When laser propagates through the atmosphere, it encounters atmospheric turbulence that distorts the laser wavefront, resulting in turbulence effects such as beam drift, spot expansion, scintillation, and fluctuations in the angle of arrival. In studies about these turbulence effects, Fried’s atmospheric coherence length (r₀) expresses changes in both the amplitude and phase of light waves and serves as a representation of the overall intensity of atmospheric turbulence, making it a standard metric for quantitative description. We propose the use of the differential image motion monitor (DIMM) to measure this parameter. During the measurement, relevant parameters, including the device’s exposure time, must be specified. Since the turbulence freezing time varies across different regions, the selected exposure time for measuring atmospheric coherence length also varies. The different exposure times of the equipment affect the measurement results. Therefore, we propose a dual-camera synchronous measurement scheme based on a beam-splitting prism, which is built upon the traditional DIMM, to construct a system for measuring the optimal exposure time. This system employs a beam-splitting prism with a 50∶50 splitting ratio to achieve comparative measurements of different exposure time under “atmospheric freezing” conditions. Through comparative experiments and evaluation methods based on the ratio of system uncertainty and centroid variance, the optimal exposure time is selected.

We first summarize the theory of atmospheric coherence length, the relationship between coherence length and exposure time, and the theoretical basis for selecting exposure time. Based on this, we improve the existing atmospheric coherence length measurement instrument (aperture of 280 mm, focal length of 2800 mm, sub-aperture center distance of 205 mm, sub-aperture diameter of 61 mm) by using a beam-splitting prism to achieve dual-camera synchronous measurement. Experiments are conducted using this system to collect image data generated during the process. Since the star images exhibit Gaussian spot characteristics, we propose an image processing method based on centroid extraction. This method first locates and separates the star spots using adaptive threshold segmentation and morphological operations, and then calculates the centroid position through a grayscale-weighted subpixel subdivision algorithm, achieving subpixel accuracy in star centroid extraction. Based on the centroid coordinates and theoretical formulas, we derive the atmospheric coherence length parameter. Next, we analyze the effect of different exposure time on the system’s measurement results. By comparing the centroid variance ratio and system uncertainty ratio between the experimental and control groups, we evaluate the measurement accuracy under different exposure time and determine the optimal exposure time under local measurement conditions. Finally, we compare the selected optimal exposure time measurement results with long-term measurement results under conventional exposure time, confirming that the optimal exposure time significantly improves measurement accuracy and system performance. This study provides theoretical support and experimental evidence for the accurate measurement of atmospheric coherence length and offers a reference for optimizing exposure time selection in various measurement scenarios.

In the synchronous measurement system (as shown in Fig. 1), we implement an adaptive exposure time measurement function. Through this system, we dynamically adjust the exposure time to adapt to changes in atmospheric turbulence. Statistical analysis of the star image centroid extraction (Fig. 3) demonstrates that our proposed method can accurately extract the centroid of the spots, further validating the precision and stability of centroid extraction. Regarding the selection of the optimal exposure time, we evaluate measurement performance at different exposure time through statistical analysis of system errors and centroid variance (Fig. 6). The analysis results show that when the exposure time is 5 ms, the system error is minimized, and the centroid variance fully reflects changes in atmospheric turbulence. Therefore, for measuring atmospheric coherence length in the Changchun region, we recommend using 5 ms as the standard exposure time. Subsequent long-term measurement data comparisons further validate the effectiveness of this choice. The 5 ms exposure time captures richer atmospheric turbulence signals and accurately reflects the dynamic turbulence changes at the time. This provides a reliable experimental basis for efficient measurement of the system in practical applications.

We propose a dual-camera synchronous measurement scheme based on a beam-splitting prism, constructing a system to determine the optimal exposure time for measuring atmospheric coherence length in this region. The scheme uses a beam-splitting prism to replace the original single detector with two detectors, ensuring that each detector receives the same amount of energy. By simultaneously detecting atmospheric turbulence with both detectors, we conduct comparative experiments under atmospheric freezing conditions, avoiding the impact of continuously changing turbulence on the experiment. We process the images collected by the detectors for centroid extraction and calculate the atmospheric coherence length at different exposure time. By analyzing the system’s measurement accuracy and the variance of the spot centroids, we determine the optimal exposure time. Our research results indicate that a 5 ms exposure time should be chosen for measuring local atmospheric coherence length. Under strong wind conditions, we suggest an exposure time of 4 ms to better capture high-frequency signals, while in windless conditions, we consider 6 ms. Comparative measurements of local atmospheric coherence length at these exposure time verify that the exposure time derived from our experimental conclusions can better reflect changes in atmospheric turbulence. This study is of great significance for improving the accuracy of atmospheric coherence length measurements.