to reflection at each lens surface + absorption as it passes thru the material

19

##
Transmittance equations

-reflected from surfaces

###
R = [(n2-n1)÷(n2+n1)]^2

Reflectance = (diff in indices ÷ sum of indices) squared

20

##
Transmittance equations

-transmittance at each surface

### Ts = 1 - R

21

##
Transmittance equations

-transmittance thru the medium

###
Tm = 1 - (amount/percent absorbed by lens)

Amount absorbed must be given in problem

22

##
Transmittance equations

-total transmittance thru a lens

### T = (Ts1)(Ts2)(Tm)

23