PPLs and PALs

• progressive power lenses and progressive addition lenses

  • varifocals
  • incorporate a channel of progressive power change from the distance zone at the top of the lens to the near zone at the bottom of the lens
  • intermediate area unlike bifocal
• advantages of PPLS

  • clear vision at all distances
  • more convenient than using 2/3 separate pairs
  • cosmetically they look like single vision lenses ie no visible dividing line
  • no image jump occurs as the patient looks down the lens
• disadvantages of PPLS

  • distortions in the periphery
  • limited field of view for reading
  • intermediate and reading areas in a PPL will always be smaller than those in a bifocal
  • period of adaptation required
  • transition between distance and near is narrow so accurate centration is required
  • cost
  • no control of inset
  • more horizontal movement needed when reading
• choice of lens

  • 3 types of PPL = hard, soft and firm (super soft)
• hard designs

  • large distance area
  • narow, intermediate corridor and wide reading area
  • peripheral distortions confined to limited nasal and temporal areas
  • distance virtually distortion free
  • good for previous bifocal wearers
• soft designs

  • distortion extends to distance area which affects peipheral vision
  • patient will have to move head more when viewing objects at periphery
  • larger intermediate zone
  • narrower reading area
  • good for first time presbyopes especially VDU users
• firm (super soft designs)

  • offers larger distance, intermediate and reading areas
  • ideal for all presbyopes so previous bifocal wearers and those who are non tolerant to other PPL designs
• methods of representing PAL designs

  • depending on where you look they are depicted in different ways
  • main types are iso cylinder, vector plot, iso mean power lines or just noticeable blur
  •  Iso-cylinder lines show regions of equivalent cylinder power which are easier to understand but may supply less information than a vector plot.
  • Vector plot shows the magnitude and direction of astigmatism
  • Iso-mean power lines show regions of equivalent sphere power
  • Just noticeable blur shows the region where patients will typically first notice blur

  o   pupil size dependent and subjective

  No method is entirely satisfactory or will predict wearer satisfaction

  
• measurements required

  • monocular distance PDs
  • pupil heights
  • select correct size and shape of frame
  • adjust frame so that it is sitting straight before taking measurements
  • pantoscopic tilt - should be approx 10 degrees
  • ensure chosen lens fits in frame - frame depth, standard lenses from pupil to bottom of lens needs to measure at least 18-20mm
• PPL markings

  • 2 types of markings for layout, power verification, dispensing and identification purposes
  • removable markings - ink stamped onto lens to identify layout, varify power and dispensing points of the lens, with perminant markings engraved onto the lens (brand identification, Add power and alignment reference markings)

  
• alignment reference markings

  • 34mm apart along 180 meridian and used to reapply ink markings for varifying horizontal alignment
  • engraved
• distance reference point

  • location on surface that provides the exact base curve which is the optimal location for verifying the distance prescription
  • inked
• fitting cross or point

  • represents point of lens which lies directly in front of pupil
  • located at the inked fitting cross marking
• prism reference point

  • represents the position for verifying prescribed prisms or prism thinning
  • is positioned exactly between the permanent alignment for reference markings
  • inked
• near reference point

  • location that provides the full add power and is the location for verifying the add power of the prescription using a focimeter
• manufacturers mark

  • represents the manufacturers logo and is always below the nasal alignment reference markings
  • engraved
• add

  • usually 2 digit number without plus sign or decimal point
  • always below the temporal alignment reference marking
  • engraved