Infrared ·Black Body Radiation

Any object with a temperature above absolute zero (-273.15 °C) emits infrared energy (heat) based on its temperature. The infrared energy emitted by an object is known as its thermal distribution.

In general, the hotter an object is, the more radiation it emits. A thermal imager is essentially a thermal sensor that detects and captures tiny temperature differences. It collects infrared radiation from objects, and creates pixels based on information about temperature differences to compose an image. Since objects are rarely at exactly the same temperature as surrounding objects, thermal imaging cameras can detect their differences and form sharp contrasts in thermal images, which is the basic principle of infrared thermal imaging

Any object has the ability to continuously radiate, absorb, and reflect electromagnetic waves. The radiated electromagnetic waves are different in each band, that is, they have a certain spectral distribution. This spectral distribution is related to the characteristics of the object itself and its temperature, so it is called thermal radiation.

Objects in real life exhibit different radiation properties. Thus, it turns out that a model volume of ideal radiation properties is first considered and then applied to actually occurring objects as a reference, this model volume is called a "black body" in radiation physics. It is unique in that, of all objects of the same temperature, it exhibits the greatest possible emission of radiation.

In order to study the law of thermal radiation that does not depend on the specific physical properties, physicists have defined this black body as a standard object for thermal radiation research.

The so-called black body means that all incident electromagnetic waves are absorbed, neither reflected nor transmitted (of course, the black body still radiates outward).

Under any conditions, an object that completely absorbs external radiation of any wavelength without any reflection, that is, an object with an absorption ratio of 1.

In blackbody radiation, the color of light varies with the temperature, and the blackbody presents a gradual change process from red, orange-red, yellow, yellow-white, white, blue-white. When the color of light emitted by a light source looks the same as that emitted by a black body at a certain temperature, the temperature of the black body is called the color temperature of the light source. The higher the temperature of the "black body", more blue and less red color of the spectrum will have.

According to Kirchhoff's radiation law, the ratio of the energy radiated by an object in thermal equilibrium to the absorption rate has nothing to do with the physical properties of the object itself, but only with the wavelength and temperature. According to Kirchhoff's radiation law, at a certain temperature, a black body must be the object with the largest radiation ability, which can be called a complete radiator.

Black body radiation refers to the radiation emitted by an ideal emitter, which emits the largest amount of radiation at a specific temperature and wavelength. At the same time, a black body is an object that can absorb all incident radiation and will not reflect any radiation, but a black body is not necessarily black. For example, the sun is a gas planet. It can be considered that the electromagnetic radiation directed at the sun is difficult to be reflected back, so the sun is considered to be a black body (absolute black body does not exist). Theoretically, the black body emits electromagnetic waves of all wavelengths on the spectrum.

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