Endoscopic imaging is an indispensable tool for visual inspection of surfaces in hard-to-access places. The conventional optical endoscope only enables imaging the regions that lie close to the focal plane, and therefore improving the depth of field (DOF) of an endoscopic system has recently attracted much attention. Here, we propose and demonstrate a simple, cost-effective, and easily available industrial endoscopic system, which is composed of a rigid tube and group of optical lenses. The outer diameter of the designed endoscopic modality presented in this article is 3 mm. By employing the wavefront coding technique, the DOF of newly-designed endoscope was extended to 2.7 mm in the proof-of-concept experiment, which is an increase of almost 10 times compared to the initial counterpart.

Ultrafast time stretch imaging offers unprecedented imaging speed and enables new discoveries in scientific research and engineering. One challenge in exploiting time stretch imaging in mid-infrared is the lack of high-quality diffractive optical elements (DOEs), which encode the image information into mid-infrared optical spectrum. This work reports the design and optimization of mid-infrared DOE with high diffraction-efficiency, broad bandwidth and large field of view. Using various typical materials with their refractive indices ranging from 1.32 to 4.06 in $3\sim 5\mathrm{\mu }\mathrm{m}$ mid-infrared band, diffraction efficiencies of single-layer and double-layer DOEs have been studied in different wavelength bands with different field of views. More importantly, by replacing the air gap of double-layer DOE with carefully selected optical materials, one optimized ${\mathrm{MgF}}_2/{\mathrm{SiO}}_2/\mathrm{Si}$ triple-layer DOE, with efficiency higher than 95% in the whole $3\sim 5\mathrm{\mu }\mathrm{m}$ mid-infrared window and field of view greater than ${75}^{\circ }$, is designed and analyzed. This new DOE device holds great potential in ultrafast mid-infrared time stretch imaging and spectroscopy.

In recent years, the endoscope is widely used in defect detection of small-size parts in complex environment. An industrial endoscope optical system was designed for aero-engine blade detection. The basic structure of the lens adopted secondary imaging, and the objective lens adopted asymmetrical inverse distance structure, which beamed the large field of view light into the small caliber cavity. The adaptor lens was adapted to the high speed camera in which the diagonal length was 42 mm, and it could magnify the intermediate image 21 times. The system was based on Zemax design software for system optimization, tolerance analysis and image quality evaluation, and finally the system realized the optical properties of large field of view (120°), small caliber (3 mm) and high temperature resistance (25 ℃～180 ℃). Due to the higher requirements of the field of view, caliber and adaptor magnification, the aspherical surface was reasonably introduced to improve the imaging quality of the system. The entrance pupil diameter was increased to 0.5 mm, the system spatial cut-off frequency was at 17 lp·mm⁻¹, the full-field modulation transfer function value was greater than 0.28, and the maximum distortion value is less than 21.2%.

An off-axis four-mirror optical system has the advantages of a wide field of view (FOV) and a small telephoto ratio. However, it will bring difficulties in assembly and detection. Here we report an off-axis four-mirror free-form telescope with a long focal length and a wide field of view based on two integration mirrors. The initial structure of a coaxial four-mirror optical system is established based on the Seidel aberration theory. A Zernike Fringe free-form surface is introduced to correct aberrations. By gradually increasing the entrance pupil diameter and the FOV, we finally obtain an off-axis four-mirror telescope with a FOV of 0.4^∘×20^∘, an F-number of 11.5, a long focal length of 2000 mm, and a volume of 360×400×600mm³. In addition, four mirrors remain coaxially aligned, remarkably facilitating detection and adjustment. The proposed off-axis four-mirror telescope, with double integration mirrors, holds great potential for application in aerospace remote sensing observations.

Optical imaging system offers best performance across a range of applications such as machine vision, medical examination, security, and quality inspection in a production line. A typical imaging system is commonly sensitive to the defocus variations in the object plane location, which largely loses the transversal information of volume and brings inaccuracy to the comprehensive vision. Therefore, improvement of depth-of-field (DOF) with high resolution imaging remains a challenge for the conventional imaging system. Here we introduce a newly designed optical imaging system, mainly being composed of a standard Tessar lens and a non-absorptive rectangularly separable phase mask, whose corresponding optical point spread function is greatly invariant to defocus. An extended DOF image is restored by using the Tikhonov regularization algorithm and deconvolving the response function of the optical system. The proof-of-the-concept experimental results verify significantly extended DOF exceeding 40 times compared to the common optical imaging counterparts. Future work includes the use of a durable and high-transmittance phase mask to develop various large DOF optical configurations without loss of resolution.

Due to the difficulty of correcting chromatic aberration (CA) in telephoto cameras, recent studies have combined image algorithms with simple optical structures, such as single-spherical lenses, for high-quality photogra-phy, moving away from complex optics. However, this approach often struggles to comprehensively address compounded issues arising from optical aberrations of simple optical systems, including defocus blur and multi-channel misalignment. To tackle this challenge, this manuscript presents an approach for developing a telephoto imaging system by leveraging the distinct characteristics of axial and lateral chromatic aberrations (ACA, LCA) over the visible spectrum. The optical design is limited to a specific wavelength range to preserve high-frequency information of the green channel. A cross-channel fitting method is presented to suppress the LCA. Subsequently, the powerful capabilities of deep learning are utilized to correct ACA, defocus blur, and other residual optical aberrations. Simulation experiments demonstrate the effectiveness of the proposed approach in mitigating the CA inherent in telephoto systems, thereby delivering high-quality imaging results over the whole visible waveband. (c) 2023 Optica Publishing Group