Physiological Optics I Flashcards
Direct ophthalmoscope
◦ Smaller FOV
◦ Decreased depth of focus
◦ Increased mag
◦ Upright image
Indirect ophthalmoscope
◦ Larger FOV
◦ Increased depth of focus
◦ Smaller mag
◦ Inverted image
How ophthalmoscopes work
- create a conjugate image of the patient’s retina on the examiner’s retina.
- BIO: a condensing lens is used to form intermediate, inverted, real image. Another lens (or accommodation) is needed to see the intermediate image, which is located about 1 arms length away from examiner
Lensometer
• measures back vertex power and prismatic properties of lenses
• How it works
◦ Uses telescope system: look through standard lens and lens of interest (test lens)
‣ Parallel light is leaving the test lens
‣ Light from the standard lens must converge at F, the primary focal point of the test lens
‣ Now we move the target until it happen
Math for lensometer
X=f^2Fv
Hand neutralization
- Similar in purpose to a lensometer
- Utilizes with or against motion to determine the power of the lens
- Minus lens: two prisms stacked apex to apex: with motion
- Plus lens: two prisms stacked base to base: against motion
- No power: no prismatic effect=no apparent motion
- Thin lenses: to determine the power of an unknown lens, add another lens power to neutralize the apparent motion effect. When motion is stopped, The power of the known lens is equal in magnitude and opposite in sign to that of the unknown lens.
Keratometer
- measures the radius of curvature of the center of the cornea
- Cornea acts as convex mirror. One measures the size of the reflected image
- To convert from radius to power: F=337.5/r
- Same principles can be used to determine curvature of a non spherical cornea
How to use lensometer
- focus eye piece
- Blur by turning wheel in plus direction then slowly step backwards until sphere lines are clear (skinny lines)
- Adjust axis to clear sphere lines
- Turn wheel in minus direction until the cylinder lines are clear. Difference between this power and the power at which the sphere lines were clear is the cylinder power
Lensometer and prism
• verifies amount and direction of prism
• Compare location of cross hairs formed by sphere and cylinder lines with the location of a “bulls-eye” image of concentric circles
◦ Crosshairs to the left of the center of bulls-eye=BO (OD) or BI(OS)
◦ Crosshairs to the right of the center of the bulls-eye=BI (OD) or BO (OS)
◦ Crosshairs fall above the bulls-eye=BU
◦ Crosshairs below the bulls eye=BD
Radiuscope
• measures the radius of curvature of RGP CL
• How it works
◦ Scope forms a target at some point P between the viewer and the CL of interest. Light from the image will reflect off the CL and form another image at some point Q
◦ P is located att he surface of the lens. This is because the object distance is zero
◦ P is located at the center of the curvature of the CL. The object distance is the radius of curvature
• Take home summary
◦ Simply need to move the CL from one such location to the other and measure how far it had moved. This distance is the radius of curvature
‣ The distance between the two positions of focus (the clear images)
Lens clock
• measures the sag of a lens
• Can be specified in terms of a lens power using other properties of the lens
• How it works
◦ Adjusting movable pin
◦ Based on sag value and assumed value of n, some lens clocks give a power readout
◦ May need calibrated
• Math for incorrectly calibrated lens clock: FL=(nL-1)/(nLC-1)xFLC
Slit lamp biomicroscope
◦ Keplarian telescope makes up eyepiece
◦ Inverting prism correct upside down image
◦ Galilean telescope is used to further magnify the image
◦ Objective lens
◦ A complex illuminating system
◦ A binocular viewing system creates stereoscopic images, creating 3D view
Image of fundus lens
Create real inverted lenses
Higher power fundus lens=
Lower mag/increased FOV
Fundus lens creates a
◦ Placing lens in front of eye creates telescope system
◦ Essentially creating a reverse telescope
◦ Assume power of eye is +60D ◦ If a +60D fundus lens is placed in front of the eye to create the reverse telescope, M=-(+60)/(+60)=-1x (true size of the image and its inverted) ‣ This is why we can directly measure the optic nerve head with a +60D
Image and mag of +90D
-(60/90)=-.67x
Minified and inverted
+20D mag and image
-(60/20)=-3X
Magnified and inverted
+78D image and mag
-(60/78)=-0.77x
Inverted and minified
+60D lens image and mag
-(60/60)=-1X
True size and inverted
-55D (hruby lens) image and mag
-(60/-55)=+1.09
Magnified and upright
Galilean telescope
Minimum coverage area of ANSI for glasses
◦ Ellipse of 40mm by 33mm centered on the geometric center of the lens
Markings on lens: H
Smaller heads (reduced min coverage size)
markings on lenses: +
Imapact rating
Markings on lenses: W
Shade number (welders)
Markings on lenses: U
Scale number for UV filters
Markings on lenses: R
IR filters
Markings on lenses: L
Visible filter
Markings on lenses: V
Variable tints
What margins must be on the frame for ANSI
manufacture logos, impact mark, and standard mark
ANSI Z87.1 safety standards for impact
- high mass impact (drop ball test): pointed projectile, 500mg dropped from 50”
- High velocity impact: steel ball, 0.25” in diameter, fired at 150ft/s
ANSI Tolerance for -6.50-+6.50
+/- 0.13
Sphere power tolerance for >+/- 6.50
+/- 2% sphere power
Cyl power tolerance ANSI: <2.00
+/-0.13
cyl power tolerance ANSI: 2-4.50D
+/-0.15
Cyl power tolerance for >4.50D ANSI
4% of cyl power
Axis tolerance for cyl <0.25
+/-14
Axis tolerance for 0.25-0.50
+/-7
Axis tolerance for 0.50-0.75
+/- 5
Axis tolerance for 0.75-1.50
+/-3
Axis tolerance for >1.50
+/-2
Plano convex and plan concave lens
one surface is flat and the other is curved
Biconvex and biconcave
both surfaces convex or concave
Equiconvex and equiconcave
half of the total power is due to the front surface and half is due to the back surface
Meniscus
convex front and concave back
Plano cylinder
one flat surface and one cylinder surface
Toric
one toric surface and one spherical surface
Base curves
• specified in terms of power
• Always on the front surface for single vision lenses
• Spherical lens: front surface
• Plus cylinder: BC is flatter on the front. The other curve is the cross curve, the back curve is the sphere curve
• Minus cyl: back flatter curve is the toric BC. The other back curve is the cross curve
• BC for CL
◦ On the BACK
Lens thickness
- minus lens is thicker in the periphery than in the center, while a plus lens is thicker in the center than the periphery
- Note that for a lens with one flat surface, s1 or s2 is zero
- When calculating edge thickness, it is good to draw a picture of the lens. Then you do not have to worry about the sign convention for si, but can instead use geometry
+ lens: tc=te+S1+S2
- lens: tc=te-S1+S2
Thick lens:surface one radius and power
R1=h^2/2Sfi
F=(n2-n1)/r1
Thick lens surface 2 radius and power
- r2=h^2/2Sfi
- F=(n3-n2)/r2
Isothikcness curves
• curves drawn on paper cross to show the curves on which thickness is the same
Geometrical center
point on the datum line halfway between the two vertical lines which ar tangent to the edges
Eye size or lens size
horizontal length of box. Eye size refers to frame, lens size refers to lens itself
B distance
vertical length of the box
DBL
Bridge size
shortest horizontal distance between the lenses
GCD or frame PD
the longest diameter of the lens
A+DBL
Effective diameter
the longest diameter of the lens
Decentration per lens (d)
-d=(frame PD-wearer’s PD)/2
Frame PD=A + DBL
Wearers PD
distance between the center of one pupil to the center of the other pupil
Minimum blank size
the smallest size lens blank needed to make the lens.
M=ED+2(d)+2mm
ED=effective diamter
D=decentration (mm)
MRP
• Major reference point (MRP): point on the lens through which the line of sight (visual axis) passes.
Distance between OC and edge of flat top bifocal segment (28mm or less)
5mm
Distance between OCand the edge of bifocal segment for a flat top (35mm)
4.5mm
Distance between the OC and edge of the bifocal segment for a franklin (executive) seg
0mm. The OC is located at the seg line
Distance between OC and edge of bifocal segment for a round seg
-r, the radius of the sag
Progressives
- Hard design: short corridor and/or high add power
* Soft design: long corridor and/or low add power
Trifocals
• 1/2 the add power in the intermediate zone
Adjusting height of the segment: if the seg appears too high on the patient
◦ Increase panto tilt ◦ Decrease vertex distance ◦ Spread the pads ◦ Move pads up by adjusting pad arms ◦ Stretch the bridge
If the seg appears too low on the patient
◦ Narrow the pads ◦ Move pads down by adjusting the pad arms ◦ Increase vertex distance ◦ Reduce panto tilt ◦ Shrink the bridge
Lensometry and bifocals
• turn glasses around backwards in the lensometer
• Measure the distance power; only the sphere needed
• Measure the near power
• The difference in the near and distance powers gives the add power
• Must flip the lenses in the lensometer if the power is higher
◦ In practice we can estimate without turning the lenses around in the lensometer.
Seg width
longest horizontal dimension of the segment
Seg depth
longest vertical dimension of the segment
Seg height
distance from the lowest point on the lens to the top of the seg
Seg drop
vertical distance between the MRP and the top of the seg
Inset
distance from the GC to the MRP.
(Frame PD-dist PD)/2
Seg inset
accounting for near PD. Distance from the MRP to the center of the seg.
(Dist PD-near PD)/2
Total inset
inset of seg as measure from the OC of the lens. Distance from the GC to the center of the seg.
(Frame PD-near PD)/2
Glasses fall down nose
pull in temples, bend down temple tips, and/or pull in nose pads to tighten the fit
One lens feels closer to the face than the other
straighten the temples
Glasses touch the cheek
reduce panto, narrow the bridge or pads to raise frame, and/or narrow the bridge pads to increase the vertex distance
Glasses too close to face
narrow the pads, shrink the bridge, or decrease face form to move the lenses further away
Frames sit too low on face
narrow the bridge, add pads, or lower the vertical position of the pads to move the frames up on the face
Crown glass RI/Abbe
1.523/58.9
CR-39 RI and Abbe
1.498/58
Polycarb RI and abbe
1.586/30
Trivex R.I. and abbe
1.53/44
Monochromatic aberrations
• distort image quality (spherical, coma, radial astigmatism) or deform the image plane (curvature of field and distortion)
SA
◦ Based on paraxial approximation not always being accurate
Marginal rays
◦ Rays in the periphery
◦ Bent more than central rays and therefore focus closer to the lens compared to central rays
Longitudinal spherical aberrations
◦ Marginal rays focus to a different point compared to paraxial rays
◦ A point object is no longer forming a point image
◦ LSA occurs for both on and off axis points
◦ Contributes to nocturnal myopia
Coma
◦ Only for off axis point
◦ Asymmetric comet shaped blur
◦ Results from the fact that magnification is varies
Aspheric lenses
◦ In very high powered lenses, spherical aberrations are a problem where they are not a problem in lower powered lenses. We have to compensate by Rxing aspheric lenses which modify the surface without changing the power
◦ -23.00 to +7.00D
◦ Flatten the lens reduces magnification
◦ Reduce the weight of the lens
◦ Progressive lenses
Pupil size and mono aberrations
◦ Both coma and longitudinal SA increase with the square of the system aperture
◦ Lateral SA increases with the cube of the aperture
◦ Increase in pupil size typically leads to a decrease in image quality owing to increased aberrations
Radial astigmatism
◦ Oblique or marginal astigmatism
◦ Rays hitting the lens or interface obliquely
◦ Tangential rays and saggital rays are altered asymmetrically
◦ A flat object plane yields an asymmetrically warped image plane
◦ Reduce by picking the correct base curve
‣ Picking correct BC can minimize
• Curvature of field
• Radial astigmatism
Terschnig eclipse
◦ Base curve vs Fv plot
◦ Best value for BC for eliminating oblique astigmatism
‣ Wollaston and Oswalt Curve
• We use Oswalt curve to pick BC (Flatter of the two)
◦ Limits=upper limit is +7.50D. Use aspheric above this. Lower is 22D
Curvature of field (power error)
◦ image points formed by all points on the paper do not lie in a plane
◦ Quality of an image on a flat screen decreases for larger distances
◦ Relation to RA: curvature of field is intertwined with radial astigmatism where there is a different warping along two principle axes
Petzval surface
◦ Image surface created by a system with no radial astigmatism
◦ Still warped due to curvature of field
◦ Curvature of field will present in an ophthalmic lens system any time the petzval surface doe snot correspond to the far point sphere of the eye
Designing lenses: radial astigmatism vs curvature of field
◦ Lenses that have corrected for radial astigmatism, curvature of field, or both fall under the heading “corrected curve” lenses
◦ Point focal lens: a lens corrected completely for radial astigmatism; curvature of field is uncorrected
◦ Percival form lens: a lens corrected completely for curvature of field; radial astigmatism is uncorrected
◦ Lenses are typically designed as a compromise between point focal and percival form so that both radial astigmatism and curvature of field are partially corrected.
Distortion
◦ Magnification of a point object depends on the objects distance from the optical axis.
◦ Barrel=minus lenses
◦ Puncushion=plus lenses
◦ Minimized by a combination of lenses known as an orthoscopic doublet
Chromatic aberrations
- Rectus from the fact that the RI n is slightly dependent on wavelength.
- Shorter wavelengths (blue) bend more as they pass through an interface or optical system than do longer wavelengths
- The image will vary in location (longitudinal chromatic aberrations) and size (lateral chromatic aberration
- Patient’s wearing high powered lenses may see colored fringes around objects because of this.
- Chromatic aberrations underlay the use of RG balance in refractions.
LCA
◦ Series of point images along the axis
◦ Inversely related to v, the Abbe value
TCA
◦ Different size images depending on the wavelength
◦ Related to prismatic effect
◦ More harmful to vision
◦ Inversely related to v
Abbe number
◦ Quantifies chromatic aberrations
◦ Inversely related to the magnitude of chromatic aberration
◦ Math for LCA: CA=F/v
◦ Math for TCA (chromatic power): CA=dF/v
Achromatic doublet
Combine a positive lens of one material with a negative lens of another material to elimate CA. The resulting lens is called an achromatic doublet.
◦ Lens with Abbe number combined with a lens of another Abbe number eliminates CA
◦ The ratio of the v values equals the ratio of the powers
Lens materials with low abbe values
Use shorter vertex distances
‣ Use monocular PD
‣ Include sufficient panto tilt
Most concerning aberrations in aopthalmic lenses in order
Oblique astigmatism
Curvature of field
Distortion
Light properties and prisms
- light bends towards the base
* Image shifts towards the apex
Units of prism
◦ Prism diopter
◦ A prism with a power of xZ will shift a beam of incoming light x cm on a wall 1m away.
Prism=y(cm)/x(m)
Prism power and apex angle
◦ We can find the deviating angle with: d=A(n-1) (DAN-1)
◦ A is the apex angle in degrees, and d is the deviation angle in degrees
Prism power and thickness
◦ math: prism=100(g(n-1))/l
◦ g is the difference in thickness between the apex and the base, l is the apec to base length.
Prism effectivity
◦ Effective power of prism depends on the location of the prism in relation to the patient’s eye
Prism orientations
BU
BD
BI
BO
Combining prisms
Vector addition
Prism=square root of H^2+V^2
• vertical prism combination ◦ If same direction in each eye, subtract ◦ If opposite directions, add • Horizontal prism combo ◦ Same direction=add ◦ Opposite direction-subtract
Prentice’s rule
• associate a prism power to each point on a lens • Uses ◦ Decentration ◦ Vertical imbalance-2 eyes ◦ Image jump-add type+power ◦ Total prismatic effect-1 eye • Math: prism=dF
Decentering and Prentice’s rule
◦ Plus lenses: the decentering direction correspond to the base direction of induced prism
◦ Minus lenses: the decentering direction corresponds to the apex direction of the induced prism
Oblique meridans and Prentice’s rule
◦ Can not use prentice’s rule with so called sin-squared formula, which estimates the power in the oblique meridian
◦ This combination can be used when the decentering (or looking) is only vertical or only horizontal
◦ Useful for vertical imbalance
◦ If given a lens that is both decentered horizontally and vertically, use prentice’s rule for each direction individually. Then the total prism is the vector of the two sums
Vertical prism effect
• vertical prism induced in each eye
• If these amounts differ, there is a vertical imbalance
• Results from different vertical prismatic effects in each eye
• To find vertical imbalance, we want the DIFFERENCE in the amount of vertical prism in each eye
• Correcting vertical imbalance
◦ Slab off=more mins lens gets more BU (BUMM)
◦ Dissimilar Segs= optical centers of the R and L segs at diff places
◦ CL
◦ Fresnel Prism=attached to the lens
Image jump
- sudden displacement of an image at the bifocal line
- Due to added prismatic effect that comes from the distance portion of the lens
- When the eyes cross the bifocal line, the prismatic effect is changed by an additional amount
Total prismatic effect
• we must sum the prism induced from looking away from the seg OC and the prism induced from looking away from the distance OC.
Compares the retinal image size in an uncorrected eye with the retinal image size in a corrected eye
Spectacle magnification
Corrected image size/uncorrected image size
Spec mag for thick lenses
◦ Need shape and power for this equation
◦ SM= (shape Factor) x (power factor)
◦ Where shape factor = Ms=1/(1-(t/n)F1)
◦ Where t is the central thickness of the lens (in m), F1 is the front surface power of the spectacle lens, n is the index of refraction of the lens
◦ Where power factor= Mp=1/(1-hFv) [h=vertex+3mm]
◦ Where h is the distance between the back surface of the lens and the entrance pupil of the eye (mm), and Fv is the back vertex power of the lens
Relative spectacle magnification
used to compare retinal image of a corrected eye with the retinal image of a standard eye. Standard eye defined to be +60D
• Math: RSM=Ia/Is
◦ Where Ia is the retinal image size in a corrected ametropic eye and Is is the retinal image size in the standard eye
Axial ametropia best corrected with
Specs
Knapp’s law
Rule for RSM. RSM=1 if a thin lens is placed at the primary focal point of the eye
Refractive ametropia best corrected with
CL
Things that increases SM for plus lenses
Increased h (vertex)
Increases thickness
Increased BC
Decreased n
Things that increase SM for minus lenses
Decreased h (vertex)
Increased thickness
Decreased n
Increased BC
Retinal image size for uncorrected axial ametropes
◦ Myope image size > emmetropia size> hyperope size
Retinal image size for corrected axial ametropes
◦ CL yield the same results as above, spectacles make RSM 1
Retinal image size for uncorrected refractive ametropes
◦ All retinal image sizes are the same
Retinal image size for corrected refractive ametropia
◦ CL make RSM=1, spectacles result in larger image for hyperope, and smaller image for the myope.
Aniseikonia
• difference in the size or shape of the images seen by the left and right eye
Anatomical aniseikonia
discrepancy in density of PR
Induced aniseikonia
optics of the corrected eye, difference in spec magnification
Meridonial aniseikonia
difference in cyl power. Prominent in one meridian
◦ A vertical object might appear to be tilted to a patient with meridonial aniseikonia
For a small difference in RSM
Rx equal BC and equal thickness
For large difference in RSM
Rx thin, flat lenses for the eye with the highest RSM, Rx thickener, steeper Lenses for the eye with the lowest RSM
If a patient has a small amount of ametropia in one eye while there ther eye has more than 4D of ametropia, the ametropia in the bad eye is likely ____. There fore, ____ should minimize aniseikonia
Axial
Specs
For every 1.00D power differnece, there will be appx ____ aniseikonia
1%
3% is a problem (around 3D difference)
Anisometropia
• refractive state of the left eye differs from the right eye, usually by more than 1D
◦ Pts with myopic anisometropia have poor uncorrected distance acuities in both eyes, with the most myopic eye having the worst acuity
◦ Pts with hyperopic anisometropia have relatively good uncorrected distance acuity in both eyes, but they may have eyestrain
Distance at which a hyperopic anisometropia can see clearly with both eyes
• Accommodation occurs equally in both eyes. Therefore there is no distance at which a hyperopic anisometrope can see clearly with both eyes. In myopia aniso, a person will typically learn to use the more myopic eye for near vision and the less myopic eye for distance. Hyperopic aniso will likely use the same eye (the least hyperopic) for all distances, putting at risk for amblyopia in early childhood
Antimetropia
◦ One eye is hyperopic and one eye is myopic
Properties of absorptive lenses
• loss of light when passing through the lens
◦ When light passes through a lens, it is reflected at both surfaces and also absorbed by the lens material
Transmittance
measures the amount of light energy that gets through an optical system. Ranges from 0-
When light hits a lens it is lost in two ways
- Reflected at the front and rear surface
R=((n2-n1)/(N2+n1))^2
-this would occur at each surface, to convert to tranmisttance, use Ts=1-R - Absorbed by the medium: assume the amount transmitted (not absorbed) by the medium is Tm=1-(amount of light absorbed by the lens). For a lens, we combine those 2 factors: T=(Ts1)(Ts2)(Tm)
There T is the total transmittance through the lens
Power of the CL is given as the
Back vertex power (Fv)
Fv=F2+(F1)/(1-(t/n)F1)
• if a center thickness is provided in the problem, always treat a contact lens as a thick lens and calculate the back vertex power. Otherwise, you can approximate the CL as a thin lens
Vertex distance
• effective power of a lens changes depending on where the lens is located in front of the eye
• Plus lenses effectively become weaker as they move closer to the cornea, thus hyperopes will require more plus power
• Minus lenses effectively become stronger as they move closer, myopes rewuire less minus power in CL
• For both plus and minus lense, mor eplus power is necessary when the lens is moved from the spectacle plane to the cornea (CL have more plus power than specs)
• Vertex equation:
Fc=Fs/(1-dFs)
• the effective vergence equation should be used to determine the power at the corneal plane for any spec Rx > +/-4.00D
Lacrimal lens of gas perm lenses
◦ A layer of tears is formed between the GP CL and the cornea. It has a certain power that is dependent on the front surface of the cornea and the back surface of the CL.
◦ Exploded system: consider the CL in air, the lacrimal lens in air, and the cornea in air.
◦ Refraction and GP CL
‣ PLL+PGP+PC=corneal plane refraction
‣ This equation must hold for a CL to perfectly correct an ametropic eye. We must account for the lacrimal lens when Rxing GPCLs
Lacrimal lens power
‣ Determined by the surface of the cornea and the back surface of the GPCL
‣ Back surface of the lacrimal lens has a radius of curvature that is equal to the radius of curvature of the cornea
‣ Front surface of the lacrimal lens has a radius of curvature that is equal to the BC of the CL
‣ Tear lens power=337.5/r(mm)
SAM FAP
‣ If the BC of the CL equals the curvature of the cornea, there will be a total lacrimal lens power of 0
‣ If the BC of the CL is steeper than the curvature of the cornea, the lacrimal lens will be plus power
‣ If the BC of the CL is flatter than the cornea, the lacrimal lens will be minus power
‣ Whenever the BC of the GPCL is changed, the power of the lacrimal lens will change; the power of the GPCL must also be modified in order to compensate for the change in the lacrimal lens power
• Steeper add minus, flatter add plus (SAMFAP)
‣ For every 0.1mm change in the BC of the GPCL, the power of the lacrimal lens and the total power of the GPCL will change by appx 0.50D.
3 step method to GPCL problems
‣ Convert to diopters (337.5/r)
‣ Tear lens: GPBC-corneaBC=tear lens
‣ Solve GPCL equation: P(LL)+P(GP)+P(C)
OZD of GPCL
Usable area of optics in CL
Increased OZD
Increased SAg=steeper
Decreased OZD
Decreased sag=flatter
For every 0.4mm change in OZD
The BC should be adjusted by 0.25D in order to maintain the same fitting relationship between the GPCL and the cornea
Overall diameter (OAD) GPCL
uncurved distance of the CL from edge to edge
◦ Average is 9.4-9.6mm
◦ Adjusted in 0.4mm steps
Lacrimal lens and corneal astigmatism
• lacrimal lens will completely correct the corneal astigmatism
Residual astigmatism on GPCL
◦ The total amount of astigmatism in the eye is equal to the amount of corneal astigmatism plus the amount of internal astigmatism (lenticular). Residual astigmatism is the amount of astigmatism not corrected by a GP
◦ For a GPCL, the amount of residual astigmatism is simply the amount of astigmatism not attributable to the cornea
◦ Patients can often tolerate <1.00D WTR astigmatism and < 0.75D ATR or oblique astigmatism. If more than this is present, fit with a toric GPCL
Javal’s rule
◦ Allows one to empirically predict the total astigmatism correction at the spectacle plane using the results from keratometry.
◦ A(RX)=1.25Ac+(-0.50D x 090)
◦ Ac is the corneal astigmatism, add 0.50D because the average amount of internal astigmatism is roughly -0.50Dx090
Over refraction nand GPCL
◦ Determine the BVA with CL fit, and to help empirically determine the total power of the CL and the amount of residual astigmatism
◦ If the over refraction for a spehiercal GPCL is not Plano, simply add the OR to the CL power in order to determin the new CL power. Note that the OR will already take into account the power of the lacrimal lens.
Peripheral curves of GPCL
◦ Allow alignment between the CL edge and the peripheral cornea
◦ prevent the edge of the CL from bearing on the cornea during movement of the CL
◦ Promote tear exchange underneath the CL to maintain adequate metabolism
◦ Support tear meniscus at the edge of the CL to promote CL centration
Edge thickness in GPCL
ideal thickness to promote lid attachment is the edge thickness of a -3.00D GPCL
◦ More plus than -1.50D tend to drop inferiorly. A Plano/minus carrier lenticular may be added to thicken the edge
◦ More minus than -5.00D edges are too thick and ride high. A plus lenticular or CN beveling added to decrease edge thickness
Edge lift and GPCL
distance between the peripheral edge of the GPCL and the cornea. Changed in 0.1mm steps
◦ Excessive edge lift: excessive peripheral pooling of NaFL. Results in decreased CL centration, increased awareness of CL on eye, and cornea desiccation (3 and 9 O’clock)
◦ Inadequate edge lift: minimal pooling of NaFL. Results in debris trapped underneath the CL, poor CL movement with a greater risk of CL adherence, inadequate tear exchange, and vascularized limbal keratitis
Center thickness of GPCL
influences O2 transmissibility, flexure, and center of gravity. Changed in 0.03mm steps
◦ A thinner CT as greater oxygen tranmission, better centration, more flexure
◦ Thicker CT has less oxygen transmission and less flexure, drops inferiorly
◦ Higher DK lenses require thicker CT in order to mimize flexure
Center of gravity of GPCL
◦ The more posterior the center of gravity, the better the centration of the CL
Spherical GPCL and astigmatism
can be fit on corneal astigmatism because of the lacrimal lens
• Large amounts of corneal astigmatisms may lead to a poor fit with a spherical GP
◦ Tori GPCL may be better
Bitoric GPCL
◦ Corneal astigmatism >2.50D is necessary to have a toric back surface
◦ A toric front surface is also required in addition to the toric back surface (bitoric) in patients where the spectacle astigmatism is not equal to 1.5 x corneal astigmatism
◦ A back surface GPCL provides more astigmatism correction that what is expected based on the BC measurement. A toric front surface is necessary in order to compensate for the additional astigmatism correction provided by the toric back surface, resulting in a bitoric CL.
◦ Fitting bitoric on-K: usually too tight. Most recommend fitting 0.25D flatter than K
Saddle fit bitoric
equal alignemtn in both principle meridians. Fit 0.25D flatter. Most appropriate in ATR/oblique
Low toric stimulation bitoric GPCL
fitting on K to 0.25D flatter than K for the flat K, and 0,75-1.00D flatter than steep K.
• This is the most popular fitting philosophy for bitoric GPCL
Back surface toric GPCL
◦ Indicated in patients >2.5oD corneal astigmatism AND the magnitude of spectacle astigmatism is eucalyptus to 1.5x corneal astigmatism
◦ In pts with spec astigmatism=1.5x corneal astigmatism, the extra cyl correction provided by the toric back surface perfectly correct for the total amount of astigmatism in the eye
◦ The rule that spec astigmatism=1.5x corneal astigmatism is usually met in pts with ATR astigmatism
Front surface toric GPCL
◦ Spherical back surface and a toric front surface
◦ Indicated for patients with corneal astigmatism < 2.50D and with an unacceptable magnitude of residual astigmatism
◦ The front toric surface will correct the residual astigmatism
◦ More susceptible to rotation on the eye, must use prism ballast
Aspheric GPCL
◦ Progressively flatten towards periphery
◦ Improved alignment, better centration and comfort, decreased SA and decreased tear exchange
◦ Back surface aspheric
‣ ATR astigmatism, irregualr corneal astigmatism, or bordering corneal astigmatism
◦ Front surface aspheric
‣ Excessive residual astigmatism or patients who have hypercritical VA demands
◦ Main disadvantage is decreased tear exchange
Multifocal GPCL
◦ Simultaneous design: both distance and near power are located within the pupil at the same time
‣ Centration is critical!
• Dependent on the size of the pupil
◦ translating deisgn
‣ Lens is segmented
‣ Prism ballasted
‣ Crescent, executive bifocal, trifocal designs
Gas permeability of GPCL
• P=Dk
• Oxygen permeability is essential to prevent excessive corneal edema
• High Dk values (51-99) or hyper Dk values (>100) for extended wear
• Low Dk (25-50) daily wear
• Total amount of gas that actually passes through= transmissivity (T)
T=(P/t)=Dk/t. T=thickness in cm
• the amount of oxygen transmitted through the CL increases when the permeability of the CL material increases, and the thickness decreases
GPCL materials
◦ GPCL contain little water, oxygen permeability is dependent on the concentration of silicone/fluorine within the CL material.
◦ PMMA
‣ DK=0, original material
‣ Corneal edema, complete lack of O2
◦ Silicone acrylate (SA):
‣ DK=12-56
‣ Flexure, deposits, and poor wetability
◦ Fluoroscilicone acrylate (FSA)
‣ Dk=18-163
‣ Better wetability, less deposit, less flexure, less dryness
Selecting GPCL materials
◦ Patients previously fit with PMMA should be refit into low DK materials
◦ Myopes: low Dk (25-50) for daily wear, high Dk (51-99) for extended wear
◦ Hyperopes: high Dk for daily wear, hyper-DK (>100) for extended wear
Corneal edema and GPCL
◦ Reduce CL wear time
◦ Fit pt with a looser CL
◦ Choose higher DK CL material
Fitting philosophy of GPCL
◦ Lid attached- interacts with superior eyelid
◦ Interpalpebral—superior eyelid located above limbus
Parameters of spherical GPCL
<2.50D corneal astigmatism)
Parameters of front surface toric GPCL
(<2.50D corn astig, residual astigmatism)
Back surface toric parameters for GPCL
corneal astig >2.50D and spec astigmatism=1.5x corneal astigmatism
Bitoric GPCL parameters
> 2.50D corneal astigmatism and spec astigmatism that does not equal 1.5x corneal
Parameters of aspheric GPCL
ATR or borderline astigmatism
Steeper GPCL needed to maintain an adequate fit with _____ astounds of corneal astigmatism and in order to minimize flexure
Higher
Higher DK=
Higher oxygen, more flexure, increased CT
-high DK extended wear, low DK daily wear
Flexure
- ONLY WHEN ON THE EYE
- Plus GPCL tend to maintain shape, as do minus GP corneal CL with a center thickness greater than 0.13mm
- Flexure increases the amount of residual astigmatism
- Can be seen with keratometry
- Only occurs when ON THE EYE
Increased flexure
Increased thickness > 1.50D cyl Increased OZD Increased DK Steeper BC
Keratometry readings and flexure
◦ K readings are spherical—CL is maintain shape
◦ K readings are toric—CL is exhibiting flexure
Warpage
• Warpage=ON and OFF the EYE
◦ Reduced VA
◦ Permanently-induced toricity of the GPCL
◦ Result of excessive digital cleaning.
◦ It occurs when the GPCL is on OR off the eye
◦ Differentiated from flexure using a radiuscope
‣ Warped GP will have toric BC
‣ GP that is flexing on the eye will have a spherical BC
◦ can alter the magnification properties and increase the effects of marginal astigmatism
SCL
• total flexure
• Lacrimal lens power is always Plano
• Do not correct corneal astigmatism
• Making the BC flatter or steeper than K will not change the power of the soft CL
• One exception to the rule of SCL conforming to the shape of the cornea:
◦ It is possible that a very thick SCL used to correct aphakia may form a minus power lacrimal lens
• In general, pts can be fit with spherical SCL if they have <1.00D WTR astigmatism or <0.75D ATR or oblique astigmatism
Soft toric CL
indicated for >0.75D ATR/oblique pr 1.00D WTR residual astigmatism
◦ Front surface is most commonly the toric one
◦ Prism ballasting: BD prism in bottom to reduce rotation . Greater prism needed for smaller diameter
◦ Periballasting: BD prism out side of the OZ o
Dynamic stabilization of SCL
Both inferior and superior portions of the lens are thinned
Eccentric lenticualtion of SCL
inferior and superior removed, thinner edge
Truncation of SCL
portion of the lower CL removed
LARS
◦ Doctors view
◦ Left ADD (clockwise)
◦ Right subtract (counterclockwise)
◦ Only works if the lens consistently rotates to the same postion on the eye
SCL
Fitting guidelines for SCL
◦ BC selection: SCL are typically fir with a BC that is 4D flatter than K
◦ Diameter: HVID + 3mm
SCL adequate fit
good centration. Extend 1.5mm beyond limbus, will move 0.25-1mm with blink, and will habe 1mm of lag with superior, temporal, and nasal gaze
Tight SCL fit
good centration, Good comfort, will not move with blink or digital rotation
Loose SCL fit
poor centration, poor comfort, excessive movement. CL may be wrinkled, easily dislodged
Toric adequate fit SCL
well centered, extend 1.5mm beyond limbus, move no more than 5 degrees, quickly returns, stable OR
Tight toric fit SCL
different orientation each time its on eye, minimal rotation
Loose toric fit, SCL
excessive rotation, slow movement of the CL to original position after blink, unstable OR
Group 1 SCL
Lower water, nonionic
Group 2 SCL
High water, nonionic
Group 3 SCL
Low water, ionic
Group 4 SCL
High water, ionic
Group 5 SCL
SiHY
Deposits are more likely in which groups of SCL
With higher water content that are ionic
Groups 3 and 4
Rigidity of SCL
◦ Stiffness
◦ Depends on thickness and the modulus of elasticity
◦ As water increases, modulus decreases
◦ Benefits of high modulus: better handling, easier removal, easier to tell where it is
◦ Drawbacks: GPC, SEALs, LEH, mucin balls, edge fluting
High water in SCL
high permeability (high DK) for hydrogel
low silicone content=low permeability (DK) for silicone
Accommodation and CL
◦ Hyperopes accommodate less with CL
◦ Myopes accommodate more with CL
◦ Pt with presbyopia and myopia may need a reading add earlier when wearing CL
Magnification and SCL
◦ Hyperopia=less mag in CL
◦ Myopes=increased mag in CL
◦ If the most amreopic eye has axial ametropia, spectacles will minimize the induced aniseikonia. If the most ametropic eye has refractive ametropia, CL will minimize the induced aniseikonia
Vergence and CL
◦ Less vergence in CL for hyperope; less induced BO
◦ Nor evergence in CL for myope; less induced BI
Relative distance mag
Old distance/new distance
Relative size mag
New size/old size
Angular magnification
- result of an increase in the retinal image size of an object by introducing an optical system between the object and the eye
- Unlike RDM and RSM, angular magnification is NOT a result of a change in the location or the size of an object
- Hand held magnifiers, stand magnifiers, and/or telescopes
Difference between linear and angular mag
- ML compares the ratio of the object size to the image size formed by an optical system. Math: ML=L/L1 or Hi/Ho
- MA compares the ratio of the original retinal image size to the retinal image size when the object is viewed through an optical system
HHM
◦ High plus lens
◦ Typically places in front of th eye so that the object of interest is located at the primary focal point of the magnifier
◦ Also known as a collimating magnifier (light leaves the system in form of plane waves
◦ Accommodation (and thus an ADD) are NOT required in order for the patient to view the object clearly though the HHM because the image is formed by plane waves (zero image vergence)
Total mag of a HHM
MT=|u|F
U=dist btwene object and eye
Moving the HHM object system anywhere in front of the eye
and MT will remain the same (moving the magnifier-same MAG and decreased FOV
Most magnifiers are labeeld based on
standard working distance of 25cm, referred to as effective magnification
‣ Math: M=(0.25)(F)=F/4
Stand magnifier
◦ High plus powered lens
◦ Object to lens distance is fixed
◦ Object is located inside the primary focal point
◦ Creates an upright, magnified, and virtual image
‣ Math: MT=mM (lat magxRDM)
◦ Patient must wear an add
‣ Stand/Add combo math: M=Fe/4
◦ Max magnification: Mmax=F/4 + 1
◦ Require the patient to accommodate or wear an add in order to see the image clearly
Telescopes
- magnify distance objects
- Objective and ocular lens set up so the secondary focal point of the objective lens coincides with the primary focal point of the ocular lens.
- Plane light waves enter and plane light waves leave (parallel in, parallel out)
Tube length of telescope
D=Fob+Foc
Magnification of telescopes
-Foc/Fobj
Can also be detmeined using the entrance nad exit pupil diameters
M=Dent/Dex
Telescope labeling
AXB, A is magnification and B is the objective lens diameter
FOV of telescopes
dependent on the diameter of the exit pupil and the diameter of the entrance pupil of the patients eye
◦ Telescopes with a given magnification, increasing the entrance pupil diameter (size of the objective lens) will increase the exit pupil diameter, and therefore the FOV
◦ Will continue to increases as the exit pupil diameter of the telescope is increased up until the exit pupil becomes larger than the size of the patient’s pupil. The patients pupil diameter becomes the limiting factor.
◦ FOV can also be increased by aligning the exit pupil of the telescope as close as possible to the patients entrance pupil.
Keplarian telescope
◦ Objective and ocular are plus lenses
◦ Objective lens forms a real, magnified, and inverted image
◦ Longer tube length and heavier
◦ Exit pupil: located outside the telescope larger FOV
◦ Be more easily aligned with the entrance pupil of the patient’s eye
◦ Exit pupil is small
◦ Images appear dim
◦ Have higher magnification
Galilean telescope
◦ Mag up to 4x
◦ Increasing the power increases the overall magnification, but also reduces the tube length. There is a limit to how much of the power of the ocular lens can be increased and the tube length decreased before the ocular lens physically touches the objective lens in the telescope
exit pupil location of Gali Allen
Inside telescope
Exit pupi location of keplarian
Outside telescope
Image of Galilean
Upright
Mag
Vitreal
Image of Hep
Mag
Inverted
Real
Telemicroscope
◦ Used to view near objects
◦ 2 solutions to eliminate the need for accommodation when viewing a near object with a telescope
‣ Adjust the tube length
‣ Add a reading cap to the objective side of the telescope
◦ A reading cap acts as a hand held magnifier. Object is located at the primary focal point of the reading cap. Neutralize the incoming divergent waves from the object, allowing plane waves to enter, so zero accommodation required.
◦ A combination of a telescope and a reading cap is known as a telemicroscope
Total mag of telemicroscope
M=M(reading cap)x M(telescopes_
Mage of reading cap for a telemicroscope
M=rF
Spec mounted telescope: center fit
continuous viewing, watching sports, concerts, movies, TV
‣ Minimal training
‣ For patients with dexterity or cognitive issues
‣ Should not walk around while wearing a center fit spec mounted scope
Spec mounted telescope: bioptic fit
upper portion, good for spotting. Used for classroom work, driving, traveling in airports, at he grocery store
‣ Significant training required
‣ Poor dexterity or cognitive issues are generally not good candidates for this
◦ A reading cap can be added to a spectacle mounted telescope so the patient can view near objects
Reversed telescope
◦ Expand VF, good for RP and glaucoma ◦ Accomplished by turning the scope around so that the patient looks through the objective lens rather than the ocular lens. ◦ Will minify objects ◦ Must have good central VA ◦ Often hand held ◦ Used for spotting objects ◦ 2.5X to 4x galilean most often used ◦ Beyond 4x, minification is too small ◦ Minus lenses can also be used to expand the FOV by minifying objects, similar to reverse telescopes ‣ Held at arms length ‣ For spotting ‣ -5 to -10D lenses used
2 types o f VF problems that cause low vision
◦ 1. Hemianopsia or visual neglect due to stroke
◦ 2. VF constriction (glaucoma/RP)
VF enhancement: mirrors
for temporal visual field loss. Placed on the nasal side of the eye with the defect so that the temporal field is reflected into the mirror
◦ They often cause confusion
◦ Often induce scotomas
◦ Cause nausea and poor cosmesis
Prisms for VF enhancement
◦ Used to relocate the missing area of the VF into the patient’s line of sight.
◦ Binocular or monocular
◦ Base towards the side of the defect
◦ 1 degree=2PD
Most effective method for VF enhancement for hemianopsia
Scanning technique
Symptoms of central vision loss
◦ Generally due to macular disease
◦ Difficulty with near tasks, reading signs, and recognizing faces, they may also have poor contrast sensitivity
◦ Require magnification, glare control
◦ Do not have trouble with mobility
Peripheral visual field loss symptoms
◦ RP, advanced glaucoma
◦ Poor mobility, poor night vision, and glare
Normal vision
20/12 to 20/25
Near normal vision
20/30 to 20/60
◦ A patient with 20/60 BCVA cannot resolve 1M at 1m, but can resolve it a 1/3m
Moderate low vision
20/70 to 20/160
◦ Because the patient can read at a distance of 25cm or less, in order to see 1M print, it is often difficult for the patient to maintain binocular vision due to increased demand on convergence
◦ Require BI prism to reduce the demand on convergence
◦ In general, when patients rewuire reading glasses > +4.00D, prescribe BI prism to reduce the convergence demand. The amount of BI prism per eye is 2 + D (the sphere power of the eye).
‣ +8.00D readers = 10 PD BI
Reading glasses for glasses >4D
◦ Require BI prism to reduce the demand on convergence
◦ In general, when patients rewuire reading glasses > +4.00D, prescribe BI prism to reduce the convergence demand. The amount of BI prism per eye is 2 + D (the sphere power of the eye).
‣ +8.00D readers = 10 PD BI
Severe low vision
20/200-20/400
◦ Usually use monocular viewing
◦ Considered legally blind in the USA
Legal blindness
◦ Patient cannot read any letters on the 20/100 line in the better seeing eye
◦ VF diameter is 20 degrees or less in the better seeing eye
Profound low vision
20/500 to 20/1000
◦ In order to read 1M print, the patient must hold the reading material VERY close to the eye (typically < 5cm), causing reading to be very difficult
Near blindness
Worse than 20/1000
Blindness
NLP
Cortical blindness
acute, bilateral blindness due to damage to the visual cortex rather than the eyes themselves; common causes include stroke, trauma, surgery, lesions, and congential causes.
Charts used in low vision
• Continuous text charts (Lighthouse) should be used to provide a functional assessment of patients reading abiltiy
VA and M notation
◦ Lighthouse charts are used in low vision use the M unit
◦ 1M is defined as the size of a symbol that subetneds 5 arcminutes at 1m. Equivalent to snellen 20/20
◦ Most newspaper size print at 40cm is 1M
◦ Finding snellen equivalent:
(Test d)/(M dist)=20/x
◦ 1M newspaper print at 40cm is equivalent to 20/50 snellen acuity
0.40/1M=20/X
X=50
Low vision case history
◦ Visual goals
◦ Ocular history: what diseases is the patient suffering from, and what type of vision loss is it causing (central scotoma)
◦ Co-morbidities: dexterity issues
◦ Glare/lighting/contrast issues
BCVA for low vision
the patients central and eccentric viewing BCVA should be assessed at distance and near; recall that central BCVA
◦ EDTRS (Lighthouse), Bailey-Lovie chart, Feinbloom chart
JND for low vision trial frame refraction
JND=snellen denominator/100
◦ The JND is the smallest lens power change that the patient can detect. JND=the denominator of the 20 ft snellen acuity/100
Trial fram refraction
◦ Watch for eccentric viewing
◦ Patient can easily turn their head and eyes in order to sue eccentric viewing if they have scotomas
◦ Eccentric viewing occurs when the patient uses a retinal point other than the fovea to view the object. The point is often located adjacent to the scotoma and is known as the preferred retinal locus (PRL)
◦ More accurate than the phoropter
Determining the amount of mag required for low vision patient s
Distance mag=what you got/what you need
The general formula for detecting the magnitude of near mag required buy a patient is
Near mag=what you got/what you need
Kestanbaum’s rule
Inverse of the patients distance VA to determine the starting add power
‣ A patient presents with distance BCVA 20/200. The predicted add power is 200/20=+10.00D
‣ Not the most accurate to determine the near mag
Lighthouse method of near mag
the predicted add is equal to ((patients current near VA)/(patients Goan near VA)) x working distance (in diopters)
‣ May require more mag if they have reduced contrast sensitive
Near devices for low vision
‣ Stand magnifiers ‣ HHM ‣ High plus readers ‣ Head borne/spectacle mounted magnifiers • Good for longer working distances ‣ Electronic magnifiers
Distance devices for low vision
‣ Hand held telescopes
‣ Center fit spectacle mounted
‣ Bioptic telescope
Nonoptical aids for low vision
◦ Large print or audio books, a scanner, talking watch, talking blood glucose meter, talking alarm clocks, bump dots, typoscope, high contrast dish ware
Contrast sensitivity in low vision
◦ Most patients with low vision also have reduced contrast sensitivity and increased glare.
◦ Pelli-Robson chart: letters of uniform size
◦ Vistech Contrast Test System: series of sine wave gratings of various contrasts and frequencies.
◦ Bailey-Lovie Chart
◦ Enhancing contrast and decreasing glare help to improve a patient’s mobility. Can use filters to help
‣ Neutral density filters: no effect on contrast, gets rid of glare
‣ Blue blockers (amber tints): reduction in glare AND enhanced contrast
VF testing for low vision
Peripheral field loss and central field loss
