# How does this ISS vision-testing instrument work? What exactly is being measured, and how?

This tweet by Surrey Satellite engineer Enguerran Petitfils says

Astronaut Serena Auñón-Chancellor Examines Her Eyes via NASA https://ift.tt/2NmEt6k

What is this device exactly, how does it work, and who makes it? There is a logo on the side which says OIS which presumably just stands for Ocular Ischemic Syndrome:

...the constellation of ocular signs and symptoms secondary to severe, chronic arterial hypoperfusion to the eye.

The link in the tweet leads to today's NASA post Astronaut Serena Auñón-Chancellor Examines Her Eyes, which does not really answer this question.

Question: What exactly is being measured, which of the "constellation of ocular signs and symptom..." are being detected, and how?

The link says it is a funduscope, meaning she can image her retina with it. It is basically a camera with a very specialized optics package that allows for taking photos through the lens of the eye. See https://en.wikipedia.org/wiki/Ophthalmoscopy for examples of retinal imaging.

Astronaut Serena Auñón-Chancellor, a member of the Expedition 56 crew currently aboard the International Space Station, examined her eye with a Funduscope with remote support from doctors on the ground. (emphasis added)

There seems to be something about spaceflight that impairs vision (https://www.nasa.gov/mission_pages/station/research/experiments/1038.html) and this may be connected to experiments trying to find a cause.

the OIS logo seems to belong to a (defunct?) manufacturer of opthalmography equipment: google "ois winstation"

Hypoperfusion (meaning less blood flow than necessary) is detectable on funduscopy images, especially if they have access to pictures taken before the flight for comparison.

The link i gave above, about the immpaired vision after spaceflights, mentions ocular flattening - a usual funduscope would not be able to give measurements on that, but this one has an unusally long tube, so maybe the optics allow for more measurement modes?

Other pictures with the same instrument cite a 'ocular health' experiment that is ongoing on ISS: https://www.nasa.gov/mission_pages/station/research/experiments/204.html;