The INL’s Matched Index of Refraction (MIR) flow facility is the largest index-matching flow facility in the world. The MIR resembles a wind tunnel but flows light mineral oil instead. By precisely controlling the temperature of the oil and immersed quartz glass models / test sections, their indices of refraction are matched for specific wavelengths of light. For the INL’s configuration, typical operating temperatures lie between 23°C - 26°C. With index matching, a model physically perturbs the flow field but optically is invisible. Taking advantage of this, the INL MIR facility has and uses state-of-the-art optical velocity measurement techniques (such as Particle Image Velocimetry and Laser Doppler Velocimetry) mounted on a multi-axis computer-controlled traversing system. Since the MIR’s measurement techniques are non-intrusive, there is no possibility of a probe or other instrumentation potentially disturbing the flow of interest. These techniques, combined with the INL MIR’s large size, allow accurate high-resolution velocity field measurements very close to, around, and even inside complicated geometric models – measurements that would be difficult or even impossible by any other means. The INL MIR is dedicated to improving our understanding of fundamental physical phenomena and providing experimental measurements for assessment and validation of computational fluid dynamics and nuclear reactor system safety codes.