Radiation thermometry is a widely utilized method offering several advantages, such as noncontact measurement, rapid response, large dynamic range, and passive operation. It is extensively applied across various fields, including industry and biomedicine. To meet the high-temperature measurement requirements of sectors like glass manufacturing, metallurgy, rocket engines, and gas turbines, visible and near-infrared spectral bands are typically employed to mitigate emissivity deviations. These bands are suitable for measuring temperatures ranging from several hundred to a few thousand degrees Celsius. To accurately determine the target temperature and emissivity, a narrow spectral band is required. Building upon the gray body assumption, dual-band colorimetric thermometry reduces the influence of emissivity and environmental factors. Both single-band thermometry and colorimetric thermometry typically use narrowband imaging, and narrower bands improve thermometry precision without compromising detector sensitivity. However, limitations in infrared detector sensitivity present challenges for narrowband or multiband thermometry. Traditionally, dual-band infrared colorimetric thermometry has relied on cooled IRFPAs. Recent advancements in uncooled infrared detector materials and manufacturing processes have brought uncooled MWIR and LWIR IRFPAs to a practical level of functionality. Moreover, colorimetric thermometry based on uncooled IRFPAs shows promising prospects in terms of size, longevity, and reliability. This research investigates MWIR and LWIR colorimetric thermometry using a wideband uncooled IRFPA, focusing on the applicability of the gray body assumption and addressing detector response drift to enhance the accuracy of radiation thermometry for room-temperature targets.

This study focuses on applying wideband MWIR and LWIR colorimetric imaging thermometry and introduces improvements to existing methods. First, through theoretical derivation and applying the gray body assumption to segmented MWIR and LWIR bands, it is demonstrated that the ratio of the radiative emissivities between the two bands is the key determinant of thermometry accuracy. Other influencing factors can be eliminated through design or calibration. The spectral radiative emissivities of common materials are analyzed to illustrate the applicability of the gray-body assumption in MWIR and LWIR bands (Fig. 1). Spectral emissivity data for natural and man-made materials were sourced from the MODIS UCSB emissivity library, established by the University of California, Santa Barbara, covering a range of materials including water, soil, vegetation, and man-made materials. These materials cover the spectral emissivity ranges for both MWIR and LWIR (3?15 μm). In addition, we conducted our measurements on 11 materials (Figs. 2 and 3). The spectral emissivities of all these materials were analyzed to determine the ratio of LWIR to MWIR emissivities. Due to response drift in uncooled infrared detectors—caused by random core temperature fluctuations related to operational duration—which can result in discrepancies between the detector’s operational state and its calibration, a correction for this drift in uncooled IRFPA thermometry was implemented. A revised model for MWIR and LWIR infrared colorimetric imaging thermometry was developed (Fig. 5). The accuracy of this colorimetric imaging thermometry was experimentally compared with that of single-band radiometric thermometry. Bias drift was mitigated using electrical tape and water. The measured samples included white ceramic tiles, cement, glass, and ginkgo leaves (Fig. 9).

The colorimetric thermometry method, based on the gray body assumption, leads to a colorimetric imaging thermometry process (Equation 4). This process involves calibrating the dual-band output signal ratio relative to temperature using a temperature-adjustable blackbody, establishing the functional relationship Q₁(T). By combining the emissivity ratio k_ε and the detector’s response ratio Q(T) to the scene, the scene temperature T can be determined. The potential for further simplification of colorimetric thermometry based on the gray body assumption depends on the emissivity ratio k_ε between LWIR and MWIR for different materials. Analysis reveals that for water and vegetation, k_ε is relatively stable around 1, while for other materials, k_ε shows significant fluctuations. Therefore, for water and vegetation, the thermometry model can be simplified by assuming k_ε=1. For materials like soil and artificial materials, the specific emissivity ratio k_ε must be considered to achieve higher thermometry precision (Fig. 4). Calibration using an approximate gray body with a known temperature T and emissivity ratio k_ε helps calibrate the temperature and effectively eliminate additional bias drift caused by the detector’s baffle correction and temperature drift, thereby enhancing thermometry accuracy. Comparative experimental results with single-band radiometric thermometry indicate that the relative error in dual-band colorimetric thermometry remains stable within 5%, with an average reduction of 3?4 percentage points compared to single-band thermometry (Fig. 10 and Table 1). This demonstrates the higher precision of the dual-band colorimetric method.

This study investigates the MWIR and LWIR colorimetric imaging thermometry method using a wideband uncooled IRFPA. After analyzing the spectral emissivities of common materials, we confirm the applicability of the gray body assumption for MWIR and LWIR. Subsequently, the response drift of the uncooled IRFPA used in thermometry is corrected, and a revised model for MWIR and LWIR colorimetric imaging thermometry is introduced. Finally, real-world thermometry experiments are conducted to compare the accuracy of dual-band colorimetric thermometry with that of single-band radiometric thermometry. The results demonstrate that the wideband dual-band colorimetric imaging thermometry method for MWIR and LWIR achieves greater precision, with an average reduction in relative error of 3?4 percentage points compared to single-band thermometry. By combining a wideband IRFPA with appropriate filters, this method allows for the miniaturization of imaging thermometry equipment, improving the precision of radiometric thermometry, and shows great potential for widespread application.