Large engineering structures such as tracks, dams, and tunnels are subjected to loads and environmental effects during their service life, leading to varying degrees of deformation. Even minor deformations in these structures can have severe consequences, including safety hazards and significant economic losses. Therefore, precise and continuous deformation measurement methods are essential to support safety monitoring, mechanical analysis, and risk assessment. Recent advances in experimental mechanics, computer science, and photogrammetry have led to the widespread use of vision-based non-contact measurement techniques, valued for their cost-effectiveness, versatility, and adaptability. Over decades of development, these methods have evolved into robust tools for deformation measurement in large structures. Current methods include contact-based measurement, laser ranging, digital image correlation (DIC), and structured light techniques. However, each has limitations: contact-based methods require physical interaction with the structure, resulting in low efficiency and high costs. Laser ranging instruments offer millimeter-level accuracy, but they struggle to meet the precision requirements of small-scale deformation measurements and cannot measure angular deformations effectively. DIC methods necessitate speckling or marking the surface, which alters the structure’s appearance. Structured light methods are restricted by their short working range. To address these limitations, a high-precision, easy-to-install, and cost-effective measurement system is urgently needed for measuring small deformations in large-scale structures.

To address the challenges of existing methods, we propose a novel approach that integrates the principles of optical lever and videometrics. First, the system structure and optical path diagram of this method are presented. Based on the application scenario, the measurement method and process are designed. Specifically, the optical path consists of a laser light source, a bidirectional mirror, and an imaging screen. During the structural deformation process, the camera continuously captures images of the laser spot projected on the imaging screen. The center coordinates of the spot are then extracted using a dedicated algorithm. By applying the geometric constraints of optical path propagation, the changes in the spot position are used to establish a relationship between the structure’s deformations (both distance and angular changes) and the spot’s actual movement. Structural deformations are ultimately calculated through a nonlinear iterative solution algorithm. The approach combines the broad adaptability of camera-based measurement techniques with the high precision of the optical lever method, enabling continuous and real-time monitoring of small deformations in large structures. The proposed method achieves high-precision measurements of millimeter-level displacements and small-angle deformations in large-scale structures.

The feasibility and accuracy of the proposed method are verified through both simulations and practical experiments. In the simulation phase, random errors such as Gaussian-distributed reference measurement errors and laser beam angle deviations are introduced to simulate actual measurement conditions (Table 1). In addition, the influence of errors in spot center extraction on deformation measurement accuracy is analyzed. Through these simulations, the relationship among spacing errors, angular measurement errors, and spot center extraction errors is determined. The influence of system parameter setting errors on measurement accuracy is also investigated (Fig. 4). Subsequently, a practical measurement system is established to validate the accuracy of the proposed method. Experimental results show that the average displacement error is 0.0536 mm, and the average angular error is 0.000638° (Tables 2, 3, and 4). Overall, the results demonstrate that the proposed deformation measurement method achieves high-precision displacement and angular measurements, effectively meeting the requirements for small deformation measurement in large structures, such as tracks.

To address the limitations of traditional videometric techniques in measuring small deformations in large structures, we propose a novel method that combines optical lever principles with videometrics. This method transforms small deformations, which are difficult to measure directly, into pixel changes that are easier to quantify. By employing a subpixel-based spot extraction algorithm and a nonlinear iterative solution algorithm, the method achieves higher measurement precision. Simulation experiments and practical measurements verify that the proposed method not only enhances measurement accuracy but also demonstrates excellent adaptability and stability. The experimental results show that the method enables continuous, real-time monitoring of small deformations in large structures. Furthermore, the measurement accuracy for both displacement and angular deformations meets the requirements for engineering applications. Future work will focus on further optimization of the system design, expanding its application in diverse real-world environments, and exploring its potential integration into automated and intelligent monitoring systems.