Airman 1st Class Nichole Porrini, 28th Security Forces Squadron flight member, looks through a phoropter during an examination at the Optometry clinic at Ellsworth Air Force Base, S.D., Dec. 13, 2011. Optometrists use phoropters to measure the refractive status of each eye and determine the correct prescription. (U.S. Air Force photo by Airman 1st Class Zachary Hada/Released)