Pantoscopic Tilt (°)?-Facial Wrap (°)?

[jofelk]

Darryl Meister,
How are tilt and wrap determined when using the
Lens Tilt & Wrap Compensation Form

from Opticampus?

[Darryl Meister]

The tilt and wrap compensation are computed from "thin" lens equations derived by Dr. Michael Keating. Essentially, the prescription is put into a dioptric power matrix, a single effective tilt axis is determined from the combined wrap and pantoscopic tilt, the prescription is temporarily rotated with respect to the effective tilt axis, a new power matrix is solved from oblique astigmatism equations performed on the original power matrix, and finally the system is rotated back.

These calculations are also based on thin lens equations. While the results are quite accurate, exact ray-tracing methods will be even more precise. It is also important to note that these calculations are based upon lens tilt, which is sometimes difficult to measure; eyecare professionals can generally only measure frame tilt, while lens is also affected by lens form and decentration. I plan to incorporate some additional functionality for calculating lens tilt from additional job parameters later. I may also make a program I wrote available at some point that does this.

[jofelk]

Since a large percentage of wrap frames are similar in shape (rectangular) and size(58mm-62mm) and curve (8 base), could a generic set of numbers be placed in your equation? A number for panascopic tilt and a number for facial wrap? Could both these numbers be vastly different among frames? How can these numbers MEASURED by the average optician?

[Darryl Meister]

Since a large percentage of wrap frames are similar in shape (rectangular) and size(58mm-62mm) and curve (8 base), could a generic set of numbers be placed in your equation?

If you're certain that the parameters (including tilt) are comparable this might be an option; otherwise you'd probably be better off using the individual values. You could also note the values for the common wrap frames you use on a cheatsheet for quick reference.

How can these numbers MEASURED by the average optician?

I don't know that the "average" optician would bother attempting to measure them in the first place. There are various inexpensive tools available for measuring pantoscopic tilt, which needs to be measured on the actual wearer. Really, you need little more than a protractor with a plumb line attached to it.

Facial wrap can be approximately measured by placing a PD ruler across the eyewire aperture so that the ruler is touching the nasal and temporal rims of the frame, and noting the angle that the ruler makes with a plane parallel to the frame front. Though, ideally, the value should be provided by the frame manufacturer (at least until subsequent adjustments).

Once you know the frame wrap, you must calculate the actual tilt of the optical axis of the lens. Assuming that the lens is edged with a guided mini bevel, this will depend primarily on the front surface of the lens and the amount of decentration.

Here is a program I recently wrote that performs the necesssary calculations:

Rx Compensator 2.0

This program is based on 'thin' lens approximations, but it's generally quite accurate. However, programs using exact ray-tracing methods (like SOLA's Spazio process) will be even more precise.

[jimsite66]

Why can't I access your rx compensator 2.0?

When I click on it, I am denied access. Is it still up and running? Maybe I am too new of a poster here?

[Barry Santini]

Facial wrap can be approximately measured by placing a PD ruler across the eyewire aperture so that the ruler is touching the nasal and temporal rims of the frame, and noting the angle that the ruler makes with a plane parallel to the frame front. Though, ideally, the value should be provided by the frame manufacturer (at least until subsequent adjustments

Thank you again, Darryl, for the time and effort to produce and explain these compensators.

Dispensers should be aware of other factors affecting wrap computation:

1. Even refractions done at 20 feet (let alone 10) are undercorrected for driving by 0.16D
2. Frame wrap angle is often *not* simple lens wrap measurements, as is often taught. There is a distinct, but significant difference between *where* the wrap angle acutally initiates, i.e. the centerline bisecting the DBL *or* the nasal edge of the lens *or* in between.
This has a direct effect on determining the effective *blocking* "PD" in order to compensate for critical centering of progressives. It also effects the degree of how particular PD is "narrowed", i.e., narrow PDs are less affected by wrap narrowing than wider PDs. I'm not aware of any frame company that takes this into account.
3. Frame-company supplied wrap angles are often not what you will find *after* insertion and adjustment...particularly on people with eyelash clearance issues.
4. Wrap frames *only* stretch open over time (from wear), so *always* figure less wrap angle, rather than more.
5. Just becuase the frame comes with an 8 BC DOES NOT mean you *have* to use an 8BC. In fact, it is surprising to note that their is not much correlation between frame wrap angle and BC used in the planos.
6. You can often use a BC 1-1.5D flatter than what you'll find in the planos. Of course, using such a flatter curve impacts calculations for wrap angle, but, especially in minus, makes the back curves less steep.
7. I find the main problems with wraps using non-optimized, spherical BCs is NOT the lack of peripheral optimization. It is the individual *starting* Rx is not "on the money" (before calculations are applied) and unnoted phorias that are negatively impacted by the BASE IN that wraps often require.
8. The jury is still ou, IMHO, about *any* progressives and client success. It seems the more they want them, thye better their end result.

FWIW

Barry

[Darryl Meister]

Why can't I access your rx compensator 2.0?

You should access this version: Rx Compensator v2.6.

[Jacqui]

Thanks Darryl

[essegn]

What is the exact formula for power (wrap and pantoscopic angles included) ?

[obxeyeguy]

What is the exact formula for power (wrap and pantoscopic angles included) ?

Use this, it's all done for you, courtesy of Darryl.:cheers:

http://www.opticampus.com/tools/tilt.php

[essegn]

Thanks, but i knew about that calculator ... i am just wondering to have exact formula for that

[Darryl Meister]

The following article by Blendowske provides a nice description of the mathematics involved for thin lenses: Oblique Central Refraction in Tilted Spherocylindrical Lenses. Note that, even for thin lenses, the mathematics can be rather involved.

My program also calculates lens tilt from frame tilt using a few additional factors, such as decentration, since these tilt equations actually rely on lens tilt, not frame tilt, which is an important distinction.

[cmhargrove]

Darryl Meister,
How are tilt and wrap determined when using the
Lens Tilt & Wrap Compensation Form

from Opticampus?

Hello jofelk,
We have been doing a lot of wraps over the past few years, and it is honestly one of the most profitable segments of our business (if done correctly). First, we charge a wrap fee of $50.00. We will wrap any lens, any material, and RX (up to about + or - 6.50).

1. We generally default the "wrap" figure to 15 degrees.
2. Next, we pop out a lens and have the patient look forward in their relaxed viewing position and use a magnetic angle finder (pictured at the bottom of the page) to get the proper "tilt" value. This angle finder can be purchased at any hardware store for about $7-$10 bucks.
Just touch the flat part of the angle finder to top and bottom of the eyewire until the needle settles. Repeat, and average if necessary.
I never compensate a tilt over 15 degrees. If it is slightly greater, I will stop at 15.
3. The most important step comes next. We create a custom foam lap (down to the 100dredths of a diopter) to nail the RX as exactly as possible. Most lab programs will allow you to bypass your standard tooling and create an "exact" tool.
4. When you check these finished lenses (uncut) on your lensometer, make sure you are verifying by the compensated values (with prism).
5. Final inspection will be done in the frame, at the proper "wearing angle" and not flat in the lensometer. This will be the first time yo actually get the lenses on the proper power, pd, and prism.

Then, bingo! You're done.

Here is a picture of a basic magnetic angle finder:

[Stephen Tobin]

Hello Darryl,
In using your program, I am momentarily stumped. Can you tell me what the term "PA" stands for in the prescription line?
Also, I assume the pantoscopic and facial wrap parameters are in degrees. Is this correct?
Thank you,
Steve Tobin

[WFruit]

Hello Darryl,
In using your program, I am momentarily stumped. Can you tell me what the term "PA" stands for in the prescription line?
Also, I assume the pantoscopic and facial wrap parameters are in degrees. Is this correct?
Thank you,
Steve Tobin

PA: Pupil distance above the optical center

And yes, pantoscopic and facial wrap parameter are in degrees. Also note that for retroscopic tilt (highly unlikely, but you never know) you can enter a negative value for the pantoscopic tilt.


Also note that explainations for all fields and exactly what the program does can be found by pressing the help button.

[Darryl Meister]

Yes, WFruit, is exactly right.

Best regards,
Darryl

[The Spectacle]

on sv lenses does the pa value compensate for the oc to be on the 180 or should it be blocked like the seg height of a progressive?

[Darryl Meister]

If you go into the "Options" menu, you can set the optical center at either position.

Best regards,
Darryl

[The Spectacle]

Thanks a bunch you are Awesome!!!