Thermal radiation infrared region

Photochromism can be defined as a reversible change between species having different absorption spectra, the initial change being induced by the action of electromagnetic radiation, usually in the ultraviolet, visible and infrared regions. The product is generally thermodynamically less stable, and consequently the reverse reaction is spontaneous and thermally induced. 

Passive optical techniques exploit the natural illumination in the environment (e.g., thermal radiation from the elements in the scene, the Sun, the cloud itself) to replace the active laser beam. In the infrared spectral region, where CW agents have the most characteristic spectral properties, solar radiation contributes little compared with thermal self-emission. As long as the suspect cloud or surface is not in complete thermal equilibrium with the environment, that is, as long as there is a temperature difference between the target and elements in the scene, there are measurable spectral differences between emissions from elements (pixels) on and off the target. 

Luminescence, the emission of electromagnetic radiation in excess of that thermally generated, has been observed in several azides. The emission typically occurs in the ultraviolet, visible, or infrared regions of the electromagnetic spectrum. Some external stimulus is required prior to luminescent emission. The stimulus can take the form of photons, energetic particles, electric field, mechanical energy, or energy available from chemical reaction. 

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