Vertical Imbalance Compensation Form

[essegn]

Hi,

I'm working in small lab and i would like to have theoretical background. During my studies i found a lot of tasks about prisms and i'm really confused..

I found the followng calculator:

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

With prescription of

R: +2.00
L: +3.00

it calculates Prism compensation 1.00 base up in right eye.

1. Fot what it is good to do compensation for right eye? It is obvious, because of smaller dpt power, but when we produce these lenses in the optical centre (optical centre lies in geometrical centre) we found zero value of prism in each lens

2. Does the Rx program somehow counts on it and modify the calculation for prism compensation ?

3. What does it mean: reading level distance ?

Many thanks

Peter

[YrahG]

Hi,

I'm working in small lab and i would like to have theoretical background. During my studies i found a lot of tasks about prisms and i'm really confused..

I found the followng calculator:

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

With prescription of

R: +2.00
L: +3.00

it calculates Prism compensation 1.00 base up in right eye.

1. Fot what it is good to do compensation for right eye? It is obvious, because of smaller dpt power, but when we produce these lenses in the optical centre (optical centre lies in geometrical centre) we found zero value of prism in each lens

2. Does the Rx program somehow counts on it and modify the calculation for prism compensation ?

3. What does it mean: reading level distance ?

Many thanks

Peter

1. the prism imbalance is caused when the patient looks off center, like when reading th epatient will gaze down, this dioptric difference will cause a prism imbalance. 1.00 diopters won't usually cause any problems.

2. I don't know what program you are using.

3. Reading level distance is the distance from the Distance Reference Point (DRP) to the Near Reference Point (NRP) in your example above with a 1.00 imbalance the value was 10mm.

[drk]

It's for deciding whether you need slab-off, methinks.

[Rafael]

http://www.optiboard.com/forums/show...ting-Cross-NRP


http://www.zeiss.de/4125680f0052ec92...256cb1004b7529

[Darryl Meister]

As the others have indicated, "vertical imbalance" in this context refers to the difference in vertical prism induced by the distance prescription while looking through the lower portion of the lens during viewing activities such as reading. Differences in prescription power will introduce differences in vertical prism as the eyes gaze away from the distance optical centers to read. This vertical prism imbalance can result in various symptoms for the wearer if it is large enough to cause significant disjunctive movement between the two eyes, a condition known as spectacle-induced anisophoria.

The OptiCampus calculator determines the vertical prism in the near region of the lens necessary to compensate for this difference in vertical prism, relieving the anisophoria. Typically, this vertical prism compensation will be applied as a slab-off by bicentrically grinding one lens surface, although there are other methods of reducing vertical prism imbalance at near as well, such as the use of Fresnel Press-On Optics, dissimilar segments, and so on.

[essegn]

If i understand right, if the vertical prism is more than app. 1.5 pdpt it is necessary to employ slab-off.

1. This calculation is useful only for bifocal and multifocal lenses, why the addition is not included in that formula ? Because in some cases is not equal for the both eyes.

2. If the prismatic diopter is more than 1.5 pdpt and slab-off is not used, is there any possibility that wearer can beccome accustomed to it after some time ?

3. Does usually the function of slab-off included in Rx program of well-know machine producers or it is the feature that must the person (operator) to do it during the calculation ?

[Darryl Meister]

If i understand right, if the vertical prism is more than app. 1.5 pdpt it is necessary to employ slab-off

Studies have shown that unwanted vertical prism at near below 1.5 prism diopters seldom results in symptoms (eyestrain, double vision, etcetera). Also, it is difficult to produce a clean slab-off line with low amounts of prism. You might be better off going with dissimilar segments if you want to correct less than 1.5 prism diopters.

Of course, a wearer can be asymptomatic with no problems even with extremely high levels of vertical prism imbalance, if he or she has impaired binocular vision, such as amblyopia or "lazy eye."

This calculation is useful only for bifocal and multifocal lenses, why the addition is not included in that formula ? Because in some cases is not equal for the both eyes.

1) Because, with only rare exception, the addition powers are equal and 2) because the differences in prism due to differences in Add power are generally very small, particularly since the line of sight is seldom more than a few millimeters above or below the optical center of the segment.

If the prismatic diopter is more than 1.5 pdpt and slab-off is not used, is there any possibility that wearer can beccome accustomed to it after some time ?

Yes, some wearers may eventually develop a physiological coping mechansim known as "gaze specific prism adaption," which is essentially a tolerance to the spectacle-induced anisophoria. And, again, if the wearer has impaired binocular vision, he or she may not benefit from a vertical imbalance correction.

When in doubt, however, you are better off applying the slab-off.

Does usually the function of slab-off included in Rx program of well-know machine producers or it is the feature that must the person (operator) to do it during the calculation ?

I believe that many laboratory surfacing programs have a slab-off calculation routine included in the software.

[Rafael]

Dear essegn, wich lab software are you using?

PM i can give you a excel sheet for this. You can use then the amout of needed prism the excel sheet give you in every RX Calculation software, to calculate the proper additional thickness you will need for process the slab off (especially on minus lenses).

Backside Slab Off on Hard Lap Tools Howto:

1) use the excel sheet to determine the amout of Slab Prism

2) Calculate the RX with the additional needed prism from excel sheet (Base 90)

3) Determine additional thickess on minus lenses don´t go under 1.8mm @ B270

4) Note the Center thickness

5) Take away the additional slab prism out and calculate the normal RX with the noted center thickness

6) Block the lenses how you will do it normally

7) Generate the Lens with the additional Prism from the Excel sheet Base 90, take care you put the lens in the correct direction!!! (Tell me wich generator you use)

8) Surface the normal way, but don´t deblock the lens

9) Fill up the Lens with resin, wait the time you need for surfacing

10) Generate the lens again (take care correct direction), but don´t generate to the end E Value, let it thicker. As far the Slab Line apears, reduce the material removal (0,5 mm is ok).

11) Control the Slab Line, if not straigt you can correct it with the generator. With little prism in Base 0 or 180. Or you correct it till smoothing, with pressure on the lens Base 0 or 180.

12) Smooth the Line down as you will design you slab.
I make Bifocal on the segment line
Progressives 3 to 4 mm under prism reference point.
But take into account the stock remove on you polishers, depends on you polish

13) Polish the Lens and control it

14) deblock, remove epoxy.....

[essegn]

Dear essegn, wich lab software are you using?

-------Bifocal lenses are being produced on Satisloh line and progressives on Optotech Freeform machine, so we are using the RX program from mentioned companies.

PM i can give you a excel sheet for this. You can use then the amout of needed prism the excel sheet give you in every RX Calculation software, to calculate the proper additional thickness you will need for process the slab off (especially on minus lenses).

------I'm not sure if you think the xls that you sent me before > http://www.optiboard.com/forums/show...ting-Cross-NRP

------As you wrote to calculate additional thickness.. Doesn't the vertical imbalance mean that we should decrease the thickness - and that means slab-off ?

---- Is the slab off the same for bifocal and progressive lenses ?

Backside Slab Off on Hard Lap Tools Howto:

1) use the excel sheet to determine the amout of Slab Prism

2) Calculate the RX with the additional needed prism from excel sheet (Base 90)

---- if you think the mentioned xls file, i don't know which parameter do you mean..
---- i can see there> Prism fitting cross,Prism imbalance Fitting cross, Prism imbalance NRP
---- do you mean to set the additional needed prism to our RX program? If so, we have the only possibility to set it to the collum: Prism or Prism base

3) Determine additional thickess on minus lenses don´t go under 1.8mm @ B270

---- how can I determine additional thickness? We have set in our database the center thickness of minus lense to 1.7

4) Note the Center thickness

5) Take away the additional slab prism out and calculate the normal RX with the noted center thickness

------ point 2 above..

6) Block the lenses how you will do it normally

7) Generate the Lens with the additional Prism from the Excel sheet Base 90, take care you put the lens in the correct direction!!! (Tell me wich generator you use)

----- we have two generators - Satisloh (convetional line) and Optotech (Freeform)

8) Surface the normal way, but don´t deblock the lens

9) Fill up the Lens with resin, wait the time you need for surfacing

------ what does it mean full up the lens with resin ?

10) Generate the lens again (take care correct direction), but don´t generate to the end E Value, let it thicker. As far the Slab Line apears, reduce the material removal (0,5 mm is ok).

----- i can't imagine how the slab line look like
----- could we get the slab line on bifocal and mulifocal as well?

11) Control the Slab Line, if not straigt you can correct it with the generator. With little prism in Base 0 or 180. Or you correct it till smoothing, with pressure on the lens Base 0 or 180.

----- what kind of correction do you mean ?

12) Smooth the Line down as you will design you slab.
I make Bifocal on the segment line
Progressives 3 to 4 mm under prism reference point.
But take into account the stock remove on you polishers, depends on you polish

---- what does it mean to do bifocal on the segment line? moving of optical centre to the top of segment ?
---- progresice 3 to 4 mm under prism reference point? Do you mean two lenses with different dpt power have the same prism under 3-4mm under PRP ?

13) Polish the Lens and control it

14) deblock, remove epoxy.....

--------------

I need to say, that we haven't got any problems with vertical imbalance compensation. I believe that our RX programs have included this feature. All this thread is only about to learn and understand this part of it.

Thank you very much

Peter

[drk]

I tip my hat to Darryl who exhibits strong understanding of visual science.

From my clinician's perspective, it would be rare that vertical imbalances of 1.5^ require slab off; a high fitting height is the best approach.

In fact, I don't even begin to think about vertical imbalance on downgaze until 2^ (and it's easier to remember than a rational number :)).

[Rafael]

Dear essegn,
a)I know only the RXPert from Satisloh, here i can help you. And this software have not the possibility to calculate slab off in automatic.
b)It´s not the same excel sheet, a few more formulas. But i have it only in german, must translate it first.
c)Slab of means bi centric grinding. You need the additional thickness on minus lenses because you generate the lens 2 times with the same curve, but at another angle to each other. And sometimes the lens get to thin and is not solid so you must control the thickness.
The process for bifocal & pal is the same.
2) yes you can put the value in the column for you additional prism with direction.
3) The standard value is uninteresting in this moment, if you add the prism in you programm you will get an new ct thickness. Than you have the lens shape and determine the et thickness of the lens shape in Base 270. If it thinner than 1.8, you have to increase it. Than you get the new ct thickness. And this you need for see above point: c)
7) On LOH Vpro/V95/VFT i can tell you how to put the lens in the machine. If you put it wrong into the machine, you will slab from the wrong site.
9/10) you need to fill up the epoxy resin after 1 Surfacing, thats needed to generate the second curve with another angle to the first curve. If you do so and generate the second time, there will apear the slab line between the epoxy and the first surface. Work for both bifocal & pal.
11) Sometimes the slabline is not straight to the segment line or the pal markings, so you must corect it. You can make it with the generator, adding a prism with a base direction, so you can get it straigt or you make the same with adding pressure on a point that you will correct througt the smoothing.
12) You move the slab line, because the thickness of the epoxy/lens decline through the stock removal. On high prisms its get slower, on low prism it gets faster and as Darryl said harder to control the line. It will get wavy.

[Darryl Meister]

11) Sometimes the slabline is not straight to the segment line or the pal markings, so you must corect it. You can make it with the generator, adding a prism with a base direction, so you can get it straigt or you make the same with adding pressure on a point that you will correct througt the smoothing.

Yes, you can adjust the vertical position and orientation of the slab-off line slightly while fining or smoothing. Fining the lens longer will move the slab-off line further. Pressing on one side of the lens blank or the other during the fining stage can adjust the orientation of the slab-off line slightly, if it is tilted with respect to the segment ledge.

Also, the slab-off line will "move" from either the top of the lens blank or the bottom of the lens blank toward the intended position at the bifocal ledge as lens stock is removed, depending upon whether you generated the upper distance zone curve first or the lower near zone curve first. You want to ensure that the slab-off line doesn't move entirely to the segment ledge while generating, since the fining process will move it slightly as well.