Buyer’s Guide for Pyrometers

What is a pyrometer used for?

Pyrometers are vital instruments in various industries, offering accurate, non-contact temperature readings across a broad spectrum—from monitoring cold chains to managing high-temperature processes in glass and metal production. By providing reliable temperature data, they help identify thermal irregularities, which boosts efficiency, enhances safety, and aids in cost management.

In the realm of industrial process monitoring, pyrometers track the temperatures of metal, glass, and plastic products during their manufacturing. In metallurgy, short-wavelength pyrometers play a crucial role in processes that involve molten metals, forging, hardening, annealing, and rolling mills. The data they collect feeds directly into closed-loop control systems, optimizing heating and cooling cycles, refining process models, and ultimately increasing throughput, yield, and profitability.

When it comes to induction heating applications, pyrometers allow for continuous monitoring without any physical contact, which means they won’t interfere with strong electromagnetic fields. They can also set off alarms if the measured temperatures stray from predetermined limits, ensuring consistent process control.

In the semiconductor sector, precise and stable temperature monitoring is essential at nearly every stage of production—from chemical vapor deposition and atomic layer deposition to polysilicon and crystal growth, and finally, wafer polishing.

Additionally, pyrometers are employed to examine mechanical and electrical systems for early signs of friction, overheating, misalignment, or bearing wear. Spotting these problems early on enables proactive maintenance, which can extend the lifespan of equipment and prevent costly damage or unexpected downtime.

How does an infrared thermometer work?

Unlike your typical thermometers that need to touch the object to get a reading, pyrometers can measure temperature from afar. Essentially, a pyrometer’s main job is to gauge the temperature of an object without any physical contact, using the infrared radiation that the object gives off. This infrared radiation is produced by anything with a temperature above absolute zero (-273.15 °C) and exists beyond the visible light spectrum. As this radiation moves through the atmosphere, its journey is significantly affected by its wavelength. The atmosphere has areas where the radiation is heavily absorbed, mixed with spots that allow it to pass through easily, which we call atmospheric windows. For non-contact temperature measurements, infrared radiation usually ranges from 0.5 to 14 μm in wavelength, covering a variety of electromagnetic waves categorized into long, mid, and short infrared wavelengths.

Ideal blackbody radiators are the ultimate emitters of infrared energy, radiating at all wavelengths without any reflection or transmission. In reality, though, very few objects can live up to this perfect standard, often giving off less radiation even at the same temperature. Infrared sensors are designed to pick up the radiation emitted from an object’s surface, along with any reflected radiation from the environment and possibly even transmitted infrared radiation.

The emissivity of a material is all about how it radiates heat compared to a perfect blackbody, and it can range anywhere from zero to one. This property is influenced by a bunch of factors, including the type of material, its surface texture, temperature, wavelength, and the angle at which you’re measuring. Emissivity is super important for getting accurate temperature readings, so it’s essential to adjust it based on what you’re doing and to think carefully about which measurement device to use.

When it comes to infrared radiation, the Stefan–Boltzmann law comes into play, linking the intensity of radiation emitted by an object to its temperature and emissivity. Pyrometers, which are used for these measurements, come in various spectral ranges and can be classified as either single-color or two-color ratio pyrometers.

All pyrometers are made up of three main parts: an optical system, a detector, and the electronics. When you want to measure the surface temperature of an object, the optical system captures the energy that the object emits and focuses it onto the detector, which is very sensitive to infrared light. Depending on the specific infrared wavelengths, the sensor detectors can use various technologies, like thermopiles or infrared photodetectors. The electrical signal generated by the detector is influenced by the temperature of the object being measured, its surrounding temperature, and how sensitive the detector is. This weak electrical signal is then amplified and converted into a digital format by the pyrometer’s electronics. The digitized data is continuously adjusted using calibration data. After that, you get the temperature readings, and depending on the interfaces available, there may be additional signal processing for either analog or digital outputs.

Infrared pyrometers come with a variety of spectral ranges, measurement ranges, speeds, sensitivities, and resolutions, catering to different budget levels. There are also various performance class options to help you choose the right model for your specific needs. The following discussion will help you identify and select the pyrometer that’s best suited for your application.

Step 1: Choose if a single and two-color measurement principle is preferred.

In general, there are two main types of pyrometers: single-color and two-color pyrometers. You might also hear them called ratio pyrometers or dual-wavelength pyrometers.

According to Wien’s displacement law, the peak wavelength of radiation emitted by a black body is inversely related to its temperature. As the temperature of an object rises, the radiation it emits increases across the entire wavelength spectrum, and the peak of that emitted radiation shifts to shorter wavelengths.

Single-color pyrometers, which are intensity-based, measure the power of the infrared radiation emitted within a specific bandwidth. On the other hand, ratio-based pyrometers look at the infrared radiation at two closely spaced wavelengths and assess the relationship between the two intensities. The bandwidths of these two colors can either overlap partially or be completely separate.

One of the great things about ratio pyrometers is that they can provide reliable measurements even when the emissivity is unknown or fluctuating with temperature. This is true as long as both wavelength signals are affected in the same way by changes in emissivity or the process itself. This ability to deliver consistent and accurate readings, even in tough conditions, is a standout feature of two-color pyrometers.

These ratio pyrometers come in handy when the measuring spot doesn’t fully cover the object or when the object is smaller than the pyrometer’s spot size. They’re also useful in situations where the optical path of the pyrometer is affected by changing transmissions, like dust, steam, dirt, or windows. You’ll often find these devices in challenging industrial settings, such as metal processing, where the emissivity is unknown and varies but does so equally at both wavelengths.

However, it’s worth noting that these devices typically have a temperature measurement range that skews toward the higher end, tend to be bulkier, and are generally more expensive than traditional single-color pyrometers.

Step 2: Select the optimal spectral range of the pyrometer.

Different detector technologies pick up infrared energy at specific wavelengths or wavebands, which can really affect the measurement results. Materials that have non-shiny surfaces usually show high and fairly consistent emissivity, particularly in the long-wave spectral ranges. On the other hand, metallic materials tend to have lower emissivity that changes with the surface texture and drops off at higher wavelengths. This variability can make it tricky to get consistent and reliable measurement results.

Choosing the right temperature measurement device means taking into account the wavelength of infrared radiation and the emissivity of the material within a certain temperature range. To achieve the best accuracy, the spectral range of an infrared pyrometer should match the material’s peak emissivity, with a focus on short-wave measurements where radiation intensity tends to be at its highest. Companies like Optris design the wavelength ranges of their infrared thermometers to meet specific measurement needs, organizing materials and pyrometer options to ensure they can adapt as required.

Step 3: Select the correct measurement range, resolution, and speed of the pyrometer

The temperature range of an object is influenced by its ability to reach both extremely cold and hot conditions. This range is defined by the coldest and hottest temperatures that are encountered in a specific measurement or application. For instance, short-wave sensors can be calibrated to measure temperatures as high as 3000 °C. However, the limited dynamic range of electronic components and the non-linear characteristics of Planck’s radiation can make it challenging to measure very low and very high temperatures. As a result, different types of sensors can have varying temperature ranges. Additionally, the material composition of the object often plays a significant role in determining the spectral range, which in turn affects the measurement range.

Another important factor to consider with pyrometers is temperature resolution, which is referred to as NETD (Noise Equivalent Temperature Difference). This value indicates the smallest temperature differences that the infrared sensor can detect. It largely depends on the sensor’s inherent noise level; a sensor with lower noise will provide better thermal resolution. It’s crucial to distinguish between temperature resolution or sensitivity and temperature accuracy. While temperature resolution tells us the smallest detectable temperature difference in a scene or over time, temperature accuracy measures how precisely the sensor can determine an object’s actual temperature. So, even if a pyrometer has high temperature resolution and can pick up on subtle heat changes, the difference between the measured and actual temperatures can often exceed the thermal resolution. Nevertheless, many pyrometers come with a high-sensitivity option that sacrifices measurement range for improved temperature resolution.

Due to the design of the detector and the electronics, pyrometers can have different response times. If you need to track rapid events, short to mid-wavelength infrared detector materials in pyrometers allow for shorter exposure times, while long-wavelength detectors are generally slower. Fast pyrometers can respond to temperature changes in as little as 0.3 ms, making them ideal for high-speed measurements.

Step 4: Choose the infrared sensor’s optic.

When it comes to measuring setups, one important factor to consider is the geometry and the ratio of the sensor’s distance to the spot size. To get accurate measurements, especially with single-color pyrometers, the spot size needs to be smaller than the target.

The distance-to-spot size ratio, often referred to as the D:S ratio, is an essential performance metric for infrared pyrometers. In the realm of long wavelengths, today’s high-performance pyrometers usually max out at a ratio of 75:1, mainly due to the limitations of detector technology. This means that the diameter of the spot being measured is just 1/75th of the distance from the pyrometer to the object. For example, if a pyrometer is set up 75 meters away from an object, it will measure the temperature of a spot that has a diameter of 1 meter. Essentially, the D:S ratio tells us how far away the pyrometer can be from a target of a certain size while still delivering an accurate temperature reading, as long as the spot size is smaller than the target itself.

To further tailor the sensor to specific application needs, there are various optic options available. Two popular choices are standard-focus and close-focus lenses. The key difference between them lies in the shape of the beam path and their focal distances. Standard focus lenses, which have a concave beam path, are optimized for longer measurement distances, while close-focus lenses, with a convex beam path, are designed for shorter distances. These close-focus lenses can either be built into the pyrometer or purchased as accessories. Additionally, some high-performance pyrometers come equipped with manual or motorized focus adjustments, allowing for easy adaptation of the focal length.

Step 5: Ensure the infrared pyrometer works in the industrial environment with the right accessories.

Pyrometer accessories are essential for ensuring that infrared thermometers perform at their best and stay protected in tough industrial settings where they face heat, dust, vibrations, shocks, smoke, and moisture. Even though these accessories are vital for success, they often get overlooked when it comes to budgeting. A wide variety of accessories, tailored for each infrared sensor, guarantees optimal performance and protection across different industrial applications.

While small sensing heads can handle ambient temperatures up to 250°C without needing cooling, cooling jackets are available for those hotter situations, reaching up to 315°C. In high ambient temperature scenarios, special connection and signal cables become necessary. Optris pyrometers are well-known for their impressive resistance to thermal shock, thanks to their innovative design and manufacturing techniques. However, adding a more robust housing can improve repeatability in environments with significant thermal fluctuations or temperature gradients around the sensor.

In industrial settings, air purge collars are vital for keeping the optics clean by stopping dirt and dust from settling. Laminar airflow helps maintain a dust-free area around the lens, ensuring a clear view.

Adjustable mounting brackets, standard pipe adapters, sighting tubes, flanges, and windows all help with seamless mechanical integration into industrial applications. Together, these accessories enhance the reliability and lifespan of infrared thermometers in demanding industrial environments.

Step 6: Consider how to integrate the infrared pyrometer into the control loop of the industrial process.

There are plenty of ways to incorporate infrared sensors into machines or industrial processes. In today’s industrial environments, pyrometers fit right into control loops, providing infrared temperature readings without the need for extra computer software. These sensors usually come with several analog outputs that can be scaled individually, giving you a straightforward live data feed for various systems. Plus, Optris pyrometers are compatible with a wide range of industrial fieldbus systems and communication protocols, such as EthernetIP, Ethernet, USB, ModbusTCP, CANBus, RS485, RS232, EtherCAT, IO-Link, ProfiBus, and Profinet, making them perfect for industrial manufacturers for straightforward integration into machinery. However, to set everything up, configure, and record data, you often need firmware updates for computer-based software, which helps ensure that every infrared sensor is user-friendly and functions at its best. Depending on the bandwidth, cost, integration, and expertise required for each application, the communication protocols should be carefully considered by the process engineer in charge ahead of time.

Step 7: Choose a vendor or manufacturer with experienced application support and service.

Choosing a partner for the best services and support is just as important as selecting the right infrared sensor—especially with complex technologies like infrared sensing. Reliable manufacturers focus on delivering long-term performance and dependable operation, but strong application support is equally critical. Skilled support engineers can advise on measurement methods, conduct tests, and optimize temperature monitoring strategies. When repairs, troubleshooting, or training are needed, responsive aftersales service and readily available technical expertise become essential. These factors should weigh heavily in the decision when selecting an infrared pyrometer.