|Kim, Yunseop "james"|
Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/15/2011
Publication Date: 12/18/2011
Citation: Kim, Y., Glenn, D.M., Park, J., Ngugi, H.K., Lehman, B.L. 2011. Characteristics of active spectral sensor for plant sensing. Transactions of the ASABE. 55(1):293-301. Interpretive Summary: Measuring plant stress in its early stages is critical in maximizing yield and quality of any crop. Several types of non-contact sensors were used to measure and analyze the light reflected from apple trees grown with a range of water stress levels and environmental conditions, particularly relative humidity and plant wetness. We found that high humidity and leaf wetness could significantly alter the amount and quality of light reflected from apple trees. These evaluations illustrate the potentials and limitations of the active spectral sensor for plant sensing and provide a guideline to understanding the sensor performance in order to improve the measurement accuracy.
Technical Abstract: 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 conditions critically affect sensor response. Active spectral 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, sensor performance characteristics must be well understood and examined in different target conditions of plant foliage in order to validate data and increase the accuracy. In this paper, the performance of a commercial active spectral sensor, GreenSeeker, was evaluated to study the effects of: 1) partial canopy coverage, 2) target off-center, 3) standoff distance, 4) target surface tilting, 5) wetness of target surface, 6) illumination and temperature, 7) bidirectional angle, and 8) diurnal solar radiation. Experiments examined a valid range of sensor responses and indentified a major effect of relative humidity that was amplified by moistened surfaces resulting in an increase of Normalized Difference Vegetation Index (NDVI) response up to 41 percent. These evaluations illustrate the potentials and limitations of the active spectral sensor for plant sensing and provide a guideline to understanding the sensor performance in order to improve the measurement accuracy.