Typically, eye exams require the use of either a phoropter (the big thing with the multiple lenses in which the patient picks the clearest option) or an aberrometer (which measure an eye's characteristics with a laser). Those are both large and expensive systems, and are not available to many who need refractive lenses, especially in the developing world.
There are lots of different proposed solutions, but this might be one of the cheapest. The system uses merely a cellphone and a plastic attachment that clips on to the cellphone's screen. The estimated cost of that attachment is a mere $1 to $2, and cellphones are becoming more and more widespread, even in poorer areas.
Essentially, the patient looks through the plastic attachment's lens at the cellphone's screen, and presses the phone's up or down arrow until certain sets of parallel lines overlap. More in-depth description:
The prototype system Raskar and his students developed as a result of that insight has an array of tiny lenses and a grid of pinholes that, combined with the software on the phone, “forces the user to focus at different depths” so the eye’s focusing ability can be measured. Essentially, Raskar explains, the test works by transforming any blurriness produced by aberrations in the eye into an array of separate lines or dots instead of a fuzzy blob, which makes it easier for the user to identify the discrepancy clearly. Rather than estimating which of two views looks sharper, as in conventional eye tests, the user adjusts the display to make the separate lines or dots come together and overlap, which corresponds to bringing the view into sharp focus. The underlying principle is similar to that used by new “adaptive optics” systems that have recently allowed ground-based telescopes to exceed the performance of the Hubble Space Telescope; these sometimes use the same kind of Shack-Hartmann sensors used in eye testing aberrometers.
The group plans to commercialize the product under the name PerfectSight, and eventually expand their creation to include more advanced detection (for cataracts, for instance). First, they'll field test the device--first in Boston, then in developing countries--and then hopefully head into production. The two billion people worldwide who could benefit from the invention should be excited.