# Optical Forums > Ophthalmic Optics >  Fitting PALs with prescribed prism

## Joann Raytar

> From Clifford W. Brooks, OD Indiana University School of Optometry Bloomington, IN 
> 
> In a recent Optical Dispensing News, Rajesh Wadhwa brought up the problems encountered when prescribed prism is used with progressive addition lenses. Many people with prescribed prism have problems with their progressive lenses because the monocular PDs and fitting cross-heights are measured in the empty spectacle frames without the prism in place. When the prescribed prism is added, the eyes do not "point" in the same direction as they did during lens fitting measurements. 
> 
> To avoid this problem, the person measuring for progressive lenses needs to anticipate the amount the eyes will be displaced by the prism. This amount is about 0.3 mm for every diopter of prescribed prism. The eye will turn in the direction of the prism apex -- away from the base direction. 
> 
> For example, let's take a patient with 5.00 D of prescribed base-in prism for each eye. The patient is measured for progressives using a pupilometer (or ruler) without this prism in place. Monocular PDs are found to be 30 mm for the left eye and 30.5 mm for the right eye. Because of the way the eyes will turn with the prism in place, the monocular PD measurements would need to be changed to 31.5 mm for the right eye and 32 mm for the right eye. 
> 
> The same compensation is used for fitting-cross height in the presence of prescribed vertical prism. 
> ...


OK, if I am following this correctly, if you fit a patient wearing prescribed prism without compensating they are going to end up looking outside the channel.  Are there folks out there that follow the above method and what are your tips or thoughts about the whole process?

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## David Wilson

Jo,
Brooks is, of course, correct. Not that I would dream of disagreeing with him. The rule also applies to single vision aspherics where it is important that the aspheric surface is correctly positioned (with the principal axis of the lens coinciding with the eye's center of rotation). If an aspheric has prescribed prism then the rule that you have quoted from Brooks should also be applied.
Regards
David Wilson

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## Joann Raytar

David,

Would you believe that I never thought about the same problem when fitting aspherics.  Thank you for mentioning that!

Prior to reading the article I had considered displacement when a person is wearing prescribed prism.  I learned my lesson the hard way.  I had a patient that I fit with a progressive using the same techniques I would apply to any other patient.  When he came in to pick up he couldn't see well in the distance but could read and see his computer monitor just fine.  I double checked everything including monocular PD's and seg heights and everything indicated that the lenses were fit correctly, this was while I was sitting at a table directly across from him.  I got up and walked a few feet away to get a towel to clean the layout marks off the lenses and looked over at him to say something.  That is when the problem was disclosed.  When I stood a few feet away his left eye wandered out when he focused on me.  After a careful refit with me stepping back a bit to check the new layout the problem was fixed.

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## Darryl Meister

Yes, Cliff is _The Man_. The important point to remember is that prescribed prism changes the apparent location of an object in order to allow (or cause) the eye to rotate into a new position.

If you consider the fact that 1 diopter of prism (PD) refers to a displacement of 1 cm at a distance of 1 m:

PD = Displacement (cm) / Distance (m)

The distance from the spectacle lens to the center of rotation of the eye is 0.027 m. This means that 1 diopter of prism will cause the line of sight to be displaced towards the apex by,

1 PD = Displacement (cm) / 0.027 (m)

Displacement = 0.027 (cm) = 0.27 mm

at the plane of the spectacle lens. This gives us our 1/3 mm for every 1 diopter of prism compensation rule. Below 3 PD, which equates to 1 mm of compensation, the rule is pretty inconsequential. Dr. Brooks gives a more thorough treatment of the subject in his textbook, including a discussion of how to compensate for the prism induced by decentration of your initial fitting cross compensation. This is not entirely unlike a discussion on the effects of time travel and whether you can actually change the present by traveling into the past.  ;)

Best regards,
Darryl

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## Win C

Does adding prescribed prism to a progressive lens make the the lens thicker? 

For example a patient with a Rx of -5.00D and add of 2.00D with 5 prism BD on one eye.

Will the prism make the lens thicker? 

How will the prescribed prism affect the thickness of a SV lens with this prescription? 

Thanks.

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## lensgrinder

Generally you would grind base down prism in a progressive to equalize the thickness, this is called prism thinning.  5 Diopters would add thickness though, it would depend on the base curve and add power as to how much.  It would also add thickness to a SV.

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## chip anderson

Don't

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