5 - Sizes, Absorption

Question 1

Spectacle magnification

-equation/what are we comparing

Answer 1

SM = Ig ÷ I0

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

Comparing retinal image size, corrected and uncorrected with specs

Question 2

Spectacle magnification for thick lenses

• 2 contributors
• equations (3)

Answer 2

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

Question 3

Spectacle magnification trends
-for PLUS lenses
—incr h (vertex distance)
—incr t
—incr BC
—incr n

Answer 3

SM:
Incr (only one diff)
Incr
Incr
Decr

Question 4

Spectacle magnification trends
-for MINUS lenses
—incr h (vertex distance)
—incr t
—incr BC
—incr n

Answer 4

SM:
Decr (only one diff)
Incr
Incr
Decr

Question 5

Relative spectacle magnification

-equation/what are we comparing

Answer 5

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

Question 6

Relative spectacle magnification

-axial ammetropes

Answer 6

Best corrected with spectacles (Knapp’s Law)

Question 7

Relative spectacle magnification

-refractive ametropes

Answer 7

Best corrected with contact lenses

Question 8

Knapp’s Law

Answer 8

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

Question 9

Comparative retinal image sizes

-uncorrected axial ametrope

Answer 9

Myope image size > emmetrope > hyperope

Think projector and screen

Question 10

Comparative retinal image sizes

-uncorrected refractive ametrope

Answer 10

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

Question 11

Comparative retinal image sizes

-corrected refractive ametrope

Answer 11

With specs: larger image for hyperopes, smaller for myopes

Question 12

Aniseikonia

-describe anatomical aniseikonia

Answer 12

Due to anatomical asymmetry, such as discrepancy in density of PRs
-e.g. wet AMD, mac edema/Irvine-Gass, ERM

Question 13

Aniseikonia

-describe induced aniseikonia

Answer 13

Due to optics of corrected eye

-esp difference in spectacle mag

Question 14

Aniseikonia

-describe meridonial aniseikonia

Answer 14

Due to differences in cyl power

• effect is prominent in one meridian
• vertical obect may appear to be tilted

Question 15

Aniseikonia

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

Answer 15

1D = 1% aniseikonia

Problems start ~3%

Question 16

Anisometropia

• describe
• major concern

Answer 16

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

Question 17

Anisometropia

-describe antimetropia

Answer 17

One eye is hyperopic, the other myopic

Question 18

Loss of light passing thru a lens

Answer 18

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

Question 19

Transmittance equations

-reflected from surfaces

Answer 19

R = [(n2-n1)÷(n2+n1)]^2

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

Question 20

Transmittance equations

-transmittance at each surface

Answer 20

Ts = 1 - R

Question 21

Transmittance equations

-transmittance thru the medium

Answer 21

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

Amount absorbed must be given in problem

Question 22

Transmittance equations

-total transmittance thru a lens

Answer 22

T = (Ts1)(Ts2)(Tm)

Question 23

Ideal thin film

• what
• equation

Answer 23

Minimizes reflection

nƒ = √(n1nL)

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