Hillary Clinton testifies prism in glasses fresnelIn January 2013 Hillary Clinton testified before the Senate and House Foreign Relations/Affairs Committees regarding the incident occurring at the Benghazi embassy on September 11, 2012.  Her testimony was delayed due to her experiencing a serious fall at home, for which she was hospitalized.  Many noticed she was wearing glasses with very odd-looking lenses during the testimony.  Several eye doctors have noted, correctly, that lenses of that type are used to correct double vision.  Since Mrs. Clinton would immediately become the Democratic front-runner for the presidency once she announced such an intention, many “across-the-aisle” are implying lately that she may not be physically qualified to handle the office. 

Prisms are put in eyeglass prescriptions to help a patient avoid seeing double vision when the eyes are misaligned.  Specifically, Fresnel prisms (the type I believe Mrs. Clinton was wearing) are plastic press-on prisms.  You can cut them out to match the patient’s frame and physically stick them to each lens; they are clear plastic with parallel lines running through them.  The doctor orients the prism in the direction needed to “move the world” for the misaligned eye and thus eliminate the double vision.  (Non-fresnel) Prism can also be cut into a regular spectacle lens if desired.  This type of prism does not have lines in it, but if the degree is high it can create one lens edge much thicker than the other, creating a cosmetic issue for the patient. 

Fresnels are ideal in two cases: 1) When you expect the eye deviation to be of a temporary nature, thus saving the patient the inconvenience and expense of cutting the prism into the actual spectacle lens (you simply press them on the lenses while the patient needs them, then peel them off once they don’t), and 2) When the deviation is so large you either cannot physically cut the power into the lens and still fit the frame, or the cosmetic effect would be intolerable to the patient.  Put another way, you can correct higher amounts of physical eye misalignment with Fresnel prisms than with regular prisms cut into the spectacle lens itself.

In a nutshell, MANY conditions can cause double vision, including traumatic brain injury, stroke, thyroid disease, and (usually temporary) palsies of cranial nerves 3, 4, and 6 (which control the muscles of the eye and are most often seen in hypertension and diabetes).  Putting politics aside, I believe it is unfair to Mrs. Clinton to jump to conclusions regarding her need for the prism; we don’t know if she had something serious happen, like a stroke, or a more benign nerve palsy.  To my knowledge she has not been seen publicly since with the prism, which is a good sign health-wise.  Most importantly, she has not yet declared her candidacy, which means she should enjoy the same rights of privacy regarding her medical records as any other citizen.  I’ll let the politicians debate what an actual presidential candidate should disclose.

Anyway, since it’s in the news, I saw a good chance to talk about Fresnel prisms.