Crops absorb parts of the total incoming solar radiation as a source of energy to produce biomass (photosynthesis). The amount of near-infrared radiation that is reflected by plants is a good indicator of the condition and healthiness of the crop. A healthy crop reflects most of the near-infrared radiation, whereas a crop under stressed conditions adsorbs this type of radiation. This information can be used for early warning detection of crop stress or failure. The human eye cannot see in the near-infrared and crop stress in its initial phase is therefore often observed too late.