9 Prism diopters Base Out per eye?

**jeffsoptical** · 08-16-2007, 12:10 PM

I just received a Rx with a ton of prism.

OD: -1.50 -.50 x 075 (9 Prism Base Out)
OS: -1.75 -.25 x 042 (9 Prism Base Out)

Add +2.00 ou

I'm going to put her in a Varilux Ellipse 1.60 Trans. 5

PD 27.5/25.5

I've always multiplied 0.3 x Prism diopters= how much to change pd or seg hgt.

9 x .3=2.7mm to move the pd in.

I think though .25 is exactly correct to use.

9 x .25=2.25 or 2mm to move pd in.

Which one .25 or .3 is correct?

This seems like a lot to change a PD.

Please give advice.

Thanks

Jeff:cheers:

**chip anderson** · 08-16-2007, 12:30 PM

I'm sure I'll get a lot of flack for this but I think you are out of your mind attempting any type of progressive with this type of Rx.

**drk** · 08-16-2007, 12:34 PM

I'm sorry, help me out! Why are you changing the p.d.?

**Fezz** · 08-16-2007, 12:37 PM

Originally Posted by drk

I'm sorry, help me out! Why are you changing the p.d.?

What he said!

**For-Life** · 08-16-2007, 12:38 PM

Originally Posted by chip anderson

I'm sure I'll get a lot of flack for this but I think you are out of your mind attempting any type of progressive with this type of Rx.

I will support you on this one.

If the patient is not currently wearing a PAL, do not put him or her in one. If you do, go for an atoric one.

Plus, if there are any alterations, a FT is a lot cheaper to fix. Also, why high index?

**braheem24** · 08-16-2007, 12:43 PM

I think this thread will help.

http://www.optiboard.com/forums/showthread.php?t=24571

**DragonLensmanWV** · 08-16-2007, 12:49 PM

I just fit a pair last month that had 8 base out OU. He had Duane's retraction. He was also wearing a progressive when he came in. We fit him in Hoya 1.70 index GPW. He really loved them. I called the ophthalmologist to congratulate him on his acumen in the Rx, and he was a little nonplussed. He said in 23 years, he'd gotten lots of complaints, but no one had ever called him to compliment him.

The reason you want high index is..8 base out ou.
With an Rx like that, you might be hard put to find a blank thick enough to grind that much prism in CR-39. And they will be thinner, yes?

I don't understand why you are wanting to change the PD.

**jeffsoptical** · 08-16-2007, 12:53 PM

Originally Posted by drk

I'm sorry, help me out! Why are you changing the p.d.?

The reason I'm changing the p.d. is because for every diopter of prism the eye moves .25 or .3 mm toward the apex or away from the base. If you don't change the pd the eye will not be in the right spot for using a PAL.

The woman is currently in a PAL with 5 diopters base out ou.

Thanks for the advice

Jeff

**DragonLensmanWV** · 08-16-2007, 01:24 PM

Originally Posted by jeffsoptical

The reason I'm changing the p.d. is because for every diopter of prism the eye moves .25 or .3 mm toward the apex or away from the base. If you don't change the pd the eye will not be in the right spot for using a PAL.

The woman is currently in a PAL with 5 diopters base out ou.

Thanks for the advice

Jeff

You're over-thinking this, I think.
I think you're confusing physical location of the center with optical location of the center.
The progressive will be in the right spot if you don't change the pd. The prism is what moves the light but if you narrow the pd the corridor will be displaced nasally, resulting in poor vision everywhere. For this high-dollar job you want to get it right.
My advice (having just done one similar) is to use the actual measured pd and let the prism do the work.
Make sure you match the pd she is currently wearing, hopefully it will be the same as you measured.

**jeffsoptical** · 08-16-2007, 01:35 PM

Originally Posted by DragonLensmanWV

You're over-thinking this, I think.
I think you're confusing physical location of the center with optical location of the center.
The progressive will be in the right spot if you don't change the pd. The prism is what moves the light but if you narrow the pd the corridor will be displaced nasally, resulting in poor vision everywhere. For this high-dollar job you want to get it right.
My advice (having just done one similar) is to use the actual measured pd and let the prism do the work.
Make sure you match the pd she is currently wearing, hopefully it will be the same as you measured.

The prism moves light, but it also moves the location of the pupils. When a patient is seeing double the doctor prescribes prism and this corrects the vision imbalance.

**gemstone** · 08-16-2007, 02:36 PM

Originally Posted by jeffsoptical

I just received a Rx with a ton of prism.

OD: -1.50 -.50 x 075 (9 Prism Base Out)
OS: -1.75 -.25 x 042 (9 Prism Base Out)

Add +2.00 ou

I'm going to put her in a Varilux Ellipse 1.60 Trans. 5

PD 27.5/25.5

I've always multiplied 0.3 x Prism diopters= how much to change pd or seg hgt.

9 x .3=2.7mm to move the pd in.

I think though .25 is exactly correct to use.

9 x .25=2.25 or 2mm to move pd in.

Which one .25 or .3 is correct?

This seems like a lot to change a PD.

Please give advice.

Thanks

Jeff:cheers:

I have never heard of this in many years.

**HarryChiling** · 08-16-2007, 02:37 PM

Originally Posted by jeffsoptical

The reason I'm changing the p.d. is because for every diopter of prism the eye moves .25 or .3 mm toward the apex or away from the base. If you don't change the pd the eye will not be in the right spot for using a PAL.

The woman is currently in a PAL with 5 diopters base out ou.

Thanks for the advice

Jeff

If you look at a prism as

1D_prism = 1cm/1m = 1cm/100cm

so for 9 diopters of prism

9D_prism = 9cm / 100cm

If you were to look at the abgle that the light deviates

tan(dev) = 9cm / 100cm
tan(dev) = 0.09
dev = 5.14^o

now the vertex distance of a pair of glasses averages about 13mm and the center of rotation lies abotu 13.5mm from the cornea so together you have 26.5mm

tan(5.14) = PD_comp / 26.5mm
PD_comp = 26.5mm * tan(5.14)
PD_comp = 2.38

If you were to further divide that by the 9D you would get

2.38 / 9 = 0.26

There is your answer my friend.

**gemstone** · 08-16-2007, 02:45 PM

Originally Posted by HarryChiling

If you look at a prism as

1D_prism = 1cm/1m = 1cm/100cm

so for 9 diopters of prism

9D_prism = 9cm / 100cm

If you were to look at the abgle that the light deviates

tan(dev) = 9cm / 100cm
tan(dev) = 0.09
dev = 5.14^o

now the vertex distance of a pair of glasses averages about 13mm and the globe diameter averages about 27mm so you have 40mm from the back of the lens to the retina

tan(5.14) = PD_comp / 40mm
PD_comp = 40mm * tan(5.14)
PD_comp = 3.6

If you were to further divide that by the 9D you would get

3.6 / 9 = 0.4

There is your answer my friend.

Are you saying you need to ajust the PD .4 mm for 9 diopters of prism?

**chip anderson** · 08-16-2007, 02:54 PM

I think Harry is saying, don't move it. Nobody can cut a lens and mount it to .4mm.

Chipo

**HarryChiling** · 08-16-2007, 02:57 PM

I actually made a mistake in there. The total amount to compensate would be

2.38mm

however if you wanted to create a shortcut for each diopter

2.38 / 9 = 0.26mm per diopter

(changed from the origianl post sorry)

**jeffsoptical** · 08-16-2007, 03:10 PM

Originally Posted by HarryChiling

If you look at a prism as

1D_prism = 1cm/1m = 1cm/100cm

so for 9 diopters of prism

9D_prism = 9cm / 100cm

If you were to look at the abgle that the light deviates

tan(dev) = 9cm / 100cm
tan(dev) = 0.09
dev = 5.14^o

now the vertex distance of a pair of glasses averages about 13mm and the globe diameter averages about 27mm so you have 40mm from the back of the lens to the retina

tan(5.14) = PD_comp / 40mm
PD_comp = 40mm * tan(5.14)
PD_comp = 3.6

If you were to further divide that by the 9D you would get

3.6 / 9 = 0.4

There is your answer my friend.

Thanks Harry. I just found it in the second edition of System for Ophthalmic Dispensing. You are right, but they assumed 30mm from the back surface of the lenses to the center of rotation of the eyes. They came up with .29 or .30.

Jeff

**HarryChiling** · 08-16-2007, 03:17 PM

Yeah if you were to change from the corrected 26.5mm to 30mm

tan(5.14) = PDcomp / 26.5mm
PDcomp = 26.5mm * tan(5.14)
PDcomp = 2.38

If you were to further divide that by the 9D you would get

2.38 / 9 = 0.26

tan(5.14) = PDcomp / 30mm
PDcomp = 30mm * tan(5.14)
PDcomp = 2.7
If you were to further divide that by the 9D you would get

2.7 / 9 = 0.30

So you really can't go too wrong either way. It's actually pretty simple geometry and worth doing whenever you have high amounts of prism.

**obxeyeguy** · 08-16-2007, 03:53 PM

It appears that that is the one new thing I learned today. In all my years, I have never compensated a Pd on any prism job.

**Andrew Weiss** · 08-16-2007, 04:03 PM

Originally Posted by obxeyeguy

It appears that that is the one new thing I learned today. In all my years, I have never compensated a Pd on any prism job.

Same. Thanks for the education, all.

**obxeyeguy** · 08-16-2007, 04:17 PM

So help me totally get this. The new lay-out PD would be 25/23 or 25.5/23.5 depending on 2.38 interpretation??

**jeffsoptical** · 08-16-2007, 04:19 PM

Originally Posted by gemstone

Are you saying you need to ajust the PD .4 mm for 9 diopters of prism?

No you would multiply .3 x 9=2.7mm moved in for each eye.

Original pd 27.5/25.5

New pd 25/23

Jeff

**gemstone** · 08-16-2007, 04:40 PM

Originally Posted by obxeyeguy

It appears that that is the one new thing I learned today. In all my years, I have never compensated a Pd on any prism job.

A new one on me too. Wonder if they know about that at Durham Tech yet.

**HarryChiling** · 08-16-2007, 04:56 PM

Originally Posted by gemstone

A new one on me too. Wonder if they know about that at Durham Tech yet.

They should, it's apparently in their book.

**Fezz** · 08-16-2007, 05:32 PM

This all sounds great. It really does. But, what were the old PDs and how will moving or changing them affect adaption?

**lensgrinder** · 08-16-2007, 05:51 PM

Originally Posted by gemstone

A new one on me too. Wonder if they know about that at Durham Tech yet.

We just figured it out yesterday.;)

Originally Posted by HarryChiling

They should, it's apparently in their book.

Well it is not our book, it is Dr. Brooks' book.;)

Do not forget to reduce the prism amout by:

0.25 X -1.96 = 0.49^ BI OD
0.25 X -1.86 = 0.47^ BI OS

So reduce the prism by 0.5^ OU for a total of 8.5^BO each eye.