During a recent NASA broadcast, I noticed something in the background:
It looks like a Snellen chart to me, but why would they have that on board? If that's not it, what is it then?
Eyes do strange things in microgravity (when you consider they're deformable bags of fluid, this isn't too surprising). This report outlines the changes that can be identified after just a short parabolic flight. Eye test charts provide a way to investigate this without requiring heavy equipment or specialists.
This study appears to be an on-going project to study this on the ISS (Mar 13 - Sep 16), and is probably the reason that chart's there...
NASA has been studying the effects of microgravity on astronauts' eyes for at least a few years. This article from Space.com from 2012 talks about some of the findings from that time.
They say it's a high priority, so having the ability to perform tests aboard the ISS makes sense, especially for the astronauts who are up there for a year.