R RACK A floor-standing structure primarily designed for and capa- ble of supporting equipment. A frame upon which one or more units of equipment are mounted. RACK AND STACK An ATE that relies on system integration of applicable units of test equipment. RADIAN Metric unit of measurement for plane angles. RADIANCE Radiant power, in a given direction, per unit solid angle per unit of projected area of the source, as viewed from the given direction. Note: radiance is usually expressed in watts per steradian per square meter.