It is important to research the unloading trajectory of the belt conveyor for designing the transfer chute. At present, there are many methods to draw the unloading trajectory. However, they are not completely unified. Also, some methods utilize insufficient theories so that the simulated trajectory does not match the actual trajectory. To accurately draw the unloading trajectory of the conveyor, this paper uses EDEM (Discrete Element Method) software to simulate discharge trajectories of a specific belt conveyor at different speeds compared with the CEMA (Conveyor Equipment Manufacturers Association) method and the method by calculating the discharge trajectory which is drawn according to the parabolic equation. This paper also analyses the advantages and disadvantages of different methods. The analysis shows that the discharge trajectory drawn by the CEMA method has the same width from top to bottom. While the actual discharge trajectory is bell-shaped from top to bottom due to the upper and lower stratification of the material speed. When the belt speed is low, the separation speed should be used to calculate the separation angle. The unloading trajectory drawn by calculating the parabolic equation is more consistent with the actual situation. But when the belt speed is low, the calculation with the belt conveyor is not reasonable. In this case, the material height should be calculated by the separation speed.

Pointer meters are widely used in transformer substations and other places because of their good robustness. Generally, the reading of pointer instrument cannot be read automatically, and can only rely on manual reading. With the development of robot and computer vision technology, it is possible to use inspection robot to obtain the reading of pointer instrument. The automatic reading of the analog meter should first detect the meter from the image, and then identify its reading. For the instability of traditional instrument detection, an analog meter automatic reading method based on YOLOv5s detector is proposed in this paper. Hough transform algorithm is used to detect the pointer and range, and then the meter reading is realized based on the angle method. Experiments show that the average processing time of a single image is 0.3s, and the reading error is less than 5%, which can meet the requirements of automatic reading in patrol inspection.

In order to improve the reconstruction quality of hologram, the iterative layer-oriented angular spectrum optimization (ILASO) algorithm is proposed in this paper, which combines non-iterative and iterative methods to achieve full color holographic three-dimensional (3D) display. In this paper, the hologram is generated by layer-oriented angular spectrum diffraction, and then the hologram is optimized by error feedback iteration, so as to reduce the speckle noise and improve the reconstruction quality of the hologram. Simulation results show that the holographic reconstructed image has strong detail representation ability, weak speckle noise and small crosstalk, which is suitable for complex 3D scenes and has excellent holographic 3D display ability.

The authors make the following corrections to the published paper [...]

This paper presents a method of haze removal and computer-generated holographic display of degraded images in coal mine. Firstly, the image enhancement of underground coal mine is realized by using the dark channel prior haze removal algorithm, which greatly weakens the shielding of coal dust and water mist in the roadway environment. Next, using the computer-generated hologram algorithm based on angular spectrum diffraction, the phase only hologram is generated with the haze removal image as the input. The peak signal-to-noise ratio (PSNR) of the reconstructed image of the red channel is 65.47dB, the PSNR of the reconstructed image of the green channel is 64.98dB, the PSNR of the reconstructed image of the blue channel is 65.78dB, and the average PSNR is 65.41dB. The simulation results show that high-quality reconstructed image can be obtained by combining dark channel prior and computer-generated hologram, and the image enhancement of underground coal mine is realized.

The classic Denisyuk recording method is commonly used to reproduce three-dimensional (3D) image of volume holography, but its diffraction efficiency is low, and white light irradiation is required to obtain high-brightness reproduced image. Aiming at the above problem, the diffraction characteristics of transmissive and reflective volume holographic gratings are analyzed by Kogelnik’s coupled wave theory. A two-step volume holographic recording method is proposed. Firstly, the transmissive volume hologram is recorded and then the reconstructed image is transferred to the reflective volume hologram. Finally, through this method, the brightness and field of view of the reproduced image are improved, and a clear and bright 3D image can be observed under natural light.

Belt conveyor is one of the main transportation equipment in coal mine. The belt is easy to tear in production. If the tear damage of belt surface cannot be detected in time, it may lead to serious production accidents. In this paper, a belt tear detection method based on industrial camera monitoring is proposed, which can identify the belt tear in time and output the quantitative evaluation result. After filtering the image, Canny edge detection algorithm is used to identify the tear region. A sliding window is used to evaluate the degree of damage area and further determine the control of belt conveyor. Experiments show that the average processing time of a single frame image is 0.4s, which can meet the needs of real-time detection in the production.

We propose a full-color see-through three-dimensional (3D) display method based on volume holography. This method is based on real object interference, avoiding the device limitation of spatial light modulator (SLM). The volume holography has a slim and compact structure, which realizes 3D display through one single layer of photopolymer. We analyzed the recording mechanism of volume holographic gratings, diffraction characteristics, and influencing factors of refractive index modulation through Kogelnik's coupled-wave theory and the monomer diffusion model of photopolymer. We built a multiplexing full-color reflective volume holographic recording optical system and conducted simultaneous exposure experiment. Under the illumination of white light, full-color 3D image can be reconstructed. Experimental results show that the average diffraction efficiency is about 53%, and the grating fringe pitch is less than 0.3 μm. The reconstructed image of volume holography has high diffraction efficiency, high resolution, strong stereo perception, and large observing angle, which provides a technical reference for augmented reality.

An optical encryption method based on computer generated holograms printing of photopolymer is presented. Fraunhofer diffraction is performed based on the Gerchberg-Saxton algorithm, and a hologram of the Advanced Encryption Standard encrypted Quick Response code is generated to record the ciphertext. The holograms of the key and the three-dimensional image are generated by the angular spectrum diffraction algorithm. The experimental results show that large-size encrypted Quick Response (QR) code and miniature keys can be printed in photopolymers, which has good application prospects in optical encryption. This method has the advantages of high-density storage, high speed, large fault tolerance, and anti-peeping.

With the development of holography and nanomanufacturing technology, metasurfaces are playing an increasingly important role in the field of holography. We designed a silicon cylindrical structure based on the Huygens metasurface. By exciting the Mie electric dipole and magnetic dipole resonance of the silicon cylindrical structure, a high transmission efficiency of 84% can be achieved at a wavelength of 633 nm, and a full phase coverage of 0-2π can be obtained by adjusting the radius of the silicon cylindrical. We used the angular spectrum algorithm to obtain the phase distribution relationship between the object image and the metasurface, and designed the arrangement of the silicon cylinder metasurface. The simulation obtained a high-fidelity hologram, and the structure has a high transmission efficiency around the 633 nm spectrum. This method can realize metasurface holography with high transmission efficiency, and it can be applied in holographic imaging.

Monomer diffusion models are important in describing the formation mechanism of the volume holographic grating in photopolymers. The most representative of these models is the first-order diffusion model, which neglects the influence of the exposure intensity threshold and dissolved oxygen in a polymer. This model causes a significant deviation between the theoretical simulation and the experimental results. Therefore, we propose a modified monomer diffusion model, which is more consistent with experimental results on refractive index modulation than other models.

In order to improve the effect, clarity and resolution of anti-counterfeiting, this paper proposes a method of reflection volume holographic three-dimensional (3D) anti-counterfeiting. This method adopts reflection volume holographic interference recording of 3D object to achieve 3D anti-counterfeiting. The principle of reflection volume holography was analyzed by using Kogelnik's coupled wave theory. A reflection volume holographic anti-counterfeiting recording optical path based on a monochromatic laser was constructed. Photopolymer is used as the volume holographic recording material, and a metal school badge is used as the object. Experimental results show that the reflection holographic gratings can record and reproduce 3D object, and the 3D shape of the reproduced image is consistent with the object, which improves the effect of anti-counterfeiting.

Efficient and low-cost solar-energy collection has become the focus of many research works. This paper proposes a recording method and an experimental verification of a wide-band, large-angle, and high concentration-ratio volume-holographic grating for solar concentration. We applied the Kogelnik coupled-wave theory and photopolymer diffusion model to analyse the formation mechanism and influencing factors on the diffraction efficiency of monochromatic volume-holographic gratings. We design and construct a three-color laser-interference system to record three monochromatic volume-holographic gratings. The best recording conditions are determined by experiment and simulation. A trichromatic volume-holographic grating is obtained by gluing the three monochromatic gratings together. The experimental results show that the trichromatic volume-holographic grating with a working angle of 6.7° and a working band of visible light has a light concentration ratio of 149.2 under an illumination of the combined recorded three-color beams, and that under sunlight is 27.2. We find that the proposed trichromatic volume-holographic grating for light concentration offers the advantages of wide band and high light concentration ratio, which provide a reference for solar concentration.

Volume holograms can achieve concentration of sunlight, but it has shortcomings such as small concentration angle, dispersion, and complex production process of the volume holography material, which is difficult to mass produce. This paper proposes a method based on the metasurface concentrating sunlight. We analyzed the mechanism of volume holographic light focusing by Kogelnik coupling wave theory, which is greatly limited by wavelength selectivity. Utilizing the metasurface control mechanism on the wave front phase, the array arrangement that meets the light concentrating effect was designed. Simulation analysis shows that the metasurface can achieve light concentration in the visible light band with low dispersion.

A new double-layer sunlight concentration system, where each layer is divided into two regions, is proposed, and the system has four volume holograms. Since the four holograms convert light in different directions, the interlayer crosstalk is reduced, and the system has a high concentration ratio. The simulation results show that the concentration system can achieve a 30° operation angle range. The holograms are fabricated on photopolymer substrates, and the left half of the system is implemented using two holograms. The characteristics of the left half of the system are assessed. The agreement of the simulation and experimental results on diffraction efficiency validates the proposed method. The tested monochromatic concentration ratio can achieve a record of 418.8, and the concentration ratio under sunlight is 5.38. The experiment results of light use efficiency are close to the simulation with non-crosstalk, which indicates that the interlayer crosstalk is small.

Volume holograms can record and reproduce three-dimensional object. Compared with planar holograms, volume holograms can improve the resolution, stereo perception and realism of anti-counterfeiting, and realize three-dimensional anti-counterfeiting. In order to improve the diffraction efficiency and anti-counterfeiting effect of volume holographic anti-counterfeiting, this paper proposes a reflection volume holographic three-dimensional anti-counterfeiting method based on photopolymer. The diffraction characteristics and influencing factors are analyzed by Kogelnik’s coupled wave theory. Based on Piazzolla’s monomer diffusion model, the effects of exposure energy and exposure intensity on refractive index modulation and diffraction efficiency were studied. The simulation results show that the reflection volume holograms can achieve a greater refractive index modulation, with higher diffraction efficiency, stronger wavelength selectivity, which enhances the effect of three-dimensional anti-counterfeiting.