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Infrared Thermography: Operating Principle and Applications

09-08-2019

Infrared thermography is a non-contact sensor technology used for measuring the surface temperature of objects. As it utilizes a non-contact measurement method, it exerts zero influence on the measurement objects. It also has a very long service life and requires little maintenance. An infrared thermography sensor can be used for in-line monitoring as it is compact and has a very good response time so it can pick up dynamic movements fast.

Several types of infrared thermography sensors are available in the market; thermal imaging camera, infrared thermometer and ratio pyrometer. They all operated based on a similar principle. These sensors are commonly available with different operating wavelengths for measurements in different applications. The thermal imaging camera offers an additional opportunity to visualize the heatmap of the surface temperature of objects in images which provides an added value for quality checking in the manufacturing sector.

Infrared operating principle

The infrared sensors measure temperature based on infrared radiation. Infrared radiation is a part of electromagnetic radiation that is emitted by every object at a temperature above absolute zero. The radiation emitted by the object is proportional to its intrinsic temperature. Through the optics in the thermographer, the beams are focused on a detector element that generates an electrical signal proportional to radiation. The signals are processed and amplified to generate output signals that directly translate to temperature. This data can be displayed on a computer or sent to a control system via a connecting interface.

Infrared temperature measurement is based on Planck’s radiation law. It describes the correlation of spectral radiation of a black body into space depending on its temperature and wavelength. The equation has been simplified as per Stefan-Boltzmann’s law to define the relationship between the electrical signal (U) from the detector and object temperature (T).

U~εT4

The correlation above shows the significance of the object emissivity (ε) on object temperature. A black body has an emissivity value of 1, but in reality, the emissivity of real objects is less than 1 as it emits less infrared radiation than the black body. Emissivity highly depends on the materials, surface temperature, wavelength and measuring arrangement. Measurement errors of up to 10% may occur as a result of using the wrong emissivity value.

Effect of temperature on infrared sensors

Infrared sensors are based on the fact that any person or object that has a temperature above absolute zero emits radiant energy. If the object fails  to specify the correct value of materials emissivity or ability to emit infrared energy, it will definitely produce errors in the measured value. While the emissivity of the material is known at certain temperature range, it is also a dynamically changing property that largely depends on the surrounding radiant temperature and the wavelength of the measuring instrument. Understanding all parameters in the measuring instrument is crucial to make the correct temperature measurement.

Moreover, If the upper ambient temperature of infrared sensor exceeds, the sensor will begin to provide erratic or incorrect temperature indications. If the sensor gets extremely hot, it will be destroyed.

Infrared thermography selection

Selecting the appropriate technology for infrared thermography should be based on the operating wavelength where measurement error is at the minimum. It may vary depending on the type of object being measured.

Furthermore, the selection of infrared sensors greatly depends on its accuracy which influenced by the measurement spot size, especially when measuring temperature of small objects. This is often referred to as “distance-to-spot ratio” which specifies the diameter of the area being measured in response to the distance of sensor from the target.

For instance, an infrared sensor with distance-to-spot ratio of 5:1 will measure approximately 1cm of diameter when the sensor is located 5cm away from the target.

Metals

For example, measuring the temperature of metallic materials is suggested to be done at a wavelength in which the metals exhibit a relatively high emissivity. The shortest possible wavelength should be used as measurement errors increase with correlation to the wavelength. The optimum is generally achieved at a wavelength of 0.8 – 1 µm.

Plastics

For plastic materials, optimal results are achieved when measurements are taken at a wavelength where the transmissivity of plastics is negligible. The thickness of plastics should also be taken into consideration as thin materials are more transmissive than thick plastics. The optimal wavelength range can be determined by testing the plastics materials. This can be from 3.43 µm for thin plastics or between 8 and 14 µm for thicker ones.

Glass

Using an infrared thermography sensor to measure glass temperature requires careful selection of wavelength range to facilitate the measurement of the temperature of different layers of glass surfaces. Generally, a shorter wavelength of 1 µm to 3.9 µm is appropriate for measuring the temperature of the deeper layer. For surface temperature, an operating wavelength of 5 µm is recommended. A longer wavelength between 8 and 14 µm can be considered if the glass temperature is low enough. However, the glass emissivity should be adjusted to around 0.85 to compensate for the reflection effect.

Applications of Infrared Thermography in industry

Test and measurement

The infrared technology offers fast measuring and precise temperature measurement that is crucial for the research and testing applications in the industrial and academic laboratory. Researchers have reportedly used infrared cameras, infrared thermometers and multi-colour pyrometers to capture the temperature of individual particles in coal combustion reactors in an attempt to understand the transient phenomena behind combusting coal particles to practically design a new low-emission burner technology. This infrared technology can capture the transient changes that occur in milliseconds which is unable to be measured with a conventional thermocouple. In industrial environments, it is a qualified and approved sensor technology to measure, monitor and control process temperature.

Injection molding


Temperature determines the quality of the finished products in injection molding or 3D-printing applications. Getting the temperature correct is crucial to ensure that the manufactured products are free of defects. This particularly holds true for the manufacturing of automotive products where tiny dents or defects are not tolerated. The thermal imager has been used in an online system to measure the surface temperature distribution of the products. It can be integrated with the PLC system to automatically reject the assemblies if the measured temperature is not within the standardized range.

 

 

Manufacturing


In the manufacturing industry, the temperature is one of the parameters that greatly affects the quality of the manufactured products. For example, the quality of the finished glass products highly depends on the temperature of the molten glass. A special thermal imager for glass temperature monitoring has been developed for this measurement purpose. In the metals industry, monitoring the temperature of the molten metals is essential to correctly maintain the process temperature during casting. This requires a special infrared sensor that offers a suitable spectral range and fast response.

 

 

Condition monitoring

Thermal imaging camera is one of the most sought-after technology as a preventive maintenance tool in industrial process applications. It can be used for condition monitoring applications to detect hot spots on crucial process equipment such as pumps. Hot spots generally indicate problems in the machinery which could be caused by loose screws, wear of bearings or overheating. Early detection of problems allows the repair and maintenance of equipment before a catastrophic failure occurs which may save the company lives and money.

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