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Folded optical path has been widely applied in Virtual Reality (Pancake VR) display in recent years. Compared with the traditional straight‐through and Fresnel optical path, the Pancake has an obvious advantage in a smaller size and clearer virtual image. However, the Pancake inevitably increases stray light because of polarized film. AS we all known, the processingof polarized film is always imperfect. Stray light greatly reduces the contrast ratio even if taking high‐contrasting Mini‐LED backlight LCD as panel, and seriously affects the user experience. In this paper, we firstly analyze the causes of stray light in the Pancake, and then propose an effective optical designing method. Besides, we optimize of the Pancake fabrication process for higher contrast, by analyzing the influence of reflectance of lens and tube. [ABSTRACT FROM AUTHOR]

We research measurement conditions that allow omitting stray light blocking masks in camera‐based halo measurements of FALD LCDs. The stray light induced error is reduced by using high magnifications and stray light optimized hardware and test patterns. Our findings simplify halo measurements and enable simultaneous peak luminance and uniformity measurements. [ABSTRACT FROM AUTHOR]

Nuclear fusion is a potential source of energy that could supply the growing needs of the world population for millions of years. Several experimental thermonuclear fusion devices try to understand and control the nuclear fusion process. A very interesting diagnostic called Thomson scattering (TS) is performed in the Spanish fusion device TJ-II. This diagnostic takes images to measure the temperature and density profiles of the plasma, which is heated to very high temperatures to produce fusion plasma. Each image captures spectra of laser light scattered by the plasma under different conditions. Unfortunately, some images are corrupted by noise called stray light that affects the measurement of the profiles. In this work, we propose the use of deep learning models to reduce the stray light that appears in the diagnostic. The proposed approach utilizes a Pix2Pix neural network, which is an image-to-image translation based on a generative adversarial network (GAN). This network learns to translateimages affected by stray light to images without stray light. This allows for the effective removal of the noise that affects the measurements of the TS diagnostic, avoiding the need for manual image processing adjustments. The proposed method shows a better performance, reducing the noise up to 98 % inimages, which surpassesprevious works that obtained 85 % for the validation dataset. [ABSTRACT FROM AUTHOR]

Folded optical path designs have been widely applied in Virtual Reality (Pancake VR) displays in recent years. Compared with the traditional straight‐through and Fresnel optical path design, the Pancake design has obvious advantages in smaller size and clearer virtual image. However, the Pancake optical path uses polarization devices to fold the optical path and, light undergoes multiple refractions and reflections between lens groups, thus inevitably introducing stray light. This stray light greatly reduces the contrast ratio even if taking high‐contrasting Mini‐LED backlight LCD as panel, and seriously affects the user experience. In this paper, we first analyze the causes of stray light in the Pancake design and then propose an effective optical design method. In addition, an effective process scheme for improving image contrast is proposed by analyzing the impact of lens and tube reflectivity. Further, a prototype of the Pancake VR optical module is successfully implemented with low stray light and high contrast ratio. [ABSTRACT FROM AUTHOR]

In the VR optical system, the pancake optics faces the problem of ghost, which is more serious than the conventional aspherical/Fresnel optics. The causes of pancake ghosts are different, thus the performance of pancake ghosts in the image is also different. Some of them can be clearly imaged, but some exist in the form of light spots. The presence of these ghosts seriously affects the image contrast. In this paper, ghost measurement and simulation methods are proposed, which obtains the Virtual image distance, position and brightness of the ghost image, and the cause of the ghost image is obtained by combining the simulation and measurement. Finally, ghost intensity was reduced from 4.68% to 2.39% using the ghost image mitigation methods. A pancake optical system was optimized and prototype was demonstrated, with a field of view of 108°, an exit pupil diameter of 10mm, and a ghost image intensity of less than 2.39%. [ABSTRACT FROM AUTHOR]

To evaluate the ability of space optical systems to suppress off-axis stray light, this paper proposes a stray light testing method for large-field of view, multispectral spatial optical systems based on point source transmittance (PST). And a stray light testing platform was developed using a high-brightness simulated light source, large-aperture off-axis reflective collimator, high-precision positioning mechanism and a double column tank to evaluate the stray light PST index of spatial optical system. On the basis of theoretical analyses, a set of calibration lenses and stray light elimination structures such as hoods, baffle and stop are designed for the accuracy calibration of stray light testing systems. The theoretical PST values of the calibration lens at different off-axis angles are analyzed by Trace Pro software simulation and compared with the measured values to calibrate the accuracy of the system. The testing results show that the PST measurement range of the system reaches $10^{-3}\sim 10^{-10}$ when the off-axis angles of the calibration lens are in the range of $\pm 5^{\circ }\sim \pm 60^{\circ }$ . The stray light test system has the advantages of wide working band, high automation and large dynamic range, and its test results can be used in the correction of lens hood and other applications.

Large depth of field (DOF) and stereo photography are challenging yet rewarding areas of research in close-up photography. In this study, a compound-eye imaging system based on a discrete microlens array (MLA) was implemented for close-range thin imaging. A compact imaging system with a total length of 3.5 mm and a DOF of 7 mm was realized using two planar aspherical MLAs in a hexagonal arrangement. A new three-layer structure and discrete arrangement of sublenses were proposed to suppress stray light and enable the spatial refocusing method, which restores image information at different object depths. The system is successfully fabricated, and the system performance is carefully investigated. Our system offers a large depth of field, high resolution, and portability, making it ideal for close-up photography applications requiring a short conjugate distance and small device volume, while also addressing the issue of crosstalk between adjacent channels. [ABSTRACT FROM AUTHOR]

Stray light (SL) control is an important aspect in the development of optical instruments. Iterations are necessary between design and analysis phases, where ray tracing simulations are performed for performance prediction. This process involves trial and error, requiring to be able to perform rapid evaluation of SL properties. The limitation is that accurate SL simulations require sending many rays, which can be time consuming. In this paper, we use deep learning to improve the accuracy of SL maps even when obtained with very few rays. Two different deep learning methods are used, an autoencoder and an EAM. The training process is performed by generating a large database of artificial SL maps, with different noise levels reproduced with a Poisson distribution. Once the training completed, we show that the autoencoder performs the best and improves significantly the accuracy of SL maps. Even with extremely small number of rays, it recovers complex SL patterns which are not visible on raw ray traced maps. This method thus enables more efficient iterations between design and analysis. It is also useful for developing SL correction algorithms, as it requires tracing SL maps under large number of illumination conditions in a reasonable amount of time.

Scattered light from the objective lens, directly exposed to the intense sunlight, is a dominant source of stray light in internally occulted coronagraphs. The variable stray light, such as the scatter from dust on the objective lens, can produce varying scattering backgrounds in coronal images, significantly impacting image quality and data analysis. Using data acquired by the Lijiang 10-cm Coronagraph, the quantitative relationship between the distribution of dust on the objective lens and the resulting scattering background is analyzed. Two empirical models for the scattering background are derived, and used to correct the raw coronal data. The second model, which depends on three parameters and performs better, shows that the scattering-background distribution varies with angle, weakens with increasing height, and enhances with increasing dust level on the objective lens. Moreover, we find that the dust on the center of the objective lens can contribute more significantly to the scattering background than on the edge. This study not only quantitatively confirms the significant impact of the stray light produced by dust on the objective lens of the coronagraph, but also corrects the coronal data with this stray light for the first time. Correcting for dust-scattered light is crucial for the high-precision calibration of ground-based coronagraph data, enabling a more accurate analysis of coronal structures. Furthermore, our model is envisioned to support the provision of reliable observational data for future routine coronal magnetic-field measurements using ground-based coronagraphs. [ABSTRACT FROM AUTHOR]

To treat certain vitreoretinal diseases, the vitreous body, a hydrogel composed of mostly collagen and hyaluronic acid, must be removed. After vitrectomy surgery, the vitreous cavity is filled with an endotamponade. Previously, pre-clinical hydrogel-based vitreous body substitutes either made from uncrosslinked monomers (1st generation), preformed crosslinked polymers (2nd generation), or in situ gelating polymers (3rd generation) have been developed. Forward light scattering is a measure of Stray light induced by optical media, when increased, causing visual disturbance and glare. During pinhole surgery, the hydrogels are injected into the vitreous cavity through a small 23G-cannula. The aim of this study was to assess if and to what extent forward light scattering is induced by vitreous body replacement hydrogels and if Stray light differs between different generations of vitreous body hydrogel replacements due to the different gelation mechanisms and fragmentation during injection. A modified C-Quant setup was used to objectively determine forward light scattering. In this study, we found that the 1st and 3rd generation vitreous body replacements show very low stray light levels even after injection (2.8 +/− 0.4 deg2/sr and 0.2 +/− 0.2 deg2/sr, respectively) as gel fragmentation and generation of interfaces is circumvented. The 2nd generation preformed hydrogels showed a permanent increase in stray light after injection that will most likely lead to symptoms such as glare when used in patients (11.9 +/− 0.9 deg2/sr). Stray light of the 2nd generation hydrogels was 3- and 2-fold increased compared to juvenile and aged vitreous bodies, respectively. In conclusion, this significant downside in the forward light scattering of the 2nd generation hydrogels should be kept in mind when developing vitreous body replacement strategies, as any source of stray light should be minimized in patients with retinal comorbidities. [ABSTRACT FROM AUTHOR]

Nuclear fusion is a potential source of energy that could supply the growing needs of the world population for millions of years. Several experimental thermonuclear fusion devices try to understand and control the nuclear fusion process. A very interesting diagnostic called Thomson scattering (TS) is performed in the Spanish fusion device TJ-II. This diagnostic takes images to measure the temperature and density profiles of the plasma, which is heated to very high temperatures to produce fusion plasma. Each image captures spectra of laser light scattered by the plasma under different conditions. Unfortunately, some images are corrupted by noise called stray light that affects the measurement of the profiles. In this work, we propose the use of deep learning models to reduce the stray light that appears in the diagnostic. The proposed approach utilizes a Pix2Pix neural network, which is an image-to-image translation based on a generative adversarial network (GAN). This network learns to translateimages affected by stray light to images without stray light. This allows for the effective removal of the noise that affects the measurements of the TS diagnostic, avoiding the need for manual image processing adjustments. The proposed method shows a better performance, reducing the noise up to 98% inimages, which surpassesprevious works that obtained 85% for the validation dataset.

This paper addresses the issue of reduced image quality due to annular turning marks formed by single-point diamond turning (SPDT) during the processing of metal-based mirrors and infrared lenses. An ideal single-point diamond turning marks diffraction action model to quantitatively analyze the impact of turning marks diffraction on imaging quality degradation is proposed. Based on this model, a fast estimation algorithm for the optical modulation transfer function of the system under turning marks diffraction (TMTF) is proposed. The results show that the TMTF algorithm achieves high computational accuracy, with a relative error of only 3% in diffraction efficiency, while being hundreds of times faster than rigorous coupled wave analysis (RCWA). This method is significant for reducing manufacturing costs and improving production efficiency, as it avoids the problem of being unable to compute large-size optical systems due to computational resource and time constraints. [ABSTRACT FROM AUTHOR]

The interference caused by stray light leads to the invalid attitude of star sensors in orbit, thus affecting the attitude control of satellites. In order to overcome this problem, this paper proposes a fast star-detection algorithm with strong stray-light suppression ability. The first step in the proposed method is stray-light suppression. The highlighted pixels are unified and then erosion and dilation operations based on a large template are performed. Using the background image only, which is filled with stray light, the cleaner star image is obtained by subtracting the background from the unified image. The second step in the proposed method is binarization. The binary star image is obtained by using a line-segment strategy combined with a local threshold. The third step in the proposed method is star labeling. It comprises connected-domain labeling based on the preordering of pixels and the calculation of centroid coordinates of stars in each connected domain. The experimental results show that the proposed algorithm extracts the stars stably under the interference of different stray lights. The proposed method consumes less resources, and the output delay is only 18.256 us. Moreover, the successful identification rate is 98% and the attitude accuracy of the X and Y axes is better than 5″(3σ) when the star sensor works at the speed of zero. [ABSTRACT FROM AUTHOR]

Wide field and long exposure time can effectively improve the ability of space surveillance telescope to detect weak space targets. However, this is easily affected by stray light, resulting in the effective target submerged in the background of stray light. Based on the basic theory of Mathematical morphology, this paper proposes an accurate and highly robust method (ETH) for stray light suppression of space surveillance telescope, which is called the enhanced Top-Hat transform. Firstly, we define the Generalized Top-Hat transform according to the traditional Top-Hat transform. Secondly, we improve the background estimation of the traditional opening, introduce the closing to realize the multi-scale micro adjustment, and analyze the influence of the multi-scale micro adjustment on the stray light suppression and space weak targets segmentation. Finally, we introduce the noise suppression factor to reduce the residual noise in the stray light. In the field experiment, the method can effectively eliminate the interference of stray light, and greatly improves the local signal-to-noise ratio of space targets. It also shows that in a very low signal-to-noise ratio, this method can accurately and effectively segment space weak targets, and has strong robustness.

Large depth of field (DOF) and stereo photography are challenging yet rewarding areas of research in close-up photography. In this study, a compound-eye imaging system based on a discrete microlens array (MLA) was implemented for close-range thin imaging. A compact imaging system with a total length of 3.5 mm and a DOF of 7 mm was realized using two planar aspherical MLAs in a hexagonal arrangement. A new three-layer structure and discrete arrangement of sublenses were proposed to suppress stray light and enable the spatial refocusing method, which restores image information at different object depths. The system is successfully fabricated, and the system performance is carefully investigated. Our system offers a large depth of field, high resolution, and portability, making it ideal for close-up photography applications requiring a short conjugate distance and small device volume, while also addressing the issue of crosstalk between adjacent channels.

Contrast ratio is the ratio of the brightest part to the darkest part of the image, which can reflect the picture hierarchy. In VR head‐mounted display (HMD), many scenes need to realistically simulate rooms or night environments. High contrast ratio helps to better display details in dark scenes, giving people the ultimate sense of immersion. There are many ways to measure contrast ratio, but still no effective standard to evaluate this indicator. In particular, many VR HMD adopt the emerging solution of folded optical path pancake. Due to the refraction and reflection of light on different interfaces, the pancake module will cause "ghosting", then affect the visual experience. Therefore, it becomes our focus to evaluate the contrast ratio of pancake VR HMD. We present a way to evaluate the contrast ratio of VR HMD by a near‐eye display measurement system, and compares the measurement results of different test methods. It can systematically and objectively evaluate the contrast of VR head‐mounted display devices and provide a useful reference for VR device evaluation. [ABSTRACT FROM AUTHOR]

Space environment surveillance is very important for space security, which is easy to be disturbed by stray light and hot pixels, and the image background presents a certain degree of nonuniformity. The existing methods can not achieve the accurate segmentation of weak targets while correcting the nonuniform background. To solve this problem, this paper presents an accurate and robust correction method for the wide-field surveillance camera, called the enhanced new top-hat transform (ENTHT). Firstly, we analyze the formation mechanism and influence of the nonuniformity background from multiple dimensions. Secondly, because of the dependence and limitations of the background suppression effect of the new top-hat transform on the selection of structural elements, we improve the new top-hat transform by designing a noise structure element (NSE). Finally, we analyze the performance and advantages of the ENTHT method. In the field experiment, the method can accurately correct the complex space nonuniform background, eliminate the stray light and hot pixels, and realize the accurate segmentation of weak targets. In the complex space environment, it brings great help to space-moving target recognition and tracking. [ABSTRACT FROM AUTHOR]

The microlens array (MLA) system can aid in realizing fast beam deflection owing to the lateral displacement between arrays. The MLA system has the advantages of miniaturization and good functionality. However, during system operation, crosstalk beams are generated between each microlens array unit, introducing additional stray light, thus affecting the imaging contrast of the system. Therefore, this study uses the matrix operation method to trace the paraxial ray to trace the optical system and analyzes the generation mechanism of crosstalk stray light in the MLA system. Furthermore, this study proposes a crosstalk suppression method based on a stop array to reasonably suppress stray light. Finally, an example of an infrared array scanning infrared optical system is considered so as to verify the correctness and feasibility of the proposed crosstalk stray light suppression method. Therefore, this paper introduces the stray light suppression principle to guide the optical design process of the system, providing a theoretical basis for the design and analysis of the microlens array scanning and search system. [ABSTRACT FROM AUTHOR]

The lateral displacement between the microlens arrays can realize the rapid deflection of a beam; however, in this process, crosstalk beams will inevitably occur between the microlens units, which will affect the imaging quality of the system. For this reason, we propose a crosstalk elimination design method based on the combination of multilayer stop arrays, establish a simulation model, use a geometric optical object image relationship to trace the characteristic light of a single group of microlenses, analyze the relationship between the structural parameters of the microlens array and the stop array size, and comprehensively discuss the light transmission of the microlens array under various conditions. An example of an infrared system is simulated using the optical design analysis software Fred, verifying the effectiveness of this design method in suppressing the crosstalk beams generated in the dynamic transmission process, ensuring independent propagation between microlens units, and realizing crosstalk-free imaging. [ABSTRACT FROM AUTHOR]

Star trackers are currently the most accurate sensors for determining the attitude of a spacecraft. These sensors comprise not only highly capable optical detectors and processor units but also complicated software solvers. One of the main solvers employed in star trackers is image segmentation. In this study, the aim is to develop a hybrid image segmentation method which is a combination of both global thresholding and density-based spatial clustering of applications with noise (DBSCAN) method to increase detection probability of the stars in heavy noise. Secondly, a sorting algorithm is added to list the detected stars in terms of their brightness to increase the efficiency of the star tracking algorithm. Then, this new approach and two different conventional segmentation methods are applied to the Orion star constellation image polluted with Gaussian, salt and pepper, and uneven background noises. The resulting images of these segmentation methods are compared in terms of denoising capabilities. Although computationally more expensive, the proposed DBSCAN-based hybrid method displays a background pixel recovery performance of 99.5%, compared to Otsu global thresholding and adaptive thresholding methods' 73.5% and 79.9% recovery values, respectively. Additionally, it has been demonstrated that the sorting algorithm successfully listed the detected stars in accordance with their brightness. [ABSTRACT FROM AUTHOR]

To treat certain vitreoretinal diseases, the vitreous body, a hydrogel composed of mostly collagen and hyaluronic acid, must be removed. After vitrectomy surgery, the vitreous cavity is filled with an endotamponade. Previously, pre-clinical hydrogel-based vitreous body substitutes either made from uncrosslinked monomers (1st generation), preformed crosslinked polymers (2nd generation), or in situ gelating polymers (3rd generation) have been developed. Forward light scattering is a measure of Stray light induced by optical media, when increased, causing visual disturbance and glare. During pinhole surgery, the hydrogels are injected into the vitreous cavity through a small 23G-cannula. The aim of this study was to assess if and to what extent forward light scattering is induced by vitreous body replacement hydrogels and if Stray light differs between different generations of vitreous body hydrogel replacements due to the different gelation mechanisms and fragmentation during injection. A modified C-Quant setup was used to objectively determine forward light scattering. In this study, we found that the 1st and 3rd generation vitreous body replacements show very low stray light levels even after injection (2.8 +/− 0.4 deg2/sr and 0.2 +/− 0.2 deg2/sr, respectively) as gel fragmentation and generation of interfaces is circumvented. The 2nd generation preformed hydrogels showed a permanent increase in stray light after injection that will most likely lead to symptoms such as glare when used in patients (11.9 +/− 0.9 deg2/sr). Stray light of the 2nd generation hydrogels was 3- and 2-fold increased compared to juvenile and aged vitreous bodies, respectively. In conclusion, this significant downside in the forward light scattering of the 2nd generation hydrogels should be kept in mind when developing vitreous body replacement strategies, as any source of stray light should be minimized in patients with retinal comorbidities.

Recently, we developed a simple theoretical model for the estimation of the irradiance distribution at the focal plane of commercial off-the-shelf (COTS) camera lenses in case of laser illumination. The purpose of such a model is to predict the incapacitation of imaging sensors when irradiated by laser light. The model is based on closed-form equations that comprise mainly standard parameters of the laser dazzle scenario and those of the main devices involved (laser source, camera lens and imaging sensor). However, the model also includes three non-standard parameters, which describe the scattering of light within the camera lens. In previous work, we have performed measurements to derive these typically unknown scatter parameters for a collection of camera lenses of the Double-Gauss type. In this publication, we compare calculations based on our theoretical model and the measured scatter parameters with the outcome of stray light simulations performed with the optical design software FRED in order to validate the reliability of our theoretical model and of the derived scatter parameters. [ABSTRACT FROM AUTHOR]

Large screen LED displays are becoming increasingly popular. The human eye has a resolution of 1/60 degree, so at very close viewing the LED chips may be more easily seen because LED screens have a larger pixel pitch than traditional display technologies, unlike OLEDs and LCDs. In large LED displays, the perceived luminance of the pixels is highly dependent on the viewing distance and FOV. The paper examines the perceived pixel luminance of a large screen LED display at different viewing distances and field of views. Additionally, stray light was examined and discussed. The results indicated that the perceived pixel luminance can be significantly different under different viewing distances and FOVs. For future LED display performance evaluation, it would be advantageous to have a measuring device with high resolution that mimics the human eye's angular resolution in perceived pixel luminance. [ABSTRACT FROM AUTHOR]

Most background suppression algorithms are weakly robust due to the complexity and fluctuation of the star image's background. In this paper, a background suppression algorithm for stray lights in star images is proposed, which is named BSC-Net (Background Suppression Convolutional Network) and consist of two parts: "Background Suppression Part" and "Foreground Retention Part". The former part achieves background suppression by extracting features from various receptive fields, while the latter part achieves foreground retention by merging multi-scale features. Through this two-part design, BSC-Net can compensate for blurring and distortion of the foreground caused by background suppression, which is not achievable in other methods. At the same time, a blended loss function of smooth_L1&Structure Similarity Index Measure (SSIM) is introduced to hasten the network convergence and avoid image distortion. Based on the BSC-Net and the loss function, a dataset consisting of real images will be used for training and testing. Finally, experiments show that BSC-Net achieves the best results and the largest Signal-to-Noise Ratio (SNR) improvement in different backgrounds, which is fast, practical and efficient, and can tackle the shortcomings of existing methods. [ABSTRACT FROM AUTHOR]

Grating groove functions scalar theory can describe the light source through the grating interference and diffraction effect of the physical phenomenon of splitting light. It can be used to represent the distribution of the complex amplitude of the illumination light source on the diffraction screen under the action of the diffraction grating and display the information on the diffracted light position and intensity distribution. As a result, the effect of groove error on the intensity of grating ghost lines and stray light is analyzed. This paper presents an energy solution and analysis of the grating periodic groove error utilizing the grating scalar diffraction theory. Based on this, the effect of the core components of the grating ruling machines, such as screw and worm gear, on the stray light and ghost line of the grating is analyzed. Finally, the random groove error is analyzed. The analysis results show that the worm gear error is an important reason for the appearance of ghost lines in the machine ruling grating, and the lead screw error and random grooving error are the main reasons for the stray light produced by the grating. We optimize the worm gear, lead screw, and error compensation system of the grating ruling machine to effectively improve the ruling quality of the grating. [ABSTRACT FROM AUTHOR]

• A LC system was constructed with a miniaturized photometric detector and a microfluidic chip. • The Z-shaped cell design increases the effective optical path length. • The Z-shaped cell and column are on the same chip without a pipeline connection. • The microchip can be filled with cation exchange resin and C18 particles. In recent years, miniaturized analytical instruments have been developing to meet the needs of portable and rapid analysis. The key of miniaturized analytical equipment is the miniaturization and integration of functional modules. This paper aims to develop a miniaturized photometric detector and separation microfluidic chip for a liquid chromatography (LC) system. The detector uses a light-emitting diode to emit ultraviolet light, which is collimated by an internal double lens. A Z-shaped flow cell with a long optical path is designed and fabricated in the separation microfluidic chip with a three-layer structure, which provides a tubing-free connection between the separation and detection unit. Detector performance is evaluated using hemoglobin (Hb) samples, with an upper limit of detection linearity (95 %) of 0.345 AU and stray light level as low as 0.08 %. Additionally, the microchip channel can be filled with cation exchange resin and C18 particles. Finally, an ion LC system and a reversed-phase LC system were constructed based on the miniaturized photometric detector and two microchips with different packed columns, respectively, and were successfully used in the separation and detection of two metabolic markers (glycated hemoglobin or bilirubin). The results of this study are expected to facilitate the development of a portable LC system and their application in community health services and family health management of chronic diseases. [ABSTRACT FROM AUTHOR]

• An analysis was conducted on the backward reflection of the telescope at the Tianqin Station. • A solution has been proposed for the secondary mirror backward reflection stray light of the Cassegrain telescope. • The effectiveness of the method was verified through ray tracing simulation and experiments with single photon detectors and cameras respectively. In the coaxial satellite laser ranging system or the other telescope-based measurement systems, the emitted beam produces backward reflection at the center of the secondary mirror, which affects the detector in the system. To reduce the impact of straylight, a new device for reduce the backward reflection of the secondary mirror has been proposed. The specific parameters of the device have been determined through numerical calculation and computer simulation. The device has been implemented in a 1.2 m laser ranging system and has been experimentally proven to reduce the laser straylight by 32.4 % with the single photon detector and there is an increase in the camera's contrast for target imaging, making it more effective to measuring the space target. [ABSTRACT FROM AUTHOR]

The interference caused by stray light leads to the invalid attitude of star sensors in orbit, thus affecting the attitude control of satellites. In order to overcome this problem, this paper proposes a fast star-detection algorithm with strong stray-light suppression ability. The first step in the proposed method is stray-light suppression. The highlighted pixels are unified and then erosion and dilation operations based on a large template are performed. Using the background image only, which is filled with stray light, the cleaner star image is obtained by subtracting the background from the unified image. The second step in the proposed method is binarization. The binary star image is obtained by using a line-segment strategy combined with a local threshold. The third step in the proposed method is star labeling. It comprises connected-domain labeling based on the preordering of pixels and the calculation of centroid coordinates of stars in each connected domain. The experimental results show that the proposed algorithm extracts the stars stably under the interference of different stray lights. The proposed method consumes less resources, and the output delay is only 18.256 us. Moreover, the successful identification rate is 98% and the attitude accuracy of the X and Y axes is better than 5″(3σ) when the star sensor works at the speed of zero.

This paper addresses the issue of reduced image quality due to annular turning marks formed by single-point diamond turning (SPDT) during the processing of metal-based mirrors and infrared lenses. An ideal single-point diamond turning marks diffraction action model to quantitatively analyze the impact of turning marks diffraction on imaging quality degradation is proposed. Based on this model, a fast estimation algorithm for the optical modulation transfer function of the system under turning marks diffraction (TMTF) is proposed. The results show that the TMTF algorithm achieves high computational accuracy, with a relative error of only 3% in diffraction efficiency, while being hundreds of times faster than rigorous coupled wave analysis (RCWA). This method is significant for reducing manufacturing costs and improving production efficiency, as it avoids the problem of being unable to compute large-size optical systems due to computational resource and time constraints.

Space-based wide-field surveillance systems are of great significance in maintaining the security of space resources by avoiding collisions between space targets. However, their performance is hindered by stray light phenomena. The nonuniform background noise caused by stray light significantly hampers subsequent target detection, leading to a high frequency of false alarms. To solve this problem, we propose a robust and accurate nonuniform background elimination method based on image block self-adaptive gray-scale morphology (IBSGM). First, we define two kinds of structural operators with different sizes and domains, which make full use of the difference between the target pixels and surrounding background pixels. Then, we block the original surveillance image and find the size of the largest target in each block by the minimum bounding rectangle method to determine the optimal size of the structural operator suitable for each block. Finally, we perform morphological processing using the defined structural operators to eliminate nonuniform backgrounds from images. Experimental results on simulated and real image datasets demonstrate that the proposed IBSGM method has higher precision in eliminating the nonuniform background when compared to other methods. [ABSTRACT FROM AUTHOR]

The Thermal Infrared Sensor 2 (TIRS-2) payload for the Landsat 9 mission closely follows the design of the TIRS instrument currently flying aboard Landsat 8. The TIRS-2 instrument, however, incorporates an important design change to mitigate the stray light issue that plagued the TIRS instrument. Shortly after the launch of Landsat 8 in 2013, calibration errors due to stray light artifacts were observed in Earth imagery from TIRS with magnitudes of 4% (10.8 $\mu \text{m}$ band) and 8% (12.0 $\mu \text{m}$ band). Out-of-field scans of the Moon were conducted to map the angles from which off-axis radiance was detected on the focal plane arrays. Optical modeling, constrained by reverse ray traces of the lunar data, identified the primary scattering sources within the TIRS telescope, and these results informed the locations and design of mitigating baffles for TIRS-2. The effect of the modifications to the TIRS-2 instrument was tested preflight through thermal vacuum (TVAC) characterization tests, and the optical models were updated to be consistent with the measured data. Preliminary assessments indicated at least an order of magnitude reduction of the total signal due to scattering in TIRS-2. On-orbit lunar scans provided the final confirmations and demonstrated that the new design changes to TIRS-2 have reduced the primary out-of-field scattering by over 40x from the original TIRS design bringing the total scattering to 1% or less. More importantly, Earth imagery produced by Landsat 9 TIRS-2 does not show any stray-light-related artifacts, as was prevalent in the Landsat 8 TIRS imagery. [ABSTRACT FROM AUTHOR]

Although it is beneficial to use an optical simulator to design a mid-air imaging system, the use of a simulator requires optical knowledge, and it cannot be handled by non-specialists. To create a design assistance system that can be used by non-specialists, we demonstrate three methods: a method for extracting mid-air images and images of stray light from computer graphics rendered images, a method for calculating the visible range of mid-air images, and an evaluation of design parameters. First, a mid-air image and an image of stray light are extracted by considering the differences of images rendered using different numbers of bounces at each camera position, after which the visible range of the mid-air image is calculated. In addition, other parameters, such as the distance between a micro-mirror array plate (MMAP) and the extracted mid-air image, are adjusted in detail and evaluated by considering the visible range of the obtained mid-air image. Moreover, to demonstrate the effectiveness of the proposed method, the design of the previous research was improved to eliminate images of stray light, and we reduced the size of the existing system. Unlike other conventional approaches, using the visible range of the extracted mid-air images and without the need for a visual check, our proposed method enables the extraction of mid-air images and unwanted light, and the evaluation of optical systems. This technique can be applied to improve the design of optical systems and in the examination of the related applications. [ABSTRACT FROM AUTHOR]

The wide-field surveillance camera plays a critical role in space debris detection and the visible light situational awareness required for early warning. However, stray light in such a system has always been a serious problem. Current methods cannot effectively eliminate the interference of stray light, which directly lead to the inability to accurately segment the target and background, which greatly reduce the accuracy of target recognition. To solve this problem, we proposed an accurate stray light elimination method based on recursion multi-scale gray-scale morphology (RMGM). First, we defined two structural operators with different domains. These two structural operators can make full use of the difference information between the target region and the surrounding background region, which is the basic premise to ensure high-precision correction. Then we used the two structural operators with different domains to perform morphological processing on the surveillance image to eliminate stray light. Finally, in order to ensure that targets with different sizes in the surveillance image are not lost, we adopt a recursion multi-scale method. We increase the size of structural operator and perform the morphological operation again on the estimated and eliminated stray light non-uniform background in order to retrieve the lost larger size target. In addition, we add an automatic decision mechanism on the recursion multi-scale method by using corresponding threshold judgment. Further experimental results on real captured image datasets show that compared with other methods, the proposed RMGM method can simultaneously have high-precision stray light elimination effect, high-precision target retention rate, and faster computation time.

• Four types of beam splitters in human eye wavefront sensing optical setup are compared from aspects of spot-field image quality and energy transfer efficiency. • Stray light is traced to the source and effectively eliminated according to the causes. • Polarizing beam splitters shows stray light suppression and increase the energy transfer efficiency to 64.9%, higher than 43.2% from the non-polarizing beam splitter plate. • Polarizing beam splitter plates are the optimal choice to merge the laser beam into the main path for human eye wavefront sensing. Beam splitters significantly affect the measurement accuracy of human eye wavefront sensing. To compare the beam splitters in human eye wavefront sensing optical path from aspects of spot-field image quality and energy transfer efficiency, the effect of polarizing beam splitter cubes, polarizing beam splitter plates, non-polarizing beam splitter cube and non-polarizing beam splitter plates on the sensing performance of the measurement path are studied. The stray light affecting spot-field image quality is traced to the source and effectively eliminated according to the causes respectively. The transfer efficiency of the measurement path built with polarizing optics is also studied. The application of polarizing beam splitters shows stray light suppression and increases the energy transfer efficiency to 64.9%, higher than 43.2% from the non-polarizing beam splitter plate. The experimental results show that a polarizing beam splitter plate is the optimal choice to merge the laser beam into the main path for human eye wavefront sensing. [ABSTRACT FROM AUTHOR]

Space environment surveillance is very important for space security, which is easy to be disturbed by stray light and hot pixels, and the image background presents a certain degree of nonuniformity. The existing methods can not achieve the accurate segmentation of weak targets while correcting the nonuniform background. To solve this problem, this paper presents an accurate and robust correction method for the wide-field surveillance camera, called the enhanced new top-hat transform (ENTHT). Firstly, we analyze the formation mechanism and influence of the nonuniformity background from multiple dimensions. Secondly, because of the dependence and limitations of the background suppression effect of the new top-hat transform on the selection of structural elements, we improve the new top-hat transform by designing a noise structure element (NSE). Finally, we analyze the performance and advantages of the ENTHT method. In the field experiment, the method can accurately correct the complex space nonuniform background, eliminate the stray light and hot pixels, and realize the accurate segmentation of weak targets. In the complex space environment, it brings great help to space-moving target recognition and tracking.

The microlens array (MLA) system can aid in realizing fast beam deflection owing to the lateral displacement between arrays. The MLA system has the advantages of miniaturization and good functionality. However, during system operation, crosstalk beams are generated between each microlens array unit, introducing additional stray light, thus affecting the imaging contrast of the system. Therefore, this study uses the matrix operation method to trace the paraxial ray to trace the optical system and analyzes the generation mechanism of crosstalk stray light in the MLA system. Furthermore, this study proposes a crosstalk suppression method based on a stop array to reasonably suppress stray light. Finally, an example of an infrared array scanning infrared optical system is considered so as to verify the correctness and feasibility of the proposed crosstalk stray light suppression method. Therefore, this paper introduces the stray light suppression principle to guide the optical design process of the system, providing a theoretical basis for the design and analysis of the microlens array scanning and search system.

The lateral displacement between the microlens arrays can realize the rapid deflection of a beam; however, in this process, crosstalk beams will inevitably occur between the microlens units, which will affect the imaging quality of the system. For this reason, we propose a crosstalk elimination design method based on the combination of multilayer stop arrays, establish a simulation model, use a geometric optical object image relationship to trace the characteristic light of a single group of microlenses, analyze the relationship between the structural parameters of the microlens array and the stop array size, and comprehensively discuss the light transmission of the microlens array under various conditions. An example of an infrared system is simulated using the optical design analysis software Fred, verifying the effectiveness of this design method in suppressing the crosstalk beams generated in the dynamic transmission process, ensuring independent propagation between microlens units, and realizing crosstalk-free imaging.

Stray light is a critical aspect for high performance optical instruments. When stray light control by design is insufficient to reach the performance requirement, correction by post-processing must be considered. This situation is encountered, for example, in the case of the Earth observation instrument 3MI, whose stray light properties are complex due to the presence of many ghosts distributed on the detector array. We implement an iterative correction method and discuss its convergence properties. Spatial and field binning can be employed to reduce the computation time but at the cost of a decreased performance. Interpolation of the stray light properties is required to achieve high performance correction. For that, two methods are proposed and tested. The first interpolate the stray light in the field domain while the second applies a scaling operation based on a local symmetry assumption. Ultimately, the scaling method is selected and a stray light reduction by a factor of 58 is obtained at 2σ (129 at 1σ) for an extended scene illumination. [ABSTRACT FROM AUTHOR]

• Ray tracing analysis was conducted to reduce stray light of OROCHI onboard the MMX. • OROCHI's optical system is simulated and its CCD reflectance was measured. • Our techniques are incorporated into the preliminary design of OROCHI. The Martian Moons eXploration (MMX) spacecraft is equipped with two cameras, i.e., the TElescopic Nadir imager for GeOmOrphology (TENGOO) and the Optical Radiometer composed Of CHromatic Imagers (OROCHI), for the scientific observation of the Martian moon Phobos. OROCHI is a wide-angle multiband camera system comprising seven cameras with different bandpass filters and one monochromatic camera in the visible and near-infrared range. Thus, OROCHI can simultaneously obtain multiband images. Previous space observations have revealed that the reflection spectra of Phobos exhibit regional differences, including a red unit and a blue unit with weak redness. Moreover, Phobos shows an absorption band near 650 nm, attributed to the red unit of Phobos. Global observations using OROCHI require the determination of such regional variations with high precision; therefore, we must reduce noise in the optical system design. In this study, to meet this requirement, we focus on stray light reduction. Stray light is a source of noise generated by reflections at surfaces of optical components (e.g., sensors, filters, lenses, and lens barrels). We design simulation models of the OROCHI optical system, measure the reflectance of its charge-coupled device (CCD) image sensor, and conduct stray light analysis via ray tracing. We estimate the ratio of the stray light intensity to the target signal intensity by deriving both the reflected (constituting stray light) and unreflected rays. We show that placing the filter away from the sensor and reducing the reflectances of the filter and lenses effectively reduce stray light. Such approaches are incorporated into the preliminary design of OROCHI. [ABSTRACT FROM AUTHOR]

Geometrical waveguide has great advantages over conventional imaging systems, like larger field of view, larger exit pupil area, lower thickness and lighter weight. Two-dimensional (2D) geometrical waveguide can expand the vertical and horizontal pupil simultaneously, thus becoming a mainstream solution for commercial applications. The transmission path of two-dimensional optical system is more complicated and therefore the stray light in two-dimensional geometrical waveguide is more complex compared with one-dimensional waveguide. In this paper, the causes of stray light in 2D geometrical waveguide are discussed based on the non-sequential ray-tracing with detailed numerical analysis and the targeted solutions are developed to effectively improve the imaging quality. Consequently, a stray-light-free design of geometrical waveguide is proposed having the field of view reaching 49° × 56° with 22.6 mm × 26 mm eye box at 20 mm eye relief.

Recently, we developed a simple theoretical model for the estimation of the irradiance distribution at the focal plane of commercial off-the-shelf (COTS) camera lenses in case of laser illumination. The purpose of such a model is to predict the incapacitation of imaging sensors when irradiated by laser light. The model is based on closed-form equations that comprise mainly standard parameters of the laser dazzle scenario and those of the main devices involved (laser source, camera lens and imaging sensor). However, the model also includes three non-standard parameters, which describe the scattering of light within the camera lens. In previous work, we have performed measurements to derive these typically unknown scatter parameters for a collection of camera lenses of the Double-Gauss type. In this publication, we compare calculations based on our theoretical model and the measured scatter parameters with the outcome of stray light simulations performed with the optical design software FRED in order to validate the reliability of our theoretical model and of the derived scatter parameters.

Grating groove functions scalar theory can describe the light source through the grating interference and diffraction effect of the physical phenomenon of splitting light. It can be used to represent the distribution of the complex amplitude of the illumination light source on the diffraction screen under the action of the diffraction grating and display the information on the diffracted light position and intensity distribution. As a result, the effect of groove error on the intensity of grating ghost lines and stray light is analyzed. This paper presents an energy solution and analysis of the grating periodic groove error utilizing the grating scalar diffraction theory. Based on this, the effect of the core components of the grating ruling machines, such as screw and worm gear, on the stray light and ghost line of the grating is analyzed. Finally, the random groove error is analyzed. The analysis results show that the worm gear error is an important reason for the appearance of ghost lines in the machine ruling grating, and the lead screw error and random grooving error are the main reasons for the stray light produced by the grating. We optimize the worm gear, lead screw, and error compensation system of the grating ruling machine to effectively improve the ruling quality of the grating.

The Ozone Mapping and Profiler Suite (OMPS) Nadir Profilers (NPs) are advanced backscatter ultraviolet (BUV) hyperspectral instruments that measure ozone profiles in the Earth atmosphere. The first NP sensor onboard the Suomi National Polar-orbiting Partnership (Suomi-NPP) satellite began its science observations on January 26, 2012, after its aperture door opened. The second OMPS NP, flying on the NOAA-20 satellite, opened its aperture door on January 8, 2018, starting science its data collection. The two NP sensors acquire Earth spectral images along their satellite flight path with a 16.7° wide swath, enabling weekly coverage of vertical ozone distribution in the Earth atmosphere. A successful thorough sensor calibration enables the NP sensors’ data records (SDRs) to meet measurement accuracy requirements. The largest error term in the albedo calibration came from the spectral wavelength calibration. This article provides SDRs accuracy analysis for both NP sensors and discusses important aspects of the SDRs performance in relation to the sensors’ characterization and calibration. [ABSTRACT FROM AUTHOR]