Temperature is one of the most frequently measured parameters, and with good reason. Temperature serves as an important indicator of the condition of an object in machinery, medical diagnostics, quality control, etc. There are many different methods for measuring temperature, including using infrared technology. Infrared has been used successfully for many years in many fields, but i
nfrared thermometry is a relatively new concept promising to reduce cost and increase reliability in temperature measurements across a wide variety of consumer and industrial applications. Some of the advantages of infrared thermometers include:
· Fast responsiveness (ms range) enables developers to collect more information per time period.
· Able to measure the temperature of objects that move, rotate, or vibrate.
· Measure high temperatures (above 1500°C) in applications where contact probes will not work or have a short lifetime.
· Do not mechanically damage or contaminate the surface (painted surfaces, food, soft plastic) of measurement.
· Do not influence the object of measurement, hence it can measure temperature in all objects such as objects with high thermal-conductivity where object temperature would change if contacted (glass, wood, small, or very thin objects).
· Measures actual product being manufactured instead of another part of the process.
· Can measure the temperature of hazardous or inaccessible objects where the use of conventional thermometers is impractical.
· Accuracy is comparable with conventional thermometers.
Principles of infrared thermometry
The underlying technology behind infrared thermometers is based on the principle that all objects emit radiation at wavelengths in the infrared region of the electromagnetic radiation spectrum. Infrared is that portion of the electromagnetic spectrum that lies beyond the visible (blue to red, 0.4-0.75 um) response of the human eye. Because IR radiation is predominantly generated by heat, it is called thermal radiation. Figure 1 shows the electromagnetic spectrum.
Every object emits radiation, depending upon its temperature. The spectral radiation characteristics of a black body can be theoretically calculated using Planck’s law. A black body is one which absorbs all electromagnetic radiation incidents on it. Planck’s law gives the intensity radiated by a black body as a function of wavelength. Figure 2 shows these for selected temperatures.
From the Figure2, it is clear that the curves never intersect, which implies that the radiation intensity at every wavelength is a strict function of the temperature. By measuring the intensity of the radiation, one can therefore determine the object’s temperature. With rising temperature, the intensity at every wavelength of the radiation spectrum increases as well. This means that one can remotely determine the temperature of an object by measuring its radiated power. If the temperature of an object increases, it will start to glow in a dark red color, with further increases in temperature changing the color light to red, bright white, etc. 6000 K is the temperature of the sun and our eyes are adapted to “detect” this radiation as white light.
If the measured body has a temperature lower than 400 °C, one needs a radiation detector which is sensitive to a much longer wavelength than those of the visible spectrum. Such a detector must be sensitive to the infrared region (also called heat radiation) around 10 µm wavelengths. There are different sensors available which are capable for accurately detecting and measuring heat radiation in the 3 to 20 µm infrared (IR) wavelength region. Thermopile is a sensor used in IR thermometer to measure temperature through the IR radiation from body.
Thermopile measures the IR radiation and provides an output signal calibrated in a variety of ranges according to customer requirements. A thermopile sensor has an IR absorber connected with a series of thermocouples. The cold contacts of the thermocouples are connected to a known reference. These thermocouples measure the object temperature. The ambient temperature of the sensor is measured using a thermistor. Usually their output is in the range of a few microvolts.