Spotting Prism – Concept
Since the early 1900’s, the field expansion strategy has been to put a prism in the patient’s blind area, and the patient uses it like a rearview mirror to give themselves field expansion. There are some benefits to this field expansion strategy, but mostly drawbacks.
Because this type of lens has been around for nearly 100 years, its variations have been marketed and trademarked under many different names – Onufryk, Inwave, VFAS (Visual Field Awareness System), SVAG (Side Vision Awareness Glasses), and the Chadwick Lens (no relation to Chadwick Optical). They all work in essentially the same way.
Spotting Prism – Benefit
A spotting prism is there to use when the patient needs it. When compared to the Peli Lens peripheral prisms which are there all the time, this can be a less confusing and less annoying alternative. When the patient feels the need to expand the field, they look into their spotting prism, and get an additional awareness of their blind field.
Spotting Prism – Drawbacks
Peripheral Vision – Think for a moment about how peripheral vision works. You see something out of the corner of your eye, and direct your focus towards it to find out what it is. What if you didn’t see that something out of the corner of your eye? Would you still look over there? Evidence shows the answer to that question is mostly no, you wouldn’t look over there. Without any cues from the peripheral vision, there’s little incentive to proactively look into the blind area.
Disturbing Double Vision – When the patient chooses to expand their field by looking through the spotting prism, they get double vision, and have to turn their head to avoid the double vision. This problem is mitigated slightly by the button prism, which allows a patient to look above or below the prism rather than turning their head.
Reduced Field Expansion – Spotting prisms require optical clarity. The optical clarity of spotting prisms reduces significantly as the prism power is increased. Because of this clarity reduction, prism power for a spotting prism rarely exceed 20 diopters, or 10 degrees.
Where the Prisms Go
The spotting prisms are placed anywhere between 2 and 10 millimeters past the patient’s pupil and towards the ear. Some doctors claim to have a proprietary system for fitting them that results in an improved success rate. At this time, we are unaware of any spotting prism systems that have been scientifically proven to outperform a placebo.