From Low Vision 1 To

Question 1

Who needs low vision aids and how does magnification help people see.

Purpose of them and 3 ways of increasing retinal image size

Answer 1

Those who can’t resolve retinal image eventhough it is focussed on the retina eg elderly pxs with armd or normal sighted people that need to see fine details eg jewellers

Mag helps people see as it makes the image bigger so there are enough photoreceptors to cover the image. Even w damaged photoreceptors.

Purpose of lva- to increase retinal image size so px can resolve details they cannot without LVA.
-increasing relative object size
-reducing relative viewing distance (simple magnifiers)
-increase angular subtense by optical means (telescope next lec)

Question 2

Wha is magnification

Answer 2

Ratio of retinal image size. Use object size to find out mag.

M= new retinal image size/old object size
Or old object distance/ new object distance

Old object distance= reciprocal of add (pxs habitual wd) and new object distance is from the focal length of magnifier.
Theta’= angular subtense

(M= theta’/theta (new object size/ old object size= h2/h1)
M=l1/l2= original wd/ new object distance (focal length of magnifier)

Mag varies from person to person, same person wont get samemag. Cant measure retinal iamge size but measure object distance or size to predict mags for individuals. Mag is a combination of increased object size and reduced viewing distance.

Question 3

increasing object size- larger print
Also what ones reducing viewing distance do

Answer 3

h1= angular subtense of theta
now double object size to h2 = doubles angular subtense
image size doubles as little error, ignored curved as retina is curved
doubling object doubles the retinal image size
magnification= new angular subtense/ old one. New object size/ old So mag1= h2/h1

Halving distance doubles angular subtense doubling retinal image size

Question 4

Reducing viewing distance so what is total mag
And what must you instruct your pxs to do

Answer 4

subtense doubling retinal image size.
M2= l1/l2. L1=original working distance. L2= focal length of the magnifier, old object distance/ new object distance (The old object distance is a reciprocal of the pxs add so their habitual wd, new from focal length of magnifier) This what simple magnifiers do.

mag is the product of the two, doubling size of object and reducing the viewing distance. So 4 times mag all together as it doubles it for each way.

Instruct px to use them with distance correction and they need to hold it at their focal length away. Old object distance is the users habitual wd so reciprocal of their add or accom they use. Bit of variation as we do have some depth of focus as well. Eg 3D add holding at 33cm etc.
if magnifier not held at focal length the mag would also depend on accom or near add being used as if it isnt held at the focal length we’ll have diverging light coming out of magnifier so you’d either accomodate or use an add. this diverging light would result in smaller and less clear image so need more accom or add to compensate for reduced mag.

Question 5

Problems with magnifiers and simple magnification nominal mag

Answer 5

Problem w mag- mag marked on magnifier not always mag the px gets. Depends on pxs habitual reading distance, where they hold the magnifier and if theres any near add or accom.

Simple magnifier- nominal magnification-
put number on mag for general idea. Nominal magnification assumes the habitual working distance of everyone is 25cm. Assumes no accom and very small distance to the eye d, assumes magnifier is held at the focal length.

Question 6

Theta without magnifier, with magnifier and also nominal magnification calculations

Answer 6

Theta without magnifier= h/q assuming small angles. (Angular mag= H is the height of the image formed by the magnifier and q is the distance of the object from the magnifier- remember -ve as in front real object)
Magnifier at focal length from object imaged formed at infinity. Theta’ (angular mag achieved by the magnifier)= h/fm. H is the height and fm is focal length of magnifier which is shorter than habitual working distance as want to bring things closer. As at infinity no extra add or accom so using distance rx.

Magnification= the power of magnifying lens/ power of original reading spectacle add (assumed to be 4)
So the nominal magnification= M=Fm/4. Fm = power of the magnifying lens. 4. So M times 4= power

Question 7

Putting a simple magnifier closer than the focal length

Answer 7

Closer objects will need add or accom.

The light is diverging so px will need accom or reading add. Image size varies with lens to object distance. Image size varies with lens to eye distance.

If the lens is moved closer to the object this reduces mag due to the magnifier alone and we will need to consider addition/ accom effects on magnification as well.

Question 8

Another equation for theta
What is the max wd for a magnifier
and also m with Q

Answer 8

Theta’= h’/ (d-l)
maximum mag is when d=0 and accom/addition reduces as d increases.

Max wd for magnifier- focal length. If object held further away than focal length light emerging from magnifier= convergent and the eye would focus that light in front of retina= blurred image if magnifier is further away from focal length.

M= -l’q/l(d-l). Q is the habitual working distance without a magnifier
so mag is dependant on the habitual working distance and the distance from the lens to the eye as well as object distance and image distance

Question 9

Trade magnification or maximum magnification

Answer 9

some manufacturers define magnification assuming the user is wearing their habitual 4.00D addition to view the image which is also at 25cm. Or viewer must be exerting 4.00D of accom.

Another value manufactures use to mark magnifiers
assumes wearer is wearing reading specs w magnifier held in contact with eye
Habitual and new image distance is 25cm
extra power must be taken into account when calculating lens sytem power
M= Fmag/4+1= Nominal magnification+1

Question 10

E equivalent power- which F value should we use

Answer 10

what F value should we use- thin lenses all equal but most magnifiers are thick lenses so should use equivalent power.
This takes into account principle planes so principle plane to the principal focus.
Positive bi convex lens- bvp would be greater than equivalent power
can’t measure using foci meter, need to work out surface powers with lens measure and go from there. Manufacturers often quote the bvp so stated mag is higher.

Feq= F1+F2-t/n times F1F2
Or
Fe= Fm+ Fa- d times Fm times Fa

if magnifier is held at the focal length away from the eye then d is the focal length of the magnifier which means that Fe=Fm. Makes sense as no accom or add then
d= separation of the magnifier and spectacle len or eye
Fm= power of the magnifier
Fa= power of the add or accom

Question 11

Simple magnifier working distance max and min what is Y what is D
What is linear fov
How do get greatest fov
And low power and high power

Answer 11

mag working distance- focal length. If further away then image is blurry as it forms in front of the retina.
Minimum working distance lens to eye- depends on users accomodation as an object inside the focal point will give divergent emerging rays.

Y is linear fov, D is the diameter of lens.
Linear fov= defined as the angle subtended by the lens periphery at the image of the eyes entrance pupil.

To get the greatest fov-
large diameter
as close to eye as possible (d)
lowest power Fm

higher power- smaller diameter as lens diameter is limited by weight or thickness and peripheral aberrations and manufacturing as greater surface curvature.

Question 12

Aberrations

Answer 12

higher power greater aberrations
high plus lenses- best form lenses usually only minimise aberrations for lenses up to +7.00D
aberrations reduced by using aspheric surfaces and or lens system

Question 13

Hand magnifiers advantages and disadvantages

Answer 13

illuminated hand held or just hand held
can be used with right and left hand
convenient and familiar
Can be illuminated
lightweight, compact, portable
cheap
eye ego magnifier distance can vary without affecting the magnification if the object is at the first focal point allowing for adjustments in total working distance

disadvantages- long eye to magnifier distances=smaller fov
easily obtainable without instruction so may be using it wrong
need a steady hand
internal illumination can make it too heavy
only one free hand for other tasks unless mounted

Question 14

Stand magnifiers tell me about h them. Same equation

Why not make them to focal length

Where is the image formed

Answer 14

pxs with unsteady hands, need two hands
height of stand is typically less than the focal length of the lens so emergent light is diverging.

Why not make them to focal length- if sun goes through them paper can catch fire

Px must accomodate or use add for the image distance
this creates a magnifying system made from the combination of the magnifier and reading add or accomodation. Need to make them special glasses just for this often.e
Fe=Fm+Fa-d times Fm times Fa
Mag is also greatest when eye or spec lens is in contact with the lens this requires the highest reading add which increases mag.

The finite image distance means the image formed below the magnifier and user must be corrected or accomodate for this distance so we need to know the reading add needed to neutralise the divergence.

Question 15

Measuring the exit vergence of stand magnifiers

Answer 15

the telescope is focused on infinity so on a distant object and then its used to observe the image through the magnifier to ensure the image will only be in focus when the emerging rays from the magnifier are parallel.

Trial lenses are added until the image is clear. The exit vergence is opposite to the power of the trial lens. The trial lens power is the power of the reading add needed if the user views the image with the eye close to the magnifier.

Question 16

Examples of stand mags from lecture

Answer 16

illuminated stand magnifiers, tilt stand magnifiers, easy view hangs around neck and propped against chest- low mag as large lens. Bright field magnifier

Question 17

advantages and disadvantage os stand mags

Answer 17

stable working distance, advantageous for high power lenses or when px unsteady hands
hands free- useful for low power lenses when plenty of working room under lens eg writing, drawing, crafts etc
can also have built in illumination

disadvantages- need reading add specifically for use with the magnifier. The user must use the magnifier at the correct distance from their eyes to focus.
High powered stand magnifiers dont leave enough room under for tasks to be performed as many have stands that completely surrounds the lens allowing no access beneath and requiring internal illumination if stand not transprent
internal illumination can make stand

Question 18

1)What are the 3 ways of increasing the retinal image size with low vision aids

Answer 18

increasing object size

reducing viewing distance- needs increased accom/ reading add. If the original working distance is 50cm= 2.00D of add or accom and 2 times mag is needed then the object will have to be moved to 25cm with 4.00D add/ accom. If however the original working distance is 25cm the object will have to be moved to 12.5cm with 8.00D of accommodation or an 8.00D add to give 2 times magnification. Thats a lot they need to focus it there.

increasing angualr subtense by optical means- telescopes

Question 19

2) How do simple magnifiers work as low vision aids-

Answer 19

Simple magnifiers are high plus lenses which increase the retinal image size by decreasing the working distance at which the near task is performed. The increased retinal image size means the user is able to resolve detail they would not be able to resolve while holding the object at the original working distance.
But you have to compare the situ with the magnifier to the original situ without so we cannot give a magnifier a definitive value for the mag a user will as it depends on users original or habitual working distance and on how the magnifier is used.

If you have someone who reads at 25cm and they want 2 times mag. 25/2= 12.5 halving viewing distance doubles the mag. If using w distance correction this would be equal to the mag marked on the magnifier. they will get that nominal mag. Would need 4.00D accom or add.

If someone reading at 50cm tall and long arms, 2 times mag they would only have to move their wd to 25cm. Original wd 50cm= 2.00D of accom or add

someone w 2.00D add to help them see at 2 times mag= 4.00D
someone with 4.00D add 2 times mag= 8.00D add allows them to focus at 12.5 cm
50cm. If used that same magnifier as 25cm person if they held it at 12.5cm they would get 4.00D magnification whereas other person got 2 times mag. (Or just forget this and doing calculations Nom mag=Fm/4 1/0.08 is 12.5)

Question 20

3) What is nominal magnification-
what is trade mag

Max and min wd and whats does equivalent power do

Answer 20

Nominal magnification is the value of magnification often marked on simple magnifying lenses. It is the mag we expect if all assumptions are met. The assumptions that nominal mag are based on are that the users habitual wd is 25cm and the magnifier is held at its focal length away from the object.
M=h/q. (2=0.025 as it assumes the habitual working distance is 25cm) assumes magnifier is held at its focal length from the object- no add or accom.
M=1/Fm

trade magnification- assumes that the image is also at 25cm and so the user needs 4.00D accom or add, assumes the spectacle or eye to magnifier distance is 0 so the power of the lens system is the sum of the addition and the magnification.Fm+4/4
So trade mag= Fm/4+1

Equivalent power works out the power relative to the principle plane of the lens system. Reciprocal; pf the distance from that plane to the image. If that distance is greater than the distance from the back vertex to the image then principal power is less than bvp.

Max wd= focal length of the lens.
Min wd= hold it closer magnifier closer to object diverging rays out of mag lens so need to accom. Sot his depends on how much accom user has.

Question 21

2) Why are stand magnifiers used with an addition whereas handheld magnifiers tend to be used with the distance rx-

Answer 21

stand magnifiers have a height which is less than their focal length which means that light emerging from the magnifier is diverging as the image is closer than infinity. To focus on the image the user will need an addition or needs to accomodate.

Px’s normally instructed to wear their distance rx when using handheld and will be shown where to hold them. They are not expected to know what the focal length of their magnifier is they find it as it will be the only position which gives them a clear image if they have no accomodation.

Question 22

What do telescopic magnifying systems do, what are they used for and what are the two types
What type of system are there and what does that mean

Answer 22

Telescopic magnifying system change the angular subtense without having to change the working distance.

Used for distant and intermediate tasks as well as near tasks. But small fov means they cannot easily be used when mobile so walking around.

Two types-
astronomical/ keplerian: two positive lenses
Galilean- positive and negative lens
both afocal systems so parallel light comes in and parallel light out

Question 23

Astronomical and Galilean main difference

Answer 23

Astronomical or keplerian- positive eyepiece negative first foacl length so neg mag inverted image

Galilean- negative eyepiece, positive first focal length so positive mag and upright

Astronomical- if fe is positive d=fo’-fe but fe is neg so — makes a plus so distance increases so longer than astronomical.
Galilean- fe is positive so you minus them

Question 24

Astronomical how do they work and where do we need the lenses to be for an afocal system
What is fov determined by and whats it like in this telescope

What is the mag equation

Answer 24

Objective: positive lens- has second focal length of 0 fo’ to get parallel light coming out and then the objective forms the image at the second principle focal point. Parallel light always forms image at F’. So we need that image to be at the first principle point so light coming out of the eyepiece will be parallel. So position the eyepiece so its at the first principle focal length away from the image formed by objective= parallel light emerging from telescope.

Keplerian- inverted image, minus sign tells us the image is inverted
fov is determined by the size of exit pupil- exit view is close to the eyepiece in Kepler- closer to eyepiece= better fov. Can get right up to the eyepiece at second principle focal point.
M= theta’/theta. M=-fo’/fe’=-Fe/Fo
assume thin lenses so assume that fo=fo’ and fe=-fe’
positive eyepiece, negative first focal length, neg mag= inverted

Question 25

Galilean same question. What’s the iamge height and fe’. Explain about fov and let’s talk about distance for both Galilean and astronomical

Answer 25

negative eyepiece, positive first focal length, pos mag= upright.
second lens placed so the image forms at focal point of the eyepiece. Negative eyepiece so the image formed by the objective is at the first principle focal point of the eyepiece.

Image height still negative fe’ is also neg so ends up being the same as astronomical and theta is the same but power of the eyepiece is negative so we end up with a positive mag= upright image.

galilean is shorter than astronomical. The exit pupil is at the second principle focus somewhere inside the telescope so cant pop eye right close to the eyepiece unlike astronomical so smaller fov.

Galilean: d= fo’-fe (fe is positive) first focal length
Astronomical- d= fo’-fe (fe is negative) first focal length

Question 26

Can you accomodate through your telescope

Telescopes for intermediate or near viewing- two practical solutions

Answer 26

most people may be younger as they are tricky to use so may have accom. May need accom for intermediate or near tasks.

Telescopes for intermediate or near viewing- two practical solutions
-adding full correction for the viewing distance in front of the objective in the form of a reading cap. (Put reading cap over objective- this neutralises divergence from near object so makes rays going into the system parallel again, another lens basically so need to work out mag for lens system )

-Refocusing the telescope by increasing its length (or can refocus telescope by increasing its length though it would no longer be an afocal system if change the length of the telescope)

Question 27

Near adjustment reading cap

Answer 27

eg incident vergence of -10D can be neutralised by a reading cap of +10D. The resultant mag of the system is now the product of the magnification of the telescope and the reading cap.

Simple magnifier Fm of 25.00D is too big wouldnt want to put in specs. Could put higher add in specs but becomes too heavy and thick. Working distance bit clos as well but telescopic allows you to have larger wds with the same magnification.

Question 28

Focusing the telescope near adjustment

Answer 28

Need Fe to coincide with Fo’. Work out new separation,
Then use d value in metres with the equation

Feq= Fo +Fe -dFoFe.
Then do Feq/4 as it assumes 25cm wd if wd was different then value to divide it by would be different. (Eg 1/0.25=4)

Question 29

Telescopes FOV

Answer 29

Mag and objective lens diamter often marked on telescope. 8x20 means 8x mag and 20mm diameter objective.
greatest fov achieved when the pupil of the eye is coincident with the exit pupil which isnt possible w Galilean.

Astronomical- real exit pupil beyond the eyepiece
galilean- virtual exit pupil inside the system

Exit pupil diameter=objective lens diameter/ mag of telescope

Question 30

Measuring magnification. And what is the exit pupil

Answer 30

the exit pupil is the image of the objective aperture seen from the eyepiece

hold the telescope with eyepiece about 20cm from your eye and point it at a bright target and measure the size of the exit pupil. Measure the diameter of the objective lens. This is often marked on tege telescope with the mag.
Or by comparing a shape image

Question 31

Prismatic telescope

Answer 31

prisms shorten the telescope and make the image erect. Astronomical things are upside down which isnt good so most these things have prismatic telescope within them that turns everything the right way up.
Prisms also shorten the telescope. Mag now is still Fe/Fo. Still comes out negative value but would gibe it a positive value. Compact and handy. Prismatic monocular attached.

Question 32

Advantages of telescopic systems

Answer 32

high levels of mag can be achieved at varying wds near distant intermediate.
But small field of view and cannot be used while mobile and object of interest can be difficult to locate.

Question 33

Examples of telescopic systems

Answer 33

keeler LVA 22 near vision telescope with 5 times mag. Fixed focus. Note convergence

spectacle binoculars- Galilean telescopes. 2.0 x mag.
Distance clear lens= can adjust distance between lenses-> diff wds.

Question 34

How do telescopes work

Answer 34

Telescopes distant viewing- parallel light into telescopic system and coming back out of it=afocal system- user can use it to view distant objects while wearing their distance rx. Train timetables at a distance etc. not good when mobile as small fov.

Question 35

What would the lens sep need to be adjusted for to neutralise the accom

Answer 35

you need to move the eyepiece to a spot where it is at its principle focal length away from the image formed by the objective.

Question 36

For the system on worksheet 1,1c what other method can you use to compensate for the divergence of the emerging rays

Adv and disadv of low vision aids

Answer 36

we can use a reading cap placed over the objective with dioptric power equal and opposite to the initial vergence. This neutralises the initial divergence so we have parallel rays entering the telescope. The telescope can then be left in distance adjustment.

Remember adv disadv of all low vision aids think ab working distance, ease of use, visual field and cost. Talk to px what minimum mag do they need, what wd will they be working at, have they got steady hands, will they be able to use the aid. Instruct them on how to use it and where to hold it, do they need their distance or reading specs. How do they adjust it for diff working distances.

Question 37

In distance adjustment wha t happens with first principal focus of eyepiece and second principal focus of objective

And for calculations waht do we assume

Answer 37

The first principal focus of the eyepiece fe coincides with the second principal focus of the objective fo’

We assume thin lenses as fe=fe’ and fo=fo’ so when we work out reciprocal of Fe this gives us fe’ which is second principal focal length we just take this as opposite sign as fe=fe’ bc we want first principal focal length so remember Galilean is positive kep is neg so Galilean gets shorter and for kep you do — to get positive so you add it onto the objective focal length

Question 38

Why is it not practical for a mag to be 22.5 times in magnification

Answer 38

A normal print letter is about 2mm wide. If this was magnified by 22.5 times this would be 45mm or 4.5 cm wide which is wider than msot objective lenses so the user wont even be able to view one whole letter at a a time.

Question 39

Why dont we make telescopes with low powers of 1 objective and 5 for eyepiece as we would get mag of 5 dioptres. Why dont lva telescopes use such low power lens

Answer 39

Focal length is 1m
Focal length of eyepiece is 1/5= 0.2m -> fe’= -0.2
Distance= fo’-fe so 1- -0.2= 1.2m
Clearly not feasible for use as lva, too long

Question 40

When are two cylindrical lenses obliquely crossed

What can any pair of obliquely crossed cylinders be replaced by

Answer 40

Two cylindrical lenses are obliquely crossed if their axes are not parallel or mutually perpendicular.

Any pair of obliquely crossed cylinders can be replaced by an equivalent sphere-cylindrical combination.
When two cylinders are combined with axes obliquely crossed there is a induced spherical component S and a cylindrical one C

Question 41

When can you get obliquely crossed cylinders

Answer 41

if you get your refractive correction incorrect
if the axis of the cyl is wrong when you refract you induce obliquely crossed cylinders.
So resultant cylinders from incorrect refractive correction.

Question 42

If an eye needs a a cylinder correction of -1.00DC x 20 and is incorrectly given a correction of -0.50DC x 130

Answer 42

Resultant residual error with a spherical and cylindrical component.

Since the eye needs a correction of -1.00DC x 20 the eye itself must have a cylinder of +1.00DC x 20. Resultant combination is worked out from the +1.00

So the resultant sphere cylindrical error is the combination of the eyes cylinder and the incorrect -0.50 x 130 cylinder given.

Question 43

Why is it important to calculate the correct cyl to

Answer 43

To put on the front of a bi toric hard cl to give the correct rx

Question 44

So what is the resultant sphere cylindrical error

Answer 44

-the resultant sphero-cylindrical error will be the sum of the cylinder needed to neutralise the cylinder of the back surface plus the cylinder needed to correct for the eye’s astigmatism.

Opposite power with the same axis and combine that w the wrong correction you’ve given.

Question 45

What is a stokes lens

Answer 45

2 Plano cyls equal opposite power and you can rotate them relative to each other to get different sphere-cylindrical powers.

Mounted in holder allows them to be rotated-> sphero-cylindrical resultant power. Trial case accessory.

Question 46

Theory behind obliquely crossed cyls- and how can it be drifted

Answer 46

Theory of obliquely crossed cylinders- any combination of two or more cylinders in contact with their axes placed randomly will have the power of a single sphero-cylindrical lens.

This can be verified by placing 2 cylindrical trial lenses with their axes at an oblique angle to each other, in a focimeter which will measure two powers in mutually perpendicular meridians.

Question 47

Sum of two cyl powers equations

Answer 47

The sum of the two cylinder powers F1+F2 is equal to the sum of the two principal powers of the combination S+ (S+C)= 2S+C
F1+F2= 2S+C
Rearranging gives S= ½(F1+F2-C) or C=F1+F2-2S
this means the spherical power of the resultant combination can be easily calculated once the cylinder power has been calculated or the thvice versa

Question 48

Astigmatic decomposition method

Answer 48

any cylinder can be broken down into 2 components. Since any 2 cylinders in contact w their axes at random is equivalent to a single sphero-cylinder then any single cylinder can be replaced by two separate cylinder with axes in any specified direction.

Basically gives you a right angled triangle.

Break each component down and add them all together
Mean sphere is the same. C cos from trig.
Resultants add up all of them components

Sign of Cr is the Same as the sign of Sum of Co.
Resultant cyl
Resultant axis
Resultant sphere
Resultant sphere cyl= resultant sphere/ resultant cyl x resultant axis

Question 49

2 cyl lenses what is the angle between them

What is the sum of max and min powers

If resultant axis is minus what do we do

Minimum power we would measure if it was accurate on focimeter how do we get axis and max as well

Answer 49

2 cyl lenses angle between them always 90 degrees

Sum of max and min power= sum of 2 separate cyls . F1+F2= 2S+C

Minus axis add 180 to it.

Power on focimeter minimum= add resultant axis+90
Max= resultant axis

Question 50

Talk about eye protection- legal and work related home, spectacles goggles and face shields

Answer 50

Eye protection- legal responsibility of employer to provide these but employees responsibility to wear the specs when requested. As an optom their hobbies and things safety wise must be noted and specs may be required for them.
Work related- flying particles, chemicals, fumes, dust, radiation, home- diy, gardening, hobbies, sports, play, cars etc. optoms have a duty of care to warn pxs of possible risks and advise on protection. Drilling, grinding welding strumming working with solvents= risk of eye injuries. Sports squash and badminton and children eye protection can be recommended for monocular children.

spectacles protect eye and offer limited protection to orbital cavities
goggles to protect eyes and orbital cavities
faceshield- provide both eye and face protection eg welding

Question 51

Standard glass vs cr39

Answer 51

Standard glass- not a safety lens material, impact causes the lens to shatter into very sharp edged fragments, high index glass is more brittle than standard glass.

CR39- traditionally thought to be safe as on impact it fractures into blunt pieces, but small high energy impact can cause CR39 to fracture into sharp pieces so not suitable for some sports eg squash and badminton. Classified sd low impact by british standards.

Question 52

Examines of mechanical protection

Answer 52

laminated glass- no longer used for spectacle lenses
heat toughened glass, chemically toughened glass
CR39= limited protection
polycarbonate
trivex

Question 53

Heat toughened glass how is it made

Answer 53

glass is heated up to a high temp and then cooled really quickly by air jets or by immersion in oil which leads to a compression envelope forms making the lens tougher so its more impact resistant and can absorb more energy.

So the interior is under high tensile stress so central tension. (Rapid cooling of lens exterior and slower cooling of interior produces compression envelope around the lens cooled by immersion in oil, thinner than standard heat treated lenes)

Question 54

Chemically toughened glass how is it made

Answer 54

compression envelope produced by ion exchange at the lens surface instead of heat. Lens dipped into salt bath at high temp for 16 hours, potassium ions exchanged for lithium or sodium ions in the glass and since potassium ions are larger they induce a compressive stress in glass matrix-> comp env.

toughened glass is harder than standard glass more resistant to scratches, if it breaks now it is more likely to fracture into less harmful cubes of glass. If the opposite surface remains under compression or the force just neutralises the compression the lens doesnt fracture.

Question 55

Chemical toughened vs heat toughened

Answer 55

Chemically toughened- can be processed in large batches, no need for thicker lens, tougher lenses (larger compression envelopes) does not alter photochromic properties, no strain pattern, tougher can be made thinner, cheaper, better.

Heat toughened- processing time depends on lens weight so suitable only for small batches, thicker than standard glass, not as tough as chemically toughened, reduces activity of photochromic salts and darkens lenses, strain pattern allows confirmation that lenses have been toughened. But this most often used as easier and doesnt require storing reactive chemicals

Strain patterns can be viewed through cross polarised filter. Pos lens looks like a x in middle thick one, neg like a square in middle and then cross coming from it.

Question 56

Polycarboante

Answer 56

thermoplastic, injection moulded under high pressure and cooling, clarity is not as good as traditional moulded plastics.

Its elasticity helps it absorb energy so excellent impact resistance.
Keep it away from acetone from cleaning/ work environment which reduces the impact resistance.

Polycarbonate has a low abbé number of 30 so poor peripheral performance, higher refractive index.
Absorbs 100 percent uv less than 380
Surface scratches very easily as its soft and any coating on cr39 and polycarbonate and the application of MAR reduces resistance as well as edge pressure so lenses are held in frames a bit more looser and lens pushed out on impact. Needs special machinery and causes clogging.

Question 57

trivex

Answer 57

Expensive, new plastic lens made from a urethane based monomer, strong thin and light material with very high impact resistance equivalent to polycarbonate.

Moulded in similar way to regular plastic lenses claiming to give better clarity than for injection moulded polycarbonate.

Better abbe number of 43-45
density is much thinner
absorbs uv less than 400nm.
There is also a mid index version of this with a n of 1.60 so can be thinner .
Much better peripheral performance and absorbs all uv light including blue light so these have a yellow tinge to them, better scratch resistance compared to polycarbonate. 0.4 compared to 0.2. But cr39 has 1 scratch resistance.

Question 58

All 3 comparisons

Answer 58

Trivex, poly, cr39
Abbé number- cr39 highest. 45, 31, 58
Scratch resistant- cr39 best. 0.4, 0.2, 1.0
Impact resistance- pass, pass, fail. Cr39 fails
Density- cr39 densest. 1.11, 1.2, 1.32
Refrac index. Polycarboante highest. 1.53, 1.59, 1.49
Chemical resistance- good, poor, good. Polycarb poor
Surfacing- good, poor, excellent. Polycarb is poor
Tint rate- 1.0, 1.5, 2.0
Typical centre thickness- 1.0, 1.5, 2.0. Trivex thinnest
Uv absorption- all under 400, under 380, not 100 percent

Question 59

is it crucial that a polycarbonate lens is not glazed too tightly in the frame and why

Answer 59

the impact resistance of a polycarbonate lens is due in part to their elasticity ie their ability to distort so they can absorb more impact energy than they would if they were rigid.

If a polycarbonate lens is held too tightly in the frame then it loses the ability to distort ie the force applied by the frame reduces its elasticity and therefore its impact resistance and it may no longer meet safety standards.

Question 60

Impact tests
Low energy what is it in m/s amd medium and high

How are these impact tests conducted

Answer 60

all lenses used for safety must pass basic level of testing-increased robustness. All appliances used for safety must possess impact resistance features

low-energy impact (45m/s)= specs
medium energy impact (120 m/s)=goggles/face shields
High energy impact (190 m/s) face shields

Note- impact tests conducted w steel balls of various weights and velocities as projectiles.

Question 61

Why wear sunglasses.

What is a tinted lens

Answer 61

Why wear sunglasses- protection from glare/ radiation (UV, IR, short wavelength visible) also as a fashion accessory. Reduce eye strain and increase visual perception.

Tinted lens- a lens that has a noticeable colour in tramission
the colour of the tint is determined by the spectral absorption, a neutral density tint absorbs all wavelengths equally and appears grey.

Question 62

UV
UVC, UVB and UVA, IR
What does spectral power depend on

Which is the most damaging and where does uv come from

Answer 62

Solar radiation-
UVC- less than 280mm = absorbed by atmosphere
UV B- 280-315nm
UV A- 315-380nm
IR- More than 780nm - doesnt do much damage at sea level
Spectral power distribution depends on the time of day and year, variations of reflections of light according to terrain eg sand snow concrete woodland etc and contributes to the light perception.

UVC is most damaging as the lower the wavelength higher the frequency more energy. This is absorbed by atmosphere though so luckily doesnt get through.

Snow sand concrete can bounce uv back to us so not only from above it also comes from under you. Levels of IR at sea level are not considered damaging to eyes.

UVB is more damaging but doesnt get to the back of eye which is good. UVA gets to the back of the eye.
Dw about IR as IR that occurs naturally isnt damaging.

Question 63

Ocular transmission - what wavelengths get through and which has greater potential to cause damage and does it and why

Answer 63

UVB has greater energy so more potential for ocular damage but UVA penetrates more deeply into the eye

Cornea- transmits down to 290- Let’s through all uvb and uva
Aqueous humour- same
Crystalline lens protects our eyes from a young age- young 310 and old 375. So blocks out most uvb letting a little through but lets through uva. Older blocks baso all uv
Vitreous humour- 290 as well

In the phakic eye less than 1 percent of uv light reaches the retina

Question 64

UVA and UVB what does it cause
What does IR cause
Occupational

Lenses with nuclear sclerosis
Blue light with cataracts

Sxs of acute corneal damage from excess uv

Answer 64

UVA and UVB- damage to the cornea, implicated in cataract (cortical starts at bottom) and ARMD.

IR- hazardous is over 1400nm which can lead to glass blowers cataract. Naturally occuring isnt damaging unless staring at sun= macular hole/ solar retinopathy.

Consider occupational requirements so uv eg uv clinics, arc lights, tv/ film/ welding. Or IR: furnace workers.

Lenses with nuclear sclerosis can absorb uv up to 400nm so protects you.

Blue light becomes attenuated with cataracts. World may be too bright for them after removal so may have to put slight yellow tint in their specs to absorb a bit of that blue light.

Uv- sx of acute corneal damage from excess uv-
foreign body sensation, severe pain, lacrimation, blepharospasm
sign of chronic corneal damage from excessive uv- pterygium

Question 65

Classifications of tints. Range of luminous transmittance requirement for driving and road use

Answer 65

0- clear or very light- 80-100
1- light tint- 43-80
2-medium dark- 18-43
3- dark tint- 8-18
4- very dark tint- 3-8

4- not suitable for driving
Rest may not be suitable for driving at night

spec lenses for driving during the day should have minimum luminous transmittance of 8 percent.
Spec lenses for driving at night should have min luminous transmittance of 75 percent.

A minimum relative visual attenuation coefficient Q is also needed as well but we will come onto that.

Question 66

The next graph- max value of spectral transmittance Tf (wavelengths)
max value of solar UVA transmittance and the range of luminous transmittance percentage from visible spectral range

What does Tv tell youu

Answer 66

Tv= transmission over the whole visible spectrum. Tells you what transmission of uv is allowed for each category of tint. Cut off at 18 percent where the uv transmission has to be lower rather than the same over the whole of the visible spectrum.

So up until 18 percent for UVA the uv must be the same or less than the transmission or the range of luminous transmitance eg 66 percent the max value of uva transmitance needs to be 66 percent or less as well over the whole visible spectrum.
Under 18 percent UVA must be half that of the transmittance.

UVB always needs to be a tenth of range of luminous transmitance. 0.1Tv

UVB damaging dont want to let through so for all categories the transmittance has to be a tenth of the visible spectrum. 18 percent transmission UVA has to be a half of the visible spectrum or less.

Question 67

Explain the Q value

Answer 67

A minimum relative visual attenuation coefficient Q is specified for signal colours which is diff for spectacles including Plano sunglasses and motorcycle helmet visors.
How well you can see traffic lights if pass Q test.

Q= Tsign/ Tv
Tv- luminous transmittance of the sun glare filter standard illumination
Tsign- luminous transmittance of the sunglare filter for the spectral power distribution of the traffic light signal.

So Q tells you whether the tint would allow the wearer to see traffic lights. A minimum visual attenuation coefficient Q is specified for signal colours.

Question 68

What are the q values

Answer 68

Q Specs and visors
Red- 0.8. 0.8
Yellow-0.6 0.8
Green- 0.6 0.6
Blue- 0.6 0.4

Question 69

How do we plot these things

Answer 69

spectrophotometer
plots transmission for each wavelength puts lens in machine. Also works out q values. Too much attenuation in specific wavelengths may reduce the visibility of traffic signals below suitable limits. If transmission gives a fail in Q value the lenses are unsuitable for driving. Should have a warning on them.

Motorcycle visors and goggles- most of them acc fail the q value which means they arent actually legal
coloured visors are for daytime use only, not road legal in uk and many other regions. So you need to make them sign a waiver.

Question 70

Non standardised claims for uv protection

Answer 70

even if the lenses filter out 100 percent of UVA and B there isnt a 100 percent protection as the uv light will be reflected into the eye from the back surface so wrap around sunglasses= better protection.

Question 71

Untinted and tinted lens materials graph form. Spot the lens

Answer 71

For tinted lens materials-
crown glass lens not transmitting 100 percent at reflects light at some surfaces and transmits some uv light down to about 340 so you’d need uv filter.

Series 2- has a uv filter on it and brings it to 400 so blocks out uv

series 3- green has a bit of a tint on it but still transmits uv so need filter

series 4- tint that cuts out basically all the green and blue si would fail the q test and not able to see the green traffic light.

PLS 540 transmits more in the red and yellow with an overall low transmission so we get a dark red-brown colour. If transmitting more light over the same range the tint would appear orange.

Wavelengths being transmitted equally over visible spectrum= grey tint

Untainted- trivex is the best and cuts all uv out,
then polycarbonate and crown glass and cr39 still lets some through so need uv filter on them.

Question 72

Order of colours, wavelengths and numbers

Question 73

1)Compare and contrast the properties and manufacture of chemically treated toughened glass with those of heat treated toughened glass.

Answer 73

Heated- heated to 650 degrees and has a risk of warping
not suitable for mass production
thicker than standard glass
alters photochromic properties- appears darker
can be identified by a strain pasttern

chemically treated- heated to 470 degrees
suitable for mass production
standard thickness only required
does not alter photochromic properties
produces stronger lenses than heat toughened
cannot be identified by strain pattern

Question 74

2) Is CR39 a safe material to use as eye protection and why-

3) Compare and contrast the properties of Trivex lenses vs polycarbonate

4) List four hazards to the eyes your px may encounter outside work

5) Is it crucial that a polycarboante lens is not glazed too tightly in the frame and why-

Answer 74

2) No, its only classed as low impact resistant by BS EN standards. Thought safe bc fractures into blunt pieces but small high energy impact eg produced by squash ball or shuttlecock can cause it to fracture into sharp pieces.

3)polycarbonate- low constringence (29), meets relevant impact resistance standards, poor scratch resistance, typical centre thickness is 1.5mm, poor chemical resistance, not tintable, poor surfacing, cuts out UVA

trivex- higher constringence than polycarbonate (43-45) meets relevant impact resistant standards, better scratch resistance, better scratch resistance, typical centre thickness 1mm, good chemical resistance, fast tint rate, good surfacing, cuts out UVA and some blue light.

4)strimming/ hedge cutting/ badminton/ squash/ car maintenance (angle grinding, welding, battery acid, working with solvents, hammering, drilling, chipping paint off, blow torch)

the impact resistance of polycarbonate lenses is due in part to their elasticity. Ie their ability to distort so they can absorb more impact energy than they would if they were rigid. If a polycarbonate lens is held too tightly by the frame it loses the ability to distort. Ie the forces applied by the frame reduces its elasticity and therefore its impact resistance and it may no longer meet safety standards.

Question 75

6) Why do british and European standards for sunglare filters not have recommended maximum transmittance values for UVC radiation-

7) Why would trivex make a good lens material for tinted specs-

8) 8) What are the minimum luminance transmissions for spectacles used for driving at day and night and why do you need to know

Answer 75

as UVC radiation is absorbed by the earth’s atmosphere and so there is no UVC at sea level.

7) as it absorbs all radiation up to about 400mm and so blocks out all harmful uv radiation

8) 8 percent for day and 75 percent for night. Need to be aware of so we can advise our pxs that dark tints are not suitable for driving at night and very dark tints are not suitable for driving at all.

Question 76

9) For spectral transmittance curves what does the Q value tell you-

5) When dispensing polycarbonate lenses it is advisable to measure monocular centres and heights so the lenses are accurately centered in front of the wearers pupils esp for high rxs, why?

6) Your patient works with solvents, which material would you advise for their safety specs and why-

Answer 76

The Q value tells you whether or not the sunglasses or tinted lenses are suitable for driving, especially for recognising coloured traffic signals.

5) polycarbonate has low constringence Abbé number 30, and so will have high levels of tca off axis greater the higher power of lens so its important wearer is looking through optical centre in primary gaze.

6) Trivex as it has an equal or possibly higher impact resistance to polycarbonate and better solvent resistance. If polycarbonate lenses are exposed to solvents the surface of the lens can become etched and cloudy. Some solvents eg acetone can reduce the impact of polycarbonate lenses

Question 77

Reflectance just in case

Answer 77

Reflectance at first surface and transmission for every mm step by step at the first surface P from the equation times (transmittance percentage/100) for first mm and ans times (transmittance percentage/100) for second mm. Then this answer from 2mm times by the original p reflectance/100= true answer.

Question 78

1)The transmission curve for a pair of tinted specs is shown below, what colour tint does the transmission curve represent

Answer 78

transmision is on y axis and wavelength on y axis. No lens gives you 100 percent transmittance as always a little bit of light passing through. Anything below 380 is uv light then 380-700= visible spectrum. uva first then uvb.

If just red tint it would just be transmitting red light, same with green, orange just orange. So transmitting other colours as well mixture of red orange green blue so brown tint

Question 79

b) same transmission curve= are these specs legal for driving-

Answer 79

the tint legal
average transmission is it 75 percent for at night (no) 8 percent is standard at daytime, from about 480nm they are above 8 so for most of the spectrum above that so average transmission way above 8 percent for those.

Q VALUES tell you whether that tint is going to be suitable so the wearer can see all 3 traffic lights, thats why theres 3 q values one for each traffic lgiht colour. There is low transmission of light over the blue/green part of the spectrum compared with overall transmission which may mean the wearer would not be able to see the green traffic light. We would need the q values to check

Question 80

B) The luminous transmittance for the spectral power distribution for the green traffic light is 7 percent and the luminous transmittance for the stand illumination D65 is 30 percent. What is the Q value for the green traffic light and would this fail the standard for driving.

What is tv and what is tsign

Answer 80

Tv- luminous transmittance for the stand illuminance D65
Tsign- luminous transmitance for the spectral power distribution for the green traffic light
transmission/ traffic light. 7/30= 0.23 and the q value must be 0.6 or above for the tint to be legal for driving.

Question 81

What colour tint does this transmission curve represent-

Would this tint be legal for driving at daytime or during the night and why-

Answer 81

transmits all wavelengths reasonably equally. So this is a grey. Doesn’t transmit as much light so will be dark grey. 10-20 is dark. 30-40 middle. And 70-80 light

averaging around about 20 percent so standard for daytime is 8 percent so on average yes as there is more than a 8 percent transmission over the visible spectrum on average.
not legal for night as the legal level is 75 percent transmission over the whole visible spectrum.

You need to be aware of the filter tint categories and that category 4 filters are not suitable for driving. Remember for driving at night the minimum transmittance allowed is 75 percent so even category 1 could be too dark.
Very dark tints 3-8 not legal to drive at all. So the purple tables on my notes

Question 82

1)Does this tint meet british standards for tinted specs, explain your answer and 2. Why is it necessary to reduce the uv transmission for dark tints-

Answer 82

Does it meet the standards in terms of a- the amount of light they transmit through the visible spectrum and also what they transmit in the uv range.

The overall percentage transmission over the visible spectrum is under 18 percent which is an estimate from the graph and for this transmission level the transmission over uv range of 315-280nm must be half that of the transmission in the visible spectrum. So it has to be 9 percent or below for uva and we can see that the average transmission over this range is 10 percent which is not half that of the transmission over the visible spectrum. If borderline discuss the standards to tell us why it might not pass standards or why it might. Above 18 percent need it to equal Tv for uva below 18- half Tv. Above and below 18 percent for uvb it needs to be 0.1Tv so only 10 percent of it.

UVA is from 320-380 so look at that section to see if UVA meets the standards of half or lower as lower than 18 percent.

Question 83

B)is it necessary to reduce the transmission of uv light for dark tints

Answer 83

bc the lack of visible light entering the eye will cause the pupil to dilate allowing more damaging uv light in.

Question 84

talking about uv and the transmittance

Answer 84

whatever the luminous transmitance of visible light UVB transmitance should be 0.1 times that transmitance as smaller wavelength more energy and more damaging.

If the tinted lens or sunspec has a luminous transmittance of 90 percent of visible light then uva transmitance should be 90 percent or less (so the same as it or less) so if its 80 percent then the uva transmitance should be 80 percent or less. Now if if the tinted lens or sunspec has a luminous transmitance of 10 percent of visible light then uva transmitance needs to be half or less so should be 5 percent or les.

the maximum transmission allowable in the uv spectral range depends on the luminous transmission in the visible range.

Question 85

What lenses are these
Lens 1 and 3 are clear with no filters applied what commonly used lens materails could they be made from- blocks most uv 380 and 400

2. If lens 2 is made in cr39 what does the transmission curve tell you about the lens- blocks under 400. And how is the graph lower.. tint?

3- Reduced transmission why if clear for polycarboante and trivex

Answer 85

clear lenses with no filters applied- transmission you’re getting is solely due to the lens material itself. Lens 1- both cut out all uv light as blocks it at 380 and 400 and uv goes up to 380. The only two lens materials to cut out all v light= polycarbonate and trivex so the first orange one is polycarbonate and the blue one is trivex as it cuts out light below 400nm.

2) if made from cr39 it would only cut out uv down to about 350nm so it must have a uv filter applied to it. The lens is not tinted so the reduction of transmission being a consequence of the loss due to reflection at the two lens surfaces. (So transmission is reduced in the graph due to loss due to reflection no tint in it.) as overall graph lower.

3) the overall graph of polycarboante and trivex are lower than the one for cr39 as polycarbonate and Trivex have higher refractive indices than cr39 so there will be more reflection at the lens surfaces which reduces the overall percentage transmission.

Question 86

Enhanced single vision lenses

Answer 86

fairly new and developed bc of reported increase in digital eye strain. Could be due to lots of things so BV, dry eyes, rx, then suggest enhanced single vision lenses. It’s similar to a PAL but only with a very small add between 0.4-0.8 can go as high as 1.12D.

This is for non-presbyopes. Free form so designed to give optimum performance for a range of working distances and gaze angles. Most include a blue light filtering MAR coat but take care when recommending blue light. Some pxs might find it more comfortable though.

(Eg essilor eyezen, Zeiss smartlife, Hoya sync III) Good optical performance 2 best form lenses in one.

Question 87

Blue light filers

Answer 87

Can be incorporated in the anti-reflection coating so the lenses reflect blue light away and appear to have a blue tinge to them if they reflect the blue light away.
Or it can be incorporated in the len substrate where minerals are added which absorb the blue light and the lenses appear slightly yellow in colour.

Controversial carafe taken when advising px on benefits but some pxs find them comfortable =research has shown that blue light in daylight is beneficial for slowing down myopic progression.

Question 88

Myopia control

Answer 88

hyperopic defocus of the image shell formed by standard single vision lenses for myopia is thought to trigger the growth of the eye.

Negative lenses in the periphery peripheral focus tends to focus behind the retina so we get hyperopic defocus and this peripheral blur triggers growth to reduce that peripheral blur and to match those curves. This results in an axial length increase which increases myopia stretching retina. Myopic defects gets over this in the periphery.

Question 89

DIMS lens

Answer 89

defocus incorporated multiple segments
lens marketed as miyosmart by Hoya- positive lenslets bring peripheral curves onto the retina. Not as obvious on lenses but need to fit these r lenses well.
Also Stellest by essilor= clear area in central lens to correct for rx surrounded by multiple lenslets with more positive power.

they bring those peripheral curves either onto or in front of the retina giving myopic defocus removing the trigger for growth. Initial trials with dims lens= up to 60 percent reduc in myopic progression.

Question 90

Other specialised lenses

Answer 90

driving, sports, specialised coatings eg olephobic or hydrophobic, different types of varis. People think AR coatings get dirty but they dont its because they show up due to
reflections so oleophobic is another coating that can be put ontop.

Question 91

Lenticular lenses-
Positive and negative lenses

Answer 91

best form- poor cosmetically esp w high plus
flatter forms have poor peripheral optical quality.
aspheric is better but lenticular is the cheaper version.

Shallow curves which reduces edge thickness, reduced diameter gives reduced sag. Has a central steeply curved part which is the lenticular portion and the surround which is the carrier portion. So its a positive or negative lens attached to a Plano-carrier. Shape of lenticulars follows shape of frame so better cosmetics. A second convex curve can be ground onto the margins to reduce edge thickness further.

positive lenses- centre thickness reduced with reduced sag, small fov so fitted very close to the eye.
negative lenses- reduction in sag with smaller diameter gives reduced edge thickness, reasonable fov even for smaller apertures.

Question 92

Simplest lenticulars

Answer 92

consists of a high positive or negative lens attached to a Plano carrier. Produced in moulds or similar way to solid bifocals. The carrier portion can be plano or have some power which allows the wearer to see clear enough to navigate. Standard round portion in middle for standard lenses.

Question 93

Profile lenticulars and blended lenticulars

Answer 93

Profile- The shape of the lenticular portion follows the shape of the frame aperture so better cosmetics

Blended- Rather than a sharp divison the edge of the refracting portion is blended into the plano carrier to give a sharp curve. More gradual change in power. Central aspheric area is thinner with good peripheral optics and better fov, blending zone has poor acuity but no visible divide and no ring scotoma. 3 zones, lenticular zone= useful bit, central aspheric area and blending.

Question 94

AO ful-vue blended lenticular
How is it better than standard blended lenticulars

Answer 94

Uses polynomial aspheric surface to blend between the central and peripheral zones. Gives improved performance over the standard blended lenticulars with a wider clear zone and less astigmatism in the blended zone. Possible with computer aided design and grinding techniques. HES w high rxs.

Question 95

The problem with anisometropia

Answer 95

brain has difficulty fusing images from two eyes due to differences causing diplopia, blur and asthenopia. Unequal magnification can cause diplopia. Iseikonic lenses can be prescribed to equalise the magnification. If long standing aniso dont fix so deep amblyopia not a problem.

Question 96

Iseikonic lenses
How can we manipulate SF and why cant we manipulate PF

Answer 96

size lenses, used to correct aneisokonia unequal image sizes between the two eyes, technique works by varying the form and thickness of the lens to alter the shape factor in the spectacle magnification formula SM= power factor times shape factor

Shape factor is proportional to the thickness and front surface power.
Can manipulate the shape factor of both lenses to balance the magnifying effects. But the lenses are no longer best form, can use aspheric or free form lenses to manipulate mag.

Power factor cannot be altered as this iwould change the effective power of the lens. So we can change the shape factor to increase spectacle magnification by increasing refractive index or thickness basically and front surface power.

Question 97

Balance lens and how do we calculate it

Answer 97

used if big difference in rx- undercorrected. So px has a lazy eye. Placed in front of the blind or deeply amblyopic eye.

Match lens form- for the power give average sphere which is sphere+1/2cyl. Lens should balance the one for the seeing eye in both weight and appearance.

Always match form and type excpect for progressive lenses where single vision lens can be used as a balance. And match coatings. Convention is to prescribe a spherical power equal to the average sphere of the other lens.

Question 98

Fresnel prism
What is prismatic power dependent on
What does it consist of
Available in what powers

Answer 98

prismatic power is dependent on apical angle and independent from apex to base distance.

Consists of parallel bands of equal prism supplied in flexible plastic sheets which can be cut to shape and stuck to the lens surface. Bands are v thin-> high prismatic power on thin sheet.

Available in powers up to 40 dioptres. Temporary measure as poor image quality often before deciding on final correcting prism. Can be used to correct temporary diplopia caused by multiple sclerosis.
cut to shape and adhered to lens surface when wet.

Question 99

Fresnel lens

Answer 99

series of prisms with increasing power
concentric circles of varying prism power. Give lenses of very low weight and constant thickness but poor optical quality. Available in up to +16.00D and -14.00D prism powers.

Question 100

Frosted lens

Answer 100

used to occlude an eye and prevent diplopia
eg where LVA given for near with a close working distance which the px cannot converge to. Sometimes used to obscure cosmetically unattractive blind eye or empty socket.

Question 101

Chavvase lens pinhole specs and recumbent spectacles and ptosis specs

Answer 101

Chavasse lens- similar to bathroom window used to obscure blind eye for cosmetic reasons. Nail varnish on top lof cr39 lens= same

Pinhole spectacles- single pinhole spectacles were used at one time post operative recuperation. Multiple pinholes in spectacles previously used as an alternative to prescrip lenses but didnt work.

Recumbent spectacles- prismatic spectacles which allow someone forced to lie flat in a bed to be able to read. Allows pxs to read whilst supine. Uses prism in a double reflection transferring vision 90 degrees. Double reflection so the print is seen the right way around.

Ptosis spectacles- mechanical device used to raise the upper lid in cases where px has difficulty opening eyes so ptosis. Gallery/ Lundy loop

Question 102

1)Explain how you can change lens parameters to equalise magnification in an anisometropic prescription, why cant you change the power factor (6)

Answer 102

Maginfication is a product of the power factor and the shape factor- M= PF times SF

Power factor PF= 1/(1-dFv’) Cant change this wihtout changing power of lens
shape factor SF= 1/1-(t/n)f1) To reduce the shape factor we need a larger denominator so we need to decrease the thickness and the front surface power SF is proportional to t and F1. To increase the shape factor we need to increase the thickness and or the front surface power.

So a more steeply curved thicker lens will give greater magnification than a relatively flat thinner lens

Question 103

2) Mrs A has an rx of +3.50/-1.00 x 135 and le- 8.75/-0.50 x 45 add 2.50. Her vas are 6.4.8 in re and 6/60 in le she wants varis what would you prescribe-

Answer 103

amblyopic left eye so only needs balance lens in the left give the average sphere of the right lens which is sphere+ half cyl in this case 3.50-0.50 is 3.00D
there is no need for an addition in the left eye so can prescribe single vision lens as it will look no different to the varifocal lens in the right and will reduce the cost
Care must be taken by glazing house to match any ar coatings, prescribe full rx varifocal lens for the right eye.

Question 104

3)Describe a fresnel lens and a fresnel prism

Answer 104

a fresnel lens consists of concentric circles of varying prism power. This produces a lens which is lightweight and a constant thickness but has very poor optics.

A fresnel prism consists of thin parallel bands of prism on a flexible plastic sheet and large prismatic effects can be produced on a thin sheet.
Prismatic power is determined by the apex angle so the same prismatic power is produced by lots of small prisms rather than one large one.

Question 105

4) Mrs B is a 32 year old accountant, she complains her eyes get very tired and ache after a day of work. What do you need to check and what could you recommend in terms of lenses?

Answer 105

need to know what shes doing at work, dont assume what shes doing.
ask about sxs like grittiness, burning, dryness, Tbut, staining and lids for any sign of dry eye and likely a cause of tear film dysfunction.
Check if she wears specs and check refraction, is she an undercorrected or uncorrected hyperope is there any uncorrected astigmatism.
Any issues with binocular status, fixation disparity, fusional reserves, accom and convergence. Any issues with glare due to lighting or windows could the position of the screen be altered.
If none of the above then could try enhanced single vision lenses with an MAR and blue filter. The effectiveness of these lenses must be assessed on an individual basis. try give you best form for distance and at near as well bc optics are a little bit better.

Question 106

1)Define a lenticular lens and what are they used for-

Answer 106

a lenticular lens has a refractive portion and a carrier portion which is often Plano, the refractive portion does not need to extend to the edge when mounted in the frame. They are used to reduce the weight and thickness of high power lenses.

Question 107

2) Adv and disadv of positive blended lenticulars compared to non blended-

Answer 107

for a blended lenticular the edge of the refractive portion is blended into the barrier portion to give a smooth transition, the main advantage is its cosmetic appearance since the edge of the lenticular is invisible. Blended lenticulars do not have the ring scotoma you get with standard positive lenticulars. The main disadvantage is poor optics you get in the blended zone which reduces field of view.

Question 108

3) why dont we use lenticulars as much today-

Answer 108

we now have higher refractive plastic lenses and more sophisticated aspheric surfaces which means we can get thinner lenses without them.