Corridor length is crucial to successful progressive lens dispensing. NICOLA PEAPER clears up the confusion around this concept and whether trigonometry has a role to play.
When is the long corridor short? Or maybe the question should be how short is a short corridor? And where do we measure it from?
The answer is not the same for all lens manufacturers. One’s 14mm corridor is another’s 11mm. How can this be?
Some manufacturers measure corridor length to the middle of the near reference circle and some to the top. Because the circle is 6mm in diameter, such discrepancies can arise.
If we look at a graph showing the corridor width of a standard progressive, it is affected by length and add power. The graph (below), reproduced with Mr Grant Hannaford’s permission (Academy of Advanced Ophthalmic Optics), demonstrates when the add reaches +2.50, the width of a 14mm corridor is 1.4mm. We should therefore be thankful corridors rarely come shorter than 14mm.
A short corridor may be suitable for reading a phone at 25cm to 30cm away but does not have the width and depth to cope with the intermediate demands of today.
Our love affair with short corridor progressives started in the 1990s with the small frames that were fashionable then. However, the affair continues even with the deeper frames available today. This is often explained by patients being accustomed to having a short corridor and, with a deeper frame, finding it difficult to locate the full near add if the corridor was lengthened. The problem is, how to calculate the length of corridor necessary to give the patient the same head and eye position experience when changing both script and frame?
Should corridor length be measured as a distance along the lens between the fitting cross and an arbitrary position associated with the near reference circle, or should trigonometry come into it?
Diagram 1 demonstrates how pantoscopic tilt (PT) and back vertex distance (BVD) dictate the head and eye position when reading through progressives. When considering the BVD, it shows the further away the lens from the eye, the longer the corridor needs to be to give the same head and eye position for reading clearly.
Likewise, as the PT increases, the corridor length should reduce. Practitioners may have dispensed two pairs of spectacles to the same patient only to have them prefer the visual comfort of one over another. Surely this diagram explains why they ‘felt different’ when, in theory, they were measured and dispensed to be identical.
Is it even necessary to choose a corridor length? Having seen how the intermediate portion of the corridor diminishes as the add increases, modern progressive designs move aberration around to provide more priority to the intermediate area.
Research into modern-day lifestyles, working distances and, most importantly, posture during specific tasks has resulted in suites of lenses, giving priority to different visual tasks, with corridor length being part of the design. The temptation to shorten the corridor, rather than trust the research, simply interferes with the lens design, reducing patient comfort. If patients say the reading seems too far down the lens, look to the BVD and PT. Is the lens sitting too close or is the PT too high? While these factors affect apparent corridor length, they will also increase the amount of oblique astigmatism and other aberrations, causing swim and reduced width of vision.
If in doubt, order designs tailored to the frame being dispensed. That will minimise unwanted aberrations caused by the angle that light is incident on a lens and, if two pairs are dispensed, will allow the same visual and postural experience.
I’m constantly told modern progressives are too complex and slightly incorrect measurements will cause failure. This isn’t true; it’s always taken training and skill to dispense progressives. Today’s patients expect more from their spectacles and understanding the implications of our choices enables us to fulfil their needs.
ABOUT THE AUTHOR: Nicola Peaper spent 20 years working as an optometrist in the UK. For the past 15 years she has worked within the lens manufacturing industry and is currently professional services manager for Rodenstock Australia.