What is radiation thermometer? A clear explanation of the principle and mechanism of non-contact temperature measurement.
Fundamentals of Temperature Measurement
radiation thermometer measures temperature without touching the object by detecting "thermal radiation," such as infrared radiation, emitted from the object. It is used to measure the temperature of objects that are difficult to measure with contact-type temperature sensors, such as high-temperature objects, moving objects, and objects whose temperature would be affected by touch.
However, just because it's non-contact doesn't mean that the temperature of any object can be easily and accurately measured. The measurement results can be affected by the object's material, surface condition, emissivity, measurement distance, measurement range, and surrounding environment.
Here, we will explain the basics of what radiation thermometer is, why it can measure temperature without touching it, and how it differs from a contact thermometer.
What is infrared radiation thermometer?
A thermometer that measures surface temperature without touching the object.
radiation thermometer is a non-contact thermometer that detects thermal radiation emitted from an object and determines its temperature from that energy.
Although invisible to our eyes, objects with temperature emit electromagnetic waves. Conventional radiation thermometer primarily use thermal radiation in the infrared region to measure temperature.
A major advantage is that it allows temperature measurement from a distance, as there is no need to directly attach a temperature sensor to the object. On the other hand, a typical radiation thermometer detects thermal radiation from the surface of the object, and does not directly measure the temperature inside the object.
What is the difference between radiation thermometer and an infrared thermometer?
Besides "radiation thermometer," you may also see names such as "infrared thermometer," "IR thermometer," and "pyrometer."
In English, multiple terms are used depending on the application and context, such as Radiation Thermometer, Infrared Thermometer, Infrared Radiation Thermometer, and Pyrometer. Since general non-contact temperature measurement uses the infrared region, the term "infrared thermometer" is also widely used.
Although there may be differences in terminology, in this article, we will refer to thermometers that detect thermal radiation from an object to determine its temperature non-contact as "radiation thermometer."
How can radiation thermometer measure temperature without touching it?
Objects that have a temperature emit thermal radiation
Objects with a temperature emit electromagnetic waves corresponding to that temperature. This phenomenon is called "thermal radiation."
A familiar example is the phenomenon where a highly heated metal appears red. At extremely high temperatures, some of the radiated energy becomes visible as visible light. thermal radiation exists even in temperature ranges where it is invisible to the naked eye, and radiation thermometer use this thermal radiation to measure temperature.
The energy radiated changes with temperature.
The energy radiated from an object is related to its temperature. As the temperature of an object increases, thermal radiation energy becomes stronger, and its wavelength distribution shifts towards shorter wavelengths.
The fundamental law that describes the relationship between temperature and the wavelength distribution of thermal radiation is "Planck's radiation law." radiation thermometer utilize this physical relationship between temperature and thermal radiation to determine the temperature of an object without touching it.
Actual radiation thermometer detect thermal radiation in a specific wavelength range, depending on the temperature range being measured and the properties of the object being measured.
Detect thermal radiation and convert it to temperature.
radiation thermometer is broadly configuration of three parts: a part that collects light, a part that converts light into an electrical signal, and a part that converts the electrical signal into temperature.
Object → thermal radiation → optics → detecting element → Signal processing → Temperature display /output
A common misconception here is that "radiation thermometer shines infrared light onto the object and measures the reflection."
Conventional radiation thermometer do not emit infrared radiation onto an object to measure its temperature. Instead, they determine the temperature by receiving thermal radiation emitted from the object itself.
Some models are equipped with laser markers, but the laser is primarily used to confirm the measurement location, and the laser itself does not measure the temperature.
What temperature does radiation thermometer measure?
Basically, what is being measured is the surface temperature of the object.
radiation thermometer utilize thermal radiation obtained from the surface of an object. Therefore, they are generally used as instruments to measure the surface temperature of an object.
For example, it allows for measurements of the surface of materials inside a furnace, products being transported, and heated metal surfaces without direct contact.
On the other hand, it's not possible to directly measure the internal temperature, which is not visible from the surface. When it's necessary to know the internal temperature, contact-type temperature sensors such as thermocouples or resistance thermometer may be more suitable.
It is capturing thermal radiation that is within the measurement field of view.
radiation thermometer don't necessarily measure the temperature at a single point where the laser beam hits. In reality, they detect thermal radiation coming from a certain range visible to radiation thermometer.
Generally, the measurement range changes as the distance to the object changes, so it is necessary to check the relationship between the size of the object to be measured and the measurement distance and diameter of radiation thermometer.
If the object being measured is too small relative to the measurement field of view, it may be affected by thermal radiation from the background, preventing the measurement of the intended temperature.
The measurement results will vary depending on emissivity
Even objects at the same temperature will emit thermal radiation differently depending on their material and surface condition. An important indicator that represents this difference is "emissivity."
emissivity is a value that represents the amount of thermal radiation emitted from an object, with blackbody (an ideal perfect radiator) set to 1. For most objects, it takes a value between 0 and 1, and it varies depending on factors such as surface roughness and oxidation state, even for the same material.
Metals, especially those with a glossy finish, have low emissivity and are susceptible to the effects of reflected thermal radiation from their surroundings, so caution is necessary.
Differences between radiation thermometer and contact thermometers
Thermocouples and resistance thermometer measure temperature by contacting the object being measured.
Temperature sensors such as thermocouples and resistance thermometer measure temperature by either making contact with the object or inserting it inside it.
This is suitable for situations where you want to measure the temperature inside an object, or when you want to permanently install a sensor in equipment to continuously measure the temperature.
radiation thermometer measures thermal radiation from an object.
radiation thermometer do not require the sensor to be in contact with the object being measured. Because they optically detect thermal radiation from the object, they can measure high temperatures and even moving objects from a distance.
Furthermore, unlike contact-type temperature sensors, it does not need to wait for the sensor itself to reach the same temperature as the object being measured, which means it can capture relatively rapid temperature changes.
Which one to use depends on what you are measuring.
Comparison items
Infrared Radiation Thermometers
Thermocouples, resistance thermometer, etc.
How to Measure
contactless
contact
Main measurements
surface temperature
Surface, interior, etc.
Moving object
Easy to measure
Measurement can sometimes be difficult.
High temperature target
Can be measured from a distance
The heat resistance of the sensor needs to be taken into consideration.
response
Relatively fast
It depends on the sensor structure and installation conditions.
emissivity effect
receive
I generally don't accept it.
Rather than focusing on which method is superior, it's crucial to choose the appropriate measurement method based on "what you want to measure, where, and under what conditions."
Objects that are difficult to contact at high temperatures
It allows for measurement from a distance of high-temperature objects that are difficult to directly contact with the sensor, such as materials inside a furnace, heated metals, and glass.
Moving or rotation objects
Products moving on conveyors, rotation objects, and continuously manufactured films and sheets can be measured without stopping the movement of the object.
Objects undergoing high-speed temperature changes
Because it does not require waiting for thermal equilibrium through contact, it can be used to capture temperature changes in objects that are heated and cooled in a short time.
Objects that can be affected by touching them
Non-contact measurement is also effective when the contact of a temperature sensor itself affects the temperature of the object, such as thin films or small components.
radiation thermometer are used in a variety of manufacturing process, including those for steel and metals, glass, resins and films, food products, semiconductors, and electronic components.
Some radiation thermometer
Objects with low emissivity, such as glossy metals.
Metals with a mirror-like surface have low emissivity and are therefore susceptible not only to thermal radiation from the object itself but also to the effects of reflected thermal radiation from the surroundings.
Objects smaller than the measurement field of view
If the object to be measured is too small relative to the measurement field of view, thermal radiation from the background will also be detected. When measuring small parts or thin wires, it is important to check the relationship between the measurement distance and the measurement diameter.
Measurements taken through glass or windows
Even materials that appear transparent to the human eye do not necessarily transmit infrared radiation in the same way. Because different materials transmit and absorption different wavelengths of infrared radiation, when taking measurements through a window, it is necessary to check the measurement wavelength of radiation thermometer and characteristic of the window material.
Environments containing smoke, steam, dust, etc.
If there is smoke, steam, dust, or other particles between radiation thermometer and the object being measured, thermal radiation from the object may be absorption or scattered along the way, or sight path may be blocked, which can affect the measurement results.
When using radiation thermometer, it is important to consider not only the object being measured, but also the light path to the object and the surrounding heat sources as part of the measurement conditions.
Things to check when choosing radiation thermometer
Temperature range to measure
First, determine the approximate temperature range of the object you want to measure. Each radiation thermometer has its own measurement temperature range, and different methods and models are suitable for low-temperature and high-temperature measurements.
Material and surface condition of the object
The radiation characteristic vary depending on the material, such as metal, resin, or glass. Furthermore, even with the same material, emissivity changes depending on the surface condition, such as whether it is a glossy surface, an oxidized surface, or a rough surface.
Measurement distance and measurement diameter
When using radiation thermometer, it is necessary to check the distance from the installation location to the object being measured and the size of the measurement area. It is important to use the thermometer under conditions where the object being measured fully fills the measurement field of view.
Response speed and usage environment
When detecting high-speed moving objects or rapid temperature changes, the response speed should also be checked. Furthermore, the environment in which radiation thermometer will be installed should be considered, such as around high-temperature furnaces, in dusty areas, or in places where it may be exposed to water.
For those who want to know how to choose specifically
The actual selection criteria, including emissivity, measurement distance, and D:S ratio, are explained in detail in the radiation thermometer guide.
Frequently Asked Questions about radiation thermometer
Are radiation thermometer and infrared thermometers the same thing?
A typical infrared thermometer is treated as radiation thermometer that determines temperature by detecting infrared radiation emitted from an object. Multiple names are used in products and documentation, such as "radiation thermometer," "infrared thermometer," and "IR thermometer."
Are you measuring the temperature with a laser?
Conventional radiation thermometer do not measure temperature using lasers. In models equipped with laser markers, the laser is primarily used to confirm the measurement location. The temperature measurement itself is performed by detecting thermal radiation emitted from the object being measured.
Can the internal temperature be measured with radiation thermometer?
Conventional radiation thermometer basically measure temperature based on thermal radiation obtained from the surface of an object. Therefore, they are not suitable for directly measuring the temperature inside an object.
Can you measure the temperature from a distance?
While measurements can be taken from a distance, distance alone is not sufficient. The measurement range changes depending on the distance, so ensure that the area to be measured is fully within the measurement field of view of radiation thermometer.
What is the difference between radiation thermometer and a thermal imager?
Both methods measure temperature non-contact using thermal radiation such as infrared radiation. While radiation thermometer primarily aim to measure the temperature within a set measurement field of view, thermography is characterized by its ability to capture a two-dimensional temperature distribution as an image.
To understand and use radiation thermometer correctly
radiation thermometer is a type of thermometer that can measure the temperature of an object without touching it, by utilizing thermal radiation emitted from the object.
It can be used in situations where contact-type temperature sensors are difficult to use, such as with high-temperature objects, moving objects, and objects with rapidly changing temperatures.
On the other hand, the measurement results are affected by factors such as emissivity of the object, its surface condition, the measurement distance, the measurement diameter, sight path, and the surrounding environment.
Instead of thinking, "Since it's non-contact, I can measure the temperature just by pointing it at the object," understanding how radiation thermometer captures thermal radiation from the object is the first step to accurate temperature measurement.
Next article to read
You can find detailed information about selecting radiation thermometer, troubleshooting measurement problems, and infrared measurement, tailored to your specific needs.
This section introduces a system that uses infrared radiation to measure not only temperature, but also moisture content, constituent, thickness, and other properties.
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