Many factors need to be considered during the selection of a non-contact thermal measurement device. Factors such as wavelength and emissivity are important during the measurement of temperature of specific materials or objects, while other factors such as compactness, integration capabilities, and ease of set up are also equally important.
The temperature of objects is measured by infrared pyrometers and thermal imaging cameras without making contact with the object. Hence, the temperature of moving components or objects that cannot be touched can be rapidly and reliable measured. Infrared measurement devices are not only relatively inexpensive, but also have various features and options, including software tools to facilitate integration for process control and high speed recording for research and development environments. Sensors and imagers operating at specific wavelengths can now be selected for specific materials, such as glass, ceramics, and metals.
Infrared sensors are available with various options: laser aiming, video interface or compact design with integrated controller
The Infrared Temperature Measurement System
An electromagnetic radiation is emitted from the surface of a body with a temperature higher than the absolute zero (-273.15 °C = 0 Kelvin) in proportion to the intrinsic temperature of the body. Infrared radiation is part of this intrinsic radiation and can be applied to measure the temperature of a body. This radiation penetrates the atmosphere. The beams are focused on a detector element using a lens (input optics) and an electrical signal is penetrated proportional to the radiation by the detector element. The signal is then amplified and subsequently transformed into an output signal proportional to the object temperature using successive digital signal processing.
The measuring value may be read in a display or emitted as an analogue output signal in order to establish an easy connection to control systems of the process management.
The following are the benefits of non-contact temperature measurement:
- Non-destructive measurement
- Very fast response and exposure times
- Long lasting measurement, no mechanical wear
- Measurement without inter-reaction, no influence on the measuring object
- Temperature measurements of moving or overheated objects and of objects in hazardous surroundings
Key Parameters Emissivity and Wavelength
Emissivity and wavelength are the two key parameters taken into account for accurate temperature measurement.
All bodies with a temperature higher than absolute Kelvin (-273°C) release infrared radiation in three ways: through a combination of emitted radiation, radiation reflected off from the surroundings, and by transmitting the radiation through itself. How these factors interact relies on the material of the target object. However, the emitted radiation element is the only key factor for non-contact infrared temperature measurements.
Three ways of emissivity
The relationship between the emission types is explained in the following way. If at any specific temperature, the sum of the radiation of the three emission types is equal to one, and solid bodies are assumed to transmit negligible radiation, the transmitted element can be treated as zero. Therefore, the heat energy coming from an object only consists of emitted and reflected radiation. This is the reason behind the low emission or emissivity of objects such as shiny or polished metals, as radiation from the surrounding environment is strongly reflected off (and so proportionally high) from these surfaces.
Emissivity for non-metals and metals
However, the emissivity of an object will be lower or greater when the radiated heat energy is monitored at different wavelengths. Hence, using IR cameras and pyrometers that provide temperature measurements at specific wavelengths that match the high emissivity of specific materials are capable of considerably increasing measurement accuracy and stability.
At present, around 80% of IR pyrometers and cameras available on the market operate over the wavelength band of 814 µm, meaning these instruments are only offering accurate and stable measurement on objects with high emissivity in this wavelength band. These objects are generally those with matt surfaces. However, accurate measurements on shiny or metal surfaces are not possible using instruments operating at the 8-14 µm wavelength band.
Wavelengths of non-metals and metals
Hence, when choosing a suitable instrument, it is important that the wavelength band over which the device measures is known and is ideal to the object of interest. The emissivity of the object values over this wavelength and the temperature range to be determined must also be known or estimated. If it is not possible to get the specific wavelength camera or IR sensor required to measure the material of interest from the supplier, one should find the best suited device that can meet the requirement.
IR Cameras and Sensors for Manifold Applications
A comprehensive range of IR cameras and temperature sensors is offered by Micro-Epsilon for almost every conceivable target material, including devices for low temperature matt surfaces and specific wavelength cameras and sensors for the temperature of hot metal surfaces or glass products (including very thin solar panel glass) and plastics.
Micro-Epsilon’s thermal imagers and IR temperature sensors are installed in a fixed position in a production process or R&D laboratory for monitoring the temperature profile of objects or materials of interest. The cameras from Micro-Epsilon are designed to deliver high speed, high accuracy measurements in quality, process control, and R&D applications. Also, the company supplies license-free, fully featured software as standard.
Micro-Epsilon’s thermal imagers and IR sensors are suitable for manifold applications
Criteria to Find the Optimal Measuring Device
The following criteria will help users to find the optimal measuring device for their specific applications:
Select a sensor’s temperature range as optimal as possible to achieve a high resolution of the object temperature. It is possible to adjust the measuring ranges to the measuring task manually or via digital interface.
A sensor’s maximum acceptable ambient temperature is highly important. Micro-Epsilon’s CT pyrometer series operates in up to 250°C with no need for cooling. Measuring devices operate in even higher ambient temperatures when water and air cooling are used. The optics can be maintained clean from additional dust in the atmosphere, using air purge systems.
The measuring object’s size needs to be equal to or larger than the viewing field of the measurement device to achieve accurate results. The spot diameter changes in accordance with the distance of the sensor. Unlike temperature sensors, thermal imagers feature a field of view (FOV), not just a point. There are different exchangeable lenses available for various measuring distances between the object and the camera. Therefore, users can select the optimal adjustment for their applications.
Response Time of Infrared Thermometers
Compared to contact thermometers, infrared sensors have a very small response time, ranging between 1-250 ms based on the detector of the device. The presence of the detector limits the response time in the lower range. The electronics help correcting or adjusting the response time in accordance with the application (e.g. averaging or maximum hold). Thermal imagers provide real-time thermography with a frame rate of up to 128 Hz.
Interfaces for the Signal Output
The analysis of the measuring results is supported by the interface, which includes:
- Bus, CAN and DP interfaces
- Analog outputs 0/4 - 20 mA and 0 - 1/10 V
- RS232, RS485, USB
This information has been sourced, reviewed and adapted from materials provided by Micro Epsilon.
For more information on this source, please visit Micro Epsilon.