IR-Basics
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innovative infrared technology BASIC PRINCIPLES of non-contact temperature measurement

Physical principles Physical principles With our eyes we see the world in visible light. Although visible light makes up only a small part of the radiation spectrum, the invisible light covers most of the remaining spectral range. The radiation of invisible light carries much more additional information. The infrared temperature measurement system Each body with a temperature above absolute zero (–273,15 °C = 0 Kelvin) emits electromagnetic radiation from its surface, which is proportional to its intrinsic temperature. A part of this so-called intrinsic radiation is infrared radiation,...

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innovative infrared technology The electromagnetic spectrum with the infrared area used by pyrometers. The electromagnetic radiation spectrum In a literal and physical sense, a spectrum is understood as the intensity of a mixture of electromagnetic waves that function as wavelength or frequency. The electromagnetic radiation spectrum covers a wavelength area of about 23 decimal powers and varies from sector to sector in origin, creation and application of the radiation. All types of electromagnetic radiation follow similar principles of diffraction, refraction, reflection and polarization....

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Physical principles If you point the measuring device on this hole, you can declare the temperature emitting from inside as black radiation which you can use for calibrating your measuring device. In reality, simple systems use surfaces, which are covered with pigmented paint and show absorption and emissivity values of 99 % within the required wavelength range. Usually, this is sufficient for calibrations of actual measurements. Radiation principles of a black body The radiation law by Planck shows the basic correlation for non-contact temperature measurements: It describes the spectral...

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innovative infrared technology The gray body Only few bodies meet the ideal of the black body. Many bodies emit far less radiation at the same temperature. The emissivity ε defines the relation of the actual radiation value and that of the black body. It is between zero and one. The infrared sensor receives the emitted radiation from the object surface, but also reflected radiation from the surroundings and potentially infrared radiation that has been transmitted through the black body. τ transmissivity Most bodies do not show transmissivity in infrared. Therefore the following applies:...

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Physical principles Thermal detectors With these detectors, the temperature of the sensitive element changes due to the absorption of electromagnetic radiation. The temperature chance causes a modification of the temperature-dependent property of the detector, which is electrically analyzed and serves as a measure for the absorbed energy. Radiation thermocouple elements (thermopiles) If the connection point between two different metallic materials is heated, the thermoelectrical effect results in an electrical voltage. The contact temperature measurement has been using this effect for a...

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innovative infrared technology Quantum detectors The decisive difference between quantum detectors and thermal detectors is their faster reaction on the absorbed radiation. The mode of operation of quantum detectors is based on the photo effect. The visible photons of the infrared radiation lead to an increase of the electrons into a higher energy level inside the semiconductor material. When the electrons fall back, an electric signal (voltage or power) is generated. Also, a change of the electric resistance is possible. These signals can be precisely evaluated. Quantum detectors are very...

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Emissivity and temperature measurement Emissivity and temperature measurement Emissivity is a key factor for the accurate measurement of temperatures. It depends on various influences and must be adjusted according to the application. Theoretically, emissivity depends on the material, its surface, temperature, wavelength, measuring angle and sometimes on the measuring arrangement. Many objects consisting of non-metallic material show high and relatively constant emissivity independent of their surface consistency, at least in long-wave spectral ranges. Temperature measurement of metallic...

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innovative infrared technology Temperature measurement of plastics Transmission rates of plastics vary according to wavelength. They react inversely proportional to the thickness, whereas thin materials are more transmissive than thick plastics. Optimal measurements can be carried out with wavelengths, where transmissivity is almost zero. Independent of the thickness. Polyethylene, polypropylene, nylon and polystyrene are non-transmissive at 3.43 µm; polyester, polyurethane, PTFE, FEP and polyamide are non-transmissive at 7.9 µm. For thicker and pigmented films, wavelengths between 8 and 14...

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Emissivity and temperature measurement Environmental influences The chart below shows that the transmissivity of air strongly depends on the wavelength. Areas of high damping alternate with areas of high transmissivity – the so-called atmospheric windows. The transmissivity in the long-wave atmospheric window (8 – 14 µm) is constantly high, whereas, due to the atmosphere, there are measurable reductions in the shortwave area, which may lead to false results. Typical measuring windows are 1.1 … 1.7 µm, 2 … 2.5 µm and 3 … 5 µm. Additional influencing variables are potential from heat sources...

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innovative infrared technology Calibration of infrared thermometers [1] [2] Infrared temperature measuring devices are calibrated with the aid of black bodies. These radiation sources can produce different temperatures with a high degree of stability (also see page 5, section on black bodies). For the calibration process it is important to know the exact value of the radiation temperature. The value is measured either using a contact thermometer or with a transfer normal radiation thermometer, and then used for adjusting/calibrating the infrared sensors. For calibration by the customer or...

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