# Optical Forums > Ophthalmic Optics >  Detecting prism in lensometer

## eyechicky

I have a question about reading prism in the lensometer. My understanding is that you can detect vertical imbalance (base up/down) when checking eyeglasses. But can you detect base in /out without knowing what the patient PD is? Thnk you.

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## shanbaum

> I have a question about reading prism in the lensometer. My understanding is that you can detect vertical imbalance (base up/down) when checking eyeglasses. But can you detect base in /out without knowing what the patient PD is? Thnk you.


No.  And, depending upon the RX, how asymmetric your patient's features are, and how the particular frame fits him, the vertical imbalance you measure may not be meaningful unless you know the vertical location through which the patient gazes - which is related to, but not necessarily coincident with, the PRP (Prism Reference Point) height.

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## Miti

Without knowing the Rx it is hard to tell if that up or down prism it is prescribed or it is an imbalance.
 Assuming you do not have prescribed prism you can detect vertical imbalance  for single vision and horizontal imbalance for multifocals.

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## shanbaum

> Assuming you do not have prescribed prism you can detect vertical imbalance for single vision and horizontal imbalance for multifocals.


Not if you don't know the patient's PD and fitting height - which was the question.

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## Miti

_
Assuming you do not have prescribed prism you can detect vertical imbalance for single vision and horizontal imbalance for multifocals.

_
Sorry, I did not pay enough attention. Yes,I need to know the fitting height for single vision, in order to detect vertical imbalance but , please correct me if I am wrong for progressives:

 Without knowing the PD I think I can look for horizontal imbalance if power is less than 3.37.If I do not have prescribed prism,  and I find on the prism dot more than .33 prism in or out in any lens, this is imbalance. Let's say :   
                                          R .5out
                                          L .4 out
                                   Total: .9 out and tolerance is .67
 For unmounted lenses I can not have more than.33 prism in or out.

 For power more than 3.37 I need to know the PD, because acording to ANSI standards I have 2mm tolerance ( 1mm per lens) from the prescribed PD, which I do not know.

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## Shutterbug

> Without knowing the PD I think I can look for horizontal imbalance if power is less than 3.37.If I do not have prescribed prism, and I find on the prism dot more than .33 prism in or out in any lens, this is imbalance. Let's say : 
> R .5out
> L .4 out
> Total: .9 out and tolerance is .67
> For unmounted lenses I can not have more than.33 prism in or out.
> 
> For power more than 3.37 I need to know the PD, because acording to ANSI standards I have 2mm tolerance ( 1mm per lens) from the prescribed PD, which I do not know.


 This will only tell you wheter the lens was ground correctly at the lab.  Without knowing the PD you may still have unacceptable prism.  The Prentice rule is in effect both horizontally and vertically here and you cannot know the effect on the patient without the full Rx requirements.

shutterbug

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## shanbaum

> For power more than 3.37 I need to know the PD, because acording to ANSI standards I have 2mm tolerance ( 1mm per lens) from the prescribed PD, which I do not know.


First, you're confusing the "out of tolerance" condition with the presence of imbalance - it's not correct to say that you have no imbalance because the imbalance is within the applicable tolerance.

Secondly, you seem to be assuming that, in the case of progressives (and, I would guess, other multifocals), you can determine the imbalance by measuring the prism at the lens' specified prism reference point -  in the case of progressives, the dot below the fitting cross.  However, if you don't know the PD and fitting height, you don't know if the lens, and its PRP, are properly positioned in the frame.

Of course, an error there may cause problems more serious than prism imbalance for the patient.  But it still means, you can't determine the prism imbalance in a pair of finished spectacles without knowing the layout geometry (that is, the PD and fitting height).

If you were checking an uncut lens after surfacing, as opposed to a finished pair of spectacles, your approach, if not your semantics, would be correct - though in that case, for a progressive, you actually don't need to know the PD at all (in fact, it's irrelevant), because the reference when checking the uncut lens is invariably the dot; that is, the location of the PRP is specified by the lens designer.  

For multifocals which have no specified PRP on the blank (like, Flat-Tops), you need more information to determine where the PRP is.

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## Miti

Thank you all. For sure I need  better english to explain what I am thinking.

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