Infrared Temperature Sensors

Emissivity and Wavelength - the two key parameters

Non-contact, infrared temperature measurement devices - including handheld and fixed systems - are relatively low cost methods of monitoring, controlling and managing process temperatures, helping to avoid costly production downtime. But for accurate temperature measurement using infrared sensors, users must carefully consider two key parameters: emissivity and wavelength.

Infrared thermometers measure the temperature of an object without
touching it. It is therefore possible to perform fast, reliable temperature measurements of moving, hot or difficult-to-access objects. While contact temperature sensors or probes can influence the temperature of the target object, sometimes even damage the product itself, the noncontact method ensures precision measurements without damaging the target object. Infrared sensors can also measure very high temperatures, whereas a contact sensor would either be destroyed or would have a shorter service life.

Not only are infrared devices now relatively inexpensive, they also offer numerous technical benefits and a variety of options for users, including handheld or inline process control, open connectivity to fieldbus systems and options for hazardous environments.

For accurate temperature measurement using infrared sensors, users must carefully consider two key parametersemissivity and wavelength.

All bodies above absolute Kelvin (-273°C) emit infrared radiation in three ways, via a combination of emitted radiation, radiation reflected from the surroundings, and by transmitting the radiation through itself. How these factors interact depends on the material of the measurement object. However, for non-contact infrared temperature measurements, only the emitted radiation element is important.

The relationship of the emission types to each other is best described in the following way. If at any given temperature, the sum of the radiation of the three emission types is equal to one, and it is assumed that solid bodies transmit negligible radiation, the transmitted element can be treated as zero. Therefore, the heat energy coming from an object only comprises emitted and reflected radiation.

This is why objects such as polished and shiny metals can only have a low emission, or emissivity, as radiation from the surrounding environment is strongly reflected (and so proportionally high) by these surfaces.

The emissivity of an object will vary when monitoring the radiated heat energy at different wavelengths. Therefore, developing sensors that measure temperature at specific wavelengths can significantly increase measurement stability.

Material groups can be used to describe the optimum wavelengths for highest object emissivities and therefore the most stable results. For metals, 0.8 to 2.3µm, glass 5µm, textiles and most matt surfaces 8-14µm. Thin film plastics are more complex, requiring specific wavelength sensors to be developed for polyethylene, polypropylene, Nylon and polystyrene (3.43µm). Polyester, polyurethane, Teflon, FEP and polyamide require 7.9µm. Thicker, pigmented films require 8-14µm.

When selecting an infrared temperature sensor, it is vital that the wavelength band over which the sensor measures is known, and that the correct wavelength band is used for the object to be measured.The object emissivity values over this wavelength and the temperature range to be measured must also be known or calculated.

March 2012


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