1. An active imaging system for plant growth monitoring comprising: (a) an unmanned aerial vehicle equipped with: a light emitter capable of emitting light at two or more wavelengths, the light emitter receiving a modulation signal from a modulator and emitting light modulated by the modulation signal; a first lens that projects light from the light emitter onto a crop and a second lens that images an area of the crop onto an array of sensor pixels, each pixel of the array of sensor pixels: detecting light reflected by the crop, generating a detection signal, and mixing the detection signal with the modulation signal from the modulator to obtain a reflectivity measurement of the crop; and, (b) a processor that computes a vegetative index based on reflectivity measurements obtained at two or more different wavelengths.