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The optical properties of water exacerbate the problems that arise in underwater imaging, including low contrast, color cast, noise, and haze. Pre-processing techniques are required in order to extract relevant information from these images. To solve the above problem, a new underwater image enhancement by using transmission optimization and background light estimation via Principal component analysis (TOBPCA) is proposed. Firstly, the background light of the degraded underwater image is computed using a dark channel prior; secondly, the degraded underwater RGB images are converted to CIELAB color space. Then, the transmission map of the lightness (L) component of CIELAB color space is represented and optimization of the transmission map is performed. Then, the Principal component analysis is used to fuse the background light and optimized transmission map. Finally, color correction is applied to enhance the underwater image. The proposed method is evaluated on the underwater color cast set (UCCS), underwater image quality set (UIQS), and underwater image enhancement benchmark (UIEB) Database based on Patch-based contrast quality index (PCQI), underwater color image quality evaluation (UCIQE), Underwater image quality measure (UIQM), Structural similarity index (SSIM), Peak signal to noise ratio (PSNR), and discrete entropy (DE) parameters. To verify the efficacy of the proposed method, a qualitative and quantitative comparison has been made on the UCCS, UIQS, and UIEB datasets. The result demonstrates that the proposed method outperforms the state-of-the-art method, in terms of PCQI, UCIQE, SSIM, and DE parameters. [ABSTRACT FROM AUTHOR]

A distance-independent background light estimation method is proposed for underwater overhead images. The method addresses the challenge of the absence of the farthest point in underwater overhead images by adopting a global perspective to select the optimal solution and estimate the background light by minimizing the loss function. Moreover, to enhance the information retention in the images, a translation function is employed to adjust the transmission map values within the range of [0.1, 0.95]. Additionally, the method capitalizes on the redundancy of image information and the similarity of adjacent frames, resulting in higher computational efficiency. The comparative experimental results show that the proposed method has better restoration performance on underwater images in various scenarios, especially in handling color bias and preserving information. [ABSTRACT FROM AUTHOR]

Data receiving frontends using avalanche photodiodes are used in optical free-space communications for their effective sensitivity, large detection area, and uncomplex operation. Precise control of the high voltage necessary to trigger the avalanche effect inside the photodiode depends on the semiconductor's excess noise factor, temperature, received signal power, background light, and also the subsequent thermal noise behavior of the transimpedance amplifier. Several prerequisites must be regarded and are explained in this document. We focus on the application of using avalanche photodiodes as data receivers for the on/off-keying of modulated bit streams with a 50% duty cycle. Also, experimental verification of the performance of the receiver with background light is demonstrated. [ABSTRACT FROM AUTHOR]

Underwater images have been highly required in many application scenarios, such as marine resources exploration, reservoir safety evaluation, and underwater aquaculture monitoring. But the captured images under water are usually degraded, due to the refraction, absorption, and scattering of light by the suspended particles in water. Some limitations of images can be low contrast, blurred details, and color distortion. Therefore, it is urgent for underwater image restoration. Much effort has been made on a variety of underwater image restoration using processing technology so far. However, land image restoration cannot be simply transformed the underwater images using background light estimation. The best restoration cannot be achieved in the current case, such as unnatural restoration, uneven brightness, and reddening color tone of the main body. In this study, an underwater image restoration was proposed using the adaptive fusion of double background lights and accurate estimation of the transmission map. First, two kinds of background lights of the underwater images were obtained using the maximum intensity prior background light estimation method based on underwater light attenuation and Quadtree background light estimation method based on background light flatness. Then, an adaptive fusion algorithm of double background lights was created to realize the accurate estimation of background lights in different scenes. Second, an accurate estimation of the transmission map was established to combine the new underwater dark channel prior, Reversed Saturation Map, and three-channel spectral attenuation coefficient. Furthermore, the restored underwater image was constructed using the obtained fused background light and accurate transmission map. The brightness of the image was adjusted to obtain the final underwater restored image. Finally, the experiment was carried out to verify the model using the aquaculture dataset of Guangdong Tilapia Breeding Farm and Underwater Image Enhancement Benchmark Dataset. The result showed that the improved estimation was better adapted to different underwater environments, thereby improving the contrast and color of the restored underwater image. A better restoration was achieved than the rest, such as dark channel prior, the maximum intensity prior, restoration using blur and light absorption, restoration using blue-green channel defogging, restoration using background light statistics model, and optimization of transmission map. Furthermore, the restoration was evaluated to deal with the current underwater image distortion, dim brightness, and reddish tone. The best value was also achieved among the seven objective evaluation indexes. The Peak Signal to Noise Ratio, Structural Similarity, Mean-Square Error, and Visual Information Fidelity increased by 0.52%, 2.1%, 3.4%, and 0.86%, respectively, in the evaluation indexes of reference image quality using the best underwater image restoration, compared with the second best. Correspondingly, the Natural Image Quality Evaluator, and Natural Image Quality Measure index were improved by 2.4% and 7.4%, respectively, in the evaluation index without reference image quality. Two datasets demonstrate that the proposed restoration can be expected to treat the unnatural restoration, uneven brightness, and reddish color in the traditional underwater image restoration. The finding can also provide a technical reference for underwater image restoration. [ABSTRACT FROM AUTHOR]

Using a single photon avalanche diode (SPAD) as a receiver in an optical wireless communications (OWC) system can effectively expand the transmission distance. However, the performance of the SPAD receiver is usually affected with the bit error rate (BER) lower bound determined by background light and the inter-symbol interference (ISI) distortion caused by dead time. In this paper, external time-gated technology is employed, and the SPAD is only activated within the set gate-ON time to alleviate the influence of background light and ISI distortion. The SPAD photon counting model and the communication BER model are established according to the external time-gated characteristics. Considering the functional relationship among the gate-ON time, signal light flux, background light flux and blocking time, we take the minimum BER as the optimization target, the optimal gate-ON time is derived. The numerical results show that whenever the signal light flux or the background light flux is higher, the BER performance of the time-gated mode is apparently better than the free-running mode. For example, when signal photons and background photons are 30 and 10 per symbol time, respectively, the BER of free-running SPAD converges to 0.1, while the BER of the time-gated scheme is about 10 − 6 . [ABSTRACT FROM AUTHOR]

A distance-independent background light estimation method is proposed for underwater overhead images. The method addresses the challenge of the absence of the farthest point in underwater overhead images by adopting a global perspective to select the optimal solution and estimate the background light by minimizing the loss function. Moreover, to enhance the information retention in the images, a translation function is employed to adjust the transmission map values within the range of [0.1, 0.95]. Additionally, the method capitalizes on the redundancy of image information and the similarity of adjacent frames, resulting in higher computational efficiency. The comparative experimental results show that the proposed method has better restoration performance on underwater images in various scenarios, especially in handling color bias and preserving information.

The special underwater imaging environment makes the image have problems such as blur and color deviation, which brings new challenges to underwater image restoration. Image restoration based on imaging model is one of the typical methods to improve the quality of underwater image . As an important parameter of reverse restoration, background light directly affects the effect of underwater image restoration. At present, there is little literature on the background light solution methods in underwater image restoration. In order to deeply understand the research status and development trend of underwater image restoration, this paper summarized the background light solution methods in underwater image restoration. Firstly, it briefly described the underwater model, then summarized the characteristics of background light and the classification of background light solution methods. With analyzing the principles and characteristics of various typical background light solution methods in detail, it summarized the advantages and disadvantages of various typical methods, and put forward the research prospect. [ABSTRACT FROM AUTHOR]

Quantum key distribution (QKD) enables private communication with information theoretic security. Free-space optical communication allows one to implement QKD without the limitations imposed by fiber networks such as the exponential scaling of transmission losses in optical fibers. Therefore, free-space QKD via satellite links is a promising technology to provide long-distance quantum communication connections. In free-space QKD systems, background light is the main source of noise, which has to be suppressed by means of spectral, spatial, and temporal filtering to reach a sufficiently low quantum bit error rate (QBER). Only then a quantum key can be exchanged successfully. To be able to define the requirements for a free-space QKD system, the background light must be examined more closely. Current considerations concentrate on cloud-free skies and rural environments. Free-space QKD will also take place when the sky is partly clouded and most likely also in urban environments. Here, an overview of physical causes of background light for downlink scenarios is given. Furthermore, the relation between QBER and background light is derived for a decoy-state BB84 protocol with polarization-encoded qubits to give an example of the dependency. Moreover, a setup to experimentally investigate the background light is shown. Measurement data were taken with this setup in Oberpfaffenhofen near Munich (Germany) in C-band. The measurement data are used to verify a background light simulation tool. The outcome underlines that simulation tools are sufficient for clear sky scenarios. [ABSTRACT FROM AUTHOR]

A non-contact method is proposed for measuring the surface temperature of an object using a standard radiator and standard object having optico-physical properties identical to those of the monitored object. Placed co-planarly with the monitored object in the field of view of an optoelectronic system (OES) is a standard object, the temperature of which is regulated and measured, and a standard radiator, whose normal spectral emissivity and temperature are known. In addition, the OES has its three fixed points, at which it registers the normal emissivity of the standard object, monitored object, and standard radiator (SR). The main distinction of the proposed method from the known ones is that the limits or tolerances for the nature of reflection and the quantitative parameters of reflection of the monitored object make no difference. The obtained analytical expression is an equation of the non-contact method of measuring the surface temperature of the monitored object, which can be applied for any spectral range of operation of the OES. A metrological analysis of the proposed method is made using a monochromatic optoelectronic system working at wavelengths of 0.65, 2.0, 5.0, 14.0, and 50.0 p,m for the temperature of the monitored object, T, equals to 400, 700, and 1000 K. Based on the analysis of the results the requirement for the implementation of the proposed method of measuring the surface temperature has been formulated, which says that the choice of an optoelectronic system for measuring the surface temperature of objects should be preceded by a methodological and metrological analysis of the optico-physical properties of the monitored object, the surrounding background, and the OES itself.

• A new method for isolating speckle pattern of each object with an image difference process is proposed. • A new image denoising method for effectively eliminating the interference of the background noise is proposed. • The image denoising method is robust to the interference of difference background noise condition. Speckle correlation imaging is a typical method to image through strongly scattering medium. However, limited by the optical memory effect range, this method is hard to image multiple objects that are randomly distributed on the object plane with strong background illumination. To solve this problem, we proposed a new method that allows to image multiple objects with strong background illumination. It consists of isolating speckle pattern of each object with an image difference process and eliminating the interference of the background noise with the principal components analysis (PCA) technique. Our method can realize imaging of more than two objects through the scattering medium, and compared with other methods, our method can provide better observations of the objects in the bright field scenarios. [ABSTRACT FROM AUTHOR]

Due to the influence of light absorption and scattering in the water medium, underwater images often appear with different degrees of blur, color distortion, and so on. The physical model-based method needs to establish image degradation processes and improve image quality by assuming and estimating complex parameters, which limits the generalization ability and practical application effect of these methods. To solve the above problems, this paper proposes an image enhancement method of non-uniform scattering in turbid underwater based on neural network. First, the high-frequency and low-frequency components of underwater images were obtained through the difference of gaussian (DOG) and bilateral filtering (BF), respectively. Then, for the high-frequency component, pulse coupled neural networks (PCNN) to improve the edge details of the image. For low-frequency components, it is carried out in two steps: an adaptive automatic white balance and polynomial regression (AAWBPR) algorithm is proposed, which can effectively eliminate the color bias under different color temperatures and improve the brightness of images; the adaptive color and contrast enhancement (ACCE) method is used to further enhance the color and contrast of images. Finally, Decom-Net and the improved U-Net method are used to reduce the errors caused by the non-uniform scattering effect, weaken the noise interference, and enhance the overall effect of images. Extensive experiments show that the proposed method can effectively improve the quality of turbid underwater non-uniform scattering images, enhance the contrast of images, reduce the complexity of parameters, and has good generalization performance and practical application effects. • A suppression algorithm of heterogeneous scattering in turbid water is proposed. • An adaptive automatic white balance and polynomial regression algorithm is proposed. • It reduces the image deviation, and improve the problem of complex model parameters. • It is applied to image segmentation, saliency detection, and keypoint detection. • This method can get better training results in small dataset. [ABSTRACT FROM AUTHOR]

A well-defined scale calibration in flow cytometry can improve many aspects of data acquisition such as cytometer setup, instrument comparison and sample comparison. The theory for scale calibration was proposed by Steen over two decades ago, but it has never been put into regular use due to the lack of a widely available precision light source. The introduction of such a light source, the quantiFlashTM , gave this possibility. Here, we describe how this light source can be used to characterize a cytometer's PMT performance. We, therefore, characterized the instrument's response over the entire PMT voltage range. As a consequence, we propose a practical method to characterize a cytometer's signal-to-noise ratio (SNR) and dynamic range (DNR). This allows the selection of a voltage/gain corresponding to a PMT's maximum efficiency and hence the lowest electronic noise, which can help with experiment design. We further introduced a decibel (dB) scale for the presentation of SNR and DNR values. SNR and DNR are stand-alone values that allow the direct comparison of different instruments. Finally, with this method, it becomes clear that increased SNR comes at the expense of DNR and thus the limiting factor of modern cytometers is the DNR. © 2017 International Society for Advancement of Cytometry [ABSTRACT FROM AUTHOR]

The Universe is largely transparent to $$\gamma $$ -rays in the GeV energy range, making these high-energy photons valuable for exploring energetic processes in the cosmos. After 7 years of operation, the Fermi Gamma-ray Space Telescope has produced a wealth of information about the high-energy sky. This review focuses on extragalactic $$\gamma $$ -ray sources: what has been learned about the sources themselves and about how they can be used as cosmological probes. Active galactic nuclei (blazars, radio galaxies, Seyfert galaxies) and star-forming galaxies populate the extragalactic high-energy sky. Fermi observations have demonstrated that these powerful non-thermal sources display substantial diversity in energy spectra and temporal behavior. Coupled with contemporaneous multifrequency observations, the Fermi results are enabling detailed, time-dependent modeling of the energetic particle acceleration and interaction processes that produce the $$\gamma $$ -rays, as well as providing indirect measurements of the extragalactic background light and intergalactic magnetic fields. Population studies of the $$\gamma $$ -ray source classes compared to the extragalactic $$\gamma $$ -ray background place constraints on some models of dark matter. Ongoing searches for the nature of the large number of $$\gamma $$ -ray sources without obvious counterparts at other wavelengths remain an important challenge. [ABSTRACT FROM AUTHOR]

The authors propose and demonstrate the fluorescence imaging of green fluorescence protein (GFP) expressed in a brain slice with a self-reset complementary metal–oxide–semiconductor image sensor under background light. By using a self-reset function to avoid pixel saturation, the weak fluorescence of GFP was successfully observed, even under background light. The result suggests that the sensor can be applied to in vivo imaging of laboratory animals during light–dark cycles, so that they can observe the different responses to bright and dark environments.

We find that gray solitons, dark solitons and bright solitons can stably propagate in organic polymer based on a single nonlinearity named photoisomerization. In order to obtain bright solitons in this negative nonlinear material, the wavelength of signal light should locate near the absorption valley of the organic material and meanwhile the wavelength of controlling light, a homogeneously distributed background light, should locate in the right side of absorption spectrum of the material. In addition, the background should be intense enough. All bright solitons can stably propagate in this material; nevertheless some of the gray solitons associated with photoisomerization are unstable. [ABSTRACT FROM AUTHOR]

The Universe is largely transparent to $$\gamma $$ -rays in the GeV energy range, making these high-energy photons valuable for exploring energetic processes in the cosmos. After 7 years of operation, the Fermi Gamma-ray Space Telescope has produced a wealth of information about the high-energy sky. This review focuses on extragalactic $$\gamma $$ -ray sources: what has been learned about the sources themselves and about how they can be used as cosmological probes. Active galactic nuclei (blazars, radio galaxies, Seyfert galaxies) and star-forming galaxies populate the extragalactic high-energy sky. Fermi observations have demonstrated that these powerful non-thermal sources display substantial diversity in energy spectra and temporal behavior. Coupled with contemporaneous multifrequency observations, the Fermi results are enabling detailed, time-dependent modeling of the energetic particle acceleration and interaction processes that produce the $$\gamma $$ -rays, as well as providing indirect measurements of the extragalactic background light and intergalactic magnetic fields. Population studies of the $$\gamma $$ -ray source classes compared to the extragalactic $$\gamma $$ -ray background place constraints on some models of dark matter. Ongoing searches for the nature of the large number of $$\gamma $$ -ray sources without obvious counterparts at other wavelengths remain an important challenge. [ABSTRACT FROM AUTHOR]

Wireless infrared communications can be defined as the propagation of light waves in free space using infrared radiation whose range is 400–700 nm. This range corresponds to frequencies of hundreds of terahertz, which is high for higher data rate applications. Wireless infrared is applied for higher data rates applications such as wireless computing, wireless video and wireless multimedia communication applications. Introduced by Gfeller, this field has grown with different link configurations, improved transmitter efficiency, increased receiver responsivity and various multiple access techniques for improved quality. Errors are caused because of background light, which causes degradation overall system performance. Error correction techniques are used to remove the errors caused during transmission. This study provides a brief account on field theory used for error correction in wireless infrared systems. The results are produced in terms of bit error rate and signal-to-noise ratio for various bit lengths to show the ability of encoding and decoding algorithms.