PRISM GLASSES

The Peli Lens™ also known as “EP” or Expansion Prism concept was developed by Dr. Eli Peli of Schepens Eye Research Institute in 1999. He tested this concept on several patients in his private practice with great success using 40Δ Fresnel press-on prisms (Peli 2000). Funded by NIH Grant EY014723, Chadwick Optical designed and produced a cosmetically acceptable permanent version of this concept in a prescription lens.

The Peli Lens™

 The InWave™ lens, was designed by an optical engineer at Kodak in the 1990’s.  InWave™ was in business briefly in the late 1990’s.  This lens uses a horizontal apex line incorporating a prism, base out, in the lens on the side of the VFL.  The lens used 12∆  prism.  Chadwick now simulates this lens with any amount of prism  requested.  The InWave™ design  works like the VFAS System  below, but the InWave™  style is structurally a safer design when used with eyeglass frames that have a narrow vertical dimension like the one above.

InWave™ Type,  also sometimes referred to as “Sector” or “Sectoral” Prisms or

the “Chadwick lens.  Although called the “Chadwick lens” by some, Chadwick Optical

in no manner endorses this lens.

The VFAS (Visual Field Awareness System) was introduced by Daniel Gottlieb, OD, about the same time as the InWave™ and used 18 ½ prism in the same manner as the InWave™. The round prism  reduced the thickness and weight when used in larger frames popular at that time and regaining popularity now.  Chadwick also simulates this lens with any amount of prism  requested.  This design is best used in larger frames to reduce the weight and thickness of the prism.

The VFAS Type, sometimes referred to as the Gottlieb lens or Button prism

Top View of 40D Peli Lens™  with permanent Fresnel prism inserts

The Peli Lens™  was subjected to clinical

trials over a 2-year period.  74% of the 43 patients studied found them useful and continued to wear them when the trials ended.  The results were published in the esteemed Archives of Ophthalmology, May 2008. 

Side view of a 40∆

InWave™ type lens.   It is thick, heavy and unsightly.

 

The bigger the prism amount the thicker the lens.  The image jump is also greater with larger amounts of prism as the user scans through the apex and into the prism.  The bigger prism amounts do allow more view of the peripheral field in the blind side if the user can learn to scan into the prism.

Text Box: A fitting and training protocol was developed during the clinical trials that eliminated the guesswork associated with  fitting other types of optical prism systems, thus keeping it more affordable.

 

There has been much debate on the proper placement of the InWave™ and the VFAS type prisms and the optimal prism values in an effort to develop a standard.  If a standard exists, we are not aware of it.  Each seems to be at the discretion of the doctor and fitting is done on a trial and error basis, a very time consuming process.  A great deal of time is spent in the doctor’s or vision rehabilitation therapist’s office training the user to scan into the prism.  The more training time involved, the higher the success rate, but the cost to the patient also increases. 

The strong research, documented success (74%) and the low cost approach of the Peli Lens™ would indicate that this concept should be tried first for hemianopic field expansion.   While this is currently the most successful of the optical aids,  the InWave™  and  VFAS prisms may help some individuals who do not adapt to the Peli Lens™.

 

Dr. Peli’s goal in the design of his lens was to keep the price affordable, make it as cosmetically appealing as possible and make it so easy to fit that it could be done in the office of any Optometrist or Ophthalmologist in a 1 hour visit.  The patient then wears the low cost temporary prisms at home for an adaptation period of a month and returns to the doctor for a follow-up visit.  If the patient adapts to the prisms, a pair of glasses with the permanent prisms can be ordered or the patient can continue with the temporary prisms.  These prisms expand the field by 20° and should require little or no training to use them. 

       Prism Glasses

Before the Peli Lens™, optical prism systems  (InWave™  and  VFAS prisms) have been time consuming to fit, required extensive patient training to be successful, and are costly to produce.  They have a reputation of not being worth the trouble, in fact, many professionals have formed the opinion that “prisms don’t work” based on their experience with them and other prism configurations.   This opinion is substantiated by most of the research in the field…. until the Peli Lens™ which does not operate like these other prism systems.

 

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