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, a split imaging system such that the area of the crop appears as two images of the same area, at two different wavelengths, on the 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.