Module 5: Ophthalmic Lenses Flashcards
-UVA (400-320 nm)
-UVB (320-290 nm)
-UVC (290-200 nm)
-Violet (400-446 nm)
-Yellow (560-592 nm)
-Blue (446-500 nm)
-Orange (592-650 nm)
- Green (500-560 nm)
-Red (650-750 nm)
-Infrared (750 nm +)
Spectrum of light
- disruptive to px
- rainbow/shifts of colour @peripheral
- measured in Abbe/chromatic aberration
- high abbe = less dispersion> better optics (20-60)
-very important 4 diopters +
Dispersion
- crown glass (58)
- CR-39 (58
-Trivex (43-45) - Polycarbonate (31)
Abbe lens materials
- light travels straight line@ object=bent
- bending = refraction
- light rays> lenses> slows down> bent/refracted
- 3 factors of lens bending:
1) angle of light when hits lens
2) lens material
3) shape of lens front/back
Refraction
- exact location of lens passing no bending
- placed infront of px pupil (unless prism)
Optical Center
- light travels @ diff speeds/depends on material
-air = 186,000 MSP - slower through lenses
- measured by “index of refraction”
Speed
-speed of light in air/ speed of light in lens material
- index of air = 1.00
- each lens has assigned index (1.49-1.9)
- Mid-index (1.54-1.58)
-High index (1.59 +) glass and CR-39
- higher index = thinner lens
Index of refraction
- CR-39 (1.49) = thicker
- Crown glass (1.523)
-Polycarbonate (1.59) = thinnest
Indices of refraction lens material
-all lenses reflect light
- light not reflected = transmitted thru lens
-% transmittion depends on material/colour
- high index = more reflection
- high index reflect more = transmit less light= decrease VA
- can add anti-reflective coating to help
Reflection
-two basic shapes: convex OR concave
- meniscus lenses= combination both
- power depends on curve (front/back)
- two basic forms: sphere & clyinders
Ophthalmic Lenses
-thicker center/ thinner edges
-converge light
- real positive
- plus lenses
-high plus power= thicker lens= thicker middle = faster light converges
- against motion (right to left/left to right)
-magnifies images
-hyperopia/ far-sightedness
Convex Lenses
-thicker edges/thinner center
-diverges light
-virtual negative
-minus lenses
-higher minus = thicker edges= faster light diverges
with motion (right to left/right to left)
-reduces images
-myopia/ near-sightedness
Concave lenses
-same curve over entire lens
- only 1 power cross
Sphere lenses
- multiple power crosses (flattest/steepest)
- curves 90* apart
-2 blurred & tilted lines
-Toric lens - treats astigmatism
-can be lenticular (via crystalline lens)
-mixed astig = corneal & lenticular astig - every cyl has a direction (axis)
Clyinder lenses
- meridian= edge-edge crossing OC of lens
-axis = between 1-180* - 0* is the same as 180* (always write 180)
-spherocylinder - cyl = strongest and weekest meridian (90* apart)
-also called principal/major meridians (one will become the axis)
-cyl measured by distance between both meridians
Meridians & Axis
-scissors motion (lines rotated criss-cross)
- compound/toric lenses
Cylinder lenses
- 1 major meridian has no power (distance/sphere)
Plano Cylinder Lenses
-2 major meridians have diff powers
Spherocylinder lenses
- measured in diopters
-based on focal length of lens
-Diopter (D) = 1/f (focal length of lens meters)
-D=1/2meters (D =0.05) > lens power +0.50
–2.00 D= 1/f (f=1/2)> virtual focal 0.50 meters
Lens power
- relates to * in cirlce (360*)
- only uses 0-180 (upper half circle)
- axis degress increase counter-clockwise
- OD = 0* @ nose
- OS = 0* @ temple
- with-the-rule astig = near 90*
- against-the-rule astig= near 0/180*
- oblique astig = between 45-135*
Axis
-spherical lens = power @ both meridians match
-Cylinder lens = Power @ meridians different
- sphere power at axis location
- sphere is most + value
- helps determine thickness of a lens
Optical/ Power cross
Abbreviations
- OD = oculus dexter (right eye)
- OS = oculus sinister (left eye)
- OU = oculi uterque/unitas
- OMD use this form of plus astig rx
1) add sphere and cyl together to make new sphere
2) keeo cyl value and switch to minus power
3) take axis and change by 90*
Transposition
-front/back lens each have power
-power depends on steep/flat curve is
- measured with lens gauge/clock
-middle pin depresses and changes dial
-must known lens refractive index= accuracy
- old clocks work with CR-39 (1.49)
- New clocks have 2 dials 1.49 & 1.6
- read pg 15-18
Base/Lens Curve
-critical in Rx over 7.00
- minus closer = stronger rx
- plus closer = weaker rx
- minus out = weaker rx
- plus out = stronger rx
Vertex Distance
- all lenses are composed of prism
- prism have apex (tip) and base
- prism = light bend toward base
-plus Rx prism meets base-base
-minus rx prism meets apex-apex - OC is where all prisms meet (not bending)
- treatment for strabismus
- OC in wrong place = unwanted prism (wrong PD)
- Verticle OC height error= unwanted verticle prism
- MRP = prism location ( OC without)
Optical Prism
- Base Up
- appear walking down hill
- feel shorter than normal
- objects look shorter
- top of hill/ sides sloping down
Prism Direction BU
-Base Down
- appear walking up hill
- feel taller than normal
- objects look taller
- standing inside bowl/ side sloping up
Prism Direction BD
- Base in/ Base out
-horizontal objects aren’t level
Prism Direction BI/BO
- diplopia occur when OD/OS rx don’t match > px looking down thru reading seg. (more verticle prism in one eye)
- 2 pairs of SV might be more suitable
- lenses made “slab-off” prism (bicentric grind)> BU prism put on front of lens (creates line thru lens)
- reverse slab-off=BD @ back of lens
- read pg 26/28
Verticle Imbalance/ Slab-off
-press on prism stickers ranges 1-40 diopters
- used to correct verticle imbalance @ near
-temporary to test various prisms
- high Rx that would create thick lens
- vision loss monocular (stroke px)
- can be cut to a small location on the lens
Fresnel Prism
- original lens matieral (OG)
- must be tempered= impact resistant
-scratch resistant - won’t warp
-most precise optics - heavy
- unsafe when scratched
Crown Glass
- conventional plastic
- 50% lighter than glass
- can be tinted
- can be coated without UV
-scratches easily - thickest material
- can warp
CR-39
- index 1.54-1.58
-best cosmetically 2.00-3.00 diopters power - come scratch coated
- maybe avail in aspheric for thinning/less heavy
- more affordable
- UV protection
- Lighter weight
- better Abbe
- scratches easily
-can warp - better optics with AR coating
- more expensive than CR-39
- TRIVEX= strength of poly w/ better optics
Mid-Index Plastic
-index range 1.59-1.74
- no cosmetic advantage under 3.00 D
- shock-absorbing coating = 1mm thin lens
- highest index= thinnest material
- UV protection up to 380 nm
-scratches easily
-low Abbe = colour aberration
- high cost
- weaker than trivex/poly
- needs AR for best optics
High Index Plastic
- first high index plastic
-best for children - very light weight
- good for high rx
- UV protection 380nm
- thinner than CR-39
- can be surfaced thinner
-safe- doesn’t shatter - scratches easily
- lower Abbe
-more expensive than CR-39 - less precise optics
- hard time tinting
Polycarbonate
- best for rx -10.00 +
- highest = extra thin
- scratch resistant
- 1.6 abbe/ better than plastic
- best optics
- heavy
- high index= high cost
-higer indices can’t be tempered
High index glass
- impact resistant= lens hardened
- heat OR chemicals
- CR-39 cannot be tempered
- Trivex and poly most impact resistant
-1)Heat = extreme hot> rapid cooling (air blower) - if scratched lose impact resistance
- can’t use with high index glass
2) chemical = soaking in molten salts - takes 16 hours
- lens stronger than heat method
Impact Resistance/Safety
- one power present over entire lens
- distance
- reading only
- intermediate only
Single Vision lens
- distorted peripheral in high Rx
- looks more natural/ better cosmetics
- flatter curves
- wider peripheral vision
- fits better into frames
- less weight
- large eyes (plus Rx)/ small eyes (minus Rx)
- all PAL are aspheric
- first made for apkakic post-cat px (1960)
Aspheric Lenses
-gradually flatten toward edge of lens
- lighter weight/ less buldge
Plus Aspheric Lenses
- gradually steepen towards edge of lens
- thinner edges
- reduces distortion of unwanted astig (off axis)
- eye look less reduced in size
Minus Aspheric Lenses
- great for high cyl Rx
- Aspheric @ back = optimize meridian powers> thinner edges than @ front of lens
- even wider peripheral view
- px eyes appear more normal size/shape
- PAL available
-50% thinner & lighter - better optics
Atoric Lenses
- prism 2 diff methods (OC move & grinding)
- Aspheric/Atoric = grinding/surfacing only
- OC must stay @ pupil
Prism with Aspheric/Atoric
s - check lens countour (flat =Atoric)
- check power (high rx/thin lens= aspheric)
- lens clock (change in curve readings= aspheric)
-amsler grid (curved = sphere),
(flat = atoric/aspheric)
Indentifying Aspheric/Atoric len
- adjust frame before measurements
-MPR height ratio to pantoscopic tilt (1mm lower from pupil every 2* tilt)
-MRP not lower than 5mm below pupil - Small frames should be used
- pupil should be within 3mm from frame geometric center horizontal/vertical
-pantoscopic angle not more than 10* - use monocular PD
- no more than 13mm vertex distance
- short vertex reduces magnified eyes
- cant decenter OC, prism must be ground in
Dispensing tips Atoric/Aspheric
- hyperopes +2.00 ^ (try aspheric)
- myopes -2.00 ^ ( try atoric)
- Astig -2.00 (try atoric)
- within +2.00 to -2.00 (no need for atoric)
-mix mid/high index with atoric/aspheric= thin lens
Lens selection guide
- means power covers full lens surface
- for +10.00 D and up
-must be aspheric - highest index possible
- choosing small rounded frames
- consider lenticular
Full Field Lens
- lens has power only in center 40mm
- rest of lens “carrier” = stays thin/no power
- reduces weight and thickness in high Rx
- still used for complications in cat sx left aphakic
- only available in CR-39
- 1960s “ welsh 4-drop”
- 4 diopters of flattening between edge and center
- lens looks like fried egg
Lenticular
- small round frames
- take PD and match with frame PD (eyesize+bridge)
- 2.00 to -4.00 (low) = mid index
-4.00 to -6.00 =mid/high index
-6.00 above = high index
-12.00 above try special lens (myodiscs/lenticular myodisc)
High minus lens
- very flat front surface (plano/low+/low-)
- circle minus “scoop” @ back side= power “bowl”
- outside area = carrier
- size of bowl= aperture & varies
- aperature range from 20mm-30mm around
- not cosmetically attractive
- solves weight and thickness
Myodisc lens
-myodisc lens enhanced to look better
- bowl is blended better into carrier space
-thins the edges more
-aperture ranges 28mm-42mm around
Lenticular Myodisc
- located 3mm-9mm above line MF seg
- all same height = no vertical prism
- horizontally OC is px PD
Distance OC
- near OC moved in nasally
- difference between distance PD & near PD
Seg inset
- available multiple sizes
- always measure widest of seg design (flat top)
Multifocal width
- straight top/ D-style
-28mm width most popular - fused design (glass)= line lip not felt
- one-piece (plastic) = lip ridge felt front lens
Flat Top bifocal
-kryptok
- less commonly used
- restricted field of vision
- only two diamter choices (22,24mm)
- not available in high index
- seg line less visible
-high plus Rx= reduced image jump
Round bifocal
- franklin style
- widest field of vision
- line very visible/ extends the width of entire lens
- no image jump
- thick and heavy
- not good for large frames
- no intermediate
Executive bifocal
- straight top trifocal
-most common type - less popular because PALs
-50% for intermediate power - difficult px adaptation
-intermediate ribbon depth of 8mm= intermediate tasks/jobs
Flat top trifocal
- clearer distance areas
- well-defined intermediate/reading zones
-more swimming effect periphery
Hard PAL
-less clear distance zone
-softer edges into intermediate/reading zone
- wider intermediate zone
- less swimming effect periphery
-swimming= unwanted astig
Soft PAL
- eqaul amounts astig each lens
Symmetric PAL
- pushes astig to nasal lens
- takes few days to adapt
Asymmetric PAL
- narrow space between distance/near when progressing
-vary in length/width
-1990 short corridor introduced (12mm)
Corridor
- locate 2 etched refernce markings (34mm apart)
- dot on top of markings
- below nasal etch = company logo
- below temporal etch = Add power
- every PAL has own chart
- place dots over chart> mark MRP
- includes MBS options
-Pupil height/PD can be measured via rulers
PAL template/cut out chart
- uses computer design> custom lenses
- position of wear
- can compensate frame position on face
- reduce unwated astig distortion
- require more measurements(mono distance/near PD,mono height/pupil OC, panto, vertex, frame wrap)
- 1/100th of a diopter accuracy
Freeform/digital lens
- Pink = indoor glare
- Yellow = enhances contrast (ski goggles)
- Green = sunglasses
-Brown = enhances contrast - Gray= no colour change/sunglasses
Lens tints
MASCOT
-Material
-Style
-Coating
-Tint
- plastic needs this
- keep away from heat
Scratch resistant (SR)
- 5 layers including more SR
- adheres better with SR applied first
Anti- reflection (AR)
- polycarbonate doesn’t need
- Glass doesn’t need
- photochromic doesn’t need
** CR-39 needs UV400**
UV filter
-likely for sports
- mirror
-glare
-blue blocker
Anti-fog
