Active Spectral Sensor Evaluation under Varying Conditions

Plant stress has been estimated by spectral signature using both passive and active sensors. As optical sensors measure reflected light from a target, changes in illumination characteristics critically affect sensor response. Active sensors minimize the illumination effects by producing their own illumination that is reflected from the target and measured by the detector. Although active sensors use modulated radiation that can be differentiated from ambient illumination, in order to validate data and increase the accuracy, sensor performance characteristics must be well understood and examined in different target conditions of plant leaves. The performance of an active NDVI sensor was evaluated to study the effect of: 1) partial canopy coverage, 2) target off-center, 3) standoff distance, 4) target surface tilting, 5) solar bidirectional effect, 6) temperature, and 7) diurnal radiation change. These evaluations provide a valid range of sensor measurements and a motivation to improve the measurement accuracy by using selective data that can be validated by supplemental sensors.