Week 6: Prism, Bifocals & Progressives

Question 1

One eye may be looking through the distance portion of a bifocal, while other eye is looking through near zone. How would you resolve this issue?

Answer 1

Seg height can be raised up or down to ensure both eyes reach the seg at the same time when rotating (looking) downwards

Question 2

What should you do when prescribing vertical prism for bifocals?

Answer 2

If prescribing grind prism, eye will deviate to look towards apex
- Fitting cross should be raised 0.3mm for every dioptre of base down prism
- Lowered 0.3mm for every dioptre of base up prism required

Question 3

Issue: Prescribing prism at near for bifocals, but not for distance. How would you resolve this issue?

Answer 3

• Prism at near can be achieved by decentering the near segments
• When decentering near segments, ensure reading zone area is large enough to facilitate reading

Question 4

How to prescribe prism slab-off/bi-prism bifocals

Answer 4

• Incorporate slab-off (i.e. remove base down) into the lens for the eye which needs to be more base up relative to the other eye

Question 5

What type of lenses can be used for prism slab off?

Answer 5

• Are typically only used in bifocal types which have a straight dividing line between distance & near segments
• E.g. flat-top bifocals (a) and executive bifocals (b)

Question 6

What is cemented or bonded segments

Answer 6

• These can incorporate any kind of prism or prescription modification in the distance & near portions
• Shape, size & location can be manipulated independently
• Can be used to control differential prism

Question 7

What is a progressive addition lens?

Answer 7

• A lens having optics that vary in power such that the power gradually increases from the distance, to near portions of the lens

Question 8

How are Progressive Lens Produced?

Answer 8

• Lens is produced such that the curvature of the front surface is gradually increased as we move further down the lens
• Process is known as optical modulation

Question 9

What is the ‘add’ in progressive lenses?

Answer 9

• A PAL would permit clear vision at any given distance (up to the set reading distance for that patient)

Question 10

List the different zones of a PAL

Answer 10

1. Distance Portion/Zone
2. Progressive Corridor/Zone
3. Near Portion/Zone
4. Periphery

Question 11

What is Distance Portion/Zone of a progressive?

Answer 11

• Largest section of the PAL lens, will vary in size depending on the design of the lens

Question 12

What is Progressive Corridor/Zone of a progressive?

Answer 12

• Area of gradually increasing power allowing vision at intermediate distances

Question 13

What is Near Portion/Zone of a progressive?

Answer 13

Restricted area of clear near vision
- All of the same power (the distance + add)

Question 14

What is Periphery of a progressive?

Answer 14

• Side portions which contains unwanted astigmatism

Question 15

What is unwanted cylinder/distortion in PALs?

Answer 15

• Due to the design of the PAL, this distortion cannot be avoided - it must be managed
• The distortion varies in its amount & orientation depending on the design and prescribed add of the lens
• Amount of distortion increases with an increase in the add prescribed

Question 16

What are some likely symptoms when one is looking through the periphery of a PAL?

Answer 16

• Distortion
• Blur
• Headaches
• Swimming effect
• Eyestrain

Question 17

List 3 advantages and disadvantages of a PAL

Answer 17

Advantages:
1. Cosmesis
2. All distances are clear in one pair of specs
3. Can be cheaper than separate pairs

Disadvantages:
1. Lenses are expensive
2. Distortion through lenses
3. Takes time to adapt to lenses
4. Requires careful frame selection

Question 18

Who Should Wear PALs (list 4)

Answer 18

1. Presbyopes
2. Esophoric myopes whose script is progressing
3. Accommodative strabismus
4. Pre-presbyopes with accommodative problems

Question 19

Who should not wear PALs (list 4)

Answer 19

1. Satisfied bifocal wearers
2. Limited finances
3. Monocular patients
4. Low myopes

Question 20

Where does Lowest Add power occur?

Answer 20

At distance reference point (DRP)

Question 21

Where is Highest Add power located?

Answer 21

At near reference point (NRP)

Question 22

List the two different corridor lengths and describe briefly

Answer 22

• Long corridor length = softer design & easier to adapt to
• Short corridor length = enable the wearer to reach an adequate near vision zone, particularly in a shallow eye frame

Question 23

What are the advantages & disadvantages of short corridors in PALs?

Answer 23

Advantages:
1. More near utility in smaller frames
2. Reduced eye declination during near vision

Disadvantages:
1. Less intermediate vision and mid-range utility
2. More rapidly increasing unwanted astigmatism
3. Distortions at bottom of the lens if the frame is too deep

Question 24

What does the marking and engravings represent in PALs?

Answer 24

• Locating correct areas for fitting the lens & for power measurements
• Indicating the add of the lens
• Indicating the brand and design of the PAL

Question 25

List what is on the permanent engravings on PALs

Answer 25

1. Lens reference points 2. Add 3. Near Portion/Zone 4. Lens Identification logo

Question 26

What is the 'Lens reference points' in progressive lenses?

Answer 26

Indicate the horizontal meridian of the lens, and as a guide to place further markings on the lens

Question 27

Where is the 'add' located in progressives?

Answer 27

Always located temporally on the lens

Question 28

What is the 'Near Portion/Zone' in progressives?

Answer 28

Restricted area of clear near vision

Question 29

What is the 'Lens Identification logo' in progressives?

Answer 29

Listing the lens identification logos so can identify the brand & design of the lens

Question 30

What is the 'Lens Identification logo' in progressives?

Answer 30

Listing the lens identification logos so can identify the brand & design of the lens

Question 31

List the lens markings for PALs

Answer 31

1. Distance checking zone 2. Near checking zone 3. Fitting cross 4. Major reference point (MRP)

Question 32

What is the 'distance checking zone' in progressives?

Answer 32

The distance prescription of the lens is checked through this zone using a focimeter

Question 33

What is the 'near checking zone' in progressives?

Answer 33

Near prescription can be read through this zone

Question 34

What is the 'fitting cross' in progressives?

Answer 34

The lens should be fitted such that the fitting cross sits in front of the centre of the pupil

Question 35

What is the 'major reference point (MRP)' in progressives & where is it located?

Answer 35

- Any prism contained in the lens is measured at this point - Located at the beginning of the progressive corridor

Question 36

What is prism thinning?

Answer 36

- PAL’s can become quite thick due to the increasing curvature in the lower portion of the lens - Prism thinning is used in order to make the lenses thinner & lighter - Amount of base down prism applied varies with the add of the PAL

Question 37

What are the different PAL designs?

Answer 37

1. Hard vs Soft designs 2. Monodesign vs Multidesign 3. Customised designs 4. Freeform designs

Question 38

What is hard design for PAL?

Answer 38

- Contains large distance & near zones, however unwanted cyl is highly concentrated in certain areas, making adaptation to the lens harder - The power in the progressive zone tends to increase faster, meaning the zone is shorter

Question 39

What are the advantages & disadvantages of hard design in PAL?

Answer 39

Advantages: 1. Can exclude cyl from certain areas 2. Larger delineated distance and reading zones 3. Shorter progressive channel Disadvantages: 1. Areas of concentrated distortion 2. Longer and more difficult adaptation 3. Progressive zone shorter

Question 40

What is soft design for PAL?

Answer 40

- Has smaller distance & near zones, however the transition from near to the periphery is much more gradual - Smaller amounts of cyl are spread out over a much larger area

Question 41

Advantages & disadvantages of soft design in PAL?

Answer 41

Advantages: 1. Easier, more rapid adaptation 2. Less distortion of peripherally viewed objects 3. Less ‘swim’ of objects with head movement 4. Dioptric power of unwanted cyl is lower Disadvantages: 1. Possible slightly compromised distance clarity 2. Longer progressive channel

Question 42

What is the Monodesign Lens in PALs?

Answer 42

- Chooses a basic design that is used for all add powers offered

Question 43

What is the Multidesign Lens in PALs?

Answer 43

- For a given type of PAL, a different design is used depending on the add of the lens

Question 44

Describe aspherisation in PALs

Answer 44

- Allows for flatter lens designs without increasing aberrations (notably oblique astigmatism) - Improved cosmetics with better quality vision - Traditional methods for measuring lens power will have shortcomings (i.e. focimetry) will have shortcomings, which require greater trust in the lens manufacturers

Question 45

Describe Hoyalux iD (Physiological Optics)

Answer 45

- Front surface has vertical progression - Back surface has horizontal progression

Question 46

List the different Varilux Physio (Wavefront Analysis)

Answer 46

1. Wavefront aberrometry 2. High order aberrations 3. Digital direct surfacing

Question 47

Describe Wavefront Aberrometry briefly

Answer 47

- Analyses aberrations generated by the reference spec lens surface

Question 48

Describe Digital Direct Surfacing (on back surface)

Answer 48

- Takes into account the power created by the front & back lens surfaces - Back surface is calculated point-by-point to maintain the wavefront created by the front lens surface & creates a “best form Rx” for each wearer

Question 49

What is extended focus lenses in PAL?

Answer 49

- Have a large field of view for reading, with a restricted field for either the distance, or intermediate vision (opposite to standard PALs) - A type of reading lens which decreases in power as we move towards the top of the lens

Question 50

What is Degressive/Occupational Lenses?

Answer 50

- Designed based on near add AND reduction in plus power as you move more superior in the lens

Question 51

Advantages & disadvantages of Degressive/Occupational Lenses?

Answer 51

1. ‘Soft’ design as surface astigmatism redistributed in the distance area 2. Wearer obtains a wide central field with very low aberration 3. Generally improves posture for the wearer, reducing neck & shoulder strain Disadvantages: 1. NO DISTANCE – unsuitable for driving 2. Creates ‘artificial far point’ – closer than optical infinity

Question 52

What are Anti-Fatigue Lenses and who are they suitable for?

Answer 52

- Designed as a distance vision lens with low ‘add’ Suitable for; - Emerging presbyopes - Patients with digital eye strain - Emmetropic patients who require ‘accommodative’