Topic 6: PALs

Question 1

State the design principal behind progressive additional lens (PAL)

Answer 1

First designed in 1959,

To provide full range of clear vision to a prebyope. The power on a PAL progresses slowly (increasingly more plus) from top to bottom so that wearer does not experience image jump

Question 2

What are the disadvantages of PAL

Answer 2

(1) The maximum plus of the lens is much further down than on bifocals. Wearer would have to adopt an uncomfortable head tilt if the near object is placed at eye level.
(2) near reading area is smaller compared to ones in bifocals. This can slow the reading speed of the wearer\
(3) Swimming effect while walking with PAL: when viewing surroundings, wearer often faces distortions at lens periphery

Question 3

Explain surface transformations

Answer 3

When single vision lens > bifocals&raquo_space; trifocal&raquo_space; multifocal,

Ledge is formed on both sides because each section inserted below the distance sphere is derived from spheres of smaller radiuses

Multifocal > PAL: sloping surfaces are added under each edge to fill out the unevenness. Aka unwanted cylinders because they at as cylinders in reflecting light and do not belong to original prescription. When wearer views through unwanted cylinder portions, will notice distortions. UC increases with increasing power progression

Question 4

Explain astigmatism plot

Answer 4

Demonstrates the distribution of UC on PAL

Points where UC is the same power, join to form the lines. While the darker shades represent higher UC power

Distortions increase with the increase of near ADD

Corridor width decreases as near ADD increases due to overcrowding of UC on both lens. Wearer will struggle with intermediate tasks as the area/corridor is too narrow and not useful

Question 5

Explain the different PAL design variations

Answer 5

(1) near emphasis PAL - caters to wearer with lots of near tasks
(2) short corridor PAL - wearers who want to wear shallow frame.

Question 6

Explain near emphasis PAL

Answer 6

As opposed to distance emphasis PAL (common) where distance zone has full width, distortion free and small near zone

Near emphasis: Distortion zone pushed upwards into distance zone to widen near zone area

Question 7

Explain short corridor PAL design

Answer 7

NOTE: not common. Only for frames with small b dimension

Corridor- space between fitting cross to near zone. Standard: 18-20mm

Max plus may end up chopped away during lens edging if corridor 18mm or smaller UNLESS corridor length is shortened to 12-14mm. This allows for faster progression of power and hence max plus remains in frame area

Main disadvantage: shorter corridor too narrow that intermediate zone is unusuable (recall overcrowding of UC hence excessive peripheral distortions)

Question 8

Summarise how PAL zones are inter-related

Answer 8

UC/distortions increase with increase in near ADD power

Intermediate zone/corridor is inversely proportionate to the amount of UC/near ADD eg. Greater UC, narrower corridor

Rapid progression of lens power (short corridor PAL) means narrow intermediate zone due to overcrowding UC

The size of distant, intermediate & near zones influence one another: an increase in width of one zone, will shrink the others

Question 9

state frame requirements for standard PAL

Answer 9

minimum frame height, vertex distance 12-15mm, panto tilt of 5-10degrees, wrap angle of 5-10degrees

Question 10

explain minimum frame height

Answer 10

10mm of lens area above pupillary centre for sufficient field of view for distance objects

minimum fitting height of product must be able to fit in the lens below fitting cross. (1-2mm more than corridor length

Question 11

explain vertex distance

Answer 11

most PALs designed for optimal function at vertex distance 12-15mm

big vd- small intermediate and near zone

small vd- near zone becomes hidden underneath

Question 12

explain panto tilt

Answer 12

5-10degrees panto tilt is required for efficient reading of wearer

small panto tilt- near zone as if further from wearer

big panto tilt- near zone appear hidden

Question 13

explain wrap angle

Answer 13

slide wrap angle of 5-10 degrees cause distortion zones at temporal sides to be less disturbing compared to 0 wrap angle

NOTE: wrap angle: angle between lens and horizontal

Question 14

explain corridor length selection

Answer 14

default for first timers: 18-20mm

the shorter the CL, the smaller the intermediate zone size and higher the distortions

short CLs ONLY those who want short frames (small B height) or those with experience same design

Question 15

explain the customised PAL design

Answer 15

aka as-worn optimised PAL

frame customisations of vd, panto tilt, wrap angle can be out of range

eg. bigger vd frame, can grind lens to be higher power

Question 16

name the instructions for first time PAL wearers

Answer 16

(1) glance down to read rather than chin down (PAL has different areas for different tasks)
(2) for peripheral objects, point nose towards object instead of glancing to the side to avoid seeing distortions - esp drivers & rear view mirros
(3) practice PAL at home where it is safe - changing gaze direction
(4) adaptation period of 1-2 weeks
(5) swimming effect is normal and expected but will be less noticeable when used to it
(6) floor objects to be seen using the distance portion - request they try to view floor by chin down OR glance down & head straight (near portion) to see difference
(7) no driving until adapted to PAL
(8) PAL prescription valid 2 years only. near ADD higher with age
(9) if heavily intermediate task orientated, general PAL not recommended but instead OPL

Question 17

describe OPL briefly

Answer 17

designed to widen intermediate view which is limited in PAL
&
Intermediate power shifted to eye level so they do not need to glance down

Question 18

explain differences between OPL and PAL based on power design

Answer 18

standard PAL- eg. progress from plano to +2 with +1 for intermediate tasks. lens area for +1 is smallest

OPL 1- power range is smaller: eg. +1 (intermediate is biggest view) to +2. power progression slow

OPL 2- full range eg. plano to +2 but also want bigger intermediate. similar to near-emphasised PAL, UC pushed upwards to increase intermediate area, OPL 2 requires deep frame for this power distribution

Question 19

explain OPL and its tradeoff

Answer 19

the further a wearer wants to see with his OPL, the smaller the usuable zones they will get

Question 20

state frame requirements for OPL

Answer 20

same as PAL but less critical since intermediate and near zones bigger

Question 21

instructions to first time OPL wearers

Answer 21

(1) adaptation few days to a week
(2) use OPL only when seated
(3) no driving with OPL

Question 22

explain layout of PAL power and zone areas (troubleshoot)

Answer 22

(1) lens power progress inferiorly, from fitting cross to near zone with max plus in middle of near zone
(2) intermediate and near zones much smaller compared to distortion zone

Question 23

explain perfect PAL alignment for wearer

Answer 23

PAL fitting cross in the pupil centre ensures:

(1) right powers used for distant objects
(2) distortion areas out of the way below eye level
(3) easily reach max plus power for reading easily by slight glance downwards

Question 24

explain misaligned fitting cross

Answer 24

(1) FC above visual axis due to high fitting height: distant blur since not right power for distant vision - adjust nosepads increase separation to lower lens
(2) FC below visual axis: near blur since max plus too far down - adjust nosepads reduce separation to raise lens
(3) Fc too nasal/temporal because of wrong mono PD: near blur since reading through distortion zone instead of near zone - frame adjustments otherwise remake
(4) FC tilted due to poor edging: overall blue since viewing through distortion zone and wrong cylindrical axis if px astigmatic - rectify lens tilt if frame is circular otherwise remake

Question 25

explain other common errors in PAL

Answer 25

vd & panto tilt: near reading difficulties - frame adjustment

insufficient near add/frame height too short: near blur - remake

corridor: near reading difficulties - advice of adaptation

over correction of myopia: dist & near blur - remake

Question 26

explain systemic approach to solve troubleshoot PAL

Answer 26

(1) take history to find out symptoms ie. which eye blur, distant or near task affected
(2) request wearer demonstrate how they use PAL: identify signs of unusual head posture
(3) re-enact FC to check for allignment errors
(4) check unusual wearing position like excessive/little vd or panto tilt