Need Help- (Info) on "Resolve Prism)

[bt5050]

hi all

was wondering if anyone can give me some help - and insight on my prism issue-

I have sen over the years - that the Od's understand and help opticans out in writing there rx's then some Md's

Some Background -
I have a few Md''s- most children Md's that write their rx's for prism as
od=== +2.00 - 75 x 120 ( 2.5 BU + BO @ 145
os=== +1.00 - 2.00 x 55 ( 1.0 BD + BO @ 160

Now I have no issues laying out Rx's such as
od- +2.00 -75 X 120 ( 1.00 BU + .50 BO )

but i get very confused and must outsoarce by orders when i recieve these rx's that the perscribing md uses a trial frame- has the PT spin the axis knob - and then just reads the axis locating of the totla prism - and i have issues when they don't BREAK IT OUT ---

can anyone help me with this - ? I was told there was a fomula to get the break down of the prism - in relations to laying it out - however it does not seem to work very well for me - and i am either told i am 10 or so degrees off of the prism - or whatever the case is - it is not correct-
When i spoke with my outside lab - they referred to this as resolve prism - however i am not sure if this is the correct turm -

It is no porblem for me to take a press on prism - use a protractor - and smack it on the axis they percribe - or iof we are dealing with a multifocal - i can easily place the prism on the axis in the SGX GENERATOR AND grind it --- but when they want this type of prism made in a SV finished pr of eyewear - ( no press on ) and i have to lay it out - i have issues ----

Thanks In adavnce for all you help - !!

Bt

[shanbaum]

was wondering if anyone can give me some help - and insight on my prism issue-

There's a problem with this notation, and it's one of my pet peeves; for some reason, optometrists and opticians have an historical tendency to eschew the perfectly-good degrees greater than 180 and less than 360.

The first thing to do is figure out what "2.5 BU + BO @ 145" and "1.0 BD + BO @ 160" really mean. For the right eye, "2.5 at 145" is base up and out, but for the left, "1.0 at 160" would be base up and in - so we have to mangle it to "1.0 at 340" for it to be "down and out" on the left eye.

You can convert those prism amounts and directions (expressed in "polar" notation) to "rectangular" or "Cartesian" notation with the "polar to rectangular" function found on most scientific calculators.

Or just enter it into Innovations, if you (are fortunate enough to) have that.

[HarryChiling]

the equation you are looking for is:

V=vertical prism
H=horizontal prism
P=prism
a=angle of prism

V=P*sin(a)
H=P*cos(a)

(P)^2=(V)^2+(H)^2

tan(a)=V/H

Note: ignore the signs of cos and sin

you can find all of these equations at www.technicalopticians.org , shanbaum is correct that the prism axis for "1.0 BD + BO @ 160" would actually be at 340 but if you ignore the signs of the cos and sin in the equations than all will work out well eighter way, It is an acceptable way of writing prism as long as you specify the base down part all though it leaves room for error and most opticians will not write it that way.

[Jeff Trail]

hi all


When i spoke with my outside lab - they referred to this as resolve prism - however i am not sure if this is the correct turm -


Thanks In adavnce for all you help - !!

Bt

That would be resultant prism, the resultant prism will always be a less number than if they gave it to you in a compound version.. in a compound form we are giving you two locations and where those two line converge or cross is the resultant. When you write it out in the vertical and horizontal components that is resolving the prism ie. if i was given an RX and it was in compound form than the resultant if given in an axis form would not be " OD 2@045", = Prism resolves to 1.4 BI & 1.4 BU." that seems to be where a lot of people get confused.. they want to add the resolved numbers and make it match the resultant..
BTW most every surface program will let you enter in prism by either axis or compound and do the math for you :)

Jeff "isn't Pythagoras' theorem a subway sub" Trail

gee next thing you know you will get a Tesela sling to drink (shocking):cheers:

[Ed_Optician]

Look at a simpler example of this problem and play with it in a lensometer. 1 BU and 1 BO will give you a resultant prism of 1.414 @ 135 BU & BO (1.5 is close enough) Easiest way to lay out this type of RX is to rotate the reticle of the lensometer to the appropriate prism axis have the sphere and cyl lines intersect along this line at the appropriate point in my example just shy of the 1.5 ring it is mostly practic3e and eyeballing it in hte lensometer