Prism

[RA]

Can anyone tell me what the rules are for splitting prism?
It's always the same directon horizontally and opposite vertically?

:)

[chip anderson]

Yes, if possible in the frame for cosmetic results. You can put it all in one lens or split any way your want as long as the result is the full amount of prism. This does not always suit every situation but as much as there is a rule, that's it.

Chip

[HarryChiling]

Yes, if possible in the frame for cosmetic results. You can put it all in one lens or split any way your want as long as the result is the full amount of prism. This does not always suit every situation but as much as there is a rule, that's it.

There is lots of wisdom in that response, "anyway you want" you could check to see what the difference in power is and use the prism to try and even out the thickness in a lens as well.

[braheem24]

you could check to see what the difference in power is and use the prism to try and even out the thickness in a lens as well.

i.e. more base BD in a PAL, more BI in the higher minus, more BO in the higher plus, etc

Same results in the end, but makes a prettier pair of glasses.

[HarryChiling]

nice ie braheem24 :cheers:

[gemstone]

I have put more in the down lens in progressives when spliting up and down prism.

[EyeFitWell]

Just make sure you choose the correct base direction for the required effect.
For example, take 6BO OD. You would want both eyes BO, not three BO OD and three BI OS.
6BU OD would be 3BU OD and 3BD OS.
(That's just splitting in the middle, as pp said, sometimes there is wisdom in unequal splitting.)
And I always ask the Dr before I do it, sometimes that's not an appropriate solution.

[bren_03825]

Just make sure you choose the correct base direction for the required effect.
For example, take 6BO OD. You would want both eyes BO, not three BO OD and three BI OS.
6BU OD would be 3BU OD and 3BD OS.
(That's just splitting in the middle, as pp said, sometimes there is wisdom in unequal splitting.)
And I always ask the Dr before I do it, sometimes that's not an appropriate solution.

:cheers:
No offense to fellow opticians, but I would always check with the doctor as well, I have seen some people (less than 1 in 100 probably) who cannot tolerate the prism split.

[Darryl Meister]

In cases of non-concomitant (also incomitant) strabismus, which causes the binocular deviation to differ depending upon the direction of the wearer's gaze, splitting the prism may reduce the visual benefit of the prism, though this condition is rare. In most cases, splitting the prism will result in better cosmesis and optics (by minimizing prismatic distortion and dispersion).

[RA]

Thanks for the replies. Can anyone explain the physiological aspects behind why base direction must reamin the same in the horizontal plane and must be opposite in the vertical plane?

:)

[braheem24]

They're actually both opposite, but since 'in' and 'out' is relative it sounds the same.

It's actually 0 and 180 degrees.

[chip anderson]

RA: Base direction should be opposite in both planes. Base Out O.U. means One at 0 and one at 180. If it were the same in both planes at on the horizontal they would both be at 0 or both at 180 (One at nasal and one at temporal) = net result Zero. Some might have trouble with this as with cylinder the plane runs all the way across the lens and and is equal at both nasal and temporal, hense we suffer the impression that 180 and zero are the same. 180 is actually temporal on the Right eye, 0 is nasal on the Right eye. O is nasal on the left eye and 180 is temporal on the left eye. This is why the Rx is probably written as Base out or base in O.U. to avoid someone actually putting the prism Base Nasal on one eye and base temporal on the other.

Chip

[RA]

I get it, I need to sharpen my critical thinking skills:hammer:

[braheem24]

What is "out" to your right eye is "in" to your left eye and vice versa.

Keeping it the same IS opposite.