# Optical Forums > General Optics and Eyecare Discussion Forum >  what is split prism for glasses

## Jamesye

thanks for reply

James

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## chip anderson

I guess you haven't gotten a reply since it doesn't appear on this thread.
Split prism is using half of the prescribed prism in opposing directions in each lens to achieve a more cosmetic but identical ocular result.

Chip

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## harry a saake

Also to add to Chips comment, when a doctor prescibes prism in a certain direction, it does not neccesarily have to be  an even split. example 8 degrees of prism base up OD, you could split 4 up in one eye and 4  down in the other, and that will work. now however you need to look at the powers that are actually running in the 90 degree meridian, if you have a lot more power in the one eye you will put less of the prism in that eye and more in the other, thus you might split it 5 and 3 or maybe 6 and 2, idea is to get uniform edge thickness as much as possible in both of the lenses

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## tmorse

> Also to add to Chips comment, when a doctor prescibes prism in a certain direction, it does not neccesarily have to be an even split. example 8 degrees of prism base up OD, you could split 4 up in one eye and 4 down in the other, and that will work. now however you need to look at the powers that are actually running in the 90 degree meridian, if you have a lot more power in the one eye you will put less of the prism in that eye and more in the other, thus you might split it 5 and 3 or maybe 6 and 2, idea is to get uniform edge thickness as much as possible in both of the lenses


Minor correction...the unit of measure for prism is 'prism diopter' , as in eight (8.0^) prism *diopters* BU that you would split 4.0 and 4.0.;)

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## HarryChiling

> Minor correction...the unit of measure for prism is 'prism diopter' , as in eight (8.0^) prism *diopters* BU that you would split 4.0 and 4.0.;)


You could do the same with degrees although diopters is a more common measure.

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## Jacqui

How many degrees are in a diopter ??

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## Dave Nelson

Prism diopters are based on the degrees of deviation of a light ray, measured 1 meter from the prism, on a flat surface. FYI, the same unit measured on an arc is referred to as a centrad, but is not used in an ophthalmic setting. Although prism diopter is, as Ted points out, the correct term, degrees are often used interchangeably with diopters, particularly by lab people.

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## mullo

Keep in mind when splitting prism that _vertical prism_ can be split with one lens "base up" and the other lens "base down", so when compounded they add up to the total amount of prismatic effect. 

i.e. 8^BU O.D. would be split 4^BU O.D. and 4^BD O.S. 

But when splitting *horizontal prism* the lenses would be both the same base direction (both lenses "base in" or "base out" to create the compounded prismatic effect.

i.e. 8^BI O.D. would be split 4^BI O.D. and 4^BI O.S.

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## harry a saake

MORSE, i am aware of the prism diopter thing, but i always thought it was a stupid term and i know there are those who disagree, and thats fine, however one thing is sure, if you make a split 4 up and 4 down i guess you could call it 4 roses and you would still have the same thing, BTW some of the old focimeters were actually marked in degrees

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## gunner05

And here I thought it was a way of cracking a lens when it had prism in it.  I think I may need to go back and make a few phone calls and apologies....  Damned walmart optical education...

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## TLG

It isn't real common, but you DO need to be careful about splitting prism for patients that have not had it split in the past. You cannot simply assume they will be ok to split. I had a patient with just 1.5 prism dioptors that I split and she could not see comfortably out of them. Putting it all back into one eye solved the problem.

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## tmorse

> MORSE, i am aware of the prism diopter thing, but i always thought it was a stupid term and i know there are those who disagree, and thats fine, however one thing is sure, if you make a split 4 up and 4 down i guess you could call it 4 roses and you would still have the same thing, BTW some of the old focimeters were actually marked in degrees


Harry... in 1892 Prentice gave us Prentice's Rule and the prism formula we use today. He also defined 1.0^ prism diopter as 1cm (10mm) of light ray devistion over a standard distance of 1 meter. We owe him big time for that, regardless of what you do/say in a lab setting. :D

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## harry a saake

Morse, i dont disagree that the formula is there, and i am aware of it and use it, i just happen to think that its not really that revelant to make a difference one way or the other

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## Bob MacNeil

In the lab,  prism degrees are how we calculate prism diopters.  Prism degrees are angular, we apply the lens index to get the diopterive value you are expecting.

Bob

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## tmorse

> In the lab, prism degrees are how we calculate prism diopters. Prism degrees are angular, we apply the lens index to get the diopterive value you are expecting. Bob


Sure, you are surfacing and inducing a set amount of prism in the lens. But in this case the optician is intent on splitting the prism the Dr. ordered. How the lab actually does it is not relevant to splitting it for lab ordering purposes, right??  :Confused:

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## Jacqui

Is a split prism actually as good as haveing the entire prism on one lens like the Dr. ordered?? I don't think it would be. Some of you will say yes, but stop and think for a minute and forget the fancy education and formulae.

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## EyeFitWell

It's usually best to talk to the prescriber and double check before you split the prism.  There are some circumstances when it must be only on one eye as written, but most of the time it's OK to split it.

BTW, In our lab we call it 8 redos BU, as our wholesale can't seem to get prism right the first time.  j/k ;-)

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## Jacqui

> btw, in our lab we call it 8 redos bu, as our wholesale can't seem to get prism right the first time.  J/k ;-)


roflmao !!

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## tmorse

> Is a split prism actually as good as haveing the entire prism on one lens like the Dr. ordered?? I don't think it would be. Some of you will say yes, but stop and think for a minute and forget the fancy education and formulae.


So let's hear you defend (explain) your position. 

As to "forget the fancy education and formulae???" what is this supposed to mean??

There is a separation of duties between lab techs and Opticians, and a separation of skills and a separation of knowledge. These are two (2) completely different fields and it's time that this industry acknowledges this simple fact. Opticians don't tell lab techs how to do their job. They just want a result within specifications and tolerances. 

You may know how to surface a lens with or without prism, but I don't think you really understand the concept of prism. What it is and what it does.:hammer:

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## HarryChiling

> How many degrees are in a diopter ??


*0.57o*, there is no advantage using one form of measure over another EXCEPT that the most commonly used measure is diopters so it would be TECHNICALLY correct to use diopters fro easy communication, but if I requested a deviation of 2 farthings over 100 farthings (whatever farthings are) if you understand prism you would know I would mean 2 diopters, the degree to me makes it easier when programming and also in a lab setting.




> There is a separation of duties between lab techs and Opticians, and a separation of skills and a separation of knowledge. These are two (2) completely different fields and it's time that this industry acknowledges this simple fact.


You may be right here, but at one time you were a lab tech BEFORE you were an optician and that was to make sure you understood optics.  Now it seems that either side of the house is build on a shaky foundation.

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## jherman

and often am, however, wouldn't more movement occur in a lens the had more power, making the higher power lens the better choice for the majority of prescribed prism?










> Also to add to Chips comment, when a doctor prescibes prism in a certain direction, it does not neccesarily have to be an even split. example 8 degrees of prism base up OD, you could split 4 up in one eye and 4 down in the other, and that will work. now however you need to look at the powers that are actually running in the 90 degree meridian, if you have a lot more power in the one eye you will put less of the prism in that eye and more in the other, thus you might split it 5 and 3 or maybe 6 and 2, idea is to get uniform edge thickness as much as possible in both of the lenses

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## Jacqui

> So let's hear you defend (explain) your position.


I don't think it's proper to put the one eye that is in balance out of balance. The good eye ( the one with no prism) should be left to look staight ahead and the other (the one that is out of balance) brought to the straight. 




> As to "forget the fancy education and formulae???" what is this supposed to mean??


Just what it says :) Let's use plain English so that everyone understands.




> There is a separation of duties between lab techs and Opticians, and a separation of skills and a separation of knowledge. These are two (2) completely different fields and it's time that this industry acknowledges this simple fact. Opticians don't tell lab techs how to do their job. They just want a result within specifications and tolerances.


I know there is, I've done both. :) I stick with lab work because I like to make things. I have also worked as an Optician, Ophthalmic Tech and Ophthalmic Nurse. I have ABOC and RN




> You may know how to surface a lens with or without prism, but I don't think you really understand the concept of prism. What it is and what it does.:hammer:


As an Ophthalmic Tech and Ophthalmic Nurse, I've written numerous Rx's with prism and yes I do understand principles. I wear prismatic lenses, so I also have personal experience.

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## Jacqui

> *0.57o*, there is no advantage using one form of measure over another EXCEPT that the most commonly used measure is diopters so it would be TECHNICALLY correct to use diopters fro easy communication, but if I requested a deviation of 2 farthings over 100 farthings (whatever farthings are) if you understand prism you would know I would mean 2 diopters, the degree to me makes it easier when programming and also in a lab setting..


Thank you, Harry !! I would give you a big smootchie, but Steve still hasn't put one on here. 






> You may be right here, but at one time you were a lab tech BEFORE you were an optician and that was to make sure you understood optics.  Now it seems that either side of the house is build on a shaky foundation.


Most of the good Opticians did start in the lab.

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## HarryChiling

> Thank you, Harry !! I would give you a big smootchie, but Steve still hasn't put one on here. 
> ............................................................  .............. 
> Most of the good Opticians did start in the lab.


I think that all opticians should have some lab time, it would help prevent those orders that require 10mm+ decentration per eye from coming into the lab with requests to "MAKE THEM THIN THIS TIME". It is a custom made product and if the opticians doesn't understand the lab tech and the lab tech doesn't understand the opticians we have communication issues, which cost everyone time and money.

Ted does have a very valid point about the prism diopter you will find the latest LDS does not contain a definition of degree, it refers to prism units as diopters so for communication between front house and back house should be refereed to as a prism diopter.

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## chip anderson

Prisms in optical instruments (not spectacles) are usually measured in degrees (as in the 45 degree one found in a lot of slit lamps and binoculars).   In spectacles they are prescribed, written and measured in diopters.  At small distances the difference in actuall light deviation (which is what a prism does) is miniscule but still spectacles are measured in diopters.  Just as that little fine line on the lensometer (focometer for you fererners)  measures in eights of a diopter although you will most often hear it referred to as a twelth (.12 diopter is an eighth of a diopter _or at least within .005 diopter as an eighth of a diopter is actually .125 diopter). _ We call it a twelth but that ain't what it is.  *Spectacle powers are always stated in diopters.*

Chip

Fo' you jumps on me I knows I kain't spell twelth.

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## tmorse

> Most of the good Opticians did start in the lab.


This canard must be put to rest. 

Today I would suggest that 'most of the good opticians' start by taking and passing formal opticianry schooling prior to entry into the field, so that they understand optics.:cheers:

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## HarryChiling

> This canard must be put to rest. 
> 
> Today I would suggest that 'most of the good opticians' start by taking and passing formal opticianry schooling prior to entry into the field, so that they understand optics.:cheers:

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