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A bench prototype photonic-based spectral reflectance sensor architecture for use in selective herbicide spraying systems performing noncontact spectral reflectance measurements of plants and soil is described and experimental data obtained with simulated farming vehicle traveling speeds of 7 and 22 km/h is presented. The sensor uses a three-wavelength laser diode module that sequentially emits identically-polarized laser light beams through a common aperture, along one optical path. Each laser beam enters a multi-spot beam generator which produces up to 14 parallel laser beams over a 210mm span. The intensity of the reflected light from each spot is detected by a high-speed line scan image sensor. Plant discrimination is based on calculating the slope of the spectral response between the 635nm to 670nm and 670nm to 785nm laser wavelengths. The use of finely spaced and collimated laser beam array, instead of an un-collimated light source, allows detection of narrow leaved plants with a width as small as 12mm.

Abstract:Beam deflection methods such as rotary mirrors and motorized turning optical heads suffer from a variety of electro-mechanical related problems which affect laser scanning performance. These include wobble, jitter, wear, windage and synchronization issues. A novel optical structure consisting of two concentric and cylindrical interfaces with unique optical coating properties for the static projection of a laser spot array over a wide angle is demonstrated. The resulting ray trajectory through the waveguide is modeled using linear equations. Spot size growth is modeled using previously defined ray transfer matrices for tilted optical elements. The model is validated by comparison with experimental spot size measurements for 20 transmitted beams. This novel form of spot projection can be used as the projection unit in optical sensing devices which range to multiple laser footprints.

A bench prototype photonic-based spectral reflectance sensor architecture for use in selective herbicide spraying systems performing noncontact spectral reflectance measurements of plants and soil is described and experimental data obtained with simulated farming vehicle traveling speeds of 7 and 22 km/h is presented. The sensor uses a three-wavelength laser diode module that sequentially emits identically-polarized laser light beams through a common aperture, along one optical path. Each laser beam enters a multi-spot beam generator which produces up to 14 parallel laser beams over a 210mm span. The intensity of the reflected light from each spot is detected by a high-speed line scan image sensor. Plant discrimination is based on calculating the slope of the spectral response between the 635nm to 670nm and 670nm to 785nm laser wavelengths. The use of finely spaced and collimated laser beam array, instead of an un-collimated light source, allows detection of narrow leaved plants with a width as small as 12mm.