Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
71 Cards in this Set
- Front
- Back
|
Brewster's Law
|
Angle at which light is completely polarized (w/water, polarized horizontally)
tan0=n2/n1 n2 = water or medium |
|
|
Snell's law
|
incident and refraction angles must equal 90
n1sin01 = n2sin02 For critical angle, 02=90 and sin(90)=1 |
|
|
What is L and L'?
|
L=object vergence
L'=image vergence L'=F+L |
|
|
Lateral mag related to L and h?
|
ML = L/L' = h'/h
|
|
|
Power of a single spherical refracting interface (SSRI)
|
F = (n2-n1)/r
|
|
|
Downstream vergence (vertex if 4D or more)
|
Fc=F/(1-xF)
Going from CL to spec or vice versa |
|
|
Equivalent power
|
Fe = F1 + F2 - (t/n2)F1F2
Used for 2lens combinations such as a thick lens |
|
|
Back vertex power
|
Fv = F2 + F1/(1-t/n2F1)
Used for thick lenses. CL's are technically thick lenses so use this if given a thickness. |
|
|
Front vertex power
|
Fn = F1 + F2/(1-t/n2F2)
|
|
|
Power of a mirror
|
F = 2n/r
variation of F = (n2-n1)/r |
|
|
Equation for converting from power to radius of curvature with a KERATOMETER (calibrated for 1.3375) or CONTACT LENSES (calibrated for 1.336, almost equal).
|
F = 337.5/r
r is radius of curvature in mm eg keratometer readout: 45.00 DK |
|
|
If a lens clock, which is calibrated for an index 1.5, is used to measure the power of a lens (+6.00 D), how do you find the actual power of the lens (n=1.7)?
|
Use F = (n2-n1)/r
Use equation to find the radius of curvature using calibrated index (1.5) and measured power (+6 D). Then use equation again with radius and actual index (1.7) to calculate new power. |
|
|
Sag equation
|
s = h2/2r
(h squared) h = 1/2 diameter of lens r = radius of curvature |
|
|
Distance is always in meters except for what 2 equations?
|
1) Prentice's Rule (cm/m)
2) Keratometry (F = 337.5/r) mm |
|
|
Prentice's law
|
Prism diopters (triangle) = cm/m
(cm deviated at 1 m) PD = dF (h=dist looking away from OC; F=power of lens) |
|
|
Prism power and apex angle equation
|
d = A(n-1)
d = deviation angle (degrees) n = index of prism A = apex angle |
|
|
In a lens with nasal and temporal thickness, what is the equation for the prismatic effect at the center of the lens?
|
Prism diopters = 100g(n-1)/d
g = difference in thickness of the 2 edges n = index of lens d = distance across lens (A dimension) |
|
|
Spectacle magnification (SM)
|
Shape factor= 1/(1-t/nF1)
Power factor= 1/(1-dFv) d = vertex distance + 3mm |
|
|
Know how SM is affected:
increase thickness increase vertex distance increase base curve increase index |
increase thickness:
-incr SM (+ lens) and incr SM (- lens) increase vertex distance: -incr (+) and decr (-) increase base curve: -(F1) incr (+) increase index: -decr (+) and (-) |
|
|
Fresnel's Law for reflection:
definition & equation |
Light traveling through a lens is lost at the front and back surface (requires 2 calculations). Shortcut: 4% is lost at each surface.
R = [(n2-n1)/(n2+n1)]squared |
|
|
Residual astigmatism equation
|
RA = spec cyl - K cyl
|
|
|
Javal's Rule
|
Accounts for lenticular astigmatism
spec cyl = 1.25(K cyl) + (-0.50 D x 90) Average: 0.50 ATR |
|
|
Stand Magnifier equation
How to find mag? What happens if move eye closer? |
Find total angular mag by:
Multiply lateral mag and RDM L/L' (going into & out of lens) d original object / d image (d eo / d ei) Move eye closer to stand = incr mag |
|
|
Hand-held magnifier equation
How to find mag? What happens if move eye closer? |
M = d Fe
Fe = power of lens (D) d = original eye to object dist (m) If 25 cm reading dist; M = F/4 Manufacturers have agreed to a reference distance of 25 cm. If not 25 cm away, use 25 cm to convert from labeled mag to power (D), then multiply it by new distance Move eye closer to lens = same mag but incr field of view |
|
|
How to do telemicroscope calculations
|
Treat reading cap like hand-held magnifier to find mag (M = 0.25Fe) then multiply cap mag by TS mag
If not at the focal point of the reading cap, then convert reading cap D to mag (power times reading distance in m) |
|
|
2 equations for telescope mag.
What does 5 x 45 mean? |
M = Foc/Fob = dent / dex
dent = diameter of entrance pupil 5x; 45 mm objective lens (entrance pupil) |
|
|
Vertical polarizers blocks which light rays?
|
horizontal
|
|
|
For a plus lens, where is the primary and secondary focal point?
|
Primary: left of lens
Secondary: right of lens |
|
|
What limits light in an optical systems?
What limits the field of view? |
Aperture stop limits light, and the images of it are the entrance pupil and exit pupil. Find the entrance pupil by looking through objective, and that's the image of the aperture stop.
Field stop limits the field of view, and the images of it are the entrance port and exit port. |
|
|
What kind of cyl is induce for a minus and plus lens with pantoscopic tilt?
|
Minus lens: axis 180
Plus lens: axis 90 |
|
|
Tolerances of sphere
|
Below +/- 6.50: 0.13
Above +/- 6.50: 2% of sphere power |
|
|
Tolerances of cylinder power
|
Below +/- 2.00: 0.13
2.00 - 4.50 : 0.15 Above 4.50: 4% of cyl power |
|
|
Tolerances of cyl axis
|
below 0.25: 14
0.25 - 0.50: 7 0.50 - 0.75: 5 0.75 - 1.50: 3 above 1.50: 2 |
|
|
Physical properties tolerances
thickness warpage base curve impact |
thickness: 0.3 mm
warpage: 1.00 D base curve: 0.75 D impact: resists 5/8 in steel ball from 50 in |
|
|
A frame says 54 x 18. What does that mean?
Is B up or down? Where is Effective diameter? |
54 = A (eye size, or lens size)
18 = DBL A + DBL = frame PD B is vertical Eff diameter is the longest point on a lens. |
|
|
Crown glass
Index Abbe # |
Index - 1.523
Abbe # - 58.9 Heaviest specific gravity |
|
|
Cr-39
Index Abbe # |
Index - 1.498
Abbe # - 58 |
|
|
Polycarbonate
Index Abbe # |
Index - 1.586
Abbe # - 30 |
|
|
Trivex
Index Abbe # |
Index - 1.53
Abbe # - 43-46 Lightest specific gravity but still heavier than polycarb (polycarb index is higher) |
|
|
Coma: on or off axis?
|
off axis only
Comet-shaped or teardrop |
|
|
spherical aberration: on or off axis?
|
both
|
|
|
Curvature of field: on or off axis rays?
|
off axis
Image is curved; outer parts are wrapped around closer to you |
|
|
Oblique (radial) astigmatism: on or off axis?
|
off axis
-Reduced by picking correct base curve (Ostwalt from Tscherning ellipse) -Related to "teacup and saucer" image |
|
|
Distortion: on or off axis?
|
off axis
-barrel (-) and pincushion (+) -reduce w/orthoscopic doublet |
|
|
Chromatic aberration: on or off axis?
|
both
Reduce with achromatic doublet (a plus & a minus lens). Power corresponds to ratio of Abbe #. |
|
|
Aberrations of most concern in ophthalmic lenses (in order)?
|
oblique astigmatism, curvature of field, distortion
|
|
|
Seg edge to seg optical center (note, drop is from distance optical center to seg top):
Flat top (28 mm or less) - Flat top (28 mm or less) - Flat top (larger than 35 mm) - Curve-top, panoptic, Ribbon-B - Ribbon R-seg - Franklin (executive) - Round segs - What are these #'s used for? |
Flat top (28 mm or less) - 5 mm
Flat top (28 mm or less) - 4.5 mm Flat top (larger than 35) - 0 (at seg line) Curve-top, panoptic, Ribbon-B - 4.5 Ribbon R-seg - 7 Franklin (executive) - 0 mm Round segs - r (radius of the seg) #'s used to calculate image jump in bifocals (independent of dist Rx) |
|
|
What do you use for axial ametropes? Refractive? What is this law called?
|
Axial - spectacles
Refractive - CLs Knapp's Law - Makes relative spec mag = 1 (reduced induces aniseikonia with ametropia) |
|
|
Different types of aniseikonia?
|
Def: difference in size or shape of images between OD & OS
Anatomical - photorec density Induced - difference in optics (ie spec mag) Overall - spherically symmetric diff in image size Meridional - differences in cyl power |
|
|
How do you counteract large differences in RSM between eyes?
|
Use a flat lens for the eye with the high RSM and a thicker/steeper lens for the eye with the lowest RSM
|
|
|
How much percent aniseikonia corresponds to 1.00 D difference? When does pt become symptomatic?
|
1%
3 D |
|
|
Which eye is used by myope/hyperope for distance and near with anisometropia?
|
Myopia: uses more myopic eye for near and less myopia eye for distance.
Hyperope: uses less hyperopic eye for both dist & near |
|
|
0.1 mm change in radius of curvature is equivalent to ___ D
|
0.50 D
|
|
|
How much cyl can a patient tolerate for WTR and ATR?
|
WTR: 0.75 D
ATR: 0.50 D |
|
|
What are the cutoffs for K cyl and lenticular cyl for RGP's?
|
None: spherical
K less than 2.5: spherical K more than 2.5: back toric Lenticular: front toric K less than 2.5 & lent: front toric K more than 2.5 & lent: bitoric |
|
|
How much cyl does a back toric require?
|
spec cyl 1.5 x K cyl
|
|
|
Describe how to fit a bitoric.
saddle fit low toric simulation RA SPE |
If the lens is not a SPE (Spherical Power Effect) and can't rotate w/out problems (RA more than 0.50D), then do a saddle fit.
If not SPE: -WTR (+/- 30 degrees): try low toric simulation -Not WTR: saddle fit |
|
|
Summary for RGP fitting: what numbers do you need to know?
|
2.5 is cutoff for K cyl (sphere vs bitoric)
RA: no more than 0.75D ATR or 0.50D WTR Back toric: spec cyl is 1.5 x K cyl |
|
|
If the eye needs cyl axis 90 and the CL rotates to 70:
How do you compensate? What do you expect for the O.R.? |
LARS: left - add 20
O.R.: halfway between them is 80, 45 degrees away from that on the opposite side of the new axis = 125 degrees. 30 degrees would result in 100% of the cyl power on O.R.; 15 degrees - 50% |
|
|
Levels of low vision
Normal vision Near-Normal Moderate low vision Severe low vision Legal blindness Profound low vision Near blindness Blindness |
Normal vision: 20/12 - 20/25
Near-Normal: 20/30 - 20/60 Moderate low vision: 20/70 - 20/160 Severe low vision: 20/200 - 20/400 Legal blindness: can't read 20/100, or VF 20 degrees or less in better eye Profound low vision: 20/500 - 20/1000 Near blindness: worse than 20/1000 Blindness: NLP |
|
|
What is absolute hyperopia and facultative hyperopia?
|
Absolute: hyperopia is too large to be neutralized by accomm
Facultative: hyperopia that can be neutralized by accomm |
|
|
Normal Rx of 5 yo?
|
About 1 D hyperopia
If more than 1.5, likely will be hyp at 14 yo If 0.5 - 1.25, will likely be emmetropic If 0.5, will likely be myopic |
|
|
Resolution acuity:
Recognition acuity: Minimum detectable acuity: Hyperacuity: |
Resolution acuity: distinguish pattern (cycles/degree) in uniform patch of equal luminance. Average adult: 40-60 cyc/degree
Recognition acuity: Snellen; resolve high frequencies Minimum detectable acuity: thinnest possible wire visible (average: 1 arcsec) Hyperacuity: sense direction; result of higher cortical processing |
|
|
Nott's method of ret
|
find where reflex is measured by moving retinoscope behind reading plane. Measure dioptric difference between the 2 for lag. Can't measure if lead.
|
|
|
Mohindra
|
Near ret done at 50 cm w/an eye occluded in the dark. subtract 1.25 (basically lag of accomm) to get dist Rx.
Cannot identify more than 3 D hyperopia. |
|
|
Stenopaic slit example:
Example: best when slit is horizontal. What's the axis? Horizontal slit: -0.25 Vertical slit: -1.25 |
Axis: 180
Example: -0.25 -1.00 x 180 |
|
|
On JCC, are the red dots and white dots positive or negative?
|
Red: positive meridian (or neg axis)
White: negative meridian This part finds axis and collapses interval of sturm on retina; doesn't change circle of least confusion |
|
|
Astigmatic clock dial:
How to get axis. Vertical power makes vert/hor line. |
Fog pt, if 12 and 6 clearest, multiply 6 by 30 = axis 180.
Vert = +5 (makes horizontal line) Hor = +1 (makes vert line) |
|
|
Duochrome step-down:
How much dioptric difference between green and red. If red, add +/- Can you do this w/color blindness? |
Difference: 0.50 D (red=long=further back toward retina when fogged)
If red "clearer" add minus Can do w/color blindness. |
|
|
How much of accomm does a person use comfortably?
|
Half of amp.
|
|
|
Expected amplitude of accomm according to age (avg & min):
|
Avg amp of accomm =
18 - 1/3(age) Minimum amp: 15 - 1/4(age) |
