The first published deployment of an active optical reflectance sensor (AOS) in a low-flying aircraft in 2009 catalyzed numerous developments in both sensor development and sensor platform integration. Integral to these sensors is a modulated light source composed of high power LED technology that emits high radiance polychromatic light. The sensor easily mounts to agricultural aircraft and can sense agricultural landscapes at altitudes from a few meters to altitudes exceeding 40 meters while traveling at velocities of more than 270 km/h. The rather large sensor-to-canopy measurement range allows the sensor to accurately measure ratio-based spectral reflectance indices such as the NDVI over fields with rolling terrain. Two versions of the sensor have since been developed and tested. A key advantage of airborne AOS is that they provide ratio-based index values unaffected by path radiance. This alone offers a viable, large scale sensing technique for researchers interested in plant and soil moisture investigations using the ‘reflectance index-temperature’ space concept or for the large scale, yet location specific conversion of ‘top-of-atmosphere’ vegetation indices, as derived from satellite imagery to the ‘top- of-canopy’ values