From PALS To Effectivity Flashcards
Limitations of multifocals and what are pals
lenses have a limited focussing range through each of the fixed focus zones.
May no longer be sufficient to provide range of wds for older presbyopes.
younger presbyopes dont want to wear bifocal- visible divison of segment is a sign of ageing.
What are pals-
progressive addition lenses, aka varifocals or progressive power lenses aka ppls
these give continuous clear vision over all distances
PALs can be considered to have 3 zones: distance, progression, near
power is worked in the front surface and the sph or toroidal rx is on the back surface. Its normally semi-finished as cant rotate the cyl.
Owen aves what did he do
Came up with first design of progressive lens.
Came up with using two surfaces and both surfaces reduce in radius from the top to bottom. More +ve power on front surface.
Stuck them together
problem- not able to produce commercially as each lens is unique and you would need to work individual rxs on with the design so individually would be expensive.
Maitenaz- Essel later essilor two types
designed the first commercially successful progressive lens launched by Essel later Essilor.
Varilux 1- spherical sections ‘hard design’
spherical surfaces of decreasing radius of curvature towards bottom of lens, stable distance and reading areas but there was a severe blending in intermediate and near areas giving rise to large amounts of peripheral surface astigmatism.
Aspheric curve from distance and near so we get surface astigmatism. For aspheric we want surface astigmatism to get rid of OAE but for varis we dont want that we want the powers to change spherically so we need to get rid of this.
Varilux 2- aspheric sections- ‘soft design’
elliptical surfaces for distance and near so spread out blending so bit more in distance as well so not as much peripheral astigmatism. People got on with these more, easier and quicker to adapt to. Reduced swimming feeling.
Conic sections of varying asphericity, reduced peripheral surface astig and near portion is narrower than for hard design and some surface astig in distance.
Comparing Aves and v1 and v2 again
Aves- the change is gradual no stable distance and near areas. Gradual. No distinct wide area for distance and reading. Had to make individually for all pxs, not commercial.
V1- wide area for distances, stable distance and reading areas, steep change in power, more astigmatism
V2- gradual change in power compared to v1, not as wide reading and distance narrower, doesnt reach full progression till further down progression, slower rate of change. Less astigmatism and swimming feeling people got on with these. Conic sections of varying asphericity
The design of a pal what do we need and what is the purpose fo a pal
need a length in corridor to fit that changing power in, not too long as they want to read at bottom also not too short as you have to fit that power in. DP at top, intermediate and np underneath and aberrations on the sides.
purpose of progressive lens feature- stable distance at top, stable near at bottom, progressive corridor connecting the two stable power areas.
Complete invisibility, no lines or ridges
generally progression is worked on front surface of a lens- semi finished lens. Finished rx is worked on back surface= sph/toric
Design of a pal with the power and the add
The design of pal is determined by what happens in the progression zone. The power could change in a linear way, we assume this for calculations but in actual fact, power changes quadratically.
For a linear progression of 0.133D/mm starting from oc (prism checking point). 10mm below the oc the power of the addition will be 10 times 0.133= 1.33D.
Add= length of linear progression zone times progression- D/mm
As the add increases the rate of change of power will increase if the progression length remains constant.
there will be induced prism at the NVP= P=Fc. F= total power at nvp and c= distance from optical centre (or prism checking point if no prism worked on)
How are pals made whats the idea behind them
Explain with the intermediate
Imagine PAL is two spherical surfaces with different radii. Larger radius for distance portion smaller radius for near portion.
If stuck together would be like an e style seg w visible dividing line, instead they are joined by a third aspheric surface- intermediate- where the radius of curvature decreases continuously from DP to NP and the radius at the top exactly matches DP and radius at bottom exactly matches NP. This produces an ovoid surface with no dividing lines.
How are they made- what is the first main determinant
And explain about surface astigmatism do we want it
complex aspheric surfaces to give you that change in power, only possible to produce with computer numeric control (CNC) grinding machines.
the rate of change of curvature is determined by a mathematical formula. Formula is the first main determinant of the optical performance of a PAL design.
2) power progression formula with p value gives us required tangential error we want but if we plot a field diagram of resultant sag and tang powers, we see tangential power gives required addition at each point but sag and tang powers do not match therefore we have surface astigmatism. For pals we want a spherical change in power along the progression and surface astigmatism is unwanted.
so what do they need to do to get rid of it
radius of curvature in tangential and saggital powers will not be the same. Lens designers must match them to eliminate unwanted astigmatism along the meridian line. This pushes surface astigmatism into the periphery.
The way in which powers are matched determines the amount of peripheral surface astigmatism and is the second main determinant of optical performance of a pal design.
The level of surface astig is proportional to the rate of change of power which increases with an increase in addition and a decrease in progression length.
Eliminating higher levels of astigmatism along the meridian line leads to higher resultant peripheral surface astigmatism as you pushed it out and smaller useful intermediate and near areas. Really you want less surface astig at the start so choose v complex equation so you have to push less over. Best designs are complicated and difficult to make and expensive.
Explain what the corridor of clear vision is and if you have a low add will you get away with cheaper designs
The corridor of clear vision is often taken to be the area where astig does not exceed 1.00D. More complex designs allows the lens designer to push the aberrations further out towards the periphery giving wider intermediate zones and larger reading areas.
If you have a low add you might get away with cheaper design as not as much astig to get rid of as rate of change of power wont be as high along the meridian so not a lot of peripheral astig as not a lot to be pushed over into the periphery with lower powers. And esp if you have a longer corridoor then the rate of change of power wont have to be as high. Rate of change of power increases w increase in addition and decrease in progression length.
How to measure the amount of astigmatism
Talk about the two maps of two diff types of varis
rotlex progressive lens mapping equipment- computer controlled mapping device that maps the power of the lens. Ancient+modern maps. Modern are no longer referred to as hard or soft though.
So called ‘hard design’- Varilux 1 has unwanted cyl
varilux 2 soft- can see more of a gradual change with soft, less unwanted cyl but also smaller reading areas.
note- can also do mean sphere plots showing power change.
ISO cylinder plot or mean sphere plot.
As add increases
Astigmatism increases so iso cylinder gets more together and clear reading area gets smaller. So bif to vari 3D lots to adapt to.
Iso cyl plot of a lens with the same design and progression length showing how astigmatism increases with increasing add.
better for pxs to get started w varis earlier with lower adds to get used to surface astigmatism and they wont notice it as much fi they started off with their add being higher.
Comparison of PAL designs what is intolerance often due to
Can be very difficult as diff companies use diff measurement techniques and apply diff standards to their lenses.
One company may take their corridor width to where astigmatism is less than 1.00D and another less than 1.5D. Moulds are often used of the Same design by companies when they make other claims for their lenses.
We only know if a design works for a px when they try it. Most often intolerance to PALs is due to poor dispensing or poor fitting.
Short corridoor pals vs conventionals depth and everything
conventional pals require a frame deep enough, depth of around 18mm, long enough to keep rate of change of power down and good reading and allows the progression to reach the full addition. This is minimum depth for each design. This limits the choice of frames for PAL wearers.
Short corridor pal lenses are available to fit into narrower frames with a minimum depth of around 14mm but since astigmatism increases as corridor length decreases these lenses are more difficult for px to adapt to esp for higher adds. Px should be warned ab limitations of these lenses and advised advantages of choosing a deeper frame. Rate of change of power greater in these so peripheral astigmatism is greater and they get a narrower reading area. Full reading rx is reached at a point close to the distance portion and all the reading area fits into the frame.
If you have more depth than you need, the reading area will just be higher up in the lens.
Occupational pals
general term- lens made to suit an occupation
generally designed for intermediate and near working distances only so no or very small distance rx. More comfortable as px dont have to lift their head to look at screens, distance area at top small one not suitable for driving.
For these as its just two wds, rate of change of power is decreased as intermediate to near so less peripheral astigmatism as well and wider reading and intermediate areas as less getting pushed out. Fitting procedure diff for diff manufacturers. Establish range of wds your px needs and choose the lens accordingly.
Free form lenses
allows designer limitless options in their designs so can tailor lens surface for each px rx. Can include control of magnification and differential prism for anisometropia. Best form lenses.
Each point given 3d coordinate rather than formula controlling x/y/z
produced on concave rear surface of lens along with cyl correction on the front surface being aspherical or spherical, some even use both surfaces to produce the power progression.
individual design for specific back vertex distance, pantoscopic tilt and dihedral angle when designing lenses for these individuals. Extra measurements must be taken when dispensing.
Main limitation of these lenses= cost
free form technology not limited to varis
More popular than old compensating methods eg slab off bi prism and iseikonic lenses
by matching peripheral astig for right and left lenes can reduce swimming effect
The swimming effect
experienced with pals when looking through periphery, can be a problem walking up and down stairs- greater with higher adds= greater periph astig
or large diff in rate of change of astigmatism in the right and left lens as the wearer moves their eyes from central gaze causing diff amounts of mag= causes diff in the perceived velocity and objects appear to change direction. Apparent change in direction is diff for each eye producing swimming effect
wearer adapts more quickly to low add or smaller changes in add
Cosmetic prism thinning
base down prism is worked on the concave surface along with the final rx to equalise the edge thickness of the finished lens, prism will now be equal in both eyes so no differential prism.
Top edge needs to be fat to get knife edge at bottom so prism will create knife edge at both ends, no differential prism. Cosmetic
often find some vertical prism at prism checking point. If the same in right and left= prism thinning. If any difference in vertical prism right and left eye- should prescribe vertical prism.
Dispensing pals
mark heights at centre of pupil at eye level when looking straight ahead into distance.
Frame should be adjusted first and confirm enough depth for the PAL design.
Directly face px eye level, natural head position.
px fixates practitioners left eye when marking right pupil and right eye when marking left pupil. Remove frame and replace and recheck marks.
Verifying pal rx and fit
use permenant engraved markings and a manufacturers template to remark the lenses as above in lens markings
distance checking point is the circle
prism checking point is the dot
near ceiling point is the N on the lens marking diagram
alternatives to pals- lens systems, diffractive lenses, liquid crystal lenses, gradient index materials, deformable lenses.
1)What are the two main factors which determine the design of a conventional PAL lens-
The choice of aspheric surface to produce the power progression, and the method by which the astigmatism along the meridian line is eliminated so formula
2) What does an iso cylinder plot tell us and why a re these often not a useful method when comparing PAL designs
An iso cylinder plot gives us a colour coded contour plot of the areas on the lens surface which have the same range of astigmatism. They are not always good comparison of lens designs as the methods of measuring astigmatism can vary from company to company. The plots may also be misleading if the colour ranges do not match or if the first contour on one plot represents a higher level of astigmatism than the first contour on another plot.
3) Why are short corridor PAL lenses not advisable for high additions
as surface astigmatism increases as the add increases and the corridor length decreases and you get a faster rate in change of power. The combination of a high add with short corrridoor= high levels of peripheral astigmatism and narrow intermediate and reading areas.
4) why do occupational PAL lenses generally give wider reading and intermediate zones than conventional PALS-
as there is only a small power change over the progression zone and this lower rate of change in power over the length of the corridor leads to less astigmatism in the periphery as less would then need to be pushed over, which results in wider zones of clear vision at near and intermediate.
5) What are the differences between conventional and free form PAL designs
conventional pals use a formula to determine the shape of the lens surface but for free form lenses each point on the lens surface can be determined individually.
Conventional pals are produced on the front surface, free form pals are often produced on the back surfaces or both surfaces, the pxs individual rx is often included in the design.
conventional pals usually only require mono cd, and height measurements. Free form PALs require BVD, pantascopic tilt, and sometimes the dihedral angle. Free form can be tailor made to the individuals rx and to the frame fitting but conventional pals hase the same design for a range of rxs.
6) Explain why pal wearers sometimes get a swimming effect in vision
as varifocal wearer looks through the edge of the lens the rate of change of astigmatism will differ in the right and left lens which will cause a difference in the magnification which is perceived as a difference in velocity which causes an apparent change of direction which is different for each eye.
Why do we often find vertical prism at the prism checking point, why is it usually base down prism and how will you know if its prescribed prism-
and about prism thinning and six of intoleranaece with it
due to prism thinning which is applied purely for cosmetic reasons to balance the edge thickness at the top and bottom of the lenses.
it is base down as the power of the lens becomes increasingly positive as we move towards the bottom of the lens which results in a reduced edge thickness. Applying base down prism will increase the edge thickness to compensate for the decrease due to increased positive power. Reduced edge thickness is generally desired for cosmetic reasons, but for lenses that have higher positive power towards the bottom, reducing edge thickness too much can lead to an imbalance in appearance between the top and bottom of the lenses. This can create an unbalanced look, with the bottom of the lens appearing significantly thinner than the top.To counteract this imbalance and achieve a more uniform appearance, base down prism can be applied. Base down prism increases the edge thickness at the bottom of the lens, helping to balance out the thickness between the top and bottom. So, while reducing edge thickness is generally preferred, in certain cases, like those involving high positive power at the bottom of the lens, it may be necessary to apply base down prism to maintain aesthetic balance.
How will you know if it’s prescribed prism- if its the same right and left then it isnt prescribed prism, vertically prescribed prism will be different right and left, you will have some differential prism. Otherwise there is baso no differential prism= prism thinning. So any differential prism= prescribed.
Prism thinning is applied automatically and does not have to be ordered, very occasionally a px may have a non tolerance to the prism thinning and the dispenser may ask for it to be cancelled. Symptoms of prism thinning non tolerance- exaggerated head movement for distance and near, intermediate blurred, narrow clear distance, strain or pulling. Or just not right. check by applying equal and opposite prism over the spectacles to see if there is any improvement= to cancel it out to see if it is intolerance so could just put some base up.
Dispensing scenario- Mr K, age 55. Rx= +5.00/-2.00 X 160 LE- +5.25/-2.00 X 15 add 2.25
bank manager, hobbies restoring vintage cars, golf, snooker
rx- thinning, BVD, anisometropia= remember your standard axes!! Re= 160 is symmetrical with about 20 in left eye. Remember this
Occupation- lens choice, standard varifocals occupational, free form. Bifocal. Single vision
frame choice- minimum depth, short corridor= not good optically. Edge thickness, frame shape= cyl is baso horizontal which means the widest edges will be top and bottom if spherical lens, narrow frame would reduce the edge top and bottom. Balance this with minimum depth for varifocal.
Hobbies lens choice, head position and option of reading area in relation to near vision requirements. Will he be able to look through near vision bit when looking under cars probably not. Cheap Sv pair. Safety specs as well. Wds what is involved where there head may be etc. think of problems they may encounter.
measurements- what facial measurements would you need to take for each dispense.
If differential vertical prism is base down how can we compensate for differential vertical prism in varifocals
We can compensate for the differential prism by slabbing off base down prism in the left near portion to give base up in the left near portion. Similar to bi prism bifocal
Or most likely today would use free form lenses to allow us to control differential prism and even magnification within the design.
PD
Why are they required =, how to ensure accurate measurements
If the practitioners eye is 40cm away from spectacle plane what is the approx error for every mm so for an 8mm difference whats the error if closer
And mono pds and when would you do them
For strabismus what do we do
required so optical centres are placed in front of pupil centres so no unwanted prismatic effect. Unwanted prism- visual discomfort or tired eyes, diplopia.
To ensure accurate measurements- arms length to minimise errors due to difference in pxs or practitioners pd. secure ruler on pxs forehead resting on bridge of nose. Close re and instruct px to look at le, place ruler so zero mark and re make sure 0 doesnt move and read measurement off= distance pd. Square on to px, level with px, no movement or steady hands, if fails with child go from the canthus of the eyes.
if pxs pd is a lot smaller than practitioners= over estimation. If the practitioner’s eye is 40cm from spectacle plane there will be an approx error of 1/16th of a mm for every mm difference in pd between patient and practitioner. So for an 8mm difference= 0.5 error. So divide by 16. If practitioner closer= angle of rotation of pxs eye will be greater so get a greater error thats why arms length is good. Further away= less error or overestimation due to difference. So if theres a diff greater than 8mm so you’re further back not as much difference but if closer more difference so bigger error.
Mono pds- centre ruler on bridge of nose and measure to centre of each pupil. PALs and aspherics generally measured by dotting centre of pupil on the dummy lens.
Assymetry, strabismus or amblyopia, pals, aspheric lenses, anisometropia. Progressive and aspheric also measure pupil heights. For strabismus occlude the pxs other eye to ensure correct fixation.
Near centration distance
we do not measure near pds, instead measure near centration distance in the plane of the spectacles but frames must be fitted first.
Place an object eg pen centrally and close to one of your eyes, at the pxs near working distance and ask the px to fixate it. Close your other eye and place the ruler against the frame and line up the zero point in the RE. Switch to the pxs le and read off the measurement at the relevant measurement point.
Pupillometer how does it work
uses first purkinje image to take pds, avoids inaccuracies due to parallax errors.
Px holds it like binoculars and looks through two eye pieces towards a target and the practitioner looks through the single eyepiece on the other end of it, adjusts the sliders to move the line targets so they pass through the purkinje image which marks the centre of the pupils.
Binocular and mono pds can be taken at various wds. Each eye can be occluded so mono pds can be taken as well for pxs with strabismus.
Multifocals and PALS What do you have to do
Ensure correct relative positioning, square on eyes level natural head position, frame fitted as worn- including pantoscopic and dihedral angles. Binocular pds sufficient unless aspheric or any of those other reasons above.
Mark segment top positions- generally at lower limbus or lower lid. Measure relative to the HCL.
Lower segment top- less annoyance better when walking down stairs eg d segs. Higher stp- if used mainly for near. Ask if existing wearers are happy.
Trifocals are generally fitted w top of the segment at the bottom of pupil.
pantoscopic angle between 8-12 degrees. Dot centre of pxs pupil
remove frame, remark refit and double check. Heights and mono pds measured using frame ruler or a manufacturers varifocal template
british standard heights measured relative to hcl. In practice often measured relative to lowest point on frame.
BVD british standards. What are the diff tools we have to measure it
The Varilux measuring gauge
digital methods like zeiss visufit (avatar) or apps eg Hoya visureal
distometers- tool to measure BVD. Place pad on the end of the gauge placed against the closed eyelid, plunger on end pushed in until the other arm of the gauge touches back surface of the lens and then there is a converter scale to work our rx adjustment as well.
British standards for bvd and error of measuring bvd with ruler
British standards say measure for rx over +-/5.00D
Distance between back vertex of lens and corneal apex is measured along line of sight perpendicular to spectacle front.
Measurement must be taken with 0 pantoscopic tilt, instruct px to tilt head until lens plane is perpendicular to floor and instruct to look straight ahead.
difficult if frame is thick or if the edge of the lens is thick
need to estimate position of the back vertex,
errors will occur if measurement is not taken along the optical axis of lens.
Accuracy can be improved by taking the measurement from the front surface and then deducting the centre thickness measured with a thickness gauge. BVD of glazed specs may differ if the base curve differs from the curve of the dummy lenses used to measure the BVD. Slightly.
Pantoscopic angle what is it and how do we measure it what instruments exist
important particularly for varifocal wearers+ free form as they are individually paying for it.
Pantoscopic angle is the angle in vertical plane between optical axis of the spectacle lens and the visual axis of the eye in the primary position (assumed to be horizontal)
centre of rotation distance remains constant as the eye rotates which ensures constant BVD or effective power.
Optical centres dropped 1mm for every 2 degrees of pantoscopic angle so that the optical axis passes through the centre of rotation- already incorporated in PALS if angle between 8-12 degrees. Fitting cross 4mm above the optical centre.
Measurement of pantoscopic angle- Zeiss pantoscopic gauge, E sill or Varilux measuring gauge can be used to measure BVD, panto, and dihedral angle.
Dihedral angle
Frame wrap. Horizontal angle of the lens. What I change when i heat glasses to make tighter i bring them in.
Free form progressive lenses- gives optimum performance in the as-worn position.
Mono pds and heights, BVD, pantoscopic angle, dihedral angle
manufacturers provide fitting kit with measuring gauge to help take required measurement.
Fairbanks’s facial measurement ruler and what can you measure with it
-apical radius
Lots of things on it, used to take measurements for making a frame from scratch, not commonly used, HES maybe.
For custom designed frames you need-
PD, apical radius, head width, bridge projection, bridge height, splays angle, frontal angle, DBR at 10 below crest and 15 below crest, angle of crest.
Apical radius- find the cutout that fits the bridge of the nose the best, if in between measurements choose the slightly looser one= one w the greatest radius. So various things n this ruler to help design the perfect frame.
Head width- measured just above ears using callipers or using rulers measure the width of head and frame head width is side to side at the ear point
Temple width- measured 25cm from the front of the frame or 25mm above the ear. Short end of arm against temple and separate ruler against temple on other side and note reading
Bridge projection- in line with lens plane, slider just clears lashes that’s how far your bridge comes out.
Bridge height- place ruler on crest of nose, place scale over lower limbus
Frontal angle- needle, rotates to see the angle of the nose. Line up centre line with centre part of nose, rotate pointer and measure angle. Equivalent frame measurement is frontal angle of pad.
Splay angle- Line up end of ruler against pxs nose and measure how much the nose splays out. Centre line w centre of nose and ask px to tilt head down.
