5 - Sizes, Absorption Flashcards
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
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
Spectacle magnification trends -for PLUS lenses —incr h (vertex distance) —incr t —incr BC —incr n
SM: Incr (only one diff) Incr Incr Decr
Spectacle magnification trends -for MINUS lenses —incr h (vertex distance) —incr t —incr BC —incr n
SM: Decr (only one diff) Incr Incr Decr
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
Relative spectacle magnification
-axial ammetropes
Best corrected with spectacles (Knapp’s Law)
Relative spectacle magnification
-refractive ametropes
Best corrected with contact lenses
Knapp’s Law
Essentially: if someone is an AXIAL ametrope, you want to correct them in SPECS to minimize spectacle magnification
Comparative retinal image sizes
-uncorrected axial ametrope
Myope image size > emmetrope > hyperope
Think projector and screen
Comparative retinal image sizes
-uncorrected refractive ametrope
All retinal image sizes are the same/no change in distance
Comparative retinal image sizes
-corrected refractive ametrope
With specs: larger image for hyperopes, smaller for myopes
Aniseikonia
-describe anatomical aniseikonia
Due to anatomical asymmetry, such as discrepancy in density of PRs
-e.g. wet AMD, mac edema/Irvine-Gass, ERM
Aniseikonia
-describe induced aniseikonia
Due to optics of corrected eye
-esp difference in spectacle mag
Aniseikonia
-describe meridonial aniseikonia
Due to differences in cyl power
- effect is prominent in one meridian
- vertical obect may appear to be tilted
Aniseikonia
- how many diopters power diff per percent aniseikonia
- when does this become problematic
1D = 1% aniseikonia
Problems start ~3%
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
Anisometropia
-describe antimetropia
One eye is hyperopic, the other myopic
Loss of light passing thru a lens
Due to reflection at each lens surface + absorption as it passes thru the material
Transmittance equations
-reflected from surfaces
R = [(n2-n1)÷(n2+n1)]^2
Reflectance = (diff in indices ÷ sum of indices) squared
Transmittance equations
-transmittance at each surface
Ts = 1 - R
Transmittance equations
-transmittance thru the medium
Tm = 1 - (amount/percent absorbed by lens)
Amount absorbed must be given in problem
Transmittance equations
-total transmittance thru a lens
T = (Ts1)(Ts2)(Tm)
Ideal thin film
- what
- equation
Minimizes reflection
nƒ = √(n1nL)
Index of film =√(index of initial medium (usually air)*index of lens medium)
