Ophthalmic Optics Boards Review - Dr. Mckee's Notes

How does aniso induce vertical prism imbalance?

More of a problem because of limited vertical fusional abilities of most patients

Metal Frame Materials: Monel

-Nickel and copper alloy -Mostly used for temples and bridges -Strong but difficult to bend

When does spherical aberration occur?

-Spherical aberration occurs when a pencil of light is refracted by a large aperture spherical lens *The lens creates more bending effect as you move from the center into the periphery

Metal Frame Materials: Titanium

Titanium (good; no skin reaction) Extremely strong Lightweight About 25% more expensive than other metal frames Rapidly becoming a commonly used material because of the weight, strength and rarity of skin reactions Can't be conventionally soldered for repairs

How can you minimize oblique (or marginal) astigmatism with a lens design?

To minimize oblique (or marginal) astigmatism, one could pick the optimal base curve for each Rx power -Producing a lens with the optimal base curve for a given back vertex power means almost every Rx would be custom-made (can now do this with free-form lenses) -In practice ranges of marginal astigmatism are allowed to provide a discrete number of base curve options for a given lens design

How do you calculate transmission?

Transmission = 1 - reflection = 1 - [(1-n) / (1+n)]^2

What about lenses with prism?

-Two types of prism with lenses 1. Ground-in prism Bicentric grinding Must do for low power Rxs Can make plano prism 2. Decentered prism Use Prentice's Rule Move pole of lens Can do decentered prism in office without extra lab costs

What is the lens clock equation? (low yield)

Fclock = 2s (n-1) / h^2 -Lens clock measures s by center pin movement and known half chord length (distance from center pin to out pin) for an assumed index of refraction

A +5 D lens with n = 1.5 has a minimum edge thickness of 1mm and has 2Δ at the PRP (no decentration, PRP is at GC). The lens is 40mm round in size. How thick is the lens at the GC?

First draw a picture Pick nasal edge to be the thinnest edge Assume thickness change comes from prism and a centered lens First find thickness of prism at GC Δt = εΔ QP/ [ 100 (n-1) ] Δt = 2 (20) / [100 (1.5 - 1)] Δt = 40 / 50 = 0.8mm At GC, thickness of prism = 1 + 0.8 = 1.8mm Now find thickness of lens at GC tc - tp = h2 FA / [2 (n -1)], tp = 0 tc = (0.020)2(5) / [2 (1.5 -1)] = 0.002m = 2mm Total thickness at GC = prism thickness + lens thickness = 1.8mm + 2.0mm = 3.8mm *Keep in mind to use the lens thickness equation, you have to go from lens edge to lens pole or from lens pole to point of interest in lens

What reduces or facilitates the melting of glass?

Flux - Soda (sodium carbonate) is added as a flux to decrease the melting point of the mixture to 2200°F (1200° C)

For chromatic aberration to cancel, bases have to be oriented in ___________directions

For chromatic aberration to cancel, bases have to be oriented in opposite directions ε1/V1 = ε2/V2 εtotal = ε1 - ε2

How do you calculate the power in a meridian of a toric lens?

Fp = Fs + Fc sin2α (α is angle from -cyl axis to meridian of interest) Keep in mind: sin2(0) = 0 sin2(30) = 0.25 sin2(45) = 0.5 sin2(60) = 0.75 sin2(90) = 1

How does aniso induce a horizontal prism effect?

From Prentice's Rule, if powers of right and left spectacle lenses differ, prism imbalances will occur

Metal Frame Materials: Gold

Gold The purity of gold is measured in karats Each karat (K) designates 1/24 of the total amount of pure gold 24K gold is pure, unalloyed gold 18K gold would be 18/24 pure gold and 6/24 other metals 10K gold would be 10/24 pure gold and 14/24 other metals To be labeled in karats the frame (or any other item) must use gold of at least 10K or greater

Plastic Frame Materials: polycarbonate

High strength - used for safety/sports eyewear Clear or light transparent colors Difficult to adjust and tends to hold adjustment poorly Lenses must be closely sized - not too small or too large Discolored by solvents - acetone Hypoallergenic

What is curvature and the equation?

How fast a surface changes shape: C=1/r r=radius *in meters*

Oblique Astigmatism

If we consider an extended object made up of many points we can get a feel for the image in image space

Oblique astigmatism - T and S Surface?

If we plot the tangential and sagittal image surfaces in a horizontal plane - T - steeper and S-Flatter

Three ways to make plastic photochromics

Imbibe material into surface of lens or coating (like Transitions®) Mix into plastic monomer before thermosetting occurs (like Corning SunSensors®) Add photochromic dyes to a material and coat front of lens with the material (like Zeiss PhotoFusion®)

In general, the more ____________ eye has to accommodate more for the same distance when corrected with spectacles

In general, the more plus eye has to accommodate more for the same distance when corrected with spectacles

Describe how glass lenses are undergo thermal tempering?

In thermal tempering glass lens heated to nearly softening point (1200°F) and cooled quickly by an air blast on both surfaces The heating causes the glass to expand The rapid cooling causes the outer layer of the glass to cool and become rigid quickly while the inner layer is still slightly plastic As the inner layer cools and shrinks it exerts tension on the outer layer pulling it into a highly compressive state When the whole lens cools the surface layer is under compression Maltese cross pattern with polariscope

Examination of a 15-year-old patient reveals the following: Rx: OD: -2.50 DS -0.75 DC x 090 OS: +3.25 DS -0.50 DC x 180 Keratometry: OD: 43.00 D @ 090; 43.50 D @ 180 OS: 39.00 DS Which of the following spectacle lens designs should be the MOST effective in reducing aniseikonia for this patient? Base Curve OD Center Thickness OD / Base Curve OS Center Thickness OS A. +3.75 D 2.0 mm / +7.50 D 3.6 mm B. +3.75 D 2.0 mm / +3.75 D 6.0 mm C. +5.25 D 2.5 mm / +7.50 D 3.5 mm D. +6.25 D 3.4 mm / +6.25 D 3.4 mm

Kind of a bad question with incomplete data Two possible approaches Easy way Do what you can to minimize differences in thickness and shape Equal base curves and equal center thicknesses Choice D is the only one that does this -Second approach: (hARD) Approximately 6D of anisometropia 4D is refractive (from cornea) With spectacles, expect 1% aniseikonia from Knapp's Law per diopter of anisomtropia 4D of aniseikonia For OD lens with back vertex power of -3D (spherical equivalent): Expect best-form lens base curve (F1) of +2D Expect center thickness (t) of 2mm For OS lens with back vertex power of +3D(spherical equivalent): Expect best-form lens base curve (F1) of +8D Expect center thickness of ~3.5mm

Equation for LCA (chromatic Abberation)

LCA = Fd (nf - nc)- (nd-1) = Fd / Abbe value (nf - nc) is called the dispersion or (nd - 1) dispersive power (nf - nc) is the mean dispersion (nd - 1) is the mean refractivity

Library

Library Stabilizes frame by pushing on sides of head

What is added to decrease the water solubility of glass?

Lime (calcium carbonate)

Photochromic glass lenses

Microscopic silver halide crystals added to glass mixture Once cooled, the glass matrix holds the crystals and decomposed constituents in close proximity

Minimum Blank Size

Minimum Blank Size = ED + 2x monoc.decentration This is a safe size for the lens before edging to insure the lens will cut out to fit the frame

Determine the ideal index of refraction and thickness of a single-layer AR coating for 580nm wavelength light in air applied to a 1.67 index lens.

ncoating = √nlens = √1.67 = 1.293 λcoating = λair /ncoating = 580nm / 1.293 = 449nm tcoating = λcoating / 4 = 449nm / 4 = 112nm

How is abbe value and dispersion related?

The reciprocal of the dispersion is the Abbe value 1 / [ (nf - nc) / (nd - 1) ] = (nd - 1) (nf - nc)

What are the two main groups of plastics?

Thermoplastic - can heat up and reshape (polycarbonate). Supplied as a solid Thermosetting - everything else is this; supplied in a liquid resin form and formed with UV light. *NONREVERSIBLE*

Need an achromatic lens of +5 D using crown glass of index 1.52 and nu value of 60 and flint glass of index 1.6 and nu value of 40.

To solve use the following two equations: F1/V1 = -F2/V2 FT = F1 *+* F2 F1/60 = -F2/40 -60F2 = 40F1 so, F2 = 40F1/60 -0.667F1 FT = F1 + F2 = 5 D = F1 + (-0.667F1) = 0.333F1 = 5 F1 = 15 D F2 = -0.667F1 = -0.667(+15) = -10 D

Describe the properties of thermosetting.

Usually supplied in liquid form Poured and cast in molds After catalyst added polymerization occurs Forms cross-linked 3-D structure Relationship of time to polymerization is the curing cycle Process is not reversible without melting or substantially altering the material Good dimensional stability Not as good of impact resistance as thermoplastics but better than glass -No additional impact treatment needed Examples are CR-39, epoxies, trade name Bakelite, melemine (Melmac)

What does a deeper sag tell you about the circle?

steeper and has a smaller radius of curvature

Equation to calculated lens thickness (edge or center)

tc - tp = (h^2FA)/2(n-1) EVERYTHING IN METERS!!!!!!!!! -Change in thickness varies directly with lens power -Change in thickness varies with the square of the distance from the pole to outer edge of lens (very strong effect) -h is the biggest factor with how thick the lens will be. -Change in thickness varies inversely with index of refraction

Prism thickness equation?

Δt = εΔ QP/ [ 100 (n-1) ]

How do you find the horizontal and vertical components of prisms?

Θ = 180° - 120° = 60° EV = ET sinθ = 5Δ sin60 = 5(0.886) = 4.33Δ BU, OD EH = ETcosθ = 5Δ cos60 = 5(0.5) = 2.5Δ BO Then make sure base directions make sense with your diagram Can check with √ EV2 + EH2 = 5Δ √ 4.332 + 2.52 = √ 18.75 + 6.35 = √ 25 = 5

What is the power of an opthalmic prism equation?

ε° = β°(n-1) (For n = 1.50, ε° = β° / 2) εΔ = 100 tan [β°(n-1)] = 100 tan β°(n-1)

When is best vision obtained with curvature of field?

Best vision is obtained when looking through the periphery of the lens when Petzval's surface and the patient's far point sphere are coincident This only occurs for a single Rx of ~ -20 D Usually the patient's far point sphere is MORE curved than Petzval's surface

Three main parts of a frame front?

Eyewires - encircle the lenses Bridge - connect the eyewires Endpieces - attach the eyewires to the temples

What are processed induced flaws in a lens?

Griffith flaws - can cause the lens to fail

High Plus Rxs: Aspheric Surface

-Aspheric surfaces Can't use lens clock to determine surface power Can't decenter to produce prism Any required prism must be ground at time of surfacing Patient's line of sight must be through PRP of aspheric lenses (more critical than for spherical lenses)

High Minus Rx: Fresnel lenses

-Available for plus and minus lens powers +20 D to -14 D -Available as pre-cut bifocal adds To +8 D -No cylinder power available -available in various prism amounts up to 40Δ

Reflection (Brightest)

-Brightest of all reflections (4.3% for crown glass) -Not commonly a problem for patients -The smaller the size of the lens, the less likely to be a problem -Solutions AR coat lens Fit the lens close to the face (decrease vertex distance) Slight face-form Slight pantoscopic tilt Change base curve of lens

_______________ process can produce deeper fractures that (can or cannot??) be removed during polishing

*Cutting* process can produce deeper fractures that *can* be removed during polishing -Polishing process sometimes fails to properly remove surface defects -Edging of lenses sometimes introduces defects

How do myopic patients and hyperopic patients differ when it comes to focusing through and undercorrected peripheral retina?

-Both myopic and hyperopic spectacle wearers are undercorrected in the peripheral retina -Hyperopic patients are usually undercorrected and can accommodate (add more plus) to improve peripheral image focus (NO PROBLEM) -Myopic patients are usually fully corrected so that they cannot relax accommodation (add more minus) to improve peripheral image focus (more problems when they look off axis - more distortion)

____________cylinders require a compromise in base curve selection Only for __________ can oblique astigmatism be reduced to ____________for a range of powers and then only if ______________ base curve is selected and lenses are used _____________ as assumed (viewing angles and distances) Above __________oblique astigmatism _____________ be eliminated with base curve choice ________________ surface generation allows complex surface geometries that can ______________ oblique astigmatism as well as other lens aberrations -Aspheric and atoric surfaces

*High* cylinders require a compromise in base curve selection Only for *spheres* can oblique astigmatism be reduced to *zero* for a range of powers and then only if *exact* base curve is selected and lenses are used *exactly* as assumed (viewing angles and distances) Above *+8* oblique astigmatism *cannot* be eliminated with base curve choice *Free-form* surface generation allows complex surface geometries that can *eliminate* oblique astigmatism as well as other lens aberrations -Aspheric and atoric surfaces

Transpose the following Rx into minus cylinder form: OD: -1.50 + 2.50 x 074

+1.00 - 2.50 x 164

Anisometropic patient has the following Rx: OD: +0.50DS OS: +10.00DS If the patient was refracted at a standard vertex distance of 12mm, what power would be needed in the OS contact lens at the corneal plane?

+11.50D fs = 1 / Fs = 1 / +10 = +0.1m fc = +0.1m - 0.012m = +0.088m Fc = 1 / fc = 1 / +0.088m = +11.36D or +11.50D (to nearest 0.25D)

A lens clock calibrated for index of refraction of 1.53 reads +8.50D when measuring a lens of index of refraction 1.74. What is the actual surface power of the lens?

+12D FCLOCK = (nCLOCK - 1) / r so +8.50 = (1.53 - 1)/r r = 0.0624m FTRUE = (nTRUE - 1) / r so FTRUE = (1.74 - 1)/0.0642 = +11.86D

The front surface of a lens measures +6D and the back surface measures -2.25D in the 045 meridian and -5.00D in the 135 meridian. What is the Rx of the lens?

+3.75 - 2.75 x 045

Aniso: Let's assume the patient does have a problem and needs an add. What can you do?

-2 pairs of spectacles One distance, one near Single vision or mostly near area -Compensated "R" bifocal segments No longer available -Dissimilar segments (low yield) Use different lined multifocal segments in each eye. Want segment with highest near OC in eye with most MINUS vertical meridian power -Fresnel prism Can get BD, BU or oblique angles for near or distance use -Contact lenses By reducing distance from correcting lens to primary principal plane of the eye, hyperopic and myopic patients behave similarly Since contact lenses move with the eye as it rotates, no induced prism as patient looks around field of view -Slab-off prism

Find the power in the vertical meridian of the following lens: Rx = - 3.00 - 2.00 x 060

-3.50 α = 30°; sin2(30) = 0.25 Fp = Fs + Fc sin2α = (-3.00) + (-2.00)(0.25) = -3.50D

A patient's Rx is: OD: -3.00 DS OS: -3.00 DS These lenses are in a frame with their poles located at the geometric center of each eyewire. The frame markings are 56 □ 20. The patient's distance PD is 68mm. What is the net prismatic effect when the patient looks through the spectacles?

-56 □ 20 means a frame with a horizontal dimension (eyesize or "A" dimension) of 56mm and a distance between lenses (or DBL) of 20mm -The sum of the A dimension and DBL is called the frame PD Frame PD = 56 + 20 = 76mm -The frame PD is equal to the distance between the geometric centers of the lenses in the frame -decentration = (Frame PD - Patient's PD) / 2 = (76mm - 68mm) / 2 = 8mm / 2 = 4mm = 0.4cm Next calculate prism effect in each lens at the point of interest (where the patient is looking in this case) P = dcm Fv P = 0.4cm x -3 D = -1.2∆ Since P = negative, base direction is BI For the right lens, BI and for the left lens, BI So prism effect for OD, 1.2Δ BI and for OS, 1.2Δ BI for a total prismatic imbalance of 2.4Δ BI

Prism reference point?

-ANSI Z80.1 defines prism reference point as the point on the finished lens through which the prism value is to be measured *No prism in Rx then PRP is at lens OC *Prism in Rx then PRP is the point on the lens that has the correct amount and direction of prism *Implies that the sphere, cylinder and axis are also present at that point

Plastic Photochromic Lenses advantages and disadvantages?

-Advantages Plastic materials lightweight and fog-resistant Can get various colors In direct sunlight can darken as much as glass photochromics Change state more quickly than glass photochromics -Disadvantages Some don't darken behind a car windshield as only activated by UVA All but Transitions XTRActive which is partially activated by blue visible light Organic molecules tend to "wear out" over time Organic molecules can be damaged by excessive heating in bead pan or dye pot

Drop ball test, markings, lens thickness, mounting and written warning for impact protector.

-All basic impact protector tests -Minimum thickness of 2 mm -High velocity impact test *For sample of 3 test lenses - none can fail high velocity impact test 1. Standard lens shape 2. ¼" projectile traveling at 150 fps 3. For successful test - no breakage, penetration or detached particles from ocular side of lens -Once sample passes any Rx of greater thickness by the same manufacturer, material and coating also passes - must retest sample yearly -Special markings on lenses *Manufacturer's mark and "+" followed by any special markings -Frame markings *Rx safety frames 1. Manufacturer's markings and Z87-2 on all frame parts -Non-Rx safety frames - *non-removable lenses* *Manufacturer's mark and Z87+ on frame front or one temple -Non-Rx safety frames - *removable lenses* *All parts marked with manufacturer's mark and Z87+

What is another name for oblique astigmatism and what causes it?

-Also called radial astigmatism or marginal astigmatism Due to obliquely incident light rays A major problem for ophthalmic lenses Not reduced with small pupil size

Aniseikonia Tests

-American Optical Space eikonometer Hasn't been made for some time Complicated to explain to the patient Complicated for the patient to respond Not a practical test -Prism dissociation and subjective comparison of image size differences Can refine with size lenses if needed

High Plus Rxs: Appearance

-Appearance *Enlargement of eyes >Awkward with spherical lenses >Lenticular constructed spherical lenses even worse >>Lighter and provide good performance in the center >>Can be used in a large frame >>Fried-egg appearance >Aspheric designs produce flatter lenses with less magnification effects

Vertical Displacement of Segment OC?

-As for a single vision lens, we want the distance optic axis of the lens to pass through the center of rotation of the patient's eye -Rule is drop OC of lens 1mm for every two degrees of pantoscopic tilt of lens

How to calculate specular magnification due to aniseikonia?

-Can discuss the size of retinal images *Spectacle Mangification >-Retinal image size without correction compared to retinal image size with correction t=center thickness of lens, n=index of refraction of lens material, F1=front surface power of lens, h=distance from back of lens to entrance pupil of eye and Fv=back vertex power of lens

Anisometropia

-Condition when the spherical equivalent refraction of the two eyes differs by 1.00 D or more -Affects less than 10% of patients -Can lead to several problems *Unequal accommodative demands *Prismatic imbalance in other than straight-ahead distance gaze *Unequal image sizes - aniseikonia

Prescribing Slab-off prism

-Consider when patient has more than 2.00 vertical anisometropia between the two eyes -Make an assumption about how much the patient lowers gaze to read Typically use 8 mm or 10 mm The difference in vertical meridional power x drop in gaze in cm = amount of slab-off prism If done with bi-centric grinding on back surface of lens, slab-off will be applied to the lens with the LEAST PLUS or MOST MINUS power in the vertical meridian Removing base-down prism is the same as adding base-up prism

Knapp's Law

-Correct an axial anisometrope with spectacles to reduce aniseikonia -Knapp's Law does not necessarily hold true because Correcting lens not always at the correct location Power of the eye not always equal to the standard eye Shape factor of the correcting lens not always 1 >If not = 1, + lens has magnification, -lens has minification Assumes the spacing of the retinal mosaic is the same in a hyperopic eye vs. a myopic eye - not always the case

Veiling or disability glare

-Different from the others -Caused by the scattering of light inside the eye Caused by cloudy media Causes diffuse illumination of the fovea and reduces the contrast of the central image Cataracts and uveitis

What is image jump and the equation? (HIGH YIELD)

-Differential displacement of an image at the top of the segment -Image Jump = add power x distance (in cm) from near OC to top of segment Only due to the prism induced by the segment Base down for all MF styles except Executive Greatest for Ultex-style bifocals Least for Executive bifocals (no image jump)

What is Coma?

-Effect of spherical aberration for obliquely incident rays of light *Instead of a blur circle you get a comet-shaped image

Prescribing and Lens Design

-Estimate 1% per diopter of anisometropia For example 2 D of anisometropia means 2% aniseikonia Always make parameter changes to eye with most plus power as these have the greatest initial base curve and produce the greatest effect when changed >Flatten base curve or move lens toward eye to decrease mag in eye with larger image

Other relections

-Flat-top or segmented multifocals may create reflections at the edges of the segments -Multiple ring effect near edges of high minus lenses due to internally reflected light *Tinting or painting edge of lenses can help *Hide-a-bevel can help

A patient's Rx is: OD: -2.00 DS combined with 2ΔBI OS: +4.00 DS combined with 2ΔBI A = 50mm, PD = 64mm, DBL = 20mm Calculate the decentration needed for PD, decentration needed for prism and total decentration for each lens.

-For decentration due to PD Frame PD = A + DBL = 50 + 20 = 70mm Patient's PD = 64mm Decentration = (70 - 64) / 2 = 3mm in OU -For decentration for prism *For OD eye horizontally P = dcm X Fv dcm = P / Fv = 2 / 2 = 1cm -need to move pole of lens 10mm temporal to get 2ΔBI prismatic effect where OD eye is located -For decentration for prism *For OS lens horizontally dcm = P / Fv = 2 / 4 = 0.5cm = 5mm -need to move pole of OS lens 5mm in to get 2ΔBI prismatic effect in front of OS eye

Equation for single layer AR and Ideal index of refraction of a coating

-For single-layer AR coatings *Thickness must be λ/4 λ must be the wavelength in the coating material *Ideal index of refraction of coating material ncoating = √nlens

Two broad categories of progressive lens designs

-Hard designs Short, rapidly changing in power corridor Wide distance and near portions Large amounts of astigmatism present in periphery of lens -Soft designs Longer, wider corridor Narrower distance and near portions Less peripheral astigmatism (Hard - more unwanted astigmatism induced Soft - not as much unwanted astig because the power change happens over a longer period of time (stretched out))

How does aniso affect the accomm. system?

-Hyperopic patients effectively become presbyopic sooner than emmetropes when corrected with spectacles -Myopic patients have to accommodate less when wearing spectacles for the same object distance and so delay presbyopia with spectacles -High astigmatic patients (>4D cylinder) require a different near spectacle Rx due to these effects in the two principle meridians

What is the Ideal level (comfortable) of illumination?

-Ideal level (comfortable) of illumination is 400 foot-lamberts (ftL) The illumination found under a shade tree on a typical summer day -Typical sunny Texas day in July 10,000 foot-lamberts Exceeds "comfortable" level by 25X Need a filter that transmits 4% of visible light to reduce 10,000 ftL to 400 ftL

AR Thickness Issues

-If AR coating is too thick, lens has a pale blue appearance (think optimized to transmit longer wavelengths so transmits red but reflects blue) -If AR coating is too thin, lens has a pale red/pink appearance (think optimized for shorter wavelengths so transmits blue but reflects red)

High Plus Rx: Eye Rotations?

-Increase in eye rotations Increased convergence More bifocal inset Fit bifocals higher No trifocals Short-corridor PALs

High Plus Rx: Aberrations

-Increased aberrations *From Tscherning ellipse, above +8 D no base curve combinations that will minimize oblique (or radial/marginal) astigmatism >Solution is to use aspheric optics >>Can rotate a conic section to produce surface geometry >>>Ellipse, parabola or hyperbola >>>Can use more sophisticated merit functions to create surface geometry >>>For plus lenses, continuous decrease in front surface power from center to edge >>>Lens flattens continuously from center to edge -Can use free-form optimized for Rx lenses that provide optimal asphericity

HighPlusRx: Accommodative Demand

-Increased accommodative demand Greatest effect for pre-presbyopic patients With +2.50 adds at 40 cm working distance not an issue

What do Glass photochromic lenses degree of darkening depend on?

-Intensity of UVA and visible light *Inside car vs. direct sunlight -Temperature *Cold enhances darkening speed and degree *Heat enhances lightening speed and effect -Memory effect *Retain exposure history *Can maximize initial range by exposing to direct sunlight for 10 minutes followed by storing them overnight in a dark place *Can reset by heating lenses in a bead pan and allowing to cool to room temperature -Tempering *Heat Gets darker in darkened state *Chem Fades quicker Fades lighter indoors Wider range overall

Drop ball test, markings, lens thickness, mounting and written warning for non-impact protector.

-Lenses must survive an impact from a 1 inch diameter steel ball weighing 2.4 ounces dropped from a height of 50 inches -Minimum lens thickness of 3.0 mm; 2.5 mm for >+3.00 D -Lenses must be engraved with marking identifying the manufacturer -100% of glass lenses tested -Statistically significant sample of plastic lenses tested -Lenses must be mounted in a safety frame with the Z87 markings visible -Written warning supplied with non-impact protector advising against use for high-velocity impact exposure

Drop Ball test for Dress Eyewear.

-Lenses must survive an impact from a 5/8 inch diameter steel ball weighing 0.56 ounces dropped from a height of 50 inches -All glass lenses tested (and must pass) -Minimum center thickness for glass lenses of 2mm (or 1mm at thinnest edge) -No minimum thickness for plastic lenses

High Minus Rx Lenticular design

-Lenticular designs For minus lenses, myo-disc design Center lens element, peripheral carrier lens

Reflection (less intense)

-Less intense than A (3.94% for crown glass) -Not commonly a problem for patients -If lens is small, less likely to be a problem -Solutions AR coat lens Tint lens Fit the lens close to the face (decrease vertex distance) Slight face-form Slight pantoscopic tilt Change base curve of lens

Rimless mountings

-Made of five parts A bridge Two endpieces Two temples -Lenses attached to rimless mountings by screws (or other fasteners) and holes drilled in lenses

Plastic photochromic lenses

-Made of organic molecules -UVA light causes the molecule to swell, split open, and absorb color in the visible spectrum *Color in swollen state can be tailored *In the absence of suitable light the molecule returns to a smaller, closed state and appears clear *Heat enhances return to clear state

Semi-rimless mountings

-Made of three parts A front Two temples -Lenses can be attached to front by screws (or other fasteners) and drilled holes or grooved lenses held with nylon cord from frame front

Aniseikonia: Anatomical

-May be due to an aberration of neurophysiology *Retinal photoreceptor spacing *Visual cortex representation *Accounts for 1/3 of anomalous aniseikonics *Unpredictable

If you Assume that refractive anisometropia is caused largely by corneal curvature differences between the two eyes then?

-Measure OD and OS K measurements -Take average K readings -If average K readings different by amount of anisometropia, anisometropia is refractive -If average K readings are the same, anisometropia is axial >Only exception would be patient with progressing cataract where lens is increasing in + power making the patient's Rx more myopic

Mostly commonly complained about reflection?

-Most commonly complained about reflection (especially for low Rxs - less noticeable in higher plus Rxs) -Less intense than the others discussed so far (0.17% for crown glass) -Seen only when looking at a bright source in dim illumination -Solutions AR coat Tint (note three passes through lens material)

High Minus Rx: Multiple Ring Images

-Multiple ring images *Images result from internal reflections of the edge of the lens and from light entering the edge of the lens Leave the edge frosted - no polish Hide-a-bevel Slight tint Edge painting/tinting

High Plus Rx: Moving eye vs non-moving eye?

-Non-moving eye, blind area for 360° -Moving eye, image motion effects *Jack-in-the-box effect Sudden appearance and disappearance of objects from or into the ring scotoma >Patient looking straight ahead >Perceive peripheral object or movement >Stimulus to look in that direction >As eye moves, object disappears into ring scotoma >Can suddenly reappear as object reemerges from scotoma

When is aniso a problem

-Not a problem unless anisometropia exceeds 4 D -Only a problem with lower add powers - effect disappears with +2.50 add power (no accommodation in eye, all near vergence correction done at the same location, the spectacle plane)

High Minus Rxs

-Noticeable changes Decreased image size Increased field of view Decrease in ocular rotation Decrease in convergence demand Decrease in demand for accommodation (pre-presbyopes) Increased aberrations Increased edge reflections Potential for increased temporal thickness and weight

High plus and aphakia

-Noticeable changes: Increased image size Decreased field of view Ring scotoma (Jack-in-the-box effect) Increase in ocular rotation Increase in convergence demand Increase in demand for accommodation (pre-presbyopes) Increased aberrations Potential for increased nasal thickness and weight

Discomfort glare?

-Occurs when the illumination in some part of the visual field is much higher than that to which the retina is adapted Night driving with oncoming headlights Viewing television in a dark room -Occurs when the ratio of highest illumination to background illumination exceeds 3 to 1

What is opacity and the equation?

-Opacity is the reciprocal of transmission O = 1/Transmission

What is curvature of field?

-Petzval's surface is created by the ophthalmic lens (independent of the patient's eye) -rPS = -nf' -If no oblique astigmatism present, Petzval's surface and the sagittal and tangential foci are all coincident -If oblique astigmatism present, the tangential foci are three times as far from Petzval's surface as the sagittal foci -The distance from the sagittal foci to the tangential foci is twice the distance from Petzval's surface to the sagittal foci -Both the tangential and sagittal foci fall on the same side of Petzval's surface (they never straddle it)

Aniseikonia: Optical

-Two types 1. Inherent Dependent upon the optics of the eye 2. Induced Due to the magnification properties of lenses worn to correct ametropia -Accounts for ~2/3 of anomalous aniseikonics -Fairly predictable

What is the best approach for high plus rxs?

-Pick the right frame Frame PD = Patient's PD -Pick the right lenses Aspheric lenses to minimize spherical aberration, marginal astigmatism and lens magnification effects Higher index materials will provide thinner lenses AR coatings will reduce reflections (external and internal) -Pick the right multifocal Flat-top bifocal or short corridor PAL to minimize eye movement needed to use add power -Measure carefully Need patient looking through PRP or there will be induced prism or increased aberrations Critical for single vision aspheric lenses and PALs

What is the best approach for high Minus Rx?

-Pick the right frame Frame PD = Patient's PD -Pick the right lenses Aspheric/free-form optimized for Rx lenses to minimize spherical aberration, marginal astigmatism and lens minification effects Higher index materials will provide thinner lenses AR coatings will reduce reflections (external and internal) Edge treatments to minimize ring reflections -Multifocal selection not as critical as for high hyperopes -Still need to measure carefully *Need patient looking through PRP or there will be induced prism or increased aberrations >Critical for single vision aspheric lenses and PALs

Spectacle Magnification: Power Factor

-Power factor portion If Fv negative, <1, If Fv positive, >1 -Reducing h (contacts or fit spectacles closer) reduces minification effects of - lenses AND reduced magnification effects of + lenses -If h = 0, no minification or magnification effect from power factor -If Fv = 0, no minification or magnification effect from power factor

Production of photoproducts?

-Production of photoproducts not necessarily harmful Bleaching and regeneration of rhodopsin Vitamin D production in the skin -Creation of some harmful photoproducts useful clinically Photodynamic therapy (PDT) for treatment of certain cancers and Subfoveal Exudative Age-Related Macular Degeneration

Reflection of 0.09%

-Reflection is 0.09% for crown glass -Affects patients with moderate to high minus Rxs -Solutions AR coating Change vertex distance Change base curve

Aniseidokonia?

-Relative size difference stated as percentage of magnification of a zero back vertex power magnifying lens required to equalize the two images *Relative ocular size difference >As perceived in the brain *Subjective test >Can't objectively measure individual ocular images >Can determine relative size differences between the two eyes >The result is a percentage of magnification needed to make the images appear equal in size

Rx: +2.00 -1.00 x 180 Find the location of the two line foci and the circle of least confusion

-Remember the line focus is formed 90° away from the meridian that created it -Meridian of greatest plus power is 180° (+2 D) and will form line focus closest to back of lens (vertical line focus) f'vert = 1 / Fhorz = 1 / +2 = 0.5 m or 50 cm -Meridian of least plus power is 90° (+1 D) and will form line focus furthest from the back of the lens (horizontal line focus) f'horz = 1 / Fvert = 1 / +1 = 1 m or 100 cm -The location of the circle of least confusion = dioptric average power of the two meridians = spherical equivalent 1 + 2 = +3 D 3 / 2 = +1.50 D Sphere + ½ cylinder = +2 + ½(-1) = +1.5 D f'colc = 1/Fave = 1/FSE = 1/1.5 = 0.667 m or 66.7cm (COLC = spherical equivalent of the lens)

Aniseikonia: How to sort this out?

-Rule out the easy stuff first *Insure correct Rx *Insure correct oculomotor balance *Insure patient has had time to adapt to Rx *If patient still has problems, try monocular occlusion as an in-office test procedure >If discomfort goes away, problem may be related to aniseikonia >If no difference, unlikely aniseikonia is involved

Spectacle Magnification: Shape Factor

-Shape factor or form factor portion *Almost always greater than 1 unless in the rare case F1 is negative (front surface of lens concave) ⇧ F1 = ⇧ magnification ⇧ t = ⇧ magnification ⇧ n = ⇩ magnification

Slab-off prism

-Slab-off prism (bicentric grinding) *Traditional removes BD prism from near area Goes on MOST MINUS lens (or LEAST PLUS) *Reverse - adds BD prism to near part of lens Goes on MOST PLUS lens (or LEAST MINUS) *Typically assume a 10mm drop in gaze when reading and slightly undercorrect

Distance reference point?

-That point on the lens specified by the manufacturer at which the distance sphere power, cylinder power and axis are to be measured *For spherical single vision and multifocals, the Distance Reference Point and Prism Reference Point are the same *For PALs, DRP is above PRP (because manufacturer assumes 8° of pantoscopic tilt (built-in) will be present in dispensed lenses - for every 2° of pantoscopic tilt lower distance OC by 1mm) (Single vision - DRP and PRP is the same point PAL - DRP and PRP are different)

When astigmatism is induced due to tilt, which axis is affected?

-The axis of the astigmatism is the same as the axis about which the lens is tilted -Plus lens tilted along the meridian (pantoscoptic tilt) will induce plus cyl along the 180 axis; face form will increase plus cyl in the 90 degree meridian. -The increase in cyl (plus or minus) depends on the sphere power. Plus sphere will induce plus cyl. Minus sphere will induce minus cylinder.

The dimmest of the five common reflections

-The dimmest of the five common reflections (0.08% for crown glass) -Solutions AR coating Change vertex distance Change base curve of lens Tint

High Plus Rx: Thickness Issues

-Thickness Issues tc - tp = h2 FA / [2 (n-1)] h = half cord length or distance from OC to edge in question Picking a frame with the frame PD (A + DBL) the same as the patient's distance PD means no decentration Significant nasal thickness with decentration FA fixed n = index of refraction of material Increasing n will decrease thickness change In addition, high-index materials can be made thinner than CR-39 and still pass the ANSI Z80 impact standards

The Astigmatic Clock Dial (high yield)

-To do the test first fog the patient (add plus until 20/30 line blurry -Next ask the patient if any of the lines look darker than the others

Prism Effects: How do you calculated total displacement at the reading level?

-Total Displacement = Vertical meridian distance power x distance from distance OC to reading level in cm combined with differential displacement *Total displacement depends on the distance and near portions of the lens *Useful for calculating prism imbalance at the reading level

Metal Frame Materials: Nickel Silvers

-Was the most popular material as it held shape the best of other metal alloys Being challenged by titanium because of weight and prevalence of skin reactions in some patients to nickel Very commonly used material (bad; skin reaction)

What is the zone of confusion? What are three ways that the image will differ?

-When the patient's line of sight is very near the top of the segment so that some light from above and below the segment top is entering the pupil, the patient experiences two images -The images differ in three ways Direction (because of differential displacement) In focus (differences in power) In size (differences in magnification)

How are optical plastics formed?

-combining organic ingredients with inorganic materials

What is a hybrid material plastic lens made of?

-mixtures of two monomers to form a copolymer are possible Resultant compound has some characteristics of each monomer Monomers don't have to be of the same type (thermoplastic + thermosetting possible) Examples: trade names *Trivex*, Trilogy, Pheonix, Trexa, and Tribrid

Can the eye can adapt to high ambient light levels

-the eye can adapt to high ambient light levels they can cause discomfort

Metal Frame Materials: Memory Metal

-titanium and nickel alloy -Returns to original shape after bending or twisting -Only the areas of the frame subject to bending or twisting are made of this material (bridge, top bar and temples) -Good for active patients who frequently have their frames bent or knocked off their faces -Contains nickel so some patients may experience a skin reaction

WTR vs ATR vs Oblique Axis?

0-30 and 150-180 - WTR 31-59 and 121-149 - Oblique 60-120-ATR

Describe the two approaches to solving an effective power problem.

1. Formula FNEW = FSTART / [1 - (d FSTART)] d = + if lens moved toward the eye d = - if moved away from the eye 2.Determination of focal lengths Draw initial lens position with focal length Draw new lens position To have the same effect, location of point focus must be the same Determine focal length needed at new position 1 / fNEW = power needed at new position

What are four mechanisms by which lenses fail? (Low Yield Question)

1. Fracture of front surface origin -From elastic denting of the surface -Small, light, high-velocity impact (i.e., B-B, rock hitting windshield) 2. Fracture of rear surface origin caused by flexure -Moderate weight, moderate velocity impact at thin area of minus lens 3. Edge fracture caused by flexure or flattening of the lens -Heavy, low velocity impact striking a plus lens 4. Edge fracture / failure caused by elastic wave reflecting through lens

Sagittal depth (sagitta or sag) of a surface with a 0.234mm radius of curvature using a 50mm diameter bell sagitta gauge would be:

1.3mm Sagitta = h2 / (2r) = (0.025)2 / [2 (0.234)] = 0.00134 = 1.3mm

A round -6D lens of index of refraction 1.5 is 75mm in diameter, has a 2mm center thickness. What is the edge thickness?

10.4mm tc - tp = h2FA/[2(n-1)] 0.002 - tp = (0.0375)2(-6)/[2(1.5 - 1)] = -0.008438 tp = 0.002 + 0.008438 = 0.010438m or 10.44mm

Parallel light from a distant object strikes the front of a right +5 D lens 5mm directly below the front pole of the lens. What prism power is induced?

2.5∆ BU OD P = dcm Fv = (0.5)(+5) = +2.5Δ = 2.5Δ BU, OD

A 6∆ prism made of polycarbonate (n=1.59) is 40mm long. What is the change in thickness from apex to base?

4mm Δt = εΔ QP/ [ 100 (n-1) ] = 6 (40) / [100(1.59 - 1)] = 4.07mm

Mounting

A device that holds two lenses in front of the eyes but does not encircle the lenses Two types Rimless Semi-rimless

What is the vertex (or pole) or a lens)

A point on the front or back of a lens where the optic axis penetrates the surface of the lens -pole is closer together for a minus lens

Advantage and disadvantages of a PAL?

Advantages Clear vision at all viewing distances No visible reading segment Disadvantages Astigmatism present in transition zones due to aspheric surfaces joining distance and near portions of lens - can't avoid this with conventional PALs (Can be reduced with PALs that split add power between front and back of the lens - free-form PALs)

Advantage and disadvantages of glass photochromic lenses?

Advantages (FAST AND DOESN'T WEAR OUT) Darken well in an automobile on a sunny day Indefinite life of photochromic material Scratch resistant Disadvantages Weight of glass material Fogging of glass material Speed of lightening slowed by cold temperature

What is the fresnel equation and what does it define?

Amount of light reflected by a lens surface is given by the where: n' is the index of refraction of the media in which light is about to enter n is the index of refraction of the media in which light is traveling I0 is the intensity of incident light

Example OD: +0.50 - 2.00 x 090 OS: -4.50 - 2.00 x 180 Assuming the patient drops their gaze 10 mm when reading, calculate the amount of slab-off prism necessary to eliminated vertical prism at the reading level and state to which lens the slab-off prism would be applied.

Amount? Difference in vertical meridian power = -6.50 - 0.50 = 7.00 D difference Amount = 7.00 D x 1.0 cm = 7.00∆ of slab-off O.S.

Photosensitization

An enhanced reaction to normally harmless radiation (typically UVA or visible) occurring only because of a photosensitizer Photosensitizer can be an orally ingested substance, directly applied to the body's surface or a metabolite of another substance

What is induced if a spherical lens is tilted?

Astigmatism

Bridges for metal frames usually have ______________nosepads to _______________ of the frame front and lenses

Bridges for metal frames usually have adjustable nosepads to distribute the weight of the frame front and lenses

Bridges with an extra piece of metal are called ___________ or ______________bridges

Bridges with an extra piece of metal are called double bar or top bar bridges

A patient's Rx is: OD: +2.00 DS OS: +3.00 DS When looking straight ahead at distance the patient looks through the poles of the two lenses. The patient then drops their line of sight through the lenses by 10mm when reading. What is the prismatic imbalance?

Calculate imbalance in each eye: For OD, P = dcm Fv = 1cm x (+2) = 2Δ BU OD For OS, P = dcm Fv = 1cm x (+3) = 3Δ BU OS Net imbalance = 3 - 2 = 1Δ BU OS (or 1Δ BD OD)

With decentration problems, how can you keep track of pole movement for total decentration?

Can make a table to help keep track of pole movement for total decentration

Can think of any decentered lens as a ____________ lens (___________edge thickness) with _____________ prism

Can think of any decentered lens as a centered lens (equal edge thickness) with a piggyback prism

Plastic Frame Materials: Zyl/Zylonite

Cellulose acetate (main plastic material) One of the most used materials for frames Thermoplastic Composed of cotton seed fibers and wood flakes Can be made in a variety of colors and patterns Material will expand with heating and shrink with cooling Excessive heat can burn or pit the frame surfaces Becomes brittle with age

What are two different chemistries in lens material? (Low Yield)

Clear and tinted glass, 100% KNO3 Photochromic glass, 40% NaNO3 and 60% KNO3

Comfort Cable or cable

Comfort cable or cable Coiled spring at the end of temple that wraps around the ear Can be used on plastic and metal frames

Describe how glass lenses are undergo chemical tempering?

Compression layer created by exchanging Na+ ions for K+ ions in the surface of the lens Bigger K+ molecules exert compressive forces in the outer layer of the glass lens Requires 16 hours at 850°-875°F (depends on type of glass) Time can be decreased to 2-4 hours with addition of ultrasound with better results (more uniform reaction, safer and produces stronger lenses, but takes longer)

Why are image jumps annoying for patients?

Creates a scotoma

Lens Decentration?

Decentration (total) can be thought of as made of two parts -Decentration for PD -Decentration for Prism

Aphakic patient who has been wearing contact lenses of the appropriate power decides to switch to spectacle lenses. A spectacle refraction done at the vertex distance of 12 mm is found to be +14.00 D. In switching from contact lenses to spectacles, what is the percentage change in image size?

Determine power of contact lens to use fs = 1000/14 = 71.43 mm behind lens fc = 71.43 mm - 12 mm = 59.43 mm behind cornea Fc = 1000/59.43 = +16.83 D Switching to spectacles Fc / Fs = 16.83 / 14.00 = 1.201 or 20.1% magnification

What is differential displacement at the prism level and the equation to calculate differential displacement?

Differential displacement = Add power x distance from near OC to reading level in cm A function of the bifocal segment only Similar to jump but instead of calculating prism effect at the segment top, prism effect is calculated at the reading level If the near optical center of the bifocal segment is located at the reading level, there is no differential displacement (Executive is the only one that will give you a BU prismatic effect. The rest will be BD)

What can glare cause in our field of vision?

Discomfort Temporary blurring of vision Fatigue

What are three types of glare?

Discomfort glare Specular reflection Veiling or Disability glare

Dissimilar segments problem Distance Rx: OD: -1.00DS OS: +4.00DS Add: +2.50

Distance Rx: OD: -1.00DS OS: +4.00DS Add: +2.50 Net vertical imbalance = +5.00D Need 7 (1.0cm) = 5∆ BD, OD P = F dcm Difference in near OC heights (in cm) = 7 / 2.50 = 2.8cm Need combination of segments with 28mm separation in near OC location with higher near OC on more minus vertical meridian (in this case, OD) Location of near OC below top of segment Executive: 0mm Flat-top: 5mm Round 22: 11mm (half of diameter) Ultex: 19mm Closest combo with out exceeding 28 is Executive OD and Ultex OS correcting 4.75∆ of the imbalance

What is the back vertex power and the equation?

Distance from back pole of lens to the posterior focal point Normal way of specifying lens power Same regardless of lens form Fv = (F1 / [1 - ( t / n ) F1]) + F2

What is the front vertex power (neutralizing power) and the equation?

Distance from front pole of lens to the anterior focal point FN = F1 + (F2 / [1 - ( t / n ) F2]) -*Clinical use for determining add power if multifocal is a front surface multifocal*

Effective Diameter

ED = 2 x greatest radius from GC to the edge of lens

-Early-on most PAL designers used a ___________ power variation for the entire range of add powers ___________-design Changing the surface designs from soft with adds through +1.50 and harder designs for higher add powers results in ____________ patient satisfaction __________-design

Early-on most PAL designers used a constant power variation for the entire range of add powers Mono-design Changing the surface designs from soft with adds through +1.50 and harder designs for higher add powers results in higher patient satisfaction Multi-design

Plastic Frame Materials: Optyl

Epoxy resin - thermosetting Can be made translucent or opaque by adding dye to the resin mixture Adjustments are relatively easy but unless frame is held in place until cool, shape returns to molded shape - strong memory Material will not shrink if stretched Hypoallergenic

What is lambert's law of absorption?

Equal thickness layers absorb equal fractions of light

Plastic Frame Materials: Kevlar

Extremely strong material Used for impact resistant frame material Doesn't expand or contract to any large degree Lenses must fit exactly without heating Can't really adjust frame - frame fits the patient or it doesn't Available in amber and darker, usually opaque colors Expensive

Equation for surface power (using radius)

F = (n-1) / r

What is the approximate power of a lens equation?

F1 + F2 = FT

What is the equivalent power equation and when is it used?

FE = F1 + F2 - (t/n) F1 F2 -Used to replace two lenses with one ideal lens or a thick lens with an ideal lens located at the primary and secondary principle planes

Describe the properties of thermoplastic.

Molecules arranged as long chains - 2D structure Usually supplied in pellets, granules or sheets Soften when heated Can be stretched, pressed or molded with no significant change in chemical structure Good impact resistance - no additional treatment needed When cooled, material hardens and shrinks Can be repeatedly heated, reshaped and cooled Examples PMMA (trade names Plexiglass and Lucite) Frame materials cellulose nitrate and cellulose acetate Polystyrene, nylons, vinyls and polycarbonate (trade name Lexan)

How are most ophthalmic lenses made?

Most ophthalmic lenses made by grinding *back surface shape* into material with diamond-dust cutting tools and then polishing surface to restore optical clarity

What if you want to make the coating for a range of wavelengths?

Multicoating uses various layers designed for various wavelengths to increase the range of utility for antireflective coatings

Plastic Frame Materials: Polyamide/copolyamide

Newer frame material Blend of nylons Can be made translucent Available in a broad selection of colors Can mount lenses without heating frame Requires less heat for adjustments compared to Zyl Retains adjustments well - little memory Material will not shrink if stretched Hypoallergenic

Newest technology darkens and becomes _____________ in the darkened state - Transitions® ______________

Newest technology darkens and becomes polarized in the darkened state - Transitions® Vantage

Is spherical aberration a problem for most ophthalmic lenses?

No. For most patients, the pupil size limits the effect Usually only a significant problem for aphakic (high-plus) patients

Distortion?

Not related to sharpness of the image Concerned with changing the proportions of the image compared to the object Results from magnification changes as you go further from the optic axis Lenses with no distortion are called *orthoscopic*

What is optical density and the equation?

Optical density D = log10 O = log10 1/T Optical density of a material is useful for transmission calculations Optical density D is the only quantity which is proportional to thickness Can use simple ratios to see what thickness gives a desired effect DTotal = D1 + D2 + ... + Dn

What is the equation for prentice rule? (HIGH YIELD)

P = dcm Fv

Specular Reflection?

Patches of bright light reflected off of smooth, shiny surfaces into the eye Metal Water Glass Roadways Polarized lenses can be of help for specular reflection when the surface is horizontal

Different types of photosensitizaton?

Phototoxic Immediate, erythematous, localized and short-lived Photoallergic Delayed, eczematous, persistent and wide-spread Can be clinical overlap of the two

The Abbe value has three other names

Refractive efficiency Constringence Nu (or ν) value

Riding bow

Riding bow Similar to cable temple but usually plastic Usually used for children's frames Wraps around ear

What is sag and the equation?

Sag - if you make a cord the sag is from the curvature to the cord s=h^2/2r

What is the basic component of Glass?

Silica - Melts at approximately 3000° - 3300°F. (1700° - 1850° C) depending upon the silica used

Example K measurements OD: 47.00 x 47.00 Rx: -2.50 OS: 44.50 x 44.50 Rx: Plano

Since K's are different by the amount of anisometropia, the anisometropia must be due to the corneal shape and not axial length differences Make sure the steepest cornea is with the more myopic Rx!

Example K measurements OD: 44.50 x 44.50 Rx: -2.50 OS: 44.50 x 44.50 Rx: Plano

Since K's are the same, anisometropia must be due to axial length differences between the two eyes

Skull

Skull Most common type of temple

How do spectacles affect distortion?

Spectacle lenses produce distortion Particularly a problem for high power lenses Minus lenses demonstrate barrel distortion Plus lenses demonstrate pincushion distortion

Metal Frame Materials: Stainless Steel

Stainless steel Iron and chrome Flexible and durable Some patients occasionally will experience a skin reaction to stainless steel although very uncommon Not necessarily the same patients who react to nickel

Fluorescence or photoluminescence

Substance absorbs radiation of one wavelength of emits radiation of another wavelength (usually longer)

Example Glass filter of index 1.523 1 mm thick Transmits 80% of light incident on it If transmission of 50% desired, what would be the thickness of a single glass filter of the same material?

TSurface = 1 - [(1.523 - 1)/(1.523+1)]2 = 1 - (0.2073)2 = 0.9570 TFilter = TSurface x TMedium x Tsurface 0.8 = (0.957) x TMedium x (0.957) TMedium = 0.8 / (0.957)2 = 0.8735 = qX = q1 = 0.8735 so, q = 0.8735 Want transmission to be 50% TTotal = 0.5 0.5 = TSurface x TMedium x TSurface = 0.957 x TMedium x 0.957 TMedium = 0.5 / 0.9572 = 0.5459 TMedium = qX = 0.5459 = 0.8735X = 0.5459 log 0.8735X = log 0.5459 X log 0.8735 = log 0.5459 X = log 0.5459 / log 0.8735 X = -0.2629 / -0.0587 = 4.48 mm

A prism with a refracting angle of 10° is made of CR-39 material with index of refraction of 1.50. What is the angular deviation of light in degrees and prism diopters?

The angular deviation of light: ε° = β°(n-1) = (10°)(1.5 - 1) = 5° Deviation of light in prism diopters: εΔ = 100 tan [β°(n-1)] = 100 tan [10°(1.5 - 1)] = 100 tan (5°) = 100 (0.0875) = 8.75Δ (Prism diopters - think turning degrees into prism. Angular Deviation - think degrees)

Tscherning's Ellipse?

The basis for best-form or corrected curve lens design Front surface = base curve; if you pick something (sphere) on the ellipse then you get no aberration. For High astigmatics - back surface free from are better lenses

The more _____________ eye will have a greater accommodative demand

The more hyperopic eye will have a greater accommodative demand

Far Point Sphere

The object space corresponding to the retinal image surface is also curved and is referred to as the far point sphere

Given: OD: +2.50 DS +2.00 Add 20mm round BF Lens "B" = 48mm Seg height = 16mm Reading level is 10mm below distance PRP Distance PRP at lens GC What is the bifocal jump?What is the differential displacement at the reading level?What is the total displacement at the reading level?

What is the bifocal jump? Jump = 1cm x (+2 Add) = 2ΔBD, OD What is the differential displacement at the reading level? Differential Displacement = 0.8cm x(+2 Add) = 1.6Δ BD, OD Induced prism from the distance Rx = (1)(2.5) = 2.5 BU OD Diff displacement = 1.6Δ BD OD Total displacement = 0.9Δ BU OD

Dress Eyewear

Z80.1 (2010)

Industrial Eyewear

Z87.1 (2010)

Another name for non-impact protector.

basic impact protector

What does a shallow sag tell you about the circle?

flatter and has a bigger radius of curvature

Another name for impact protector

high velocity impact protector

C=1/r; smaller r =?

larger C (steeper) (steeper=smaller)

Examination of a 15-year-old patient reveals the following: Rx: OD: -2.50 DS -0.75 DC x 090 OS: +3.25 DS -0.50 DC x 180 Keratometry: OD: 43.00 D @ 090; 43.50 D @ 180 OS: 39.00 DS Which of the following spectacle lens designs should be the MOST effective in reducing aniseikonia for this patient? Base Curve OD Center Thickness OD / Base Curve OS Center Thickness OS A. +3.75 D 2.0 mm / +7.50 D 3.6 mm B. +3.75 D 2.0 mm / +3.75 D 6.0 mm C. +5.25 D 2.5 mm / +7.50 D 3.5 mm D. +6.25 D 3.4 mm / +6.25 D 3.4 mm

nbeo explanation: -To correct aniseikonia Need to increase magnification OD Need to decrease magnification OS Consider the answer choices A. +3.75 D 2.0 mm / +7.50 D 3.6 mm This is a bad choice because it is the way the lenses would typically be made based on best-form design and would lead to at least 4% aniseikonia based on Knapp's Law B. +3.75 D 2.0 mm / +3.75 D 6.0 mm This is a bad choice because the OD lens is made as a best-form design and the OS lens is made with a flatter base-curve than usual (decreasing spec mag) but then made thicker than usual (increasing spec mag). No real net effect in mag for OS eye. C. +5.25 D 2.5 mm / +7.50 D 3.5 mm D. +6.25 D 3.4 mm / +6.25 D 3.4 mm Of these two choices D provides the most increase in magnification OD (max increase in base-curve and maximum increase in lens thickness) and most decrease in magnification OS (max decrease in base curve) so choice D is the best.