5 - Sizes, Absorption Flashcards Preview

KMK - Physiological Optics > 5 - Sizes, Absorption > Flashcards

Flashcards in 5 - Sizes, Absorption Deck (23)
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1

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

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

Comparative retinal image sizes
-uncorrected refractive ametrope

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

11

Comparative retinal image sizes
-corrected refractive ametrope

With specs: larger image for hyperopes, smaller for myopes

12

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

Aniseikonia
-describe meridonial aniseikonia

Due to differences in cyl power
-effect is prominent in one meridian
-vertical obect may appear to be tilted

15

Aniseikonia
-how many diopters power diff per percent aniseikonia
-when does this become problematic

1D = 1% aniseikonia

Problems start ~3%

16

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

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

Ideal thin film
-what
-equation

Minimizes reflection

nƒ = √(n1nL)

Index of film =√(index of initial medium (usually air)*index of lens medium)