- Radiation Pyrometer Definition: A radiation pyrometer is defined as a non-contact temperature sensor that measures temperature by detecting the thermal radiation emitted from an object.
- Measurement Capability: Radiation pyrometers are particularly useful for measuring high temperatures that are inaccessible or hazardous for traditional contact sensors.
- Types of Radiation Pyrometers: The main types include fixed focus, which does not require distance adjustments, and variable focus, which can be adjusted to focus on different distances and sizes of the target area.
- Advantages: These devices offer non-contact temperature measurement, rapid response times, and minimal interference from environmental factors.
- Applications: Radiation pyrometers are essential in industrial settings for monitoring temperatures in processes involving furnaces, molten materials, and other high-temperature environments.
A radiation pyrometer infers surface temperature from thermal radiation received at a distance. Unlike thermocouples and resistance temperature detectors RTDs, it does not touch the target. Its usable temperature range depends on the detector, spectral band, optics and target conditions, so there is no universal 750°C lower limit.
What is a Radiation Pyrometer?
A radiation pyrometer is a non-contact radiation thermometer. Its optics collect radiation from a defined target area, and its detector produces an electrical signal related to radiance in a chosen wavelength band. The instrument converts that calibrated signal to temperature using the target’s emissivity relative to a black body.
Where,
- Q is the thermal radiation in W/m$^2$
- ϵ is the emissivity of the body (0 < ϵ < 1)
- σ is the Stefan-Boltzmann constant in W/m$2$K$4$
- T is the absolute temperature in Kelvin
This Stefan-Boltzmann form describes total radiation from an idealised grey surface. A practical pyrometer senses only its specified wavelength band. Accurate conversion also depends on surface emissivity, radiation reflected by the surroundings and transmission through the atmosphere or a viewing window.
- A lens or mirror collects radiation from the specified target spot and directs it onto the receiving element.
- A receiving element converts the radiation into an electrical signal. Depending on the instrument, this detector may be a thermopile, thermocouple or semiconductor photodetector.
- Signal-processing and display electronics apply calibration data, emissivity settings and compensation before reporting temperature.
Types of Radiation Pyrometers
The diagrams below show fixed-focus and variable-focus optical arrangements. This is not a complete classification. Modern instruments are also grouped by spectral method, including single-colour, broadband and two-colour ratio pyrometers.
Fixed Focus Type Radiation Pyrometer
A fixed-focus radiation pyrometer has optics set for a specified measurement geometry. The illustrated historical design uses a long tube, a narrow front aperture and a concave mirror.
A thermocouple detector sits near the mirror’s focal plane. Radiation from the target is reflected onto its hot junction, and the resulting emf is measured by an indicator calibrated in temperature. The fixed optics do not require manual refocusing within the stated operating geometry. Distance still changes the measured spot size, however, so the target must fill the instrument’s specified field of view. Dust on the optics, an obstructed sight path or a target outside the rated geometry can bias the reading.
Variable Focus Type Radiation Pyrometer
A variable-focus radiation pyrometer allows its optical focus to be adjusted for the target distance. The illustrated design uses an adjustable polished concave mirror.
The mirror directs target radiation onto a blackened thermojunction formed around a small copper or silver disc. The operator views the target image through the eyepiece and central opening, then moves the mirror until the image coincides with the disc. Heating at the junction produces an emf for the calibrated indicator. Adjustable focus can support different target distances, but it does not remove the need for correct spot size, alignment, emissivity and a clear optical path.
Advantages and Disadvantages of Radiation Pyrometers
Radiation pyrometers are useful when contact measurement would disturb the target or expose a probe to unsuitable conditions. Their accuracy still depends on the complete measurement setup.
Some advantages are:
- Suitable models can measure temperatures beyond the safe range of many contact probes. The rated range varies by detector and wavelength.
- They can measure moving, inaccessible, electrically live or contamination-sensitive targets without physical contact.
- Many models respond quickly because the sensor does not need to reach the target’s temperature. Actual response time is model-specific.
- The sensing electronics can be kept away from a hostile target, with suitable optics or fibre used for the application.
Some disadvantages are:
- Unknown emissivity, reflected surroundings, smoke, vapour, windows, dirty optics and an undersized target can all cause error.
- Reliable readings require suitable calibration, a correct emissivity setting, clean optics and a target that fills the specified spot.
- Industrial optics, purge systems and wavelength-specific instruments can cost more and need more setup than a simple contact sensor.
Applications of Radiation Pyrometers
Radiation pyrometers are used where the target is hot, moving, remote or unsuitable for contact. Instrument wavelength and optical geometry must match the material and process.
Some examples are:
- Monitoring furnace walls, kiln loads, boilers and heat-treatment processes through a suitable sight path.
- Measuring molten metal, glass or ceramic surfaces with a spectral band suited to the material and any viewing window.
- Monitoring flames or plasma when the selected wavelength rejects unwanted gas-band or background radiation.
- Tracking moving rollers, strip, wire and conveyor products without attaching a probe.
- Measuring a defined spot on a large surface such as a pipe or wall. A spot pyrometer does not report the whole-surface average unless its field of view covers that area.
Conclusion
A radiation pyrometer measures radiation from a defined target area and converts the signal to an indicated surface temperature. Unlike contact devices such as thermocouples and resistance temperature detectors (RTDs), it does not need thermal contact. The detector and spectral band set the usable range, so selection should follow the target rather than a universal temperature threshold.
Fixed-focus and variable-focus designs describe how the optics align the target with the detector. Single-colour instruments use one spectral band. Two-colour ratio instruments compare two bands.
Non-contact operation suits hot, moving or inaccessible targets and can provide fast response. It does not eliminate measurement error. Emissivity, reflected radiation, atmospheric absorption, viewing windows, target size, optical contamination and calibration all affect the result.
A ratio pyrometer can tolerate partial signal loss when both wavelength channels are reduced equally. It can still be wrong when emissivity or transmission differs between the two bands. The target, sight path and instrument specifications must therefore be checked together.
For reliable use, choose the wavelength and temperature range for the material, confirm the distance-to-spot requirement, set or determine emissivity, keep the optics clean and verify calibration at the required uncertainty.
