1. An active imaging system for plant growth monitoring comprising: 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 receiving the modulation signal from the modulator and detecting light from the light emitter after the light has been reflected by the crop, thereby obtaining a reflectivity measurement of the crop; and, a processor that computes a vegetative index based on reflectivity measurements obtained at two or more different wavelengths.