When dealing with light and optics, it is commonplace to constrain our understanding to the visible range of the electromagnetic spectrum, i.e., the narrow interval where the human eye is able to respond. However, this anthropomorphic definition of optics is too narrow. On both sides of the visible spectrum, we find spectral ranges full of interest that also allow an analysis quite similar to that given in the visible spectrum. In the high-energy, high-frequency side, we find that ultraviolet radiation, X-rays, and g-rays are of use in astronomy, medicine, and technology because of their capability to trigger chemical reactions and generate ionized matter. On the low-frequency, low-energy side, we have witnessed the development of radiofrequency applications in telecom, radar, and microwaves. But, when reaching those portions of the spectrum closer to the visible, the use of millimeter waves, terahertz, and infrared radiation has produced a variety of devices and technologies that have strongly expanded the capabilities to sense nature in a different way. In the infrared, image-forming systems have advanced in accuracy, time response, and analysis capabilities through improved algorithms and by using hyperspectral techniques and sensor fusion. Beyond thermography, infrared image technology already distinguishes chemical compounds, and takes full advantage of the detection of the polarization of light.
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