Ophthalmic Optics Flashcards
How to correct for seg being too high?
- INC panto tilt
- DEC vertex distance
- Spread the pads
- Move pads up
- Stretch the bridge
How to correct for seg being too low?
- DEC panto tilt
- INC vertex distance
Basically bring away from face
Center Thickness equation?
CT = (S1 + ET) - S2
Thickness, warp, BC, impact tolerance for glasses?
Thickness: +/- 0.3mm
Warp: +/- 1.00D
BC: +/- 0.75D
Impact: 5/8 inch steel ball fr 50” drop
Minimum Blank Size (MBS) equation?
MBS = ED + d + 2mm
ED: effective diameter
d: decentration = Frame PD - Pt PD
Frame PD=A+DBL
Lens clock power equation?
F lens = F lens clock [ (n lens - 1)/(n lens clock - 1)]
Frame adjustment if the frame “Falls down nose”?
- Pull in temples
- Bend down temple tips
- Pull in nose pads
Frame adjustment if “one lens closer to face”?
- Straighten temple
2. Pull in temple on same side
Frame adjustment if “touching cheek”?
- Reduce panto tilt
- Narrow bridge or pads to raise frame
- Narrow bright or pads to INC vertex distance
Frame adjustment if “too close to face”?
- Narrow pads
- Shrink bridge
- DEC face form to move lenses away from face
Frame adjustment if “too low on face”?
- Narrow bridge
- Add pads
- Lower the vertical portion of the pads to move frames up
Frame adjustment if “too high on one side”?
Pull up the temple
Index of crown glass?
n=1.532
Abbe=58.9 (high)
Index of Cr-39?
n=1.498
Abbe=58 (high, light, impact resistant)
Index of Polycarb?
n=1.586
Abbe=30 (low, great impact resistant, high chromatic aberrations)
Index of Trivex?
n=1.53
Abbe=43 - 45
How do you reduced Radial Astigmatism?
Picking the CORRECT Base Curves *
NOTE: radial astigmatism is oblique or marginal astig due to rays hitting lens or interface obliquely –> power altered by the tilt of the lens
What is the upper and lower limits of aspheric correction?
< -23.00D
> +7.00D
Which two aberrations contribute to warping along 2 axises?
- Radial Astigmatism (RA)
- Curvature of Field (COF)
NOTE: COF is when a Plane Object =/= Plane Image
An image surface created by a system w/ no radial astigmatism yet still warped by COF is considered what kind of surface?
Petzval Surface
NOTE: COF will be present in any ophthalmic lens system ANY time the Petzval Surface does NOT correspond to the FAR POINT sphere of the eye –> warpage
What is the point of a point focal lens?
These lenses correct completely for radial astigmatism NOT COF
What is the point of a Percival Form Lens?
These lenses correct for COF NOT Radial Astigmatism
Pincushion is caused by plus or minus lenses?
PLUS
Barrel distortion is caused by plus or minus lenses?
MINUS
What type of qualitative aberration is inversely related to abbe value and results in a series of pt images along the axis?
Longitudinal Chromatic Aberrations (LCA)
Which qualitative aberration produces diff size images , is related to prismatic effect, creates more harm as prismatic effect inreases, and underlies the R/G spherical test in clinic?
Lateral (transverse) Chromatic aberration
What is the equation for chromatic aberration?
CA = d*F/v
d –> distance to optical center of lens
F –> lens power
v –> Abbe number
Therefore, lower abbe = MORE CA
Describe the purpose of an achromatic doublet.
Combines (+) lens of one material w (-) lens of another to eliminate CA (CA total = 0)
F total = F1 + F2 –> CA total = CA1 + CA2
0 = F1/v1 + F2/v2
What are the aberrations of most concern to least in lens materials w low abbe values?
- Oblique astigmatism
- Curvature of Field
- Distortion
NOTE: use a SHORTER vertex distance, Monocular PDs, use sufficient PANTO tilt (less than 10 degrees for HIGH powered lenses)
Prism Power & Thickness equation?
prism power = [100*g (n-1)]/L
g –> difference in thickness btwn apex & base
L –> apex to base length
Equation for Lensometer?
x = f^2(F vertex)
Where is the segment line relative to the GC for:
a. FT 28 mm (or less)
b. FT 35 mm
c. FT >35 mm
d. Curve/Panoptic
e. Executive/Franklin
f. Round Segs
a. FT 28 mm –> 5 mm below
b. FT 35 mm –> 4.5 mm below
c. FT >35 mm –> 0 mm below
d. Curve/Panoptic –> 4.5 mm below
e. Executive/Franklin –> 0 mm below
f. Round segs –> Seg radius (r)
What type of magnification compares the retinal image size in an UNCORRECTED eye w the retinal image size in a CORRECTED eye?
Spectacle Magnification
SM=(I glasses)/(I w/o glasses)
SM smaller for myopes & bigger for hyperopes
What is the spectacle magnification formula when taking shape and power factor into account?
SM = (Shape Factor)*(Power Factor)
Equation for Shape Factor?
Shape Factor=Ms=1/[(1-(t/n)*(F1’)]
Equation for Power Factor?
Power Factor=Mp=1/(1-h*Fv’)
h –> distance btwn BACK surface of the lens & entrance pupil of the eye
How does increasing vertex distance affect SM in (+) vs (-) lenses?
(+) –> INCREASES SM
(-) –> DECREASES SM
Just use the Power factor equation
What kind of magnification compares retinal image of corrected eye vs retinal image from a STANDARD eye?
Relative Spectacle Magnification
Which law is a rule for RSM in axial ametrope where RSM=1 if a thin lens is placed at the primary focal point of the eye?
Knapp’s Law
Knapp’s Rule states that lenses placed at the anterior focal point of the eye, generally 15 mm in front of the eye, will create similarly sized images on the retina, whenever the disparity between the two eyes is due to a difference in axial length of the eyes.
Axial ametrope primary focal point is about 15-17 mm in front of cornea
Provide intution as to why spectacles work better for axial ametropes
Where is the primary focal point in an axial ametrope?
15 to 17 mm in front of the cornea
What are refractive ametropes best corrected with?
Best corrected with CONTACT LENSES (no effect on magnification)
NOTE: uncorrected refractive myopes, hyperopes, emmetrope all have the same image size formed on the retina; we would want magnification to remain constant after correction –> make SM small by choosing CL
Compare the retinal image sizes for an uncorrected AXIAL ametrope:
Myope, Hyperope, Emmetrope
Myope > Emmetrope > Hyperope
NOTE: when corrected, RSM = ~1 thus its still M>E>H
Compare the retinal image sizes for an uncorrected REFRACTIVE ametrope:
Myope, Hyperope, Emmetrope
M=H=E
NOTE: when corrected w CL’s RSM=~1; spectacles result in LARGER image for Hyperopes & Smaller for Myopes
Spherical equivalent equation?
Fse=Fc/2 + Fs
Describe Meridional Aniseikonia.
Due to differences in CYL power btwn OD and OS
The effect is prominent in ONE meridian –> vertical image may appear to be tilted to pt w MERIDIONAL anisekonia
When is aniseikonia a problem?
When it is >3%
If there is a SMALL difference in Relative Spectacle Magnification, how would you prescribe?
Rx equal BC
Rx equal thickness
If there is a LARGE difference in Relative Spectacle Magnification, how would you prescribe?
Rx thin, flat lenses for eye with the HIGHEST RSM
Rx thicker, steeper lens for the eye w LOWEST RSM
If there is aniso >4.00D, how would you prescribe?
Rx spectacles to minimize aniseikonia; likely axial
If there is a small amount, how would you prescribe?
CL minimize aniseikonia bc likely refractive
If there is aniesikonia bc large astigmatism, how would you prescribe?
CL bc large amounts of astigmatism usually from the cornea
NOTE: every 1.00D power difference, there will be approx 1% aniseikonia
For every 1.00D of power difference btwn the eyes, there will be approx how much aniseikonia?
1%
Seg Width?
Longest horizontal dimension of the seg
Seg Depth?
Longest vertical dimension of the seg
Seg Height?
Dist from lowest pt on lens to top of seg
Seg Drop?
Vertical distance btwn the MRP and top of seg
Inset?
Distance from GC to the MRP
What is Seg Inset?
Accounting for near PD
Distance from GC to the MRP
Total Inset?
Distance from GC to Center Seg
I total = I seg + I
