Compound prism question

**John Allen** · 05-02-2013, 05:14 PM

I have an RX that is -2.75OD and -3.00-0.25x175. There is 5 Base out and 7 Base UP OD and 5 Base out and 7 Base Down OS. How would I calculate the axis since it is at an angle for the prism. Is there a formula.

Thanks

John

**drk** · 05-02-2013, 06:45 PM

Man, I never do this, but I'll give it a go.

Method #1: Rig that up in a lensometer and read it.
Method #2: Graph paper. Draw vectors and use a protractor.
Method #3: http://www.opticampus.com/tools/compounding.php

**SAD: FNAO** · 05-02-2013, 06:55 PM

I rememember a certain graphing chart that was out many years ago which enabled the optician to calculate the axis of the compound prism !!

**braheem24** · 05-02-2013, 07:33 PM

nevermind :)

**rbaker** · 05-02-2013, 08:16 PM

Originally Posted by SAD: FNAO

I rememember a certain graphing chart that was out many years ago which enabled the optician to calculate the axis of the compound prism !!

Polar coordinate graph paper and a pencil or a Dalton E6B computer. If you are into batteries and modern hi tech stuff go to Staples and glom onto a TI or HP calculator. Read the manual and in 30 minutes you will be computing compound oblique resultant prism to 12 decimal places.

**John Allen** · 05-03-2013, 08:56 AM

Thanks for all the help :)

**Darryl Meister** · 05-05-2013, 11:15 AM

Given horizontal P_H and vertical P_V prisms, the basic formulas are:

$P_M=\sqrt{P_V^2+P_H^2}$

for the prism magnitude P_M and:

$A=atan\left(\frac{P_V}{P_H}\right)$

for the prism angle A, relative to the horizontal 180-line.

You will then need to determine the specific prism axis from 0 to 360 degrees by considering the actual quadrant on the lens that the prism base falls within (e.g., prism down and out for the right eye would fall into the third quadrant between 180 and 270 degrees).

Best regards,
Darryl

**Jacqui** · 05-05-2013, 06:47 PM

44 years ago I started at AO, the second week there I was shown how to use a resultant prism chart. Pretty much polar coordinate with axis, etc penciled in. Still use it, but now I have one that goes all the way to 20^ WOW

**rbaker** · 05-05-2013, 06:55 PM

If you get a really really really big sheet of polar coordinate paper you can go all the way up to 100 diopters of prism!

**Darryl Meister** · 05-06-2013, 08:52 AM

Originally Posted by rbaker

f you get a really really really big sheet of polar coordinate paper you can go all the way up to 100 diopters of prism!

Otherwise, you can just divide the original horizontal and vertical prism values by single value that makes them fit on your coordinate paper, such as dividing by a factor of 10, and then multiply the final prism magnitude result by the same value to scale the result back up. Or just use a scientific calculator.

Best regards
Darryl