What is the proper way how to determine the corridor lenght at progressive lenses?
The height of the frame only or any more "sofisticated" method ?
What is the proper way how to determine the corridor lenght at progressive lenses?
The height of the frame only or any more "sofisticated" method ?
Are you referring to the proper way to measure the corridor length of a finished progressive lens or the best way to choose the corridor length of the initial lens design?
Best regards,
Darryl
Darryl J. Meister, ABOM
If the former is even possible, I'd love to hear your take on bothOriginally Posted by Darryl Meister
-Tony
(Settles into chair, and grabs popcorn................)
Corridor in my opinion is one of the most subjective elements in a progressive design. It largely comes down to a patient's own preference. If they have previously adapted to a short corridor design they will likely prefer staying in a short corridor lens. If when posed the question of how they intend to use their PAL's they mention intermediate tasks a longer corridor will likely be preferred. it must be understood that all PAL fitting is about managing trade offs, improving the intermediate will sacrifice distance or near, etc.
Of course now most digitally surfaced lenses have multiple corridors so it should be just a matter of requesting a specific length. If a patient is having a difficult time with a PAL it often is just a matter of choosing a different corridor based on their specific complaint.
it is important to note that i have found corridor lengths to be different in different designs so i generally think of them as being relative to lengths with in a specific design algorithm as opposed to comparing the numbers between two different designs.
Minkwitz anyone...
Theoretically, the length of the progressive corridor is the separation between the point that produces the lowest Add power along the corridor and the point that produces the highest Add power. The lowest Add power generally occurs at the distance reference point (DRP) of the lens, while the highest Add power is generally located at the near reference point (NRP). However, in practice, corridor length is usually defined as the distance from the fitting point of the lens to some point along the corridor producing a minimum percentage of the Add power (for example, to 85% of the Add power).What is the proper way how to determine the corridor lenght at progressive lenses? The height of the frame only or any more "sofisticated" method ?
Several factors must be taken into account when choosing the corridor length for a progressive lens design. For example, shorter corridor lengths offer the following advantages:Every additional 1 mm of corridor length requires roughly 2° of additional ocular rotation to reach the near zone. If the corridor length is too long, the wearer may not be able to reach the full Add power without awkward or uncomfortable postural adjustments. Additionally, the minimum fitting height of the progressive lens may not allow for a wide selection of frame styles.
- More near vision utility in smaller frames
- Reduced eye declination during near vision
Shorter corridor lengths, however, have certain disadvantages:Consequently, the length of the corridor should be carefully chosen in order to offer the most utility with the least amount of optical compromise. Furthermore, the rate of change in Add power along the corridor is also important. Excess Plus power around the fitting cross should be kept minimal in order to ensure clear distance vision, and the ramp up in Add power as the corridor approaches the near zone should reflect how a typical presbyope holds materials while reading.
- Less intermediate vision and mid-range utility
- More rapidly increasing levels of unwanted cylinder power and distortion
The actual choice of corridor length and the progression of the add power will depend upon the specific design goals of the lens designer. For larger optical companies, these design goals are often based upon vision science research, wearer trials, and wearer success with previous lens designs.
Best regards,
Darryl
Darryl J. Meister, ABOM
Is there any way how to measure or determine if the patient will have awkward or uncomfortable postural adjustments because of extended ocular rotation?
It has something common with pantoscopic angle or pupilar distance? (the smaller the pd the less ocular rotation will be reached ?)
I guess you don't mean the distance between manufacturer's printed circles, but rather something which represents the actual performance of the lens. I marked on the power cross-section (blue) the two positions of the circles. Do these represent the corridor length. I did the same thing with lenses from virtually all manufacturers, and I can tell you that there's no standardized approach (or, in other words: It's a real Jungle out there).
It's both science and art. In general, moderate to high myopes and advanced presbyopes will need the most attention.
No.It has something common with pantoscopic angle or pupilar distance? (the smaller the pd the less ocular rotation will be reached ?)
The only way I've been able to sort this out is to wear the lens.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Ok, if we expel high myopes and advanced presbyopes. What would be the procedure ?
Is there any article related to this topic ?
That leaves emerging presbyopes and those in between, i.e. +1.75 Adds. I'll address the former first.
My primary concern with emerging presbyopes wearing PALs for the first time is adaptibilty. The lens should have reduced amounts of swim and unwanted surface astigmatism, along with a generous distance zone. The corridor length will have to be average to slightly longer than average to minimize swim and rapid power changes. One exception might for those who only need a near correction (wears for near tasks only)- they might be happier with a short corridor lens that will increase the vertical size of the near zone and minimize posturing.
The intermediate presbyopes, or inbetweeners, require a blend of both concepts and concerns, biasing the design towards distance or near function depending on the degree of close work, night driving, etc. Most will still be asymptomatic at intermediate distances, although that will depend on the amplitude of accommodation, pupil diameter, contrast, and object distance and size.
I'll see what I can find. Use some of the terms I used above in your search, including "reading depth'. I'd like to see more discussion in this area now that most boomers are wearing Add powers at or above +2.00 D.Is there any article related to this topic ?
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
For advanced presbyopes, a compromise is always required when designing progressive lenses. The single greatest factor is unwanted astigmatism, which is proportional to the addition power of the lens. The lens designer must therefore compromise between a longer corridor that offers more intermediate utility while minimizing distortion, or a shorter corridor that provides a more accessible reading area at the expense of increased distortion and reduced intermediate utility, when the presbyope requires it most.
Best regards,
Darryl
Darryl J. Meister, ABOM
With myopes of -2.50-4.00., who say they read w/o glassses, i cut their adds from +2.50-1.75 by 0.75-1.00d, and add a chemestrie plus layer of the same amount to make them into computer/nv
They luv them!
B
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