In response to the existing convolutional neural networks to blur the algorithm, there is a problem of unclear image texture details, and this article proposes an image deblurring algorithm based on multi-local residual connection attention network. First, a convolutional layer is used to extract shallow features. Secondly, a new multi-local residual connection attentional module based on residual connection and parallel attentional mechanism is designed to eliminate image blur and extract context information. Thirdly, a pairwise connection module based on dilated convolution is used to restore details. Finally, a convolutional layer is used to reconstruct a clear image. The experimental results show that: The Peak Signal to Noise Ratio (PSNR) and Structure Similarity (SSIM) of GoPro data set are 31.83 dB and 0.9275, respectively. Both qualitative and quantitative results show that the proposed method can effectively restore texture details of blurred images, and the network performance is better than that of the comparison method.
Microchip solid state laser is an important light source for nanometer measuring instruments because of its small size and long life. In this paper, In this paper, the plano-plano half external cavity Nd: YAG and Nd: YVO4 microchip lasers are constructed. The length of the laser resonator is changed by controlling the expansion and contraction of the piezoelectric ceramic. At the same time, the longitudinal mode and wavelength are observed using F-P scanning interferometer and wavelength meter. The cavity tuning characteristics of the two microchip solid state lasers are studied, including the relationship between the cavity length and the optical power, Optical power and the variation rules of single and double longitudinal modes in the process of laser longitudinal mode sweeping through the laser outgoing linewidth. The experimental data show that single and double longitudinal modes change alternately during the cavity tuning process, and the cavity length and pump current jointly affect the laser output mode and optical power.
In order to break through the limitation of application scene caused by the limitation of single wavelength of conventional laser projector and realize the layer projection of different parts, different materials and different assembly process, a multi-color layer laser automatic focus scanning projection system is designed and established. Two wavelengths of laser are adopted as the light source, according to the different position of dichromatic mirror, two laser scanning projection schemes of multi-color co-optical axis and spectral path are proposed; and the corresponding mathematical models of optical systems are deduced. By adjusting the distance between mirror groups through the self-focusing function of the systems, the minimum light spots of different colors can be focused on the projectors at different distances. Two kinds of projection optical systems are simulated by ZEMAX optical design software; then the reliability and projection effect of the two systems are compared and analyzed. Finally, the multi-color co-optical axis projection experimental system is built. The experimental results show that on the projection plane at 3m, the spot diameter of various wavelengths of the co-optical axis system is within 0.8mm and the spot size is uniform, which can achieve the scanning and automatic focus projection function of multi-color layers.
Terahertz time-of-flight (THz-TOF) method is a new development direction in the field of thickness measurement, which has the advantages of fast, non-destructive and high precision. However, water vapor in atmospheric environment has strong absorption of terahertz wave, which limits the application of THz-TOF method in industrial field environment. In this paper, the propagation mechanism of terahertz wave in THz-TOF method in atmospheric environment is firstly analyzed, and then a method of water vapor elimination based on absorption model is proposed. Finally, two kinds of plastic plate samples are used as the object of application research. By comparing with the measurement results in dry environment, the results show that the method can effectively eliminate the interference of water vapor and realize the accurate measurement of material thickness and refractive index in atmospheric environment.
Optical fiber refractive index sensor is widely used in various complex environment monitoring. A high sensitivity refractive index sensor based on the structure of few mode fiber (FMF)–coreless fiber (CLF)–FMF is designed, and the experimental verification is carried out. The sensor consists of a section of CLF fused between two small section of FMFs to form a Mach–Zehnder interference（MZI）, for measuring external refractive index. Fiber Bragg grating (FBG) is used for temperature compensation. The interference spectrum generated by MZI structure is affected by both refractive index and temperature, while FBG is only affected by temperature. Using the refractive index and temperature sensitivity coefficient of MZI and FBG, the sensitivity matrix is constructed to realize the simultaneous measurement of refractive index and temperature. Experimental results show that the MZI refractive index sensitivity is 345.66 nm/RIU and temperature sensitivity is 0.0134 nm/°C. Meanwhile, the FBG temperature sensitivity is 0.0104 nm/°C.
For requirement of detecting, ranging and warning systems of sniper in snipping battlefield, research for optical properties of gun flame and smoke was developed. According document literature for gun flame and smoke and parameters of optical measuring instrument, a particular measuring scheme was designed to test optical properties of gun flame and smoke outfield. The test for optical of gun flame and smoke with 5.8 mm and 7.62 mm caliber ballistic rifle was carried out and relative spectral and radioactive test data was calibrated. The results of test suggest that radiation of gun flame and smoke distributes mainly at 2 μm~5 μm, the whole process lasts approximately 6 ms, the strongest radiation is at 2 ms during the whole process, and the radiant intensity of 5.8 mm gun is stronger than that of 7.62 mm at both mid-wave and long-wave infrared. All these efforts we have made will provide important gist to detect gun flame and smoke and to improve propellant for gun weapons.
The traditional interferometry method of large optical element relies on artificially changing the beam expanding lens and optical path structure according to different test samples. This method inevitably introduces some systematic errors. Therefore, a set of corresponding dual-line control scheme is proposed according to the functional requirements of the dual-optical path interferometer and the simulation experiment. Through the cooperation of Bluetooth communication, serial communication and mechanical structure, the optical path can be converted and calibrated for many times, so that the changing position of the optical element after each switch of the measurement diameter is fixed, and the real-time status is displayed in the interactive interface based on MFC(Microsoft Foundation Classes). The results show that the measurement repeatability of PV10 and RMS can reach 0.004 λ and 0.000 4 λ under 450 mm measurement diameter, and the measurement repeatability of PV10 and RMS can reach 0.000 8 λ and 0.000 16 λ under 100 mm measurement diameter. Finally, the experimental results show that the system ensures superior measurement efficiency and repeatability, which provides reference value for the research and development of dual-optical path interferometers.
In view of the phenomenon that the scale factor of the total reflection prism laser gyro changes periodically with temperature, this paper studies the influence of temperature change on the ring laser area under the condition of frequency stabilization according to the matrix optical method, obtains the relationship between the scale factor of the total reflection prism laser gyro and the frequency stabilization voltage, and points out that the beam offset is the cause of the periodic change of the scale factor with temperature. According to the relationship between the scale factor and the frequency stabilization voltage, the scale factor compensation model of the total reflection prism laser gyro is established, and the nonlinearity of the scale factor before and after compensation is compared through experiments. The results show that the nonlinearity of the scale factor is increased by more than one order of magnitude by using the compensation model proposed in this paper. The research of this paper has certain reference value for improving the performance of total reflection prism laser gyro.
The evolution between different types of pulses is the embodiment of the passive lock fiber laser rich dynamics. A hybrid mode-locked fiber laser with multiple pulse switching is reported. When the pump power is 400 mW, the switch of pulses between harmonic mode-locking, soliton molecule and soliton cluster are realized. Then, when the pump power is increased to 600 mW, the noise-like pulse output is realized, and the corresponding output power and pulse energy are 15.2 mW and 0.86 nJ, respectively. By adjusting the polarization controller, the noise-like pulse wavelength is tunable from 1895 nm to 1930 nm. The designed laser has the characteristics of mode-locked pulse switching, wavelength tunable and self-starting.
To verify the influence of optical axis pointing error on optical pose measurement accuracy and provide a theoretical basis for subsequent real-time equipment to realize pose measurement, through splitting the algorithm steps and tracing the influence of the optical axis pointing error based on the central axis method, a conclusion is drawn that the pointing error can affect the pose processing results, which is divided into two parts. The direct influence of the model intersection algorithm and the indirect influence of the dynamic benchmark outside the model are deduced and analyzed, through the combination of simulation calculation and measured data, the conclusion that pose angle error is no more than 0.1° in typical medium and long-term optical pose measurement in the situation 200" pointing error of is obtained.through the combination of simulation calculation and measured data, the conclusion that pose angle error is no more than 0.1° in typical medium and long-term optical pose measurement in the situation 200" pointing error of is obtained. It lays a theoretical foundation for the capacity expansion of subsequent range equipment.
Aiming at the influence of airborne panoramic video stream, a panoramic real-time stitching method based on embedded is proposed. Extract surf feature points in the image and generate feature descriptors. The matching degree is determined by calculating the Euclidean distance between two feature points. After affine transformation, Poisson transform is used to realize the fusion and smoothing between images. The above processes are executed concurrently on the target equipment, and customized optimization is carried out according to the characteristics of each process to realize panoramic real-time splicing. After test, this method realizes the seamless effect at the splicing joint, and the splicing speed reaches 30 Hz, which can meet the requirements of real-time display.
In the field of solar power generation such as solar water heaters and solar cells, climate factors such as rain, snow and cloudy days will seriously affect the power generation effect, and the work of solar servo system must also consume energy. Therefore, it is extremely important to quickly judge the current weather conditions and design an adaptive on-off servo system. When the weather is rainy or snowy, the system should be shut down to save energy. In view of the problems of low efficiency, poor accuracy and large amount of calculation of traditional weather recognition methods, this paper creates a weather classification set with multiple categories on the basis of public weather images, and provides a weather image recognition technology based on convolutional neural network and feature fusion. By using the traditional way to obtain the color, texture and shape of the image as the bottom feature of the whole model, it is improved on the basis of the original VGG16(Visual Geometry Group-16) model, so as to extract the deep feature of the image. Finally, the bottom feature and deep feature are fused and output on Softmax, and the total recognition rate is 94%.
Tooth surface objects like gray method is laser phase shifting interferometry foreground area is extracted tooth surface interference image, one of the important methods for the method on the basis of different artificial setting threshold and ignore the measuring efficiency and accuracy of image edge character leading to constrained problem, this paper proposes a gear interference image based on adaptive threshold method to extract foreground area.Firstly, the morphological characteristics of gear tooth surface and the difference of each edge vertex are analyzed, and the image is divided into regions.Then, according to the changing rule of edge gray value, the mask result is obtained by filting qualified pixels through neighborhood window, so as to realize foreground region extraction.Finally, the segmentation results of four groups of algorithms and traditional methods were compared according to five kinds of image evaluation indexes. The results show that the matching accuracy of the algorithm with the reference result is improved by about 3.5%～4.5%; PRI is up about 3%～4%; VOI is up about 15%～25%; GCE reduce 2.5%～3.5%; Final phase information accuracy increased by
Ships have an extremely important position in the military field. Their wakes will cause relatively obvious changes in the temperature and height of the sea surface, and they have the characteristics of long duration and difficult to eliminate. Therefore, we can more intuitively identify the ship target through simulating ship wake and the infrared radiation image of the sea surface. There is a strong military demand for intuitive identification of ship targets. Based on the data of atmospheric transmittance to 8 μm～14 μm band in different background environments simulated by MODTRAN software, we established a ship wake infrared radiation model by combining Cox-Munk slope probability distribution model and considering wave occlusion factors. Infrared wake images of different background environments and different detection distances were obtained. The simulation results show that under the same detection condition, although the radiation brightness of the ship wake decreases with the increase of the detection distance, the ship wake on the sea surface is more easily identified because the shielding effect of the rough sea surface on the infrared radiation is considerably diminished. The atmospheric transport model has a tremendous influence on the infrared imaging results. In summer, the background radiation energy is substantial and the average occlusion effect of sea surface is modest, so the infrared imaging of ship wake is clearer.
In order to achieve the aiming, identification and tracking of long-distance targets, photoelectric tracking systems often require their load systems to be continuous zoom, and have high requirements of large zoom ratio, wide band and consistency of optical axis. In this paper, the analysis of structural form and initial structural parameter of calculation of the continuous zoom system are introduced in detail, and on the basis of clarifying the relevant technical parameters, zoom ratio of 40, wide-band visible continuous zoom optical system is designed. The continuous zoom optical system is a designed to use spherical mirrors, which are easy to process, and the MTF value of each field of view is above 0.2 at 150 lp/mm, smooth cam curve, no inflection points, and good stray light control which has little impact on the system. Finally, through the location test, the system has good consistency of optical axis, which is less than 0.1mrad. Therefore, the performance indicators of the system are well met the requirements of the use of the equipment, providing reference value for the engineering implementation of the photoelectric tracking system.
In the optical components, the surface defects are often caused by photoelectric action, photothermal action and plasma action, and the surface defects can directly affect the performance of the optical system. When classifying the surface defects, the shapes of many surface defects are irregular, so it is difficult to achieve the desired effect by relying on normal pattern recognition technology. To overvcome the low precision and long time consuming in traditional surface defect detection methods, a convolutional neural network based surface defect detection method is proposed in this paper. Firstly, the surface defect image is obtained by scattering method to analyze its imaging characteristics, and the training ability of the network is strengthened by rotating the image and mirroring the amplified data set. Furthermore, the AC training network model is used to strengthen the feature acquisition power of the network without increasing the extra calculation. Finally, the Softmax classifier is used to classify the surface defects into scratch, pitting and noise. Experimental results show that the defect detection accuracy of the model used is more than 99.05%.
Aiming at the problems of unsatisfactory processing effect and long processing time in single frame infrared image with complex background, a hierarchical convolution filter detection algorithm is proposed. It is mainly divided into two parts: first, according to the characteristics of small infrared targets, a hierarchical convolution filter operator is designed to filter the image, so as to achieve the effect of increasing the efficiency of small targets in the image and suppressing the background; Secondly, the adaptive threshold method based on the maximum value is used to binarize the image to filter the background clutter, and finally extract the target to be detected. Experiments in a large number of infrared images with different backgrounds show that the performance quantization results of the algorithm are better than the performance results of the existing five typical infrared dim and small target detection algorithms, and the average processing time is only 30.42% of Laplacian of Gaussian (LoG) filter algorithm. Finally, through the experimental comparison, Hierarchical convolution filtering method can effectively solve the problem of small target detection in infrared images in different complex backgrounds.
An automated measurement setup of laser-induced damage threshold (LIDT) according to ISO 21254-1,2,3,4:2011 standards is presented. The main blocks of the setup, include pulse laser system, laser beam parameters measurement system, damage detection system, specimen holder and control system, are described. In order to control the LIDT measurements, software based on LabView programming package was created. Energy density of 0.1 J/cm2～100 J/cm2 is achieved. LIDT using the 1-on-1 test on 1064 nm AR coatings and Al reflective coatings on K9 substrates are respectively 27.09 J/cm2 and 3.21 J/cm2, relative measurement uncertainty are respectively 3.91% and 5.61%.
As an important accessory of the high and low temperature test device of the photoelectric imaging system, the optical window assembly is assembled on the side of the high and low temperature test box. Combined with the comprehensive detector of the photoelectric system outside the box, it is used to detect the performance index of the photoelectric imaging system under high and low temperature conditions. It is an important part of the high and low temperature performance detection device of the photoelectric imaging system. According to the application requirements and material analysis, multispectral ZnS is selected as the optical window material to meet the performance index detection requirements of multispectral photoelectric system; Through the heat conduction theory, the low-temperature service state of the optical window module is analyzed, focusing on the influence of the heating of the window module under the low-temperature condition on the surface shape of the window module, and the structural form to realize the micro stress assembly and the design scheme to solve the frost and fog of the window module under the low-temperature condition are put forward; The finite element calculation results are extracted, processed and combined by sigfit, and the caliber is analyzed by code v φ The wave aberration RMS of 310mm window module in the temperature range is better than that of λ/ 15. They all meet the optical performance requirements of window components and have been verified by the physical prototype. Therefore, the structural design scheme of the multispectral optical window assembly not only meets the requirements of multi band use, but also meets the requirements of low-temperature defrosting and defogging, and ensures the optical performance requirements under high and low temperature conditions.
The natural forest areas in China are widely distributed and the terrain is complex. Relying on the traditional patrol inspection method of forest rangers to prevent and control forest diseases and insect pests is inefficient, and it is difficult to find early forest diseases and insect pests in time. Therefore, the best time for prevention and control may be missed. In view of this problem, a deep learning network based on multispectral image detection of forest diseases and insect pests is designed, and a set of detection software is implemented. Through the UAV hanging flight experiment, the built deep learning network can complete the detection of infected areas in forest areas, and the detection results are analyzed.
In order to measure the reflectance of large aperture and high reflectance optical elements in high energy laser transmission system, a precision measurement system for two-dimensional scanning of large aperture optical elements was designed. The structure and working principle of the system are introduced, and the factors affecting the measurement accuracy of the system are analyzed. The influence of the systematic error of the scanning system on the measurement accuracy is theoretically analyzed. The results show that the measurement error is 10-6 magnitude when the horizontal deviation is 0.29mm perpendicular to the beam propagation direction. When the variation of cavity length is small, the rotation axis deviation can be compensated by adjusting the cavity mirror of ring-down cavity, and the system can be fine adjusted. By fitting processing of measured light intensity and time data to get the corresponding a exponential function fitting curve, and ring-down time is obtained by calculation and reflectivity, ring-down time and reflectivity is obtained by calculation, through contrast analysis shows that this kind of error analysis method can more effectively measured reflectivity of cavity mirrors and can reduce the error brought by the experimental data itself.
In near-shore scenes, under the influence of background, the probability of false detection and low detection accuracy of ship critical parts are high. To address the above problems, this paper proposes a detection network for ship critical parts based on semantic features, which named CPDNet (critical part detection network). Firstly, by optimizing the network structure and adopting attention mechanism, the feature expression ability and the perception ability of the ship's critical parts are improved. Secondly, based on semantic information, a semantic mask module is designed to reduce the impact of background on detection accuracy. In addition, the angle parameter is added to make the network applicable to targets with orientation. Finally, a ship's critical parts dataset, named CP-Ship, is constructed to verify the effectiveness of the proposed network. The experimental results on the CP-Ship dataset show that the average accuracy of the proposed network is 11.35% higher than RetinaNet. Compared with other network models, the proposed network performs well in both detection accuracy and speed.
Aiming at the adaptability of common optical path system to ambient temperature, a thermal control method of primary mirror assembly based on comprehensive heat transfer is proposed based on the temperature optical deformation characteristics. The heat transfer model of the primary mirror assembly is established, and the temperature distribution under typical conditions are analyzed; The thermal simulation of the main mirror with different materials is carried out, and the model is modified with the thermal optical test results, so that the absolute deviation between the simulation and the measured results of the temperature is less than 1.4 ℃. At the same time, the temperature gradient control threshold of the assembly is determined; On this basis, the zoning heat transfer strategy is adopted to make the main mirror assembly reach the goal of high temperature rise level and low temperature gradient. Taking a main mirror assembly as the object, the simulation and test are carried out, When the average temperature rise of the main mirror reaches more than 16 ℃, the axial temperature gradient of the mirror is ≤ 2.5 ℃, the radial and circumferential temperature gradient is ≤ 2.4 ℃, and the change of the main mirror shape is less than 0.005 λ,The results can provide an optimization idea for the overall thermal control design of the common optical path system.
Be longing to the standard infrared source, 30 ℃～420 ℃ blackbody temperature must speedily rise up to the set temperature, and keep stable. For the characteristics of large difference in temperature rising and falling power, large lag etc, Using auto tuning method of impulse response based on the Bang-Bang control, the parameters of blackbody overshoot and maximum temperature rising and falling rate have been gotten. Using compound intelligent temperature control tactics, The temperature of 30 ℃～420 ℃ blackbody can rising speedily in the first stage, when being close to the target temperature, it rising up slowly, and keeps stability in the target temperature. Experiments show that 30 ℃～420 ℃ blackbody rises to the target temperature, having no overshooting, and the temperature stability is ±0.03 ℃/min, meets the measurement technical requirements, this technical index has reached the international level of similar products.
Speckle correlation is the basis of speckle-based optical measurements and imaging recovery technologies, which determines the resolution of the opitcal system. At present, the theoretical description of speckle size (grain or resolution) is not accurate enough, and lacks of experimental verification. This paper explores the influencing factors of speckle pattern autocorrelation size, and compares it with the same numerical aperture objective focusing to reveal the 'scattering lens' properties of thin scattering medium. Through multiple sets of measurements of speckle autocorrelation and lens focusing size, the results show that the apodization function will affect their resolution, and the Abbe criterion needs to be corrected according to the specific optical path. This paper has a certain reference value for speckle-based measurement and imaging technology.
In order to meet the high-precision measurement requirement of spectral emissivity of high temperature material coating, including the stealthy materials, thermal protection materials, heat insulating coat and so on, the measuring methods for normal spectral emissivity of materials at 1 273 K～3 100 K was explored. A measurement model of normal spectral emissivity of materials was established firstly based on the emissivity definition. On this basis, the measurement facility of normal spectral emissivity of materials was built, the wavelength range is 0.7 μm to 14 μm. In order to overcome the technical difficulties of the cavity effect caused by heating the sample in the measuring device, a temperature sample furnace with a movable graphite crucible was developed, and good experimental results were obtained. The normal spectral emissivity of two samples (SiC and low emissivity coating) were measured by this facility. The results show that normal spectral emissivity of two samples decrease with wavelengths and increased with temperature. Finally, the measurement uncertainty of normal spectral emissivity of materials at high temperature was analyzed, the relative expanded uncertainty is 3.6%.
Birds and drones are two major threats to flight safety, and both are "low and slow small" flying objects. In order to protect the safety of the airport clear zone, we need to develop an early warning system with the function of anti-low and slow small flying objects. In this regard, we designed and implemented a real-time detection system for low and slow small flying objects, based on FPGA (Field Programmable Gate Array) driven camera arrays for real-time acquisition of large-field-of-view sky video, combining parity shunt algorithm and inter-frame differential algorithm for hardware operation to detect moving targets, and the system frame rate reached 17 fps@1 024x768 pixel, with an average detection accuracy of 99.69%. Gigabit Ethernet, optical fiber and switch are used to transmit the front-end runway video to the back-end tower command center, supporting 3 km long-distance transmission. Compared with the traditional software-based serial processing, the system has the advantages of high real-time, low power consumption and small size, which is suitable for deployment in practical application scenarios.
In order to measure the resonance wavelength of plasma grating, the sensitivity of grating parameters to stress is studied, a new type of stress-sensitive polydimethylsiloxane (PDMS) thin film plasma grating is proposed. Based on the principle of finite difference time domain (FDTD), a simulation model of periodic plasma grating structure is established. With the help of periodic boundary conditions, by applying stress to the grating and changing the parameters of the plasma grating (i.e. period, duty cycle and Au film thickness) to achieve the measurement of the resonance wavelength, the sensitivity of the grating parameters to the force is studied; and compare the simulation result with the theoretical value to get the relative error. The relative error is calculated by comparing the simulation result with the theoretical value. The results show that when the grating period is 0.7 μm, the duty cycle is 55%, and the gold film thickness is 0.02 μm, the response to force is most sensitive at this time; Secondly, comparing the resonance peak wavelength at different periods obtained by simulation with the theoretical calculation value, the results of the two are consistent; When the period is 0.7 μm, the wavelength of the resonance peak is 1.251 μm, and the relative error obtained by theory and simulation is less than 2%, and the result is more accurate. This method plays an important role in the fields of monochromator, spectrometer and sensor.
A general model of fiber Bragg grating (FBG) acceleration detector has been established for the two-point package model. The sensitivity and the resonance frequency's analytical expressions of the acceleration detector have been deduced theoretically, and the influence factors which affecting the sensitivity and resonance frequency have been researched deeply, the acceleration detector’s response characteristics have been discussed, which would be affected by the ratio, that the package fiber stiffness to the structure stiffness. Based on this basis, the restrictive relation between the sensitivity and the resonance frequency have been analyzed, among the equivalent mass and the ratio have been in the range of 0~100 g, 0~1 and 0~100, respectively. And the resonance frequency’s change rule within the scope of 0~500 Hz (low-medium frequency) and 0~1200 Hz (medium-high frequency) also has been studied. Furthermore, the package fibers have been chosen 10 mm and 60 mm as examples to analyze the influence to the above two mentioned, the sensitivity of each simulation would reach up to ~1 000 pm/G, and the quality factor has been introduced. This study has played a significant role in the design and comprehensive performance evaluation of the acceleration detector, and provided a theoretical reference for the optimization of structural parameters to it.
In order to solve the problem of image clarity and contrast degradation in fog scene image restoration, a single image defogging algorithm based on residual learning and guided filtering was proposed. The residual network was constructed by using foggy images and corresponding clear images. Multi-scale convolution is used to extract more detailed haze features. Taking advantage of the anisotropy of the guided filter, the image after the residual network is filtered to maintain the image edge characteristics, and a clearer fog-free image is obtained. The experimental results show that, compared with DCP algorithm, CAP algorithm, SRCNN algorithm, DehazeNet algorithm and MSCNN algorithm, On synthetic foggy images, the PSNR reaches 27.840 3/dB at the highest, the SSIM value reaches 0.979 6 at the highest, and the running time on natural foggy images reaches 0.4 s at the lowest. and the subjective evaluation and objective evaluation are better than other comparison algorithms. Proposed to fog algorithm not only to the fog effect is better, and faster, with strong practical value.