Prism

[eyechicky]

Can anyone explain how to use a prism compensator on a lensometer?

Also checking prism in progressives. Is it as simple as lining the PRP (prism reference point/MRP) up with the eyepiece and reading the amount and direction off the prism circles. I have a set I checked with no prescribed prism in rx, everything reads fine in lensometer (fitting cross ht, pd, rx) but when I line up the PRP, I get 2 prism BD in each eye. I know that the vertical imbalance is 0.00 D since the prisms are cancelling, but is this correct and should the glasses be Ok to dispense since VI=0.00. Even though the prism in eacj eye individually is over tolerance?

Thanks for your help.

[Henchos]

The prisim you have measured is a function of the amount of prisim thinning which depends on the addition of the lens,the higher the add,the higher the amount of prisim thinning,i think its normally about 2/3rds of the add.

As for the prisim compensator,its used to get your images back into the view if the prescribed prisim exceeds the amount the lensometer can display. So simply add opposite prisim to bring your image back into view and subtract the amount of prisim you have used to compensate.

[Ed_Optician]

I prefer a B&L 70/Reichart lensometer with insertet prisms 3 6 and 9

I use them to keep the image close to the center of the reticle
If an Rx calls for 6 BU I would use a 3 set base down and only read it as 3 BU otherwise the image is at the extreme edge of the reticle

Ed

[keagles2]

:hammer: I will be setting for the written and practicle exams in virginia in Nov. AND Dec. Does any one have a simpler way to get the prism evaluation down.I dont quite understand the out comes such as in the following problem-
+2.00-0.75x090 2bo
+1.00-0.50x145 2bd
Heeeeeeeelp:)

[chip anderson]

You needs 1 out in each lens. 1 up O.D. and 1 down O.S.

[keagles2]

Thank you for the answer but I need help getting to that point.Is it possible to have that explained to me via email as I have studied that but there has to a better explanation than the complicated books.
:) :)
Kathy

[Ed_Optician]

You need to line it up as 1.4 up and out in OD (45 degrees) 1.4 down and out in the OS (45 degrees) If I get a chance I will shoot a pair of pictures through the lensometer with my digital camera to better explain itEd