This image was made by using the remotely sensed data from two different wavelength "channels" of the Advanced Very High Resolution Radiometer (AVHRR) aboard a NOAA polar orbiting satellite. Different types of vegetation, and stages of its growth, are difficult to distinguish using only visible light. Basically, it all looks sorta green. But by adding in additional radiative information from the near infrared portion of the spectrum, the type and health of vegetation may be further distinguished owing to both botanical properties of the leaves and to the amount of available moisture in the soil. Scientists have invented a data processing algorithm for these two radiation "channels" called the Normalized Difference Vegetation Index (NDVI). This image, therefore, is really an image of a mathematical quantity calculated from the raw radiation data measured by the AVHRR. It is formally incorrect to say that a satellite "senses vegetation". The satellite senses upwelling radiation from which a remote sesning scientist can infer vegetation using an algorithm, such as the NDVI algorithm.