The foveated imaging simulates the characteristics of human eye imaging, which can achieve global imaging of detection targets with a large field of view and realize local high-resolution imaging for target detail discrimination. This technology has been applied to the large field of view imaging such as scene monitoring, danger detection, remote sensing, and target tracking, so as to reduce the complexity of bandwidth and optical systems. The strategies for implementing foveated imaging in the past include non-uniform sensors with variable photosensitive density for imitating the variable sampling rate of the retina, designs of foveated optical systems, calculation integration of independent imagers with different resolutions, and a single sensor with multiple channels segmented into different magnification ratios. However, the high cost of hardware, the complexity added by non-uniform sensors, and the complexity of foveated optical systems, usually make these solutions unattractive. We propose a dynamic foveated optical imaging system consisting of an object-side telecentric lens, a liquid crystal lens with a rectangular aperture, an optical sensor, and a polarizing film. The object-side telecentric lens reduces the effect of oblique incident light on the imaging of the liquid crystal lens by making the main image ray in the object plane parallel to the optical axis. By introducing a liquid crystal lens with a rectangular aperture to modulate the phase of light waves, the system achieves local high-resolution imaging, and the center of the lens can be moved in real time according to actual situations. In other words, while ensuring high-resolution imaging of the target of interest in real-time detection, the entire image plane can be scanned, and other areas have low-resolution imaging. The lightweight and small volume of the liquid crystal lens with a rectangular aperture makes the entire optical system more compact.

The same liquid crystal material (HTW137700-100, Jiangsu Hecheng Co. Ltd.) in Fig. 4 is used to fabricate a liquid crystal lens with a rectangular aperture, with a clear aperture of 5 mm×5 mm according to the structure in Fig. 1. The high impedance film material of the lens is aluminum-doped zinc oxide with a resistance of 3×10⁶ Ω/□, and the thickness of the liquid crystal cell is 30 μm. The interference fringes of the liquid crystal lens under the driving conditions in Table 1 are measured by using Mach-Zehnder interferometry (Fig. 3). It can be seen that the actual aperture of the liquid crystal lens is smaller than the clear aperture, and the position of the lens varies with the driving voltage. The ordinary refractive index of the nematic liquid crystal used in Fig. 4 is 1.513, and the extraordinary refractive index is 1.774. With a laser of wavelength 532 nm, the bright/dark changes of the liquid crystal cell with voltage are observed, and the phase delay of the liquid crystal is obtained as a function of the effective voltage. It can be seen that within the linear response range of the liquid crystal, which is between 0.6 V and 1.52 V, the phase delay is approximately linearly related to the effective voltage. When the voltage is higher than 1.52 V, the response curve is in the nonlinear region, and when the voltage is higher than 2 V, the liquid crystal tends to be saturated. Therefore, according to Eqs. (5) and (6) and the electric field simulation results in Fig. 2, the actual aperture of the liquid crystal lens is determined to be a circular area with a diameter of 1.56 mm centered at the origin of the liquid crystal lens in Fig. 3. By analyzing the effective area of the liquid crystal lens in Fig. 3, the focal length at different positions of the lens center is determined to be 8.2D, and the aberration is (0.1±0.02)λ. The wavefront map of the lens is shown in Fig. 5, from which it can be seen that the wavefront of the lens remains unchanged.

The dynamic foveated imaging system consists of a polarizer, an imaging telecentric lens group, a liquid crystal lens with a rectangular aperture, and a complementary metal-oxide-semiconductor (CMOS) device. Fig. 6 shows the experimental setup, while Fig. 7 shows the actual experimental setup. As can be seen from Fig. 6, a parallel beam of light is incident on the polarizer, with the polarization direction of the polarizer parallel to the rubbing direction of the liquid crystal lens. The polarized beam passes through the imaging telecentric lens and reaches the liquid crystal lens with a rectangular aperture. As shown in Fig. 3, the actual aperture diameter of the liquid crystal lens with a rectangular aperture is smaller than the clear aperture diameter, so part of the light reaching the CMOS is modulated, and thus a portion of the light is focused to form an image. In other words, the square aperture liquid crystal lens is located between the imaging telecentric lens group and the CMOS for focusing and real-time monitoring of the region of interest, with the focusing and real-time monitoring processes driven by an electric field, without any mechanical device. We select the driving conditions in Table 1 to drive the liquid crystal lens, with the center coordinates shown in Fig. 3. The circular area in Fig. 8 represents the actual position of the liquid crystal lens at different times, and the experimental results are shown in Fig. 9, indicating that the optical system has achieved dynamic foveated imaging. The modulation transfer function (MTF) test card ISO12233 is used to test the low resolution of the region of interest and the remaining areas (Fig. 10). It can be seen from Fig. 10 that the resolution of the foveated area is higher than that of the remaining areas.

We aim to achieve dynamic foveated imaging by introducing a liquid crystal lens to alter the optical path difference. The electric field distribution of the moving center in the liquid crystal lens with a rectangular aperture is simulated and analyzed, and the corresponding interference fringes of the liquid crystal lens under the electric field distribution are measured by using a Mach-Zehnder interferometer. Experimental results show that the dynamic foveated imaging system can achieve high-resolution imaging of the region of interest while maintaining low-resolution imaging in other areas. Compared with traditional optical systems, the dynamic foveated imaging system has the advantages of compact structure, simple optical materials, high zero-order diffraction efficiency, and small transmission bandwidth. It can also scan different fields of view and perform high-resolution imaging, which makes it suitable for applications in scanning recognition, tracking, positioning, and other fields.