Optical design and testing have numerous applications in industrial, military, consumer, and bio-medical settings. From the point of view of system output, image optics or nonimage optics are demonstrated. In general, we could say that before 1995, optical design was simply a professional design work under the assistance of optimization via computed ray tracing, although it looked so brilliant in the old days. After 1995, optical design works became more and more complicated because of some optics, which were gradually replaced by a new name: opto-mechatronics. These indicate that optics might be integrated as a module, which might involve optical design, mechanical design, electronics, optimization, and more algorism. The Sony SCD-1 pickup lens for the SACD disk was a critical sample in 2000. After 2000, the tolerance problem might no longer be an issue thanks to advances in manufacture technologies in recent years. Therefore, optics and optical systems go back to this marvelous world.

Recent research on optical design, even image optics or nonimage optics, from deep UV to the far infrared, has focused on two main areas: large optics such as NASA or military purposes with complicated polarization aberration analysis, adaptive technology, and optimization methods such as the finite element method, and micro-optics cooperating with diffractive theory and Fourier optics. More advances have been announced, such as optics integrated with holograms in order to achieve amazing real-time 3D images and laser optics integrated with semiconductors to achieve extremely large storage devices. We might expect that “Optics Next,” optics of the next generation, might be revealed in the coming years following the quick development of laser diodes, algorism, and fast response MEMs.

This feature issue is aimed at scientists, engineers, and practitioners interested in understanding how different materials and components combine with optimization to determine and influence image or nonimage system performance. In addition, new concepts and trends for optics and further optical systems will be highlighted in this special issue. Scientists and engineers from commercial, academic, and military disciplines came together to share advances in either imaging systems or nonimage systems.

In this collection of papers you will find research ranging from traditional aberration theory to advanced freeform design technology with off-axis aberration compensation, the design of small endoscopes to ophthalmoscopes, the design of a freeform micro-lens array for solar energy, and LED lighting for large public yards. From white LED applied to dental surgery to neural-network-optimized contact lenses. From a 3D statistics-based reconstruction method with high random-error tolerance for integral imaging to a near-infrared imaging system for nondestructive inspection. Some more new and interesting research areas are announced in this paper, such as diffraction of the aperture structure arrangement in a transparent AMOLED display, LCD Moiré by image-based particle swarm optimization, 2D quasi-spiral M-sequence-based DOE, single-shot digital holography, and a reconfigurable opt fluidic switch.

Dewen Cheng et al. present a solution to breaking the resolution/field-of-view (FOV) invariant to design a large FOV and high-resolution optical system, especially for a head-mounted display (HMD) system. A tiled HMD using two compact rotationally symmetrical eyepieces was designed and developed. The overall optical system is compact with high performance. The system volume is smaller than $30\mathrm{mm}\times 35\mathrm{mm}\times 30\mathrm{mm}$. Based on two ${0.61}^{\prime \prime }$ micro-display devices, the overall tiled system demonstrates a FOV of $66°(\mathrm{H})\times 32°(\mathrm{V})$ with a 7.5-mm exit pupil diameter and a 15.7-mm eye relief.

Liang Zhang et al. propose a novel high temperature strain sensor based on polyimide-coated fiber Bragg grating (FBG) and rhombus metal structures. By heating the low-softening-point glass via a micro torch, the polyimide-coated FBG could be fixed into the rhombus metal structure. Consequently, when the rhombus structure is stretched and compressed, respectively, the FBG will be subjected to a reverse state. Results showed that the proposed high temperature strain sensor could be used at a high temperature of 300°C. A resolution of $\sim 10\mathrm{\mu \epsilon }$ has been achieved experimentally. The average wavelength-strain sensitivity at 300°C is 1.821 and 1.814 pm/με, for the compress and stretch states, respectively.

Mostafa Ahmed Agour discusses a fast method for measuring the optical properties, e.g., the refractive index profile and birefringence, of fibers. It is based on recovering the phase distribution of light diffracted by a fiber sample at the recording plane from a single-shot digital hologram. Within the recovering process, an optimized approach based on the spatial carrier frequency method was utilized. In contrast to the low spatial resolution of off-axis digital holograms, the method ensures the best utilization of the camera support. Experimental results are given for illustration.

Jize Yan et al. report a measured distance error caused by double peaks in the BOTDRs (Brillouin optical time domain reflectometers). Due to the variances of the peak powers of the BSS along the fiber, the measured starting point of a step-shape frequency transition region is shifted and results in distance errors. Zero-padded short-time-Fourier transform (STFT) can restore the transition-induced double peaks in the asymmetric and deformed BSS, thus offering more accurate and quicker measurements than the conventional Lorentz-fitting method. The recovering method based on the double-peak detection and corresponding BSS deformation can be applied to calculate the real starting point, which can improve the distance accuracy of the STFT-based BOTDR system, multi-frame super-resolution and sampling limited imagery that models space objects. Spatial acuity enhancement was obtained, with a 1.8 times resolution enhancement.

ZhiTing Ye performed a study on a hollow light guide with edge-lit LED light sources that has been proposed to resolve all the issues of weight, uniformity, and efficiency simultaneously. The major approach is to modulate the LED luminous intensity profile by a cup of parabolic surface with continuously varied focal length. The modulated light-emitting profile directly makes up a sufficient uniformity on a planar surface, and extra components would not be required. The prototype is a circular planar illuminator with a diameter of 178 mm and a weight of 240 g. The experiment shows an overall efficiency of 82.37%, with a uniformity of 83.7%. The weight of the whole module is 40% lighter than that of a solid light guide with the same size.

Zacarias Malacara-Hernandez et al. present a least squares procedure to find the tilts, curvature, astigmatism, coma, and triangular astigmatism by means of measurements of the transverse aberrations, using a Hartmann or Shack–Hartmann test. The sampling points are distributed in a ring centered on the pupil of the optical system. The properties and characteristics of rings with three, four, five, six, or more sampling points are analyzed with more detail and better mathematical analysis than in previous publications.

Jun Zhu et al. describe an approach utilizing freeform optical surfaces for the realization of a novel optical function called the transverse image translation, which acts as a basic geometrical transformation for images. A freeform single lens composed of two different freeform surfaces is well-designed for the given system as the only translation element. The image of the given system is expected to be successfully translated transversely as designed when the freeform lens is properly inserted.

Dmitry Reshidko et al. study the use of potential optical benefits of having a curved image surface rather than a flat one. Curved sensor technology allows for optically faster lens solutions. Trade-offs of several relevant characteristics such as packaging, chief ray angle, image quality, and tolerance sensitivity are discussed. A comparison of a benchmark flat field lens, and an evaluation design imaging on a curved surface and working at $f/1.6$, provides useful specific insights. For a given image quality, departing from a flat imaging surface does not allow significantly reducing the total length of a lens.

Chih-Yu Wang et al. show that laser acupuncture stimulates acupoints with laser light, which is a noninvasive treatment, but one that is able to achieve a similar effect to traditional acupuncture. The implementation of a lifting–thrusting function can be achieved by moving a laser focused spot back and forth; thus the most concentrated energy, such as the tip of the acupuncture needle, moves upward and downward in the acupoint. The results show that by controlling the diopter of the lens, the focused light spot can move in the range from 4.5 to 9.5 cm; therefore, the range of lifting and thrusting for the present laser acupuncture is 5 cm. The area of the focused spot was about $6\mathrm{mm}\times {10}^{-3}\mathrm{mm}$, which is comparable to the commonly used traditional acupuncture needle tip.

Jia-Wei Chen et al. present a model of focal plane testing methods such as Shack–Hartmann wavefront sensing or phase shifting deflectometry, which are valuable tools for optical testing. In this study, a novel wavefront slope testing method uses a scanning-galvo laser, where a single-mode Gaussian beam scans the pupil of the tested optics in this system. In addition, the ray aberration is reconstructed by a four-step phase shifting measurement by modulating the angular coordinates. The measured wavefront is verified by the Fizeau interferometer in terms of Zernike polynomials.