Conventional pyrometers reach their physical limits when measuring reflective materials such as metals, glass, or coated surfaces. This is due to the low emissivity in the LWIR and MWIR ranges, which causes the measurement to be heavily influenced by ambient reflections. This results in unreliable temperature readings. One solution is imaging in the SWIR (0.9–2.5 μm) range, which enables significantly more robust measurements, as Vision & Control describes in the following article.
Every body above absolute zero emits thermal radiation, the intensity and spectrum of which depend on the temperature and the emissivity ε. Planck’s law of radiation describes the emitted spectrum as a function of temperature and wavelength:
M(λ,T) = (2πhc² / λ⁵) * 1 / (exp(hc / (λkT)) – 1)
Wien’s displacement law describes the wavelength at which the radiation intensity is at its maximum:
λ_max * T = 2.898 * 10⁻³ m*K
However, many surfaces are not ideal blackbodies. The emissivity ε describes the ratio of the radiation emitted by a real surface to that of an ideal blackbody at the same temperature. A low emissivity implies that the object reflects a large portion of the ambient radiation rather than emitting its own radiation.
SWIR cameras for measurements at temperatures of 300°C and above
Figure 1 | LWIR image (left) and visual image (right) of a metallic test specimen
While thermal imaging cameras with microbolometers typically operate in the LWIR (8–14 μm) or MWIR (5–8 μm) ranges, this range often does not provide reliable results for bare, reflective surfaces. SWIR cameras (SWIR: 0.9–2.5 μm) are significantly more advantageous in this regard, as emissivity is higher in this spectral range. The material’s intrinsic radiation outweighs ambient reflection. SWIR cameras thus enable measurements starting at approximately 300°C. SWIR cameras typically use InGaAs detectors. These sensors offer a high dynamic range and low dark current, allowing even weak radiation to be detected with precision. For the user, this means stable and reproducible measurements even on challenging surfaces. The short wavelength enables smaller pixel sizes and thus higher resolutions, which is important when analyzing temperature distributions in finely structured objects. The selection of suitable optics is crucial in this context. Telecentric vicotar objectives ensure uniform imaging across the entire field of view and minimize measurement errors at varying working distances. Additional IR long-pass or bandpass filters suppress interfering extraneous radiation and increase measurement reliability. The lens’s high transmittance and numerical aperture determine how much radiation reaches the sensor—a decisive factor for the signal-to-noise ratio and measurement speed.
Temperature Measurement in Soldering Processes
A practical example is temperature measurement on soldering materials with a process temperature of approximately 230–330°C. For sensitive electrical components, a defined temperature range must not be exceeded. Using the vicosys image processing system and a SWIR camera (e.g., the TRT033S from Lucid
Vision), this process can be monitored with precision. In combination with suitable vicotar optics and vicolux lighting, accuracies in the range of ±1.5 K have been achieved. For the user, this means lower scrap rates, more stable processes, and reliable quality assurance.
Calibration and Accuracy
However, a key challenge is absolute calibration. In practice, this is achieved through reference measurements at points with known temperatures. Alternatively, color-coded calibration rods with defined melting points or special paints with high emissivity can be used. The choice of calibration method depends on the material, geometry, and temperature range. Software corrections can compensate for emissivity deviations and automatically linearize measured values. The modular vicosys image processing system from Vision & Control enables the direct integration of SWIR cameras into industrial automation and quality assurance processes. Standardized interfaces such as EtherCAT, Profinet, or Ethernet/IP enable seamless communication with PLCs and control systems. This allows temperature profiles to be monitored in real time, automatically documented, and, if necessary, process parameters to be adjusted immediately.
Conclusion
Temperature measurement using SWIR cameras offers a precise alternative to traditional pyrometers and thermal imaging cameras—especially for reflective surfaces. High levels of accuracy can be achieved through appropriate calibration strategies, optical adjustments, and integration into automation systems. For users, this means reliable temperature monitoring even in challenging environments, lower failure rates, less rework, and improved overall equipment effectiveness.
Figure 2 | Planck's law of radiation describes the relationship between temperature and wavelength
Published in inVISION 6/2025