Design of a tube lens with a focus tunable lens for optical inspection systems

Optical inspection systems use large numerical apertures (NAs) to achieve high resolution. However, as the NA increases, the depth of field decreases significantly, causing images to blur easily with slight object shifts. Conventional methods maintain focus by mechanically moving the entire lens or specific lens groups. However, these methods increase the system volume, require high-performance motors, and necessitate precise inertial control when the lens group mass is substantial. In this study, we propose a novel tube lens design that integrates a focus tunable lens (FTL), which electronically adjusts its curvature without requiring mechanical movement. We precisely calculated the required curvature radius of the FTL for various object distances and performed optical simulations to verify the system performance before fabrication. The developed tube lens has a focal length of 300 mm and is combined with objective lenses featuring focal lengths of 30 and 60 mm. This enables magnifications of 10X and 5X, respectively. Simulation results confirmed stable image positions for object shifts of ± 50 μm and ± 0.36 mm at 10X and 5X magnifications, respectively, while maintaining diffraction-limited resolution and near-zero distortion aberration. These findings demonstrate that FTL-based focus control is highly suitable for high-precision inspection and imaging applications.