Sorry if this inquiry is basic, but my medical training is not in optics.
If a patient reports that his left eye image is slightly lower than the right eye image, and objective testing discloses a value of 1.0, does that mean that the ground right lens should have an 0.5 UP prism and left lens 0.5 DOWN prism?
It doesn't sound intuitive that the eye which sees a lower image would have a down prism.
When a ray of light encounters a prism, it deviates towards the base, and the image towards the Apex. The wavelength and frequency of these deviated colors is different, they deviate differently at different angles due to the velocity difference of the prism. I hope this helps explain it!
It might be helpful to know if the lenses are plus, minus or plano. And is this a near or distant issue (both) Presbyopia?
Chris
A Plus lens is like a prism base to base, the thicker portion of the lens in the center! Opposite for a minus lens, Apex to Alex, thinner in the center.
To answer the OPs original question, you do not need to split the prism at 1 prism diopter. Base down in the left eye works. Another consideration, if the power of the lens is sufficient, you can decenter to achieve the desired prism.
If the subject is wearing spectacle lenses, and there's sufficient power in the vertical meridian, check if the glasses are not level, dissimilar powers, or errors in the optics.
Hope this helps,
Robert Martellaro
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
There is a multiplicity of opposites in optics/optometry!
WITH NO GLASSES ON, OR LOOKING THROUGH THE OPTICAL CENTERS:
On a parallax test (cover test) if the left eye sees something as "lower" that means the eye is in a "hyper position" and it will have to make a downward version motion to center the retinal image on the fovea.
On a dissociated prism test (Von Graefe, Maddox) if the left eye sees something as "lower" than the simultaneously-observed right eye image (whatever that may be), that means the left eye is higher/in the hyper position.
The measurement via prism to make the images fuse (in a dissociated test) or to make parallax disappear (cover test) is considered the angle of deviation in prism diopters.
PRESCRIBING: Usually, but not always, the full deviation amount is prescribed in spectacle correction. Sometimes a portion of the deviation is prescribed to merely aid to the fusional vergences (as is almost always the case when prescribing for lateral phoric deviations).
Putting the entire deviation in one lens or splitting it into "base up" and "base down" between the two eyes is multi-factoral and depends on things such as amblyopia, lens thickness due to concomitant refractive error correction, habituation, etc. But in general it's a good idea.
FABRICATING: I'll let someone else take over from here...
Having said all that, patients don't report things as you presented it. I would take Robert's advice on troubleshooting the spectacle correction, first.
to make it simple - the image moves toward the apex of the prism, not the base
Thank you ALL, but I'm ignorant of most optics and do not understand most of these posts. Is there a simple answer to:
"If a patient reports that his left eye image is slightly lower than the right eye image, and objective testing discloses a value of 1.0, does that mean that the ground right lens should have an 0.5 UP prism and left lens 0.5 DOWN prism?"
No, it does not mean that. The opposite is true.
Posting the distance rx would be very helpful.Originally Posted by Astigmatism
The cause may be from the prescriptions power in the vertical meridians and the optical centers not properly placed.
Was this problem present in past pairs of glasses?
Does it go away if an older pair is worn?
Or is this unrelated to a prescription?
Last edited by Uncle Fester; 01-25-2024 at 12:29 PM.
If you want to move the image up you need base down prism.
As per the forum rules which you agreed to when registering, this forum is for eyecare professionals only. Please contact a qualified eyecare professional for any questions or help you may require.
OptiBoard Administrator
----
OptiBoard has been proudly serving the Eyecare Community since 1995.
There are currently 1 users browsing this thread. (0 members and 1 guests)
Posting Permissions
Bookmarks