Centration, PD and wrap eyewear

**Barry Santini** · 11-09-2009, 06:14 PM

I've always understood that the BLOCKING PD employed to fabricate wrap eyewear should be effectively WIDER than the GIVEN PD, because as the frame wraps from the (traditionally-flat) FRAME PLANE, it effecvtively NARROWS the PD as MEASURED/VERIFIED IN THE FRAME PLANE.

Rodentsock makes a different case, using the intersection of the patient's visual axis with the FRONT SURFACE of the lens, and concludes that the "corrected PD" (COR PD) factor to be used in blocking/fabrication is, in fact, NARROWER than the given PD.

Since the patient's visual axis intersects the REAR of the lens with respect to SEEING THROUGH A FF, rear-surface progressive corridor, I'm confused by what I should be paying attention to.

Take the poll, and please post your thoughts.

Thanks

Barry

**Barry Santini** · 11-09-2009, 06:23 PM

I can't seem to get the Rodentsock info pasted/attached to the post.

Can I email it to anyone with better computer skills, and have you attach it to this thread?

Send me an email if you can help.

Barry
bsantini@fnol.net

**Darryl Meister** · 11-12-2009, 01:24 AM

Ignoring the effects of lens thickness for now, you are correct in stating that you should widen the PD for wrap frames slightly in order to maintain the same optical center distance after edging. The monocular PD of each lens will effectively "shrink" by an amount proportional to the cosine of the wrap angle.

That said, this will only ensure that the optical center is located in front of the pupil center after edging. It will not necessarily ensure that the patient does not experience unwanted prism, since the tilt of a lens with non-zero thickness and front curvature also introduces some horizontal prism. In some cases, you may not want the optical center in front of each eye.

I'd have to review Rodenstock's material before commenting on their particular approach though.

**Rafael** · 11-13-2009, 05:26 AM

Darryl is the excel sheet correct?

**Barry Santini** · 11-13-2009, 01:55 PM

Yes, this is the approximate formula I thought to use for blocking wraps.

But...Rodenstock's approach IS JUST THE OPPOSITE! They calculate a narrower factor (COR PD)

I've been able to email Darryl the info, and await his analysis, which he will post here.

Barry

**HarryChiling** · 11-13-2009, 02:07 PM

Barry,

You mind shooting me a copy.

**Barry Santini** · 11-13-2009, 02:31 PM

No problem, Harry.

Send me an email for me to reply to.

bsantini@fnol.net

**YrahG** · 11-13-2009, 03:27 PM

A quick ray trace leads me to the PD needing to be compensated out or wider than the supplied PD. A little more in depth and the calculations is as follows:

x = pd comp (mm)
t = thickness (mm)
n = index
f = tilt

x = t*sin(f)/n

That's my quick and dirty on the subject. I have attached a [strike]PDF[/strike]JPG of my chicken scratch to view, the bottom left is where I show that the ray essentially comes out parralel (power wasn't a factor in this drawing), then angles created where the ray intersects the lens is blown up a bit on the bottom right with an explanation of the logic to come to the conclusion above. Not the most elegant but it works.

The compensation looks to be insignificant with realistic numbers being plugged in except in the plus power range where the thickness can cause the compensation to be significant.

I did not factor into this equation any effects from prism compensation as that would have an effect as well, I think in the case of wraps we get to the point where it becomes too complex to keep plugging in factors to compensate for, the level of accuracy to some extent is unnecessary.

**Rafael** · 11-13-2009, 07:36 PM

Barry, can i have a copy too.

**Barry Santini** · 11-14-2009, 08:46 AM

Sure...on it's way. Let me know what you think!

B

**HarryChiling** · 11-14-2009, 09:43 AM

Barry it looks to me like the paper shows in the first illustration and second illustration that the effect is a narrowing, therefore the compensation would be to fit it wider, the formula above looks interesting.

**Barry Santini** · 11-14-2009, 10:27 AM

Yet, Harry, Rodentsock suppiles the edging entity with a COR PD (corrected PD) value, that they say, is to be USED when blocking for edging.

This value is *always" narrower than the patient's PD. I asked the same as you for clarification, and got the response you saw.

Barry

**Darryl Meister** · 11-14-2009, 12:23 PM

I've had a look at the Rodenstock piece. As I said earlier, there are generally two considerations when decentering a lens in a steeply wrapped frame: mechanical and optical.

The Rodenstock piece just describes the mechanical aspect. When correcting for the mechanical aspect, the monocular PD decentration will need to be modified slightly in order to ensure that the optical center or prism reference point remains in front of the pupil center after edging:

$PD_{NEW}=\frac{PD_{OLD}}{\cos W}$

where W is the angle of tilt. Note that this is not a change to the desired PD; it is simply a small adjustment that a finishing laboratory would need to make in order to obtain the desired PD after edging.

This will not necessarily result in a prism-free condition for the actual wearer though, since the face-form tilt of the lens introduces unwanted horizontal prism imbalance. The prism induced by lens tilt is given approximately by:

$\Delta=100\tan W\cdot\frac{t}{n}B$

where t is the center thickness in meters, n is the refractive index, and B is the base curve.

Applying an optical compensation for this induced prism would result in a change to the desired monocular PD distances in the form of horizontal prism, which is a change that the original eye care professional should be made aware of when receiving the order.

Further, both forms of compensation may be applied simultaneously: 1) The moncoular PD may be mechanically adjusted to ensure that the prism reference point distance remains correct after edging, while 2) horizontal prism may be added to the prescription in order to overcome optically any prism induced by lens tilt.

**Barry Santini** · 11-14-2009, 01:24 PM

Thanks, Darryl & Harry.

It's obvious that, despite my prompt to them, the Rodenstock COR PD is a value forecast where the PD will END UP AFTER edging into a wrap, it no adjustment is made in blocking.

I think..(hope)..

Barry

**Optician1960** · 11-16-2009, 10:17 AM

1st, thanks for the great discussion on the blocking pd vs actual for the wraps...

Have you tried any other centration devices: Zeiss, AIT Office, Optikam? If so, thoughts?

**HarryChiling** · 11-18-2009, 09:26 AM

Originally Posted by Barry Santini

I can't seem to get the Rodentsock info pasted/attached to the post.

Can I email it to anyone with better computer skills, and have you attach it to this thread?

Here you go.

**Fezz** · 11-18-2009, 09:43 AM

Thanks Harry!

:cheers::cheers::cheers:

**Barry Santini** · 11-18-2009, 12:37 PM

I think I get it now. Despite my query that Rodentsock didn't reply consistent with my question, the COR PD is the "finished PD that would result IF the actual/real client PD was used for blocking.

Of course, the COR PD is narrower...DUH!

Barry

**Luis davila** · 11-19-2009, 01:54 AM

hi, you have three ways to do it:
First Ask for the lab to surface the prism
second you can dot up with the lensometer just putting the center in the second circle depnds if it is BO or BI and R or L.
The last one is usin this formula to find how much milimiters to descenter.
d=sph+cil(senaxis)2/Prism