Reading Compound Prism

**EyeFitWell** · 10-08-2008, 03:56 PM

So, I'm sitting at my desk yesterday and Doc asks me, "you can read prism in glasses, right?" I perk up and happily say, "yes!" "Could you read how much prism is in these?" "Sure" I say, glad to have something important to do. I get the PD from Doc.
I walk over to my lensometer and confidently place the right lens in and look through the eyepiece. Hmm...mires are completely out of view. Ok, no prob, I've seen this before, I take the lens out, twist my prism dial 3D to the right (lines are way over to my left) and put the lens back in. Look in eye piece, now the mires are visible in the lower left corner of the target. It looks like about 3.5 at 45, but I've already moved the mires to the right 3D!
So it occurs to me, No, technically I have no idea how to accurately break this into the down and out components considering that to even see the mires I have to move it over 3D which changes the angle I'd get if I ignored that fact. What now looks like 45 may have origionally been 30 if I could see it in the true position.
Long story short(er), we used the ancient autolensometer in the back (the techs use a new one which can't read that much prism).
I had guessed at about 3 down and 5out, turns out to be 4dn 6out.
So, the problem was solved for that pt. However, I am now wondering, is there a way you could determine within about 1 prism dioptor how much prism was actually in those glasses with the use of a manual lensometer only?
If so, how?

**HarryChiling** · 10-08-2008, 04:05 PM

Use the compound rule to add the original 3 you added at 180 or 360 whichever lens you have along with the 3.5 at 45 or if your truly measureing down then it would be either 225 or 315 depending on the eye. Put these two together and you have your prism power.

**FullCircle** · 10-08-2008, 04:08 PM

pssst...
http://www.laramyk.com/component/opt...oc_view/gid,3/

thanks to the wonderful folks at Laramy-K

**Jacqui** · 10-08-2008, 04:39 PM

Originally Posted by FullCircle

pssst...
http://www.laramyk.com/component/opt...oc_view/gid,3/

thanks to the wonderful folks at Laramy-K

Yes, aren't they wonderful ?!?

:cheers:

**chip anderson** · 10-08-2008, 04:42 PM

Just courious, why would you get the PD from the doc? Don't you trust yourself to take one?

**EyeFitWell** · 10-09-2008, 01:29 PM

Originally Posted by chip anderson

Just courious, why would you get the PD from the doc? Don't you trust yourself to take one?

LOL, oh, no, I just didn't have the pt in front of me and doc had the chart. That's all.

**EyeFitWell** · 10-09-2008, 01:31 PM

Originally Posted by HarryChiling

Use the compound rule to add the original 3 you added at 180 or 360 whichever lens you have along with the 3.5 at 45 or if your truly measureing down then it would be either 225 or 315 depending on the eye. Put these two together and you have your prism power.

Harry, what is the compound rule?

**HarryChiling** · 10-09-2008, 02:28 PM

Originally Posted by EyeFitWell

Harry, what is the compound rule?

I should say resolving prism to get it into the (x,y) coordinate system your used to:

http://onlineopticianry.com/wordpress/?p=34

Once you've broken down the prism into it's horizontal and vertical you just add them together and then compound them if you'd like back into the original form.

http://www.opticampus.com/tools/compounding.php

Use these resources, Darryl has spent a considerable amount of time compileing and coding those calculators to help. I have typed my fingers to the bone getting alot of that information on a site accessable to you and others. It's FREE and useful.

**EyeFitWell** · 10-09-2008, 03:02 PM

I appreciate your answers, but I don't think you really see my point. Ever read Optical Formulas Tutorial? (I'm sure you have) Ellen Stoner personally taught me this resolving prism thing. And the chart posted is helpful, but I've got that in my textbooks.
Let's use some nice whole numbers to illustrate my point:

Suppose I moved the prism dial 3D to my right, then read the prism as 3D at 45/225. Using the chart, that gives me about 2.1 Out and 2.1 Down. Add in the three and I get 5.1Out, 2.1Down.

If I take those same numbers, 5.1Out and 2.1Down and use those numbers to convert back to the compound form, I get OD5.5D&O @25.

The problem is that the angle I get after moving the prism dial over will NOT be the correct angle, and therefore will lead to incorrect results.

Right?....hmmm I think I need to think this over for a minute.

**lensgrinder** · 10-09-2008, 04:00 PM

Since you induced 3^ in and you then read 3.5 @ 225, let us start by resolving 3.5 @ 225.
$\sin(45) = \frac{v}{3.5}\\<br /> v = 2.47^\Delta\ BD$
$h = 2.47^\Delta\ BO$ since the sin and cos of 45 are the same

Now let's add the 3^ horizontally that you induced in the lensmeter. You now have 5.47 out and 2.47 down

This assumes you moved the mires horizontally only. This also assumes you put them back in the lensmeter at exactly the same place.

You will not remain at 225 if you do not induce the 3^, becuase as the horizontal prism increases and the vertical componont stays the same you get closer to 180 in this case.

**HarryChiling** · 10-09-2008, 04:58 PM

Originally Posted by lensgrinder

Since you induced 3^ in and you then read 3.5 @ 225, let us start by resolving 3.5 @ 225.
$\sin(45) = \frac{v}{3.5}\\<br /> v = 2.47^\Delta\ BD$
$h = 2.47^\Delta\ BO$ since the sin and cos of 45 are the same

Now let's add the 3^ horizontally that you induced in the lensmeter. You now have 5.47 out and 2.47 down

This assumes you moved the mires horizontally only. This also assumes you put them back in the lensmeter at exactly the same place.

You will not remain at 225 if you do not induce the 3^, becuase as the horizontal prism increases and the vertical componont stays the same you get closer to 180 in this case.

Thanks Brent been a while since I've seen a post always nice to have you checking posts. :cheers:

**HarryChiling** · 10-09-2008, 05:06 PM

Originally Posted by EyeFitWell

So, I'm sitting at my desk yesterday and Doc asks me, "you can read prism in glasses, right?" I perk up and happily say, "yes!" "Could you read how much prism is in these?" "Sure" I say, glad to have something important to do. I get the PD from Doc.
I walk over to my lensometer and confidently place the right lens in and look through the eyepiece. Hmm...mires are completely out of view. Ok, no prob, I've seen this before, I take the lens out, twist my prism dial 3D to the right (lines are way over to my left) and put the lens back in. Look in eye piece, now the mires are visible in the lower left corner of the target. It looks like about 3.5 at 45, but I've already moved the mires to the right 3D!
So it occurs to me, No, technically I have no idea how to accurately break this into the down and out components considering that to even see the mires I have to move it over 3D which changes the angle I'd get if I ignored that fact. What now looks like 45 may have origionally been 30 if I could see it in the true position.
Long story short(er), we used the ancient autolensometer in the back (the techs use a new one which can't read that much prism).
I had guessed at about 3 down and 5out, turns out to be 4dn 6out.
So, the problem was solved for that pt. However, I am now wondering, is there a way you could determine within about 1 prism dioptor how much prism was actually in those glasses with the use of a manual lensometer only?
If so, how?

You move the mires so they are 3D to the left as you mentioned or 3D BO

the mires are visible in the lower left corner which by the way is referenced as 3.5D @045 Down or 3.5D @ 225 which as Lengrinder points our resolves to 2.47D BD and 2.47 BO so then

2.47D BD

2.47D BO + 3D BO = 5.47D BO

6D @ 204 or 6D @ 024 Down

**lensgrinder** · 10-09-2008, 10:09 PM

Originally Posted by HarryChiling

Thanks Brent been a while since I've seen a post always nice to have you checking posts. :cheers:

I know it has been a while since I have posted, but I do read everyday.
It seems like I have been very busy lately, but nothing is getting accomplished!

**Kevin Howtopat** · 10-09-2008, 10:28 PM

So if a person has a radical amount of prism like this, how does that affect the measurement used for the pd? It seems like the measurement would be inaccurate if a persons eye muscles were pulling their eyes in different directions? Or would you measure from center of one eye to the other? Maybe I just answered my own question.

**HarryChiling** · 10-09-2008, 11:58 PM

Originally Posted by lensgrinder

I know it has been a while since I have posted, but I do read everyday.
It seems like I have been very busy lately, but nothing is getting accomplished!

I can't tell, www.opticaled.com that's great I like the ray trace video you've put up.