The study of target infrared radiation characteristics is a key research direction in the field of infrared detection technology and holds significant application value in acquisition and tracking, detection, and recognition. External field measurements of infrared radiation characteristics are essential for obtaining the target’s surface temperature distribution and radiation characteristics in real-world environments. Typically, imaging measurement methods are used to capture target images in various states and calculate target radiation brightness, intensity, or temperature distribution. However, practical applications face several challenges: 1) system calibration is often conducted before or after measurements, with significant changes in ambient temperature during both the measurement and calibration phases, resulting in temperature drift in the system and large errors in target characteristic calculation; 2) the slow rate of temperature rise and stabilization in the blackbody leads to a lengthy calibration cycle (typically over 30 min), with calibration coefficients becoming inaccurate due to changes in ambient temperature; 3) the dynamic target characteristics measurement process is lengthy, with continuous ambient temperature fluctuations. Therefore, traditional measurement methods still have inherent errors that need to be addressed.

In this paper, we propose an external field measurement method for target infrared radiance based on ambient temperature correction. During the calibration phase, a blackbody image is obtained using the close-range extended source method, and the ambient temperature is recorded simultaneously. The calibration equation is solved using the least squares method, establishing a relationship between the pixel gray response, target radiance, and ambient temperature of the infrared system. In the measurement phase, the blackbody image of a surface source with a known temperature is recorded, and the deviation between the retrieved and measured ambient temperature is calculated. By recording the target infrared image, ambient temperature, and atmospheric parameters, the measured value of the ambient temperature is corrected during the measurement process, improving the accuracy of target radiance measurements. The measurement method and process are shown in Figs. 2 and 3.

To verify the feasibility and accuracy of the proposed method, external field calibration and verification experiments are conducted using a refrigerated infrared system. In the calibration experiment, both the traditional calibration method and the method incorporating ambient temperature correction are used. The infrared system has a response wavelength of 4040?4120 nm, 640 pixel×512 pixel, and an integration time of 1000 μs. The blackbody size is 700 mm×700 mm, with an emissivity of 0.98. Five temperature points are set at 75, 100, 125, 150, and 175 ℃, respectively. The ambient temperature is measured using a meteorological station with an accuracy of ±0.2 ℃. The original data are shown in Table 1. Four temperature points (75, 125, 150, and 175 ℃) are used for calibration fitting (Table 2), and the average relative error of the fitting results is calculated and compared (Fig. 4). In the validation experiment, the blackbody is imaged at a horizontal distance of 280 m from the infrared system (Fig. 5). The blackbody emissivity is 0.98, and temperatures of 70 ℃ and 90 ℃ are set alternately. The solar radiometer and ground weather station are placed next to the infrared system, and the blackbody temperature, blackbody image, ambient temperature, and atmospheric parameters (transmittance and path radiation) are recorded in real time (Fig. 6 and Table 3). One of the 70 ℃ and 90 ℃ blackbodies is used as the target for measurement, and the other served as the reference blackbody for ambient temperature correction. The radiance value and relative error of the 30 pixel×30 pixel region of the blackbody image are calculated pixel by pixel using the method proposed in this paper and compared with the direct calculation results based on the calibration equation (Table 4).

In the calibration experiment, the ambient temperature varies from 3.8 ℃ to 5.5 ℃, while in the validation experiment, the ambient temperature varies from 17.1 ℃ to 17.5 ℃. The average gray level of the blackbody image at 70 ℃ in the validation experiment is 3894.42, which is higher than that of the blackbody image at 75 ℃ in the calibration experiment (3317.8), indicating that the infrared system experiences temperature drift and the response relationship has changed. The results show that the relative error in the traditional direct calculation method is generally large, possibly due to significant changes in ambient temperature, changes in the infrared system state, and inaccurate coefficients in the original calibration equation. Compared with the traditional method, the direct calculation method that considers ambient temperature has an average relative error of 13.52%, improving accuracy. The method proposed in this paper, which includes ambient temperature correction, achieves an average relative error of 7.91%, a 5.61% improvement over the direct calculation method, and yields results closer to the actual target values. The method can provide a useful reference for conducting external field measurements and theoretical research on target infrared radiation characteristics.