Topic 6: PALs Flashcards
State the design principal behind progressive additional lens (PAL)
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
What are the disadvantages of PAL
(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
Explain surface transformations
When single vision lens > bifocals»_space; trifocal»_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
Explain astigmatism plot
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
Explain the different PAL design variations
(1) near emphasis PAL - caters to wearer with lots of near tasks
(2) short corridor PAL - wearers who want to wear shallow frame.
Explain near emphasis PAL
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
Explain short corridor PAL design
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)
Summarise how PAL zones are inter-related
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
state frame requirements for standard PAL
minimum frame height, vertex distance 12-15mm, panto tilt of 5-10degrees, wrap angle of 5-10degrees
explain minimum frame height
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
explain vertex distance
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
explain panto tilt
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
explain wrap angle
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
explain corridor length selection
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
explain the customised PAL design
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
name the instructions for first time PAL wearers
(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
describe OPL briefly
designed to widen intermediate view which is limited in PAL
&
Intermediate power shifted to eye level so they do not need to glance down
explain differences between OPL and PAL based on power design
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
explain OPL and its tradeoff
the further a wearer wants to see with his OPL, the smaller the usuable zones they will get
state frame requirements for OPL
same as PAL but less critical since intermediate and near zones bigger
instructions to first time OPL wearers
(1) adaptation few days to a week
(2) use OPL only when seated
(3) no driving with OPL
explain layout of PAL power and zone areas (troubleshoot)
(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
explain perfect PAL alignment for wearer
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
explain misaligned fitting cross
(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
explain other common errors in PAL
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
explain systemic approach to solve troubleshoot PAL
(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
