Emissivity refers to the ratio of the energy radiated from the surface of an object to the energy radiated by a blackbody at the same temperature. (A blackbody is an idealized radiator that radiates all of its energy and has an emissivity of 1.00 on its surface)
The emissivity of various substances is determined by the material of the object itself, and at the same temperature, different substances will radiate different amounts of energy to the outside. As shown in the figure on the right, the electrical insulating tape is attached to the surface of the stainless steel cup, and the temperature is measured by a contact thermocouple, and the temperature at the insulating tape can be seen to be consistent with the contact temperature measurement, while the emissivity of the surface of the stainless steel cup is different from the tape, so the temperature display is quite different.
The detection target can be divided into two parts: non-metallic and metallic materials: the emissivity of most non-metallic materials (such as plastic, paint, leather, paper, etc.) is usually 0.95, and the emissivity of targets of the same material and different colors is very close, and the error usually does not exceed the measurement accuracy range; Some non-metallic materials with bright surfaces have a low emissivity (e.g., ceramic tiles, glass, etc., generally have an emissivity of 0.92).
The emissivity of a metallic material is affected by the following factors:
● Materials: Different materials have different emissivity, such as copper, which is generally higher than aluminum.
● Surface finish: Usually the emissivity of materials with rough surfaces is higher than that of smooth surfaces.
● Surface color: The surface emissivity of dark colors represented by black is higher than that of light colors.
● Surface shape: The surface has depressions, angles or uneven regularities, and the emissivity is higher than that of flat parts.
In addition, temperature (over 500°C) and detector band can also affect emissivity.
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