Hangzhou City, Zhejiang Province, China-October 15, 2026
A photovoltaic plant maintenance engineer once told us something: they were using a thermal imager to inspect modules and found several panels with abnormal temperatures. They opened them up—nothing wrong. They swapped in a known-good panel—it still showed hot on the thermal image.
After two days of troubleshooting, they finally traced it to a calibration issue with the thermal imager, not a fault in the modules.
The ending of this story is harmless enough. But if it were a chemical pipeline, a high-voltage switchgear cabinet, or a lithium battery production line, the cost of the same misjudgment would be very different.
Infrared thermal imagers are increasingly common tools in industrial inspection. But between “there’s a hot spot on the thermal image” and “there’s actually a problem here” lies a gap that many people don’t realize exists. That gap is largely determined by the thermal imager’s front-end optical system—and within that optical system, the most easily overlooked component is the filter.
Why Thermal Imagers Need Filters
Industrial-grade thermal imagers typically operate in the 8–14μm long-wave infrared band. The reason for choosing this band is straightforward: the thermal radiation peak of room-temperature objects (0–100°C) falls right here, atmospheric absorption in this band is relatively low, detection range is longer, and signals are more stable.
But the problem is that 8–14μm is not an isolated window. In real industrial environments, infrared radiation from various sources exists simultaneously—heating equipment, sunlit metal surfaces, high-temperature gas radiation. The bands covered by these interference sources overlap with the target band. If the thermal imager does not undergo precise optical filtering, the detector receives a mixture of the target object’s thermal radiation and background noise.
The temperature reading shown on the thermal image is the result of the detector integrating all incoming infrared radiation. The more background noise, the more the reading deviates from the object’s true temperature. This is one of the fundamental reasons thermal imagers “get it wrong.”
A long-pass filter is standard in the optical system of industrial thermal imagers. Its role is to block short-wave interference outside the target band, allowing only long-wave infrared above 8μm to reach the detector. The precise position of the cutoff edge and the steepness of the cutoff slope directly affect short-wave interference suppression: the steeper the cutoff edge, the narrower the transition zone, and the lower the probability of stray light mixing into the target band.
In Gas Detection, Filter Requirements Are Even More Demanding
Industrial gas leak detection is an important thermal imaging application—but what’s used here is not a general-purpose thermal imager, but an infrared gas imaging system designed specifically for the target gas’s absorption peak.
The principle works like this: gas molecules have characteristic absorption in specific infrared bands. When a gas cloud passes between the light source and the detector, infrared radiation in the corresponding band is absorbed, the signal received by the detector weakens, and a “cold zone” appears in the image at that location. The leaking gas cloud is thus visualized.
Different gases have different absorption peaks: methane at 3.3μm, CO₂ at 4.26μm, hydrogen sulfide near 3.8μm. Gas imaging systems require precise narrowband filters at the corresponding absorption peaks to extract the signal from that band alone while blocking background radiation from other bands.
The difficulty here lies in two aspects. First, background thermal radiation at industrial sites is complex. If strong radiation from other bands is not sufficiently blocked, false signals will form. Second, gas absorption peaks are often very narrow. If the filter’s center wavelength deviates by just a few nanometers—precisely missing the absorption peak—the entire system’s sensitivity drops dramatically.
MULTI IR’s mid-infrared gas detection narrowband filters hold center wavelength tolerance within ±20nm and peak transmittance above 85%. They are in stock for the characteristic absorption bands of major industrial gases including CO₂, methane, and CO. These specifications have been validated across numerous gas detection applications—not just numbers on a datasheet.
An Easily Overlooked Issue: The Effect of Temperature on Filter Performance
In industrial environments, there is a problem with optical components that is often overlooked: temperature drift.
The refractive index of optical thin films shifts slightly with temperature, causing the filter’s center wavelength and cutoff edge position to drift with ambient temperature. The standard curve measured in the lab and the actual curve under high-temperature industrial conditions may differ to some degree.
For broadband applications with moderate precision requirements, this deviation is usually within acceptable limits. But for applications like gas detection that demand extremely high wavelength precision, temperature drift can cause system sensitivity to drop under extreme temperatures. This needs to be confirmed at the selection stage: whether the supplier provides measured curves at different temperatures, and whether the temperature coefficient falls within the system design tolerance.
This issue is rarely raised during procurement. It typically only gets traced back after equipment produces anomalous data under high or low temperature conditions.
There are many reasons a thermal imager can “get it wrong”—algorithm calibration, detector aging, lens contamination are all common factors. But the filtering quality of the optical system is a foundation that is fixed from the design stage. What downstream algorithmic compensation can do is limited. Whether the signal coming in from the front end is clean is the starting point of data quality.
How clearly an instrument’s eye can see depends not only on how sensitive the detector is, but also on what the filter lets in—and what it keeps out.
MULTI IR – National-level “Little Giant” Specialized and Sophisticated Enterprise. Over 10,000 types of infrared sensitive components in stock. Top 3 globally in comprehensive strength. Lead drafter of the infrared filter industry standard.
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About us
Founded in 2007, Hangzhou MULTI IR Technology Co., Ltd. is an optoelectronic technology enterprise integrating R&D, production, and sales. Its products are widely applied in aerospace, medical care, AR/VR, display imaging, photography, and other fields, steadily holding the position of the world’s largest spot supplier of optical components.
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To view the original version on ABNewswire visit: MULTI IR Explains How Infrared Filters Affect Thermal Imager Accuracy in Industrial Applications
