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.