The ozone layer in the sky efficiently blocks ultraviolet light with a wavelength range of 0.24-0.28 μm, preventing this spectrum of solar radiation from reaching the ground. This creates the "solar blind ultraviolet" band, which has numerous advantages in civilian, military, and police applications. Especially, it plays a valuable role in criminal investigation science, particularly in retrieving physical evidence without destroying the scene. It can access and search scenes quickly while extracting high-quality images of fingerprint marks, bloodstains, semen spots, fire and explosive residues, and other physical evidence. Nevertheless, the unique application of solar blind ultraviolet (UV) bands and optical materials poses challenges that traditional optical design cannot meet. The development of aspherical and diffractive surfaces offers an effective new solution to this problem. The rapid advancement of ultra-precision machining and testing technology has made it possible to machine and test diverse aspherical surfaces with high precision. The advantages of Q-type aspherical surfaces, such as efficient optimization of optical systems and simplified processing of aspherical surfaces, are even more highly valued by the industry. Therefore, it is essential to introduce the benefits of Q-type aspheres and diffractive optical elements into the design of day-blind UV optical systems. We hope that our design results can facilitate the development and application of UV refractive-diffractive hybrid imaging optical systems.

Based on the requirements of the actual application environment, the detector selection is conducted for the day-blind UV optical system, which includes the need for stable zoom. To meet these needs, we comprehensively consider two optical materials—fused silica and calcium fluoride due to their cost and performance in the UV range. We optimize the initial structure with Zemax OpticStudio software and set Q-type aspherical and diffractive surfaces on a lens substrate. Then, we reduce the number of system lenses to five and improve image quality. Finally, we design a day-blind UV zoom hybrid optical system for criminal investigation. The image quality of the system is evaluated, and the machinability analysis of the relevant surface types and the imaging simulation for criminal detection are improved.

After the initial structure is optimized by Zemax OpticStudio software, we design a day-blind UV optical system with a configuration of five lenses. The 1st, 3rd, and 5th lenses are made of fused silica, while the 2nd and 4th lenses are made of calcium fluoride. Notably, the last lens features a Q-type asphere on the front surface and a diffractive surface on the back surface (Fig. 5 and Table 2). Machinability analysis is conducted for the parity aspheres, diffractive surfaces, and Q-type aspheres (Figs. 6-8). The evaluation of system image quality yields the following results. The modulation transfer function (MTF) exceeds 0.7 at all three focal lengths (Fig. 9). The field curvature remains below 0.3 mm across all wavebands, and distortion is less than 0.06% (Fig. 10). The cam curve of the lens exhibits a smooth trend without inflection points (Fig. 11). Tolerance analysis indicates that the system is highly feasible (Tables 6 and 7). Finally, imaging simulations are performed, which yield the desired results (Fig. 12).

Based on the ARTCAM-407UV-WOM UV detector, we design an optical system for day-blind UV zoom using a combination of Q-type aspherical surfaces and single-layer diffractive elements in the UV band. The optimized system consists of five lenses with only two materials, calcium fluoride and fused silica, and the Q-type aspherical and diffractive surfaces are set on a convex flat lens, which is easy to process and use. The MTF values at Nyquist spatial frequency of 11 lp/mm are all higher than 0.7, and the full-field-of-view distortion is less than 0.06% in the working waveband. The optical system has a simple and compact structure and has high imaging clarity and resolution, which has certain advantages in criminal investigation detection, especially in the extraction of potential traces and physical evidence.