# Optical Forums > Ophthalmic Optics >  Ground-in prism

## Neophyte Optician

If a pair of glasses is supposed to have ground-in prism (e.g. 2 prism diopters base right), when I verify it on a manual lensometer should the 2 prism diopters show up no matter which part of the lens I am looking at?

Also, how do I grind that in?

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## Fezz

The grinding-in process is done at the surfacing lab. You should be able to *detect* that a lens has prism in it when verifying any part in the lensometer. But, checking for accuracy of that prism amount has to be done correctly.

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## Leo Hadley Jr

What is the power of the lens?
Sometimes you can manually induce the prism with a stock lens. If the power is low, you need to grind it in surface.

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## Neophyte Optician

It's essentially a -6.00 lens in each eye.  I need to put 2 prism diopters in each eye.  Is it possible to get 2 prism diopters throughout the lens by grinding it into the lens?  How does Prentice's rule affect this?

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## Leo Hadley Jr

You can easily induce the prism in a -6.00 provided your frame is not abnormally huge. Just layout the lenses as normal *with the prism induced*, spot, block, and edge. Your result should be perfect.

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## gemstone

Nope, not here.

http://www.abo-ncle.org/licensing.html

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## chip anderson

I been done to seen a lot o Rx's with prism.  Ain't never seen one with prism base right on it.   I seed, base in, base up, base down, base out and at a few odd angles, and some different in the top and the bottom.  But can't see why you would have base right unless the patient had a fused neck and his head was on crooked.

Chip

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## Ory

> I been done to seen a lot o Rx's with prism. Ain't never seen one with prism base right on it. I seed, base in, base up, base down, base out and at a few odd angles, and some different in the top and the bottom. But can't see why you would have base right unless the patient had a fused neck and his head was on crooked.
> 
> Chip


There are several reasons for base right or base left (AKA Yoked prisms).  They can be used to shift the image binocularly to the null point for nystagmus, to straighten a head turn due to extraocular muscle imbalance, etc.

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## fjpod

Yoked prisms can also be used in low vision situations such as in stroke and head trauma. In this case, I would not recommend using decentration to achieve the prism. Put the OCs in the middle of the pupil and have the prism ground in.

Just my hunble opinion.

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## obxeyeguy

> Put the OCs in the middle of the pupil and have the prism ground in.
> 
> Just my hunble opinion.


Huh?  Prism is obtained by moving the OC, either by decentration or grinding, same result.  If the OC is in the middle of pupil, no prism.

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## fjpod

> Huh? Prism is obtained by moving the OC, either by decentration or grinding, same result. If the OC is in the middle of pupil, no prism.


With the OC in the middle of the pupil, there is no prism unless you grind it in, and if you do, then you will have the optical centers of the lenses in front of the pupil (which is where the best VA is) and you will also have your prism.  This would be even more important if the lenses are aspheric, which they very well might be in a -6.00.

Maybe Daryl Meister can explain it better...or maybe I'm wrong, but prism by decentration and ground in are not the same in all respects.

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## odosou

If my optics memory serves me correctly the optical center is the place where there is no prism, as measured by a lensometer.  If you dot the place in the lens where the pupils are and measure the amount of deviation that is the amount of prism in the Rx.  As far as grinding in versus decentering, a lot depends on the Rx and how much prism you can obtain by decentration.  In high Rx's this is pretty easy, in low Rx's you will have to grind it in.

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## fjpod

Odosou,

Everything you say is true, but it is my understanding, that when you decenter an aspheric -6.00, the Rx of the lens changes as you move away from the center.  So if you try to achieve prism by decentration in these cases, you might not be doing the best thing for the patient.  You might be hurting their BVA.  In particular, when prisms are yoked (bases in same direction, right, left, up or down), there is usually some visual, vestibular, or cortical pathology involved, and it would be best to insure the patient is getting the best VA through the center.

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## obxeyeguy

> With the OC in the middle of the pupil, there is no prism unless you grind it in, and if you do, then you will have the optical centers of the lenses in front of the pupil (which is where the best VA is) and you will also have your prism. This would be even more important if the lenses are aspheric, which they very well might be in a -6.00.
> 
> Maybe Daryl Meister can explain it better...or maybe I'm wrong, but prism by decentration and ground in are not the same in all respects.


Huh?  Aspheric is a different story.  Decentered and grinding accomplishes the same thing.  With any prism, the OC is NOT in front of the patients pupil.  That is called prism.  No prism, OC's are on PD.  Yes you are correct thou, you can not decenter an aspheric stock lens much to gain prism.  Non aspheric, you can't tell the difference

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## odosou

One thing about the decentering that also comes to mind, not just aspherics but also low abbe value materials, i.e. high index materials have more chromatic problems the farther from the geometric center you go. (Think about the patient that needs that MRP in Polycarb)  Grinding in would possibly help some, but you are still off the optical center so prism kind of stinks when it comes to the best optics unless dealing with glass or CR-39.

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## Darryl Meister

> Maybe Daryl Meister can explain it better


...



> With any prism, the OC is NOT in front of the patients pupil. That is called prism. No prism, OC's are on PD. Yes you are correct thou, you can not decenter an aspheric stock lens much to gain prism. Non aspheric, you can't tell the difference


It's a little more complicated than that though. You can have prism, for instance, without any "optical center." In fact, decentration to induce significant magnitudes of prism is only practical with relatively strong prescriptions.

Essentially, prism occurs whenever a plane tangent to the front surface is tilted relative to a plane tangent to the back surface at a given point, so that the surface slopes differ. Prism is directly proportional to this difference in slopes. When these two planes are parallel, on the other hand, rays of light leave the back surface traveling in the same direction that they enter the front surface. At the optical center, for instance, the planes tangent to the front and back surfaces are parallel, so no prism is produced. (Actually, this is a bit of an oversimplification, but we don't need to worry about the details.)

In order for a lens to bring rays of light to a single point focus, light striking the lens 1 cm away from the optical center must also be deflected by 1 cm, while light striking the lens 2 cm away from the optical center must be deflected by 2 cm, and so on. A plus or minus lens with power deflects rays of light by a progressively increasing degree away from the optical center because the slopes of the front and back surface become more and more tilted with respect to each other away from the center. In a minus lens with a plano front surface, for instance, the slopes of the front surface remain flat, whereas the slopes of the concave back surface become progressively steeper.

Decentering a lens simply moves the lens far enough to place a point on the lens in front of the eye where the slope of the front surface differs from the slope of the back surface by the desired amount. Alternatively, you could simply tilt the front surface relative to the back surface until the desired difference in slopes is achieved, which is how prism is "ground." The effect, in terms of the difference in surface slopes at least, is essentially the same, however. In fact, tilting a spherical surface has the effect of decentering it; if you "pry" one side of a basketball up with a stick, for instance, the ball rolls away from the stick.

That said, while the same difference in slopes can be accomplished, other differences can occur. For instance, as someone noted earlier, lenses with non-spherical surfaces will produce other optical changes. Grinding prism may also result in a thinner lens compared to decentering for prism. Further, the optical axis of the lens may be tilted by a different amount in some cases, which will produce differences in the amount of prescription change that occurs as a result of lens tilt, although the effect is probably negligible.

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## obxeyeguy

Darryl:  I'm confused.  Lets use a -2.00 sph with1 BI.  Normal decentration to obtain PD is 2in.  I take a -2.00 stock cr-39 and decenter it 3 out, resulting in 1 BI, how is this any different thickness wise (with out changeing base curve) than me ordering it "ground in"?  Same curves to get the power, same resultant prism.  Should also be the same thickness.

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## Darryl Meister

> I'm confused. Lets use a -2.00 sph with1 BI. Normal decentration to obtain PD is 2in. I take a -2.00 stock cr-39 and decenter it 3 out, resulting in 1 BI, how is this any different thickness wise (with out changeing base curve) than me ordering it "ground in"? Same curves to get the power, same resultant prism. Should also be the same thickness.


In your particular example, the minimum thickness of the edged lens is equal to the minimum thickness of the original lens blank, since the thinnest point of the original lens blank (the optical center) still falls within the frame boundary. However, this will not always be the case, particularly for plus prescriptions. Say, for instance, that you were trying to introduce base out prism again with that same -2.00 D lens in a frame with a 44 mm eyesize that normally required only 2 mm of decentration for a given wearer's normal PD (i.e., without prism for decentration). Inducing any base out prism above 4.0 prism diopters will actually move the optical center and the thinnest point of the lens outside the aperture of this frame, resulting in a lens that is thicker than necessary.

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## obxeyeguy

OK, thanks.  That I knew.

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## fjpod

So Darryl,

If this -6.00 lens was a high index, hyperaspheric (which many of us would use in a case like this), and the prism desired, in this case 2 prism diopters, was achieved with decentration...You don't think the VA would be affected to a clinically significant degree?

Thanks for your insight.

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## Darryl Meister

> You don't think the VA would be affected to a clinically significant degree?


Yes, it's entirely possible. You should generally avoid decentering to induce prism with aspheric lens designs (finished or semi-finished).

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## fjpod

> ...unless the patient had a fused neck and his head was on crooked.
> 
> Chip


I've seen this often enough.

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## fjpod

Consistent with getting the patient the best possible optical result in a case like this where the lens power is fairly high, and the prism requested is "unusual", and the likelihood that the patient will be sold a high index lens, which is likely to be aspheric, is great.. and judging that virtually all respondants to this question, except Darryl, thought that prism by decentration and ground-in are synonymous...

Would it be acceptable, in this case, for a prescriber to write "ground-in prism" or "hyper-aspheric with ground-in prism" on this Rx?

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## obxeyeguy

> and judging that virtually all respondants to this question, except Darryl, thought that prism by decentration and ground-in are synonymous...


Yes, that was me.  In my example, no one can tell the difference.  I said aspherics are a different story.


> With the OC in the middle of the pupil, there is no prism unless you grind it in, and if you do, then you will have the optical centers of the lenses in front of the pupil (which is where the best VA is) and you will also have your prism.


Still believe that the OC's are in front of the pupil with prism?

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## Leo Hadley Jr

Great replies by everyone, this is a good thread.

I would like to hear back from the original poster. I am pretty sure this person just wanted to make a pair of glasses with 2 diopters of prism.
I do not think there was head trauma or any special circumstances. The only info we have is a -6.00 lens with 2 diopters of prism which is nothing to induce. I agree with all the posts above but, we are talking about moving a stock lens 1-2mm. I doubt there will be any acuity issues no matter what material is used. Of coarse grinding the lens would give optimum results but I know my eyes could never detect the difference.

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## Darryl Meister

> Still believe that the OC's are in front of the pupil with prism?


I think he was referring to the "center" or "pole" of the aspheric surface, which should generally remain in front of the pupil, with or without prism.

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## fjpod

> I think he was referring to the "center" or "pole" of the aspheric surface, which should generally remain in front of the pupil, with or without prism.


That's why we meassure OC heights in higher powered aspheric single vision lenses.

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## HarryChiling

> That's why we meassure OC heights in higher powered aspheric single vision lenses.


It is now refered to as the DRP or PRP which should be the same in this case, not the OC.  Take a look at the lens description standards:
http://www.visionsite.org/s_vision/s...ID=253&DID=384

It helps us avoid chasing our tails over sematics.

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## jrumbaug

> If a pair of glasses is supposed to have ground-in prism (e.g. 2 prism diopters base right), when I verify it on a manual lensometer should the 2 prism diopters show up no matter which part of the lens I am looking at?
> 
> Also, how do I grind that in?


 
Answer to 1st question
example 1    rx =  plano sphere with 2 diop base out
   you will see 2 diop base out prism regardless where you measure it

example 2    rx =  -5.00 diop sphere with 2 diop base out  ( rare rx )
   you will only find 2 diop prism 4mm from the optical center. which brings up the point.... to measure horizontal prism you must have the patients PD. The practical exception is low powers with high prism. Vertical prism is not PD critical (  an exception might be high cylinder at oblique angles, example rx +0.50 - 3.50 axis 045 )

Answer to second question
 I don't know. I don't grind. But I do induce prism by decentering spherical ( not aspheric ) lens, when the rx allows it.  example .. -1.00 sphere with 0.50 prism base out, move lens 5mm, but if sphere was -0.25, you can't move most blanks 20mm.

Jim Rumbaugh

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## renee1111

> Would it be acceptable, in this case, for a prescriber to write "ground-in prism" or "hyper-aspheric with ground-in prism" on this Rx?


I thought hyper-aspheric lenses were only used in very high plus RX's (cR-39 and only come in the highest base curves, 9, 10, 12, 14)  :Confused:  I could be wrong though....

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## go_hercules

This topic caught my attention and I was hoping someone could clear this up for me since it's on the subject.  I understand that decentration will not work in some cases such as high prism and low power because the lens would not have sufficient diameter to be decentered.  So, assuming there is not enough power in a lens for decentration to yield the required prism, it would have to be ground in.  I get that, but my question is if that same low power lens were infinitely large (I guess that would be a complete sphere based on the base curve!!), then could the lens be decentered to yield the prism?  And if you did decenter a large amount, then edge the lens appropriately, would it end up being the same as a regular sized lens with ground in prism?  It just seems to me that the two lenses in this case, arrived at differently, would be identical when finished.  Any insight into this would be very much appreciated.

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## jrumbaug

> This topic caught my attention and I was hoping someone could clear this up for me since it's on the subject. I understand that decentration will not work in some cases such as high prism and low power because the lens would not have sufficient diameter to be decentered. So, assuming there is not enough power in a lens for decentration to yield the required prism, it would have to be ground in. I get that, but my question is if that same low power lens were infinitely large (I guess that would be a complete sphere based on the base curve!!), then could the lens be decentered to yield the prism? And if you did decenter a large amount, then edge the lens appropriately, would it end up being the same as a regular sized lens with ground in prism? It just seems to me that the two lenses in this case, arrived at differently, would be identical when finished. Any insight into this would be very much appreciated.


This answer to your hypothetical large lens may be a little hard to follow.
Gee.... it may even be wrong.
IF the calculated optical center does not fall within the lens THEN
.......... ground in prism is thinner
OTHERWISE
......... they are the same

In the case where the O.C. (thinnest part) would not fall within the lens, the lens could be ground with a theoretrical center thickness, thinner than standard, perhaps in some cases, even a negative center thickness. 

Jim Rumbaugh

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## HarryChiling

This topic caught my attention and I was hoping someone could clear this up for me since it's on the subject. I understand that decentration will not work in some cases such as high prism and low power because the lens would not have sufficient diameter to be decentered. 

So, assuming there is not enough power in a lens for decentration to yield the required prism, it would have to be ground in. 
*YES*

I get that, but my question is if that same low power lens were infinitely large (I guess that would be a complete sphere based on the base curve!!), then could the lens be decentered to yield the prism?
*YES and NO, it is possible to eventually run into the lens circumference running into itself before you reach teh prism, but before you get to that point it would be outside the realm of reality for vision or much else.* 

And if you did decenter a large amount, then edge the lens appropriately, would it end up being the same as a regular sized lens with ground in prism? 
*NO, since the optical center of a stock lens is set the further off the lens it goes the thicker the edges of the lenses get, where as in a ground prism situation you could speciafy the edge thickness.* 

It just seems to me that the two lenses in this case, arrived at differently, would be identical when finished. 
*NO*

Any insight into this would be very much appreciated.

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