Infrared is heat-emitting radiation.
Thermal imaging detects infrared radiation, which gives off heat. The infrared radiation emitted from a person or object is called its heat signature. This technology is used in gaming, law enforcement and by the armed forces to find hidden people or locations that emit heat, especially if this heat contrasts to the heat of the surrounding area. Plants also give off infrared radiation, which can be detected through thermal imaging.
Detecting Disease
Thermal imaging has been used to detect disease in plants, sometimes before any visible damage appears. Experiments conducted with tobacco showed increased temperature in the areas where a tobacco mosaic virus was introduced, yet fungus on beets showed lower temperatures on infected areas. These findings could facilitate early detection of pathogens and contribute to the formation of a recognizable disease heat signature, which could allow more specific identification of the pathogen and give clues about how that pathogen works.
Seed Viability
A method to quickly analyze seed viability was developed by scientists at Kew Gardens in the United Kingdom. When the seeds in question take up water, subtle changes occur in them, depending on the seed's viability. When a dry seed takes up water, the stored sugars in the seed dissolve, causing the temperature of the seed to cool. This can be detected via thermal imaging. A database of thermal signatures for plants is being constructed by the Kew scientists to aid in identifying viable seeds.
Irrigation
Thermal imaging technology is being used to pinpoint when a plant needs irrigation. A plant will close its stomata -- the tiny holes in the leaf used for water and gas exchange -- to conserve water when under water stress. When the stomata are closed, cooling of the leaf ceases, and the leaf heats up. This change of heat can be detected, and irrigation can be targeted to that plant or group of plants. Each group of plants may have a distinct heat signature that indicates its level of water stress.
Illegal Grow Operations
Police have used thermal technology in the search for illegal marijuana growing operations. The plants involved put out infrared heat, but law enforcement is often more concerned with the heat emitted by the apparatus surrounding the plants that help them grow, like heaters and lights. When a police helicopter flies over a suspected illegal growing operation, it probably won't get a heat signature from the plants, but it determines if the heat signature overall is consistent with conditions common to growing operations.
Tags: heat signature, infrared radiation, group plants, growing operations, infrared radiation which, radiation which