Almost all of the applications of remote sensing to date have been based on observing crops in distinct areas of the electromagnetic spectrum. The spectrum is represented in the figure below. The amount of energy radiating from a surface in a particular portion of the spectrum is measured by an instrument called a radiometer. Radiometers can be hand held for research purposes and monitoring small field plots or placed on board aircraft and satellites to survey entire fields, farms, or agricultural regions. Instead, the apparent reflectance of an object is best used to learn about its properties.
Reflectance is the ratio of the radiance from an object to the irradiance reaching the object.
The radiometer has special filters that can be designed so that only radiation from a specific part of the spectrum is measured. The usefulness of the visible and infrared portions of the spectrum can be seen by comparing the typical spectral responses of a crop canopy and a bare soil. That is why plants look green to our eye, as this is where they reflect the most radiation in the visible part of the spectrum. The structure of the leaves account for this relative increase in reflectance in the near-infrared. Because of these properties, measurements of reflectance in the red and NIR can be used to determine differences in crop canopy densities.