Physiologic optics (KMK) - oph characteristics of lenses, frame specifications/adjustments, aberrations/distortions, prisms
A pt's bifocal sits too high. How to adjust the frames?
(Try in order): Increase panto Decrease vertex distance Spread the pads Move the pads up Stretch the bridge
Limits to Tscherning ellipse
+7.50 (above this - must use aspheric lenses) -22.00 For n = 1.523
Where is the optical center of an executive seg?
0mm (OC at the seg line)
Seg drop
1/2(B) - seg height The vertical distance from MRP to top of seg
1PD shifts an image how much?
1PD will shift it 1cm 1 m away
Where is the optical center of a curve top/Panoptik seg?
4.5mm
ANSI impact resistance of regular (non-safety) glasses
5/8 in steel ball from 50 inches
How to calculate frame PD?
A + DBL
A vs. B measurements of a lens
A - horizontal length B - vertical length
Percival form lens
A lens that's completely corrected for curvature of field; radial astigmatism is uncorrected
Point focal lens
A lens that's completely corrected for radial astigmatism; curvature of field is uncorrected
What is an Abbe number represent
A measure of constringence (reciprocal of dispersive power of lens) Higher Abbe --> better Lower Abbe --> more chromatic aberrations
How to eliminate chromatic aberrations?
Achromatic doublet - positive lens of one material with negative lens of another material
Curvature of field - defn, when is it seen
An aberration that occurs in all lenses secondary to nature of optics A flat object will produce a curved (Petzval) image plane
Why does coma occur?
As height of incidence rays varies --> MAGNIFICATION varies --> asymmetric comet-shaped img Only for off-axis point sources
Where is the optical center of a round seg?
At the center of the seg (depends on the radius of the seg)
Where is the optical center of a Ribbon-B seg? A Ribbon R seg?
B: 4.5mm R: 7mm
Names of the curves of a plus cyl
Base curve - flatter of the front surface Cross curve - the other front surface curve Back surface - sphere curve
Names of the curves of a minus cyl
Base curve - the front surface Toric base curve - back flatter curve Cross curve - the other back surface curve
Biconvex v. equiconvex
Biconvex - both sides are convex but may have different powers Equiconvex - half the total power due to front, half from back surface
What are longitudinal and lateral chromatic aberrations related to?
Both are inversely related to Abbe number Lateral - related to prismatic effect (increases the further you get from the center of the lens)
Slab off prism
Change the shape of the more minus lens --> more BU prism To compensate for vertical imbalance
How to eliminate radial astigmatism?
Chose the proper base curves (Tscherning's ellipse)
Defn of meniscus lens
Convex front surface, concave back surface
Purpose of aspheric lenses
Correct for aberrations in high power lenses Make lens thinner and lighter Make a PAL
What causes distortion? How does it affect the image? Types? How to correct?
Different magnification across a lens. Does not result in poorer resolution or blur. Minus lens --> barrel Plus lens --> pincushion Correct via orthoscopic doublet (rarely done)
Near inset defn, how to calculate
Distance from MRP (distance PD) to near CD = (PD - near CD) / 2
Inset defn, how to calculate
Distance from geometric center to MRP = (frame PD - pt PD) / 2
Total inset defn, how to calculate
Distance from geometric center to near CD = (frame PD - near CD) / 2
How to calculate minimum blank size
ED + 2(monocular decentration) + 2mm (fudge factor) Monocular decentration = (frame PD - pt's PD) / 2
When do we worry about spherical aberr and coma?
For high power lenses Usually - not too big of a deal, since pupil only lets through paraxial rays anyways
Hard v. soft progressive design (corridor length, high/low power)
Hard design - shorter corridor, for higher powers Soft design - longer corridor, for lower powers
Where is the optical center of a flat top?
If 28mm or less - 5mm below the seg edge If 35mm - 4.5mm below seg edge If larger than 35mm - 0mm (OC at seg edge)
ANSI standards/tolerance for sphere/cyl power and axis
If sphere <6.50: 0.13 tolerance If > 6.50: 2% If cyl <2.00: 0.13 If cyl 2-4.50: 0.15 If >4.50: 4% If cyl <0.25, 14 deg tolerance 0.25-0.50, 7 0.50-0.75, 5 0.75-1.50, 3 >1.50, 2
How to calculate the amount of jump?
Jump = add power x distance from near OC to seg dividng line
How does light bend when it hits a prism?
Light towards the base Image towards the apex
How does spherical aberration change with pupil size?
Longitudinal - aberrations increase with the square of the radius Lateral - increase with the cube of the radius
Longitudinal v. lateral aberrations
Longitudinal - distance between focus points in aberration Lateral - area encompassed by the aberrations
How to lenso a bifocal?
Measure the distance front vertex power (glasses are in backwards) Then the near power Subtract the two
ANSI regulations on safety eyewear
Minimum coverage 40x33mm Has the proper markings for what it's appropriate for (H, +, W, etc) High mass impact (drop ball test) - 500g pointed projectile from 50 inches High velocity impact - 0.25in steel ball fired at 150 ft/sec
CR-39 - refractive index, Abbe number
N = 1.498 V = 58
What does a material code of 498-581 indicate?
N = 1.498, abbe value 58.1 this is CR-39
Crown glass - refractive index, Abbe number
N = 1.523 V = 58
Trivex - refractive index, Abbe number
N = 1.53 V = 43-45
Polycarb - refractive index, Abbe number
N = 1.586 V = 30 (lots of chromatic aberrations)
Pt CC glasses too low on the face. How to fix?
Narrow the bridge Add pads Lower the vertical position of pads
A pt's bifocal sits too low. How to adjust the frames?
Narrow the pads Move the pads down Increase vertex distance Reduce panto Shrink the bridge
Pt CC glasses are too close to the face. How to fix?
Narrow the pads Shrink the bridge Decrease face form
Defn of major reference point (MRP)
Point on lens through which the line of sight passes
Prism power to prism thickness equation
Prism = 100g(n-1) / l g = prism thickness l = apex to base length
Pt CC glasses fall down nose. How to fix?
Pull in the temples Bend down temple tips Pull in nose pads
How to minimize the amount of jump?
Put the near OC as close to the dividing line as possible
Pt CC glasses touch the cheek. How to fix?
Reduce panto Narrow the bridge or pads to raise the frame / increase vertex distance
Seg height v. seg depth
Seg height - from top of seg to bottom part of the frame Seg depth - vertical dimension of the seg
Tscherning ellipse
Shows the best values for BCs to eliminate oblique astigmatism, curvature of field The ellipse varies based on viewing distance, lens material Made of two curves - Wollaston and Ostwalt (the flatter one we use)
Where is the base curve located for a sphere, plus cyl, minus cyl lens, and contact lens?
Sphere - front surface Plus cyl - flatter front surface Minus cyl - front surface CL - back surface
Pt CC glasses one lens is closer than the other. How to fix?
Straighten the temples
What is the effective diameter of a lens?
The longest possible diameter of the lens Will never be smaller than A
Geometric center of a lens
The physical center of the lens (where A and B lines cross)
ANSI standards for GP metrics (?)
Thickness 0.3mm Warpage 1D BC 0.75D Impact resistance 5.8in steel ball from 50 in
Thin v. thick prism
Thin prisms - have an apical angle of less than 10 deg
In most cyl lenses - which side is the cyl found?
Toric surface is on the back surface
Total prismatic effect v. vertical imbalance
Total prismatic effect - when looking through a seg, need to account for both distance + seg prism by decentration (in a single eye) Vertical imbalance - comparing between the two eyes
Tips to minimize chromatic aberrations in low Abbe value materials
Use shorter vertex distances Take monocular PDs Include sufficient panto tilt
Oblique astigmatism
When light hits a spherical lens obliquely --> tangential/sagittal image planes
Why do spherical aberrations occur
When the assumption of point object --> point image breaks down (paraxial approximation) Results in longitudinal spherical aberration - focuses in different planes, happens in both on- and off-axis
Formula for angle of deviation of a (thin) prism
d(min) = alpha(n-1) d(min) - because deviation depends on angle of incidence alpha - apical angle
How to compound two prisms
prism power = sqrt(x^2 + y^2) Prism angle: tan(theta) = x/y X = vertical component Y = horizontal component
Sag to radius of curvature formula
s = y^2 / 2r (y = half of the lens clock diam) r = y^2 / 2s
How to resolve one prism into two directions
x = prism power(sin theta) y = prism power(cos theta)
