Physiological Optics 1 Flashcards
BIO compared to direct
BIO has a
- Larger FOV
- Less magnification
- inverted real image
- Larger depth of focus
How BIO works
Retina is the object –> Light leaves and focuses into an image between the patient and doctor. The new image becomes the object for the doctor’s eye, which is focused on the doctor’s retina.
Condensing lens forms an intermediate, inverted, real image.
The lensometer measures what
The back vertex power of the lens.
Formula for lensometer
x = (focal length of the standard lens)^2 (back vertex power of lens)
x= meters that you move the dial. Away from you = negative, myopia
Towards you = positive, hyperopia
Hand neutralization
Minus lens –> With motion
Plus lens –> Against motion
What 2 images do you see in the radiuscope
The first image is from the surface of the lens. Now set it at zero.
The second image is the center of curvature for the GP lens.
Take the difference between the 2 positions of focus to find the radius of curvature.
What does the keratometer measure
The cornea acts as a
Measures the radius of curvature of the CENTER of the cornea along certain axes
Convex mirror (minus lens)
How does a keratometer work
Keratometer projects an image onto the K. We see the reflection back. Then measure the size of the reflection by turning vertical and horizontal dials so the +’s cross.
Keratometer assumes n of the cornea is
1.3375 and treats the K like a single spherical refracting interface (SSRI)
Formula for power of kertatometry
F= 3375.5/radius in meters
Lens clock measures what
The sag of the lens
Formula to correct for different n when using the lens clock
F of pt lens = (n of lens - 1) / (n of lens clock - 1) x F of lens clock
How to calculate mag factor based on lens used (90D vs 78D)
Mag = - (F eye) / (F lens) Mag = -(F ocular) / (F objective)
F of the eye is 60D
The drop ball test is apart of which ANSI standards
Z87.1
ASNI Z87.1
Drop ball test/High mass impact: pointed projectile, 500g, dropped from 50 inches
High VELOCITY impact: Steel ball, 0.25 aches in diameter, fired at 150 feet per second
Equiconvex and equiconcave
Half of the total power is due to the front surface and half is due to the back surface
What formula connects sag to radius
Sag = h^2/ 2r
Decentration per lens
Frame pd - pt pd/ 2
Minimum blank size equation
= ED + 2(decentration) + 2mm
Major reference pt
Point on the lens through which the line of sight (visual axis) passes.
How many mm is the distance from the seg to near OC
- FT28
- Executive/Franklin
- Round
- 5mm
- 0mm
- Half of the radius. radius = 22mm? Distance is 11mm
Hard and soft progressive designs
Hard- small corridor, high add
Soft- large corridor, low add
Inset
Geometrical center(frame PD) to the pt’s distance pd
GC- pt pd/ 2
Seg inset
Pt’s distance pd to their near pd
Distance pd - Near pd / 2
Total inset per lens
Distance from geometrical center(frame pd) to near pd
GC- near PD / 2
How to measure the add power of a bifocal lens using lensometer
Measure distance front vertex and near front vertex. Subtract
N and abbe of:
- Glass
- CR9
- Poly
- Trivex
- 1.523, 58
- 1.499, 58
- 1.586,30
- 1.53,44
Coma monochromatic aberrations only occurs for what sources
Off axis
Results in asymmetric comet patch
How are longitudinal spherical aberrations formed
Marginal rays bend more than paraxial rays
The tschernig ellipse allows us to find the best base curve to reduce what types of aberrations ?
Radial/oblique/marginal
Curvature of field
*Based on the Ostwald curve, which is flatter.
How dose refractive index depend on wavelength?
ROYGBIV
Red is long, violet is short. Shorter wavelengths bend more!
Green will bend quicker than red.
Equations connecting chromatic aberrations and abbe
CA = F/Abbe
CA= dF/Abbe
What eliminates chromatic aberrations?
Achromaic doublet
Deviation power of prism
Light deviation in cm / how far away the screen is in m
Apex angle
DAN-1 equation
Deviation= apex angle (n-1)
1 degree of angle deviation in prism is how many pd
1.75 pd
Prentice’s rule
= dF
distance from OC measured in cm
How to combine prisms (Horizontal and vertical) using vector addition
= square root (horiz squared + vertical squared)
Difference between image jump and total prismatic effect
Image jump- just multiply the add x distance from seg to near OC in cm.
Total prismatic effect- Sum of the prism induced from looking away from the distance and seg OC. 1 eye.
First determine prism induced when looking away from the distance OC.
Then determine prism induced when looking away from seg OC.
Spectacle mag compares what? What is the equation
Compares the retinal image sizes of an uncorrected vs corrected eye.
Spec mag= corrected/uncorrected
Spec mag for thick lenses general equation
SM = shape factor x power factor
Shape factor equation
Shape factor=
1
____
1-(t/n)F1
*Always order same thickness and BC for aniso patients to keep shape factor the same.
Power factor equation
Power factor =
1
____
1-h(Fv)
h= vertex power + 3mm
How does SM change for a plus lens if you:
- Increase vertex distance
- increase thickness
- increase BC
- Increase n
- Increase –> more plus
- Increase
- Increase (BC = F1)
- decrease
How does SM change for a minus lens if you:
- Increase vertex distance
- increase thickness
- increase BC
- Increase n
- Decrease –> more minus
- Increase
- Increase.
- Decrease
Relative spectacle magnification
- Compres what
- Equation
Compares retinal images of corrected eye vs standard emmetropic eye
RSM= Corrected ametrope retinal image size /emmetrope retinal image size
Axial ametropes are best corrected with __ according to ___’s law
Specs, Knapps law
Refractive ametropes are best corrected with ___
CLs
*refractive ametropes all have the same size image formed on the retina. Keep mag constant with CLs.
Uncorrected axial ametropes. Do myopes or hyperopes have larger retinal images?
Myopes have largest retinal imag
How does retinal image change with corrected refractive ametropes?
Retinal image will be largest for hyperope due to increased mag.
Types of aniseikonia:
- Anatomical
- Induced
- Meridional
- Some anatomical asymmetry such as a discrepancy in the density of photoreceptors.
- Due to the optics of the corrected eye, due to a difference in spec mag.
- Due to difference in cyl power between left and right eyes. A vertical object might appear tilted,
How to prescribe lenses for:
- Small differences RSM
- Large differences in RSM
- Small differences RSM: Equal BC and equal thickness.
- Large differences in RSM: Thin, flat lens for the eye with high RSM. Prescribe thicker, steeper lens for the eye with lowest RSM.
For 1.00D of power difference, what % of aniso will there be?
What D difference usually becomes a problem?
1%
3.00D/3% usually becomes a problem
Define:
Anisokonia
Anisometropia
Anisokonia- Difference in size or shape of the images seen by the left and right eyes.
Anisometropia- Refractive state of the left eye differs from the right eye, usually by 1.00D
Who is more likely to develop amblyopia?
Hyperopic anisometrope or myopic anisometrope?
Hyperopic anisometrope because accommodation occurs equally in both eyes. Pt will be using the least hyperopic eye all the time at distance and near.
A myopic anisometrope could use the less myopic eye at distance and the more myopic eye at near.
When light falls upon a lens, how can it be lost?
Reflected by the front or back surfaces
Absorption by the material
Formula for reflectance
Reflectance = ( (n2-n1) / (n2+n1) ) ^2
This tells you how much light was reflected off the front surface. Can assume it was the same for the back surface as long as both sides are surrounded by air.
To determine transmittance, subtract this value from 1
T= 1-Reflectance
How to calculate total transmission through a lens
Multiple Transmission of the front surface x back surface x T through medium
(They would have to give you the amount absorbed by the material. 1-absorption= transmission through material)
Formula to calculate Power of the lens needed in the direct ophthalmoscope based on patient rx and Dr Rx
Power of lens in direct = patient Rx + Doctor Rx
