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In recent years, stable and photoluminescence (PL) tunable fluorescent materials known as carbon dots (CDs) have seen rapid development. Here, a simple, low-cost, and environmentally friendly approach has been developed to synthesize the orange-red CDs (OR-CDs) with paper tissues as the precursor in the presence of ethanol and concentrated sulfuric acid. The average diameter of the OR-CDs was measured at around 1.25 nm. The as-prepared OR-CDs are composed of carbon cores with the graphite structure distributed in the middle and wrapped or entangled by polymers for the outside. Moreover, the PL characteristics of the OR-CDs under the dilute solution state and the concentrated solution state, respectively, were explored. The former with the blue light showed prominent excitation-dependent characteristics in the range from 365 to 420 nm; the latter rendered the excitation-independent property with distinguished orange-red fluorescence emission. With an increase in the concentration, the emission fluorescence intensities gradually increased under the excitation wavelength of 540 nm, which can be attributed to the aggregation-induced emission property of the OR-CDs. By virtue of their excellent PL properties and low toxicity, we believe that the OR-CDs have potential applications in various optoelectronic areas, especially in white LED fields.

To assess the aging of oil-paper insulation, an accelerated aging experiment is executed. Raman spectroscopy, a nondestructive detection method with access to component identification and fault diagnosis, is used to analyze the aging of oil-paper insulation. Raman feature of oil-paper insulation aging with a close relationship with the degree of polymerization is obtained based on the concept of quadratic mutual information. By analyzing the relationship between the extracted feature and the degree of polymerization of samples at different aging degrees, the feasibility of reflecting the aging degree of oil-paper insulation is determined. By load analysis, the chemical correlation between the extracted feature and oil-paper insulation aging is clarified. For eight test samples, the prediction error of degree of polymerization based on Raman features is less than 50. These results show that the features extracted in this paper are helpful to realizing the Raman spectrum diagnosis of oil-paper insulation aging.

Fluorescent anti-counterfeiting is one of the most widely used anti-counterfeiting technologies at present. The demand to develop new anti-counterfeiting materials and technology is more and more urgent. Zinc oxide quantum dots (ZnO QDs) have superior fluorescent properties under ultraviolet light, making them a suitable replacement for traditional phosphors for anti-counterfeiting printing, which is environmentally friendly and meets the needs of sustainable development. In this paper, water-soluble ZnO QDs with an average particle size of 5.64 nm were prepared. Paper coated by ZnO QDs was obtained after ultrasonic treatment, which could emit bright yellow fluorescence when excited by ultraviolet light. As the concentration of ultrasonic solution is increased, the loading amount of ZnO QDs on the coated paper increased gradually, reaching the maximum when the concentration is increased to 1molL^−1, which then does not change with an increase in concentration. The fluorescent intensity of the coated paper was consistent with the changing trend of the loading amount. The coated paper has excellent optical stability, is easy to recycle, and provides simple identification of authenticity by ultraviolet light and anti-copy functionality. Their application in packaging and printing is of great significance to the development of complex, concealed and non-repeatable anti-counterfeiting technology.

A novel, simple, and inexpensive flexible surface-enhanced Raman-scattering (SERS) platform based on common laboratory filter paper modified with Ti_3C_2T_x flakes was reported. Ti_3C_2T_x synthesized from a Ti_3AlC_2 phase with a mixture of HCl and LiF and Ti_3C_2T_x nanosheets were characterized by the TEM, XRD, UV–Vis spectrum, and Raman spectrum. Paper-based substrate has been proven to sample on rough and irregular surfaces. Thus, Ti_3C_2T_x was further manufactured as paper substrate by the immersion method to transfer nanosheets to filter paper. SERS activity of prepared substrate was demonstrated using R6G by the same filter paper modified with and without Ti_3C_2T_x, and various concentrations of R6G were tested to prove the sensitivity of the substrates. Further detection of CV and MG certified the universality of paper substrate based on Ti_3C_2T_x nanosheets for detection of organic pollutants. The uniformity and stability were proved by CV and R6G molecules. This SERS platform combines the advantages of 2D material and flexible paper scaffolds, resulting in a highly sensitive, cost-efficient, and easy-to-manufacture large-scale flexible substrate and is expected to be used in practice.

Laser engraving technology is a type of laser processing technology, widely used for product coding, marking, and so on. A large amount of research has reported the results of metal surface engraving; however, few research results, to the best of our knowledge, have provided theoretical support for the application of paper packaging laser engraving. In this paper, the quality of paper laser engraving is investigated by experimental methods. First, various phenomena appearing in paper carving were studied, including plant fiber burning, charcoal, and edge marks; second, the main factors affecting the quality of laser engraving are researched, and the influence of laser intensity and the preset width of carving marks on the engraving quality are discussed. The results show that the engraving precision is the best when the laser power is 11 W and the preset width is small (0.26 mm). Finally, the laser engraving precision of UV coated paper is studied, and the effect of UV material melting and secondary crystallization on engraving the quality of paper laser engraving quality is discussed. When the laser power is small, the maximum and minimum values of UV film melting and secondary crystallization engraving trace are relatively small as well; further, when the laser power increases, the maximum width of engraving is basically consistent with the preset width, and the precision of laser engraving is optimal.

Airborne particulate matter has become an emerging issue globally due to environmental degradation and the health risk it causes. Volatilization of weakly adsorbed particles onto quartz filter paper (QFP) limits its performance. The adsorption of particulate matter (${{\rm PM}_{10}}$PM_10) onto QFP coated with different concentrations of graphene oxide (GO) was investigated to enhance the adsorption potential. Hummer’s method was adopted to synthesize GO. QFPs were coated with different concentrations of GO using a spin coating technique to optimize the result. The morphology and microstructure of GO-QFP were characterized by various experimental techniques, like XRD, FTIR, EDX, and SEM. GO showed considerable affinity to aerosol particles for GO-QFP weighing 5 mg/ml, whereas adsorption of the coated samples before and after was significantly reduced. The high affinity to aerosol particles was due to dominated $\pi {\text -} \pi $π-π interactions and the grooved regions formed on the GO layer. It was considered that the high surface to volume ratio of GO-QFP improves the adsorptive property of the QF and consequently enhances the performance of the filter paper.

In this paper, we show that laser speckle analysis (LSA) can provide valuable information about the structure of crumpled thin sheets. Crumpling and folding of slender objects are present in several phenomena and in various ranges of size, e.g., paper compaction, cortical folding in brains, DNA packing in viral capsids, and flower buds, to name a few. The analysis of laser speckles, both numerical and graphical, is a source of information about the activity of biological or non-biological materials, and the development of digital electronics, which brought the ease of image processing, has opened new perspectives for a spectrum of LSA applications. LSA is applied on randomly crumpled and one-, two-, and three-times folded papers, and appreciable differences in LSA parameters are observed. The methodology can be applied for easy-to-implement quantitative assessment of similar phenomena and samples.

Quantifying the coloration and luminous transmittance of eyewear filters is essential to understand their impact on human visual performance. Accurate measurements require the use of a spectrophotometer, with such equipment often being prohibitively expensive for wide-scale use. This paper details a new technique to characterize eyewear filters using only a digital camera and a sheet of white paper. Images of the paper are captured with and without the filter in front of the camera lens, and then subsequent analysis of pixel intensities allows the filter coloration and luminous transmittance to be determined. The technique has been applied to six different eyewear filters using three different camera and illumination configurations, demonstrating a reasonable match to the spectrophotometer data. This technique is suited to implementation in a smartphone app, in order to provide a low-cost and widely deployable solution to monitor the ageing of eyewear inventory.

An optical sensor for the dual sensing of oxygen (O_2) and carbon dioxide (CO_2) based on sensing films coated on filter paper is proposed. Ethyl cellulose (EC) doped with platinum(II) meso-tetrakis(pentafluorophenyl)porphyrin (PtTFPP) and 7-amino-4-trifluoromethyl coumarin serve as the oxygen sensing material and reference blue emission dye for the pH indicator, respectively. The CO_2 sensing layer includes the pH-sensitive fluorescent indicator 1-hydroxy-3,6,8-pyrenetrisulfonic acid trisodium salt immobilized within the EC. The O_2- and CO_2-sensitive materials can both be excited with a 405 nm LED, and the two emission wavelengths can be detected separately. The experimental result reveals that the optical O_2 and CO_2 sensors have sensitivities of I_N_2 /I_100%O_2 =22.8 and I_N_2 /I_100%CO_2 =3.6, respectively. The response times of the optical O_2 sensor were 15 s upon switching from nitrogen to O_2 and 41 s when moving from O_2 to nitrogen (N_2). The response times of the optical CO_2 sensor were 7 s upon switching from 100% N_2 to 100% CO_2 and 39 s when moving from 100% CO_2 to 100% N_2. The proposed optical dual sensor can be used for the simultaneous sensing of O_2 and CO_2 concentrations in environmental applications.

In situ ablation of thin metal films on fused silica substrates by picosecond class lasers was investigated as a method of characterizing the beam at the sample plane. The technique involved plotting the areas enclosed by constant fluence contours identified in optical microscope images of the ablation sites versus the logs of the pulse energies. Inconel films on commercially available neutral density filters as well as magnetron sputtered gold films were used. It was also shown that this technique could be used to calibrate real-time beam profile diagnostics against the beam at the sample plane. The contours were shown to correspond to the boundary where part or all of the film was ablated.

The objective of this work was to establish the relationship between the calculated subsurface scattered-photon distribution and the mathematical quantity known as point-spread function (PSF). Photon distribution of subsurface scattered light was calculated using the Monte Carlo method developed for describing reflectance and opacity of paper and of images printed on paper. The obtained normalized photon distribution made it possible to separate optical and mechanical components of dot gain for the paper-ink system. In the presented method of obtaining the reflectance profile of a screen element, the PSF convolves with a modelled reflectance profile of that element. It was found that the PSF can be better approximated by means of the Lorentzian function when compared to the Gaussian profile that was used in the past research on this topic.

We present a spectral model for predicting the fluorescent emission and the total reflectance of color halftones printed on optically brightened paper. By relying on extended Neugebauer models, the proposed model accounts for the attenuation by the ink halftones of both the incident exciting light in the UV wavelength range and the emerging fluorescent emission in the visible wavelength range. The total reflectance is predicted by adding the predicted fluorescent emission relative to the incident light and the pure reflectance predicted with an ink-spreading enhanced Yule-Nielsen modified Neugebauer reflectance prediction model. The predicted fluorescent emission spectrum as a function of the amounts of cyan, magenta, and yellow inks is very accurate. It can be useful to paper and ink manufacturers who would like to study in detail the contribution of the fluorescent brighteners and the attenuation of the fluorescent emission by ink halftones.

We have demonstrated an efficient speckle reduction method for laser illumination using a micro-vibrated paper screen along the projection direction. Using this method, a micro-vibrated amplitude of 0.532 μm, or a 2π phase change is sufficient to de-correlate the generated speckle pattern for a static SHG green laser beam image. When the measured speckle contrast is lowered to about 5.0%, and comparable with an LED source, a speckle-free image can be achieved. This technique will be suitable for compact and portable laser illumination equipment, image display, signal sensing devices, and so forth. Wearable and near-to-eye laser display applications are also included.

We present a noncontact method for quantitative composition and thickness monitoring of flat sheet products using terahertz time-domain spectroscopy. We apply the method to obtain simultaneous measurement of thickness and moisture content of paper sheets. The paper is modeled as an effective medium of water mixed with fibers, and model parameters are estimated from fits to the measured transmission amplitude. We demonstrate the method on two different paper samples and obtain uncertainties that are comparable with existing sensor technology. Monte Carlo simulations indicate that these uncertainties can be reduced further by at least an order of magnitude.

I propose a model for predicting the total reflectance of color halftones printed on paper incorporating fluorescent brighteners. The total reflectance is modeled as the additive superposition of the relative fluorescent emission and the pure reflectance of the color print. The fluorescent emission prediction model accounts for both the attenuation of light by the halftone within the excitation wavelength range and for the attenuation of the fluorescent emission by the same halftone within the emission wavelength range. The model's calibration relies on reflectance measurements of the optically brightened paper and of the solid colorant patches with two illuminants, one including and one excluding the UV components. The part of the model predicting the pure reflectance relies on an ink-spreading extended Clapper-Yule model. On uniformly distributed surface coverages of cyan, magenta, and yellow halftone patches, the proposed model predicts the relative fluorescent emission with a high accuracy (mean ΔE_94=0.42 under a D65 standard illuminant). For optically brightened paper exhibiting a moderate fluorescence, the total reflectance prediction improves the spectral reflectance prediction mainly for highlight color halftones, comprising a proportion of paper white above 12%. Applications include the creation of improved printer characterization tables for color management purposes and the prediction of color gamuts for new combinations of optically brightened papers and inks.

A critical factor that affects the appearance of printed paper surfaces is gloss uniformity, which is usually assessed visually. To relate gloss uniformity to nonvisual quantities, we first visually identified areas of either high or low gloss on the same sample for two different types of paper. We then measured the roughness and the reflectance of these areas. Microroughness was measured with an atomic-force microscope, and roughness was measured over a larger area with a confocal laser scanning microscope. The local reflectance of the high-gloss and the low-gloss areas was obtained from images taken with a gloss-imaging instrument and compared with the roughness of each area. This correlation is nonlinear, and roughness is insufficient to predict the local reflectance. Light-scattering measurements were made in the specular direction to map the gloss uniformity over larger areas than was possible with the gloss-imaging instrument. These maps were used to show the possibility of using both the spatial frequency and the fan filters, which together form a set of cortex filters, to analyze the variation of the gloss about the mean value and its spatial distribution on the surface in terms of spatial frequency and azimuthal orientation.

The measurement of the bonded area of pulp fibers has been an unsolved issue in paper science for more than 40 years. By the use of an established pulp fiber model, and a 4×4 transfer matrix formalism we simulated the optical behavior of pulp fibers in a modified imaging ellipsometer, and we demonstrate that there are rather strong symmetries in the ellipsometric angles Ψ and Δ when comparing single fibers, unbonded fiber crossings, and fiber–fiber bonds. Based on these symmetries we propose and test an algorithm that allows to distinguish the three cases (single fibers, unbonded fiber crossings, and fiber–fiber bonds) in the analysis of ellipsometric data.

Laser-induced breakdown spectroscopy has been performed to detect trace metal elements (Ca and Mg) in aqueous solution. In order to overcome the sensitivity drawbacks in liquid sample analysis, an absorbent paper was used as the sample support in this experiment. Calibration curves were constructed by using the standard solution with variable concentration and the limit of detection was estimated for each element. Finally this system was used to analyze three types of water samples collected from different locations in Nanjing, China and the results were compared with inductively coupled plasma atomic emission spectroscopy and showed good correlation.

It has only recently been realized that solar corona can be generated by dispersions of tree pollen grains suspended in the atmosphere, and these studies have come almost exclusively from Scandinavia. Using corona photographic and surface pollen analyses, it is shown here that paper birch trees in the interior of Alaska regularly generate solar corona during the boreal green-out in mid-May. Although near-spherical in shape, these ∼27?μm average diameter particles have three surface protrusions involved in germination that are indicated to aid in the generation of elliptical corona, for which a strong preferential particle orientation is needed. For observations at solar elevation angles of ∼35°–40°, an axis ratio of about 1.2 and average radius of 2.5° (for the second-order red band) are found. Because oriented particles of a particular shape tend to fall slower than randomly oriented ones, this microdesign promotes the lateral spread of pollen and enhances tree reproductive opportunities, an especially important trait for pioneering species.

The far-infrared properties of spruce wood are examined with a terahertz time-domain spectrometer. The solid wood is shown to exhibit both birefringence and diattenuation. The birefringence properties are sufficient for construction of a quarter-wave plate operating at 0.36 THz, and a half-wave plate operating at 0.71 THz. The origin of the birefringence is attributed to preferential fiber orientation within the wood. Similar birefringence is observed in lens paper in which the fibers are preferentially oriented in one direction.

The measurements of the absorption spectrum of pure water in a paper by Pope and Fry [Appl. Opt.36, 8710 (1997)] have been criticized by Quickenden et al. [Appl. Opt.39, 2740 (2000)]. These criticisms are answered.

A simplified stochastic model for the fiber structure of paper is introduced. The packing density and optical thickness of the fiber network are derived analytically, and their dependence on fiber characteristics can be seen. We undertake a Monte Carlo simulation of light scattering that is based on geometrical optics, using a realization of the model, which gives packing densities and optical thicknesses well in accordance with those given by the stochastic model and the scattering quantities as functions of three angles.

In recent years there has been a renewed interest in modeling the halftone microstructure to better control the colors produced in a halftone image. Diffusion of light within the paper has a significant effect on the halftone color; this effect is known as optical dot gain or the Yule–Neilsen effect. Because of diffusion, a photon may exit the paper from a different region of the halftone microstructure than that into which it entered the paper. To account rigorously for this effect requires knowledge of the paper’s point-spread function or, equivalently, the paper’s modulation transfer function (MTF). A new technique for measuring the MTF of paper—the series-expansion bar-target technique—is introduced. The method uses a bar target, but the analysis more closely resembles that of the edge-gradient technique. In the series-expansion method, bar-target image data are expanded into a Fourier series, and the paper’s MTF is given by the series-expansion coefficients. It differs from the typical bar-target analysis in that the typical method plots the amplitude of the fundamental frequency component for several targets of varying frequency, whereas the series-expansion method plots the amplitude of the fundamental and its harmonics for a single target. Two possible techniques for measuring the MTF with the bar-target series-expansion method are considered. In the first, the image of the bar target is projected onto the paper, and in the second, the bar target is placed directly on the paper, in close contact.

A method for time-resolved recording of light scattering in thin, highly scattering media is described. Subpicosecond pulses from a high-power Ti:sapphire laser are used, and single-shot recordings of the scattered light are made with a fast streak camera. The method is applied to the study of light scattering in paper, and a 1-ps resolution is demonstrated. The dependence of the light scattering on the basis of weight and density of the paper has been studied. A white-light continuum generated from the high-power pulses by the use of self phase modulation in water is used to study the wavelength dependence of the scattering process. A model for the propagation of light in paper has been developed and used in Monte Carlo simulations. The experimental results are used for testing this model, and absorption and scattering parameters are determined from that comparison.

The application of angle-resolved light scattering (ARLS) to the measurement of the surface roughness of glossy coatings on paper was investigated. To this end, ARLS patterns were measured for laser light scattered from several glossy paper samples, and these patterns were compared with those calculated using a theoretical model based on plane-wave scattering from an isotropic rough surface. Mechanical stylus profilometry data for the rms roughnesses and the autocorrelation functions of the coatings were used as input to calculate the patterns. For all the paper samples measured, as well as for all the incidence angles used, there was good agreement between the experimental and the calculated patterns when all the rms roughnesses measured by profilometry were reduced by 30%. The indication from these experiments is that ARLS may be used to determine the roughness parameters of the coatings. As a check on these results, measurements were also performed with a commercial optical surface probe; these data agreed well with both the ARLS and the stylus profilometry results.

Optical thin film methods were used to determine the refractive index and absorption coefficient of paper at 70.5,96.5, and 118.8 μm. Transmittances and reflectances were measured of paper, two quartz and two TPX plates, as well as of the paper pressed between the quartz and TPX plates. Measurements were performed at angles of incidence between 0 and 50°. The optical constants found provided a reasonable fit between the experimental points and the calculated performance. The small residual departures between the two sets of data are probably due to inhomogeneities within the paper.

Color characteristics of an RGBW (red, green, blue, white) electrophoretic display (EPD) prototype developed by Samsung Electronics are analyzed. EPD shows strong crosstalk between subpixels because of both the fringe field between subpixels and the scattering phenomena at the display surface. An RGB-to-RGBW color-decomposition algorithm optimized to EPD characteristics is developed that compensates for color deterioration due to the fringe field and scattering phenomena. For the four-color-decomposition algorithm, white is added to the primary colors to enhance the reflectance of the vivid colors while minimizing chroma loss. The psychophysical experimental result shows that images rendered with the algorithms developed in this study are preferred more than 90% of the time over those rendered with algorithms from previous studies. This research proves that, in spite of the limited physical property of EPD, the color quality can be improved dramatically through the use of well-designed color-rendering algorithms.

This article is a review of a reprint of the 1890 commemoration edition of The Scientific Papers of James Clerk Maxwell (Dover Publications, Inc., New York), $12.50. Vol. 1, 607 pp.; Vol. 2, 806 pp. One hundred and one of Maxwell’s shorter papers, articles, speeches, and reviews on electricity and magnetism, the dynamical theory of gases, theory of light, color vision and other optical problems, elastic solids, molecular physics, etc., are in this reprint; his longer treatises on electricity and magnetism, heat, and matter and motion are not included. This review is illustrated with photographs from the Maxwell family archives which have not been published before.

It is contended that the most comprehensive collection of data on the visible and near-ultraviolet spectrum of pure liquid water can still be found [Appl. Opt.38, 1216 (1999)]. The data produced [Appl. Opt.36, 8710 (1997)] from the integrated light loss in a light-scattering cavity containing water produces a useful set of data, although the error bars presented are underestimated. Furthermore, an oxidative purification step was not incorporated into the water purification procedure to remove residual traces of light-absorbing organic impurities.

Writing tests were made on commercial Electrofax paper using an all glass thin window cathode ray tube. Developed line widths from 5 mil to 15 mil could be obtained by varying beam current. Writing speeds of up to 790,000 in./sec were achieved. Electron beam current appeared to vary linearly with writing speed.

Applied Optics Editor Ron Driggers explains the new policy for considering expanded conference papers in OSA journals.

Editor-in-Chief Ron Driggers explains the standards that must be met by a simulation paper in order to be peer reviewed for publication in Applied Optics.

Editor-in-Chief Ron Driggers details the Applied Optics policy of not considering papers that have been rejected twice by OSA journals.

External obstacle detection is a significant task in transmission line inspection and is related to the safe operation of the power transmission grid. In recent years, unmanned aerial vehicles (UAVs) equipped with different devices have been widely used for transmission line inspection. However, because of the complex environment of transmission lines and weak power line textures in the obtained images, most existing methods and systems cannot meet the requirements for real-time and high-accuracy external obstacle detection of transmission lines. In this paper, a novel, to the best of our knowledge, UAV system integrated trinocular vision technology with remote sensing is developed to achieve better external obstacle detection of transmission lines in real time, which is composed of a DJ-Innovations (DJI) UAV equipped with a global positioning system (GPS), angle sensors, trinocular vision including three visible cameras with the same parameters, and a small processor with a pre-implanted software algorithm. In this paper, a new method for external obstacle detection of transmission lines is proposed to satisfy the requirements for real-time and high-accuracy practical inspection applications. First, the original trinocular images need to be rectified. Then, the rectified trinocular images are adopted to achieve three-dimensional reconstruction of power lines. Finally, based on trinocular vision, bag of feature, and GPS, the clearance distance measurement, obstacle classification, and obstacle location are realized. Experimental tests on 220 kV transmission lines reveal that our proposed system can be applied in practical inspection environments and has good performance.

High-power fiber lasers have been widely utilized in manufacturing, medical care, and many other fields. Due to mode instability, nonlinear effects, and so on, the output power of a monolithic fiber laser is limited. Coherent beam combining (CBC) of fiber lasers is a promising way to obtain higher output power. An all-fiber CBC structure with internal phase detection has a compact construction and potential for a larger fiber laser array. For the existing internal active phase control of an all-fiber structure, π phase ambiguity always occurs because of double passing the fiber path. Additional compensation is needed under this condition, and the compactness of the system will decrease. In this paper, internal phase control of an all-fiber structure based on double wavelength detection without π-ambiguity is proposed. By adding a beacon laser with a different wavelength, phase locking of a coherent fiber laser array can be achieved internally without π-ambiguity. A corresponding math model is established, and a phase matched condition is derived. The spectral width of the beacon laser is analyzed, and the result shows that it can reach tens of nanometers with a proper optical path difference. Simulations of seven, 19, and 37 beams are carried out, and the results show that the structure proposed in this paper has the ability to achieve phase control with good robustness. The control bandwidth in the simulation is better than 1 kHz. By properly designing elements, the structure is expected to achieve high-power CBC of an all-fiber structure experimentally.

In the procedure of surface defects detection of large-aperture aspheric optical elements, it is necessary to scan the surface of the element to achieve full coverage inspection. Since the curvature of the aspherical element is constantly changing from the center to the edge, it is of great difficulty to carry out efficient path planning. In addition, the machine vision system is a microscopic system with limited depth of field, and the sub-aperture imaging of aspherical elements has a visual depth along the object side. When the object depth is greater than the depth of field, out-of-focus blur will generate, so the object depth needs to be as small as possible. In response to these problems, this paper proposes a fast path planning algorithm based on the minimum object depth of a sub-aperture. To ensure minimum object depth, the machine vision system collects images along the normal direction of the sub-aperture plane. To address the problem of the surface curvatures of aspheric elements being different and the overlap coefficient difficult to determine, this paper proposes an image processing based overlap coefficient self-optimization algorithm. When scanning with full coverage of elements, there is only one connected domain in the horizontal projection image of all sub-apertures. According to this premise, the overlap coefficient is optimized through an image processing method to obtain a local optimal path planning strategy. According to the obtained path planning strategy, combining the component parameters and mechanical structure, the mapping matrix of the path planning algorithm transplanted to the detection system is calculated. Through computer programming, automatic sub-aperture acquisition is realized, and the self-edited sub-aperture stitching program is applied to reconstruct the collected sub-apertures. Our algorithm can complete path planning within 5 s, and the experimental results show that the maximum stitching misalignment error of the collected sub-apertures is no more than four pixels, and the average is one pixel. The reconstruction accuracy satisfies the needs of subsequent image processing and digital quantization.

As the level of detail in today’s images increases, so does the demand for resolution. Due to the necessity of mask stitching technology for full exposure of large array chips, we propose a mask configurable readout circuit architecture, which is suitable for large array structures. However, the stitchable readout circuit architecture has some non-ideal effects: row driver function failure and the column non-consistency problem. In our design, we solve the problem of column non-consistency after stitching. At the same time, we changed the signal transmission structure in order to avoid the row driver function failure caused by the mask stitching. In this paper, a prototype 2130×2130 CMOS image sensor is fabricated in 0.11 µm CMOS technology. The chip can capture images at 20 fps and reduce fixed pattern noise (FPN) from 3.5% to 1.5% through correction techniques. The architecture proposed in this paper is suitable for large array image sensors.

To detect the internal humidity of the transformer accurately and sensitively and eliminate the interference caused by electromagnetic fields, an interferometric measurement is proposed in this paper for the first time, to the best of our knowledge. The phase value distribution of the interferogram affected by humidity can be extracted quantitatively. The peak-to-peak value (P-P) of the phase image can reflect the humidity, according to theoretical analysis, and the main factors affecting the P-P are current and humidity. It has been tested by currents of 800 A, 1000 A, and 1200 A and different humidities. The paper reveals the relationship between humidity and P-P, proving that it is a reasonable application of real-time measurement of internal humidity in the transformer.

Editor-in-Chief Ron Driggers shares his experiences with handling rejection.

Editor-in-Chief Ron Driggers shares his experiences with handling rejection.

The spot mode has a great influence on the effect of laser paint removal, but it has not been clarified. This paper studies the influence of a Gaussian beam and a flat-top beam on surface quality. The results show that for the laser paint removal with the Gaussian beam, the surface hardness, corrosion resistance, and adhesion of marine steel improved significantly. The surface hardness increased by a factor of 20.8% compared to the original substrate, and the paint adhesion increased by 32.7%. For the flat-top beam, the surface quality of the substrate is unchanged, but the cleaning efficiency was observed to be 20% higher than that of the Gaussian beam. This paper can provide technical support for the control of surface cleaning quality in the laser paint removal process.

This feature issue includes two reviews and 34 research papers that highlight recent works in the field of computational optical sensing and imaging. Many of the works were presented at the 2021 Optica (formerly OSA) Topical Meeting on Computational Optical Sensing and Imaging, held virtually from 19 July to 23 July 2021. Papers in the feature issue cover a broad scope of computational imaging topics, such as microscopy, 3D imaging, phase retrieval, non-line-of-sight imaging, imaging through scattering media, ghost imaging, compressed sensing, and applications with new types of sensors. Deep learning approaches for computational imaging and sensing are also a focus of this feature issue.

The catadioptric panoramic imaging system may provide 360° panoramic imaging by employing the convex surface of a quadric surface with rotational symmetry as the reflector, which effectively compensates for the disadvantages of the narrow field of view in typical camera systems. First, this paper proposes a theodolite-based catadioptric camera image model based on the rotational symmetry of a catadioptric camera mirror, which simplifies the 2D modeling problem to a 1D problem. Simultaneously, the equivalence of the theodolite imaging model and the standard spherical imaging model also is demonstrated in this work. Second, this paper presents a method to calibrate the theodolite model parameters using only a single view and explains the calculation of model parameter initialization and iterative optimization steps in detail. Then, this paper demonstrates how to calibrate the theodolite model parameters using only a single view, as well as how to calculate the model parameter initialization and iterative optimization steps. Finally, simulation experiments and actual experiments have confirmed the correctness and stability of the method. The experimental results show that the average and standard deviation of the back-projection error are, respectively, 0.1983125 pixels and 0.0006153 pixels, in this model. We believe the theodolite model suggested in this paper offers a viable approach to catadioptric camera image modeling.

In this paper, theoretical closed expressions for the symbol error rate (SER) of two optical spatial modulation (OSM) schemes in a free-space optical (FSO) communication system—optical spatial pulse amplitude modulation (OSPAM) and optical spatial pulse position modulation (OSPPM)—over correlated gamma–gamma fading channels are derived. To study practical optical channels affected by atmospheric eddies, the effect of arbitrary atmospheric channel correlation is considered particularly, and it is shown that the correlation can degrade system performance. The parameters of atmospheric turbulence and the number of optical antennas are also under consideration in performance analysis of the FSO multiple input, multiple output system. All analytical results are validated with Monte Carlo simulations, which clearly illustrate the effect of spatial constellation and signal constellation on the average SER. In addition, it is shown by comparison that turbulence fluctuation and channel correlation have less influence on the OSPPM system than the OSPAM system. Overall, the framework proposed in this paper is proven to be effective and conducive to algorithm research on reducing the effect of correlated channels on OSM FSO systems.

In a previous paper [Appl. Opt. 61, C20 (2022)] it was proven that for a circular exit pupil and any optical path differences, the border of any spot diagram is integrated by the caustic surface and/or marginal rays. In this paper, the previous results are extended to annular (circular an elliptical) as well as hexagonal (single and segmented) exit pupils. Several examples of wavefronts affected by linear combinations of orthonormal Zernike aberrations are shown.

Convolutional neural networks have achieved remarkable results in the detection of X-ray luggage contraband. However, with an increase in contraband classes and substantial artificial transformation, the offline network training method has been unable to accurately detect the rapidly growing new classes of contraband. The current model cannot incrementally learn the newly appearing classes in real time without retraining the model. When the quantity of different types of contraband is not evenly distributed in the real-time detection process, the convolution neural network that is optimized by the gradient descent method will produce catastrophic forgetting, which means learning new knowledge and forgetting old knowledge, and the detection effect on the old classes will suddenly decline. To overcome this problem, this paper proposes an incremental learning method for online continuous learning of models and incrementally learns and detects new classes in the absence of old classes in the new classes. First, we perform parameter compression on the original network by distillation to ensure stable identification of the old classes. Second, the area proposal subnetwork and object detection subnetwork are incrementally learned to obtain the recognition ability of the new classes. In addition, this paper designs a new loss function, which causes the network to avoid catastrophic forgetting and stably detect the object of the new contraband classes. To reliably verify the model, this paper produces a multi-angle dataset for security perspective images. A total of 10 classes of contraband are tested, and the interference between two object detections is analyzed by model parameters. The experimental results show that the model can stably perform new contraband object learning even when there is an uneven distribution of data types.

Based on stable imaging principle of four groups and combined with optical design software, a method for initial structure calculation of zoom optical system and optical system optimization design is proposed to solve the problems of difficult initial structure calculation, large volume, and inflection point of zoom curves of a high zoom ratio optical system. First, this paper theoretically derives the four-group stable imaging principle of the zoom optical system. Then, using the derived equations and optical design software, the initial structural parameters of the four-group zoom optical system are solved to ensure that there is no inflection point in the zoom curves. Finally, a lightweight and small 40× continuous zoom optical system is designed to verify the feasibility and effectiveness of the design method proposed in this paper. The zoom optical system has the advantages of good imaging quality, lightweight, small size, smooth zoom, and no inflection point.

The high-resolution space camera needs to image the fringe target that meets the sampling frequency in the modulation transfer function test. However, blurry regions often appear in the target image, which seriously affects the visual effect and test efficiency. In this paper, by analyzing the imaging process of the rectangle striated target by a complementary metal-oxide-semiconductor transistor detector, a moiré fringe blurry degradation model of the target image is proposed. According to the degradation model, it is concluded that there are low blurry degradation regions in the image. Taking the low blurry degradation region as the reference information and the region similarity as the weight, the weighted spatial filtering and mean filtering are carried out to eliminate the blurry band in the target image. The experimental results show that compared with bilinear interpolation and guidance filter, this algorithm can effectively remove the image blur caused by the target periodic error and the angle error. The work in this paper provides a practical tool and theoretical support for subsequent space camera tests.