Shortwave Infrared Cameras for High Temperature Manufacturing and R&D Processes

The most common infrared cameras are sensitive to energy in the long-wave portion of the electromagnetic spectrum, usually defined as wavelengths between 8 and 15 micrometers (μm). These cameras are typically lower-cost handheld devices because they can detect long-wave infrared (LWIR) using uncooled sensors. Medium wave cameras (MWIR) are sensitive to wavelengths between 3 and 8μm and require cooling of the detector. These cameras are more sensitive than longwave cameras, offer higher frame rates, and are generally more expensive.

This article will focus on a high-temperature process option that few users consider: Short Wave Infrared (SWIR). SWIR cameras are sensitive to the shorter infrared wavelengths between visible light and 3µm; they have historically been dedicated to qualitative imaging applications such as laser beam profiling, silicon wafer inspection, and imaging through paint/ink layers, not practical quantitative temperature measurement applications. Now, with the addition of temperature measurement capabilities, SWIR cameras offer a win-win opportunity for anyone concerned with thermal analysis of high-temperature materials or processes.

Some high-temperature applications, such as metalworking and welding, take place in furnaces or environmental chambers designed to maintain specific atmospheric conditions or temperatures. These chambers typically have windows made of ordinary silica-based glass, which is opaque at longer wavelengths. These chambers must be equipped with windows made of materials such as zinc selenide or calcium fluoride to be transparent to LWIR or MWIR. Not only are these materials more expensive than silicon-based glass, but they may not handle the chamber's environmental conditions - heat, humidity levels, or vacuum - as well as glass.

Another consideration is that the thermal energy dissipated by the material may vary. This variation occurs not only between different materials but also between the same material at different temperatures or at different stages of processing. These changes in emissivity can affect the accuracy of temperature measurements - and this effect is more pronounced at longer wavelengths than at shorter wavelengths.

Fortunately, a SWIR camera calibrated for temperature measurements can overcome both of these challenges. Shortwave cameras can easily see through glass and can accurately measure temperature despite changes in emissivity.

In metalworking operations, it is important to know the temperature at which the material is processed. For example, when making steel plates, manufacturers start with a block of molten metal and roll it back and forth through a rolling mill to achieve the desired thickness. Steel needs to be kept at a certain temperature throughout the process because compressing it at too low a temperature can change its material properties and cause the steel to become more brittle or brittle.

Additive manufacturing, also known as 3D printing, is another process where high-temperature consistency is critical. One form of 3D printing works by depositing metal powder, which is then rapidly heated using a laser to melt and fuse it into a solid. Similar versions rely on electron beams rather than lasers to melt metallic materials. As with traditional steel fabrication, the temperature at which a material is processed affects its final properties.

People developing recipes for additive manufacturing processes need to know if their laser or electron beam has the correct power, is focused correctly, and is moving over the material at the correct speed to impart the correct melting temperature. The powder must melt enough to mix together and form a single piece with no gaps, but not drastically change its properties. Because the process is additive -- by depositing successive layers of material to build a complete part -- developers need to know how fast the previous layer is cooling to determine whether the new layer is sufficiently fused together.

In these applications, accurate temperature measurement for each process is essential to ensure that high-quality parts are produced in a consistent manner. With a temperature-calibrated SWIR camera, users can obtain a complete thermal profile of the end-to-end process. The 2D detector array clearly shows thermal patterns and gradients as the part is formed, while also providing temperature data for each pixel in the image. This is ideal for providing quantitative data for deeper analysis. With the ability to accurately measure temperatures up to 3000℃, there is no need to worry about missing critical temperature thresholds.

A temperature-calibrated SWIR camera can also be used for other applications, such as measuring the temperature of a filament through a glass bulb or studying the plasma arc between a pair of electrodes through a glass window in a chamber. Where SWIR cameras really excel is when you need to measure a target through a glass enclosure or window.

SWIR light is usually defined as being in the wavelength range of 0.9 to 1.7μm, but it can be extended to 2.5μm. These wavelengths are only slightly longer than those of visible light (0.4 to 0.7μm), but shorter than the thermal portion of the electromagnetic spectrum (3μm), which is considered MWIR. Typically, SWIR cameras use indium gallium arsenide (InGaAs) detectors, which are sensitive in the 0.9 to 1.7μm range, but other materials such as mercury cadmium telluride (MCT) and lead selenide (PbSe) have a certain SWIR band. sensitivity.

Thermal cameras detect radiant energy from objects, which varies with wavelength and temperature. Objects that are cooler emit more energy at longer wavelengths; as the object heats up, the peak energy emission shifts to shorter wavelengths. That's why the metal burners on the stove are initially dark, but glow orange when heated - the heat energy has been converted from infrared light to visible wavelengths. For example, a blackbody radiator heated to 3,000℃ emits only 1.5 units of radiation in LWIR, but in SWIR the same 3,000℃blackbody emits 100 units of radiation. Therefore, SWIR cameras are more sensitive at high temperatures.

At room temperature, most of the energy detected by a SWIR camera comes from reflected light. That's why cameras are often used in imaging applications such as seeing through paint layers in art restoration, analyzing short-wavelength lasers, or monitoring moisture levels in the produce. But once the temperature of an object rises above 300℃, thermal energy begins to be the dominant radiation. This allows radiometric calibration of SWIR cameras to measure temperatures from 300℃ to over 3,000℃.

The difference between a SWIR camera that is used only for imaging and a SWIR camera that can be used for temperature measurement is the radiometric calibration. The process of calibrating SWIR cameras is similar to MWIR and LWIR cameras. Manufacturers use a blackbody to take multiple measurements at a known temperature, radiation level, emissivity, and distance to create a table of values based on the camera's output at each temperature.

Once the camera output values for each blackbody temperature measurement are fed into the calibration software, the data is passed through a curve fitting algorithm to generate the appropriate in-band radiance and output values based on the normalized spectral response of the camera. This produces a series of calibration curves that are stored in the camera system's memory as a series of digital curve fit tables that relate radiation values to black body temperature. When the system takes a measurement, it takes the camera's output value at a given moment, enters the appropriate calibration table, and calculates the temperature.

Of course, a black body is an ideal radiator. In theory, it has 100% emissivity, which means it doesn't absorb the energy that hits it, but instead emits it all. In practice, a good blackbody for calibration might have an emissivity of 98%. Other materials have lower emissivity values. So another part of the process requires knowing the emissivity values of different materials at different temperatures. It can then be used to correct calibrated measurements for accurate temperature readings. Overestimating the emissivity of an object means underestimating its temperature.

Emissivity error has a greater effect at higher temperatures due to the relationship between temperature and wavelength. For a 500℃ object, an emissivity error of 1% has little effect in SWIR but can skew readings in LWIR by a few degrees. But at 3000℃, the same 1% emissivity error results in a temperature error of about 10 degrees in SWIR, while the temperature error in LWIR is much larger, between 70 and 80 degrees.

During high-temperature processing, emissivity values may change. For example, running a steel sheet back and forth through a rolling mill changes its surface slightly each time, and thus its emissivity. At short wavelengths, these small changes only introduce a small error in the temperature reading. At long wavelengths, the same error can drop the temperature by tens of degrees. Rather than trying to adjust for changes in emissivity on the fly, it is simpler to use the part of the spectrum that produces less error.

In additive manufacturing using metallic materials, the emissivity changes as the material is heated and rapidly cooled under different chamber and surface conditions, making it difficult for LWIR or MWIR cameras to accurately measure the temperature of the melt pool. In these types of pyrometry applications, even small changes in emissivity can lead to larger errors. A calibrated SWIR camera reduces measurement errors and provides more useful temperature feedback when trying to optimize the system.

With its wavelength and measurement advantages, a calibrated SWIR camera can be the right choice for a specific high-temperature application. SWIR cameras can see through the ordinary glass into the chamber where the high-temperature process takes place, which makes it more affordable than installing special materials for LWIR or MWIR cameras to see through. Physics also favors SWIR, because, at higher temperatures, the SWIR band emits more thermal energy. The effects of emissivity errors are also greater at longer wavelengths—another advantage of SWIR. Because this effect increases with temperature, LWIR measurements can deviate by tens of degrees as heat increases. SWIR cameras provide more accurate thermal readings when measuring hot objects, even with errors in emissivity estimates.

The above introduces the SWIR camera used in high-temperature manufacturing and the R&D process in detail. If you want to purchase a SWIR camera, please contact us.

JAVOL is a professional custom infrared imaging equipment manufacturer. Relying on multi-spectral high-sensitivity photoelectric sensor chips of advanced compound semiconductor materials, with deep learning AI algorithm as the engine, it integrates low-light night vision technology, infrared thermal imaging technology, short-wave infrared technology, and more. Spectral technology in one fusion technology, our company designs, develops and manufactures advanced imaging products and system solutions, which are widely used in machine vision, automatic driving, drone payload, high-end manufacturing, medical diagnosis, and other fields.