Color Conversion of Wide-Color-Gamut Cameras Using Optimal Training Groups.

The colorimetric conversion of wide-color-gamut cameras plays an important role in the field of wide-color-gamut displays. However, it is rather difficult for us to establish the conversion models with desired approximation accuracy in the case of wide color gamut. In this paper, we propose using an optimal method to establish the color conversion models that change the RGB space of cameras to the XYZ space of a CIEXYZ system. The method makes use of the Pearson correlation coefficient to evaluate the linear correlation between the RGB values and the XYZ values in a training group so that a training group with optimal linear correlation can be obtained. By using the training group with optimal linear correlation, the color conversion models can be established, and the desired color conversion accuracy can be obtained in the whole color space. In the experiments, the wide-color-gamut sample groups were designed and then divided into different groups according to their hue angles and chromas in the CIE1976L*a*b* space, with the Pearson correlation coefficient being used to evaluate the linearity between RGB and XYZ space. Particularly, two kinds of color conversion models employing polynomial formulas with different terms and a BP artificial neural network (BP-ANN) were trained and tested with the same sample groups. The experimental results show that the color conversion errors (CIE1976L*a*b* color difference) of the polynomial transforms with the training groups divided by hue angles can be decreased efficiently.

Colorimetric Characterization of Color Imaging System Based on Kernel Partial Least Squares.

Colorimetric characterization is the basis of color information management in color imaging systems. In this paper, we propose a colorimetric characterization method based on kernel partial least squares (KPLS) for color imaging systems. This method takes the kernel function expansion of the three-channel response values (RGB) in the device-dependent space of the imaging system as input feature vectors, and CIE-1931 XYZ as output vectors. We first establish a KPLS color-characterization model for color imaging systems. Then we determine the hyperparameters based on nested cross validation and grid search; a color space transformation model is realized. The proposed model is validated with experiments. The CIELAB, CIELUV and CIEDE2000 color differences are used as evaluation metrics. The results of the nested cross validation test for the ColorChecker SG chart show that the proposed model is superior to the weighted nonlinear regression model and the neural network model. The method proposed in this paper has good prediction accuracy.

Colorimetric characterization of the wide-color-gamut camera using the multilayer artificial neural network.

In order to realize colorimetric characterization for the wide-color-gamut camera, we propose using the multilayer artificial neural network (ML-ANN) with the error-backpropagation algorithm, to model the color conversion from the RGB space of camera to theX Y Z space of the CIEXYZ standard. In this paper, the architecture model, forward-calculation model, error-backpropagation model, and the training policy of the ML-ANN were introduced. Based on the spectral reflectance curves of the ColorChecker-SG blocks and the spectral sensitivity functions of the RGB channels of typical color cameras, the method of producing the wide-color-gamut samples for the training and testing of the ML-ANN was proposed. Meanwhile, the comparative experiment employing different polynomial transforms with the least-square method was conducted. The experimental results have shown that, with the increase of the hidden layers and the neurons in each hidden layer, the training and testing errors can be decreased obviously. The mean training errors and mean testing errors of the ML-ANN with optimal hidden layers have been decreased to 0.69 and 0.84 (color difference of CIELAB), respectively, which is much better than all the polynomial transforms, including quartic polynomial transform.

Tone Mapping Operator for High Dynamic Range Images Based on Modified iCAM06.

This study attempted to solve the problem of conventional standard display devices encountering difficulties in displaying high dynamic range (HDR) images by proposing a modified tone-mapping operator (TMO) based on the image color appearance model (iCAM06). The proposed model, called iCAM06-m, combined iCAM06 and a multi-scale enhancement algorithm to correct the chroma of images by compensating for saturation and hue drift. Subsequently, a subjective evaluation experiment was conducted to assess iCAM06-m considering other three TMOs by rating the tone mapped images. Finally, the objective and subjective evaluation results were compared and analyzed. The results confirmed the better performance of the proposed iCAM06-m. Furthermore, the chroma compensation effectively alleviated the problem of saturation reduction and hue drift in iCAM06 for HDR image tone-mapping. In addition, the introduction of multi-scale decomposition enhanced the image details and sharpness. Thus, the proposed algorithm can overcome the shortcomings of other algorithms and is a good candidate for a general purpose TMO.

Latent fingerprint residue detection method using Sagnac Fourier transform imaging spectroscopy.

We propose a new method to detect latent fingerprints and their residues based on Sagnac ultraviolet Fourier transform imaging spectroscopy. The three-dimensional data cube including two-dimensional images and spectrum dimensions can be obtained by the new hyperspectral imaging technique. The method to inhibit the redundancy from the spectra-image data is also presented, which includes the self-adaptive differential filtering, the apodization algorithm, and a fast Fourier transform method. The whole process is also discussed in detail. Not only the latent fingerprint but also its residues' distribution are provided in experimental results, and the proposed method is demonstrated.

Design method for the high optical efficiency and uniformity illumination system of the projector.

How to balance the optical efficiency, illumination uniformity and the size of the illumination system is a challenging task in projector design. In this paper, we present a mathematical model describing the relationship between optical energy of the illumination system and the optical parameters and an optimization design method considering the light intensity distribution of the light source. By using the proposed method, two illumination systems are designed with different types of the digital micromirror device chips. In addition, we also propose a non-coaxial system to solve the deformation problem caused by the large flip angle of the DMD chip and further improve the illumination uniformity based on Scheimpflug principle. The optical efficiency and illumination uniformity of the illumination systems were verified and analyzed. The results indicate that the systems designed by the proposed method can provide a good design scheme and obtain a satisfactory utilization rate of the optical energy and higher uniformity.

Color reproduction method based on neural networks and visual matching.

A new method is proposed to build up a color reproduction model based on neural networks and visual matching. Training data were obtained from a visual matching experiment and the commonly used CIECAM02 model to set up a link between the device parameters and human color perception. The process of training neural networks is presented. An experiment simulating an office environment was conducted, together with a user study, to verify the performance of the proposed method in identifying and reproducing color.

Spectral sensitivity estimation of trichromatic camera based on orthogonal test and window filtering.

The three-channel spectral sensitivity of a trichromatic camera represents the characteristics of system color space. It is a mapping bridge from the spectral information of a scene to the response value of a camera. In this paper, we propose an estimation method for three-channel spectral sensitivity of a trichromatic camera. It includes calibration experiment by orthogonal test design and the data processing by window filtering. The calibration experiment was first designed by an orthogonal table of the 9-level and 3-factor. A rough estimation model of spectral sensitivity is established on the data pairs of the system input and output in calibration experiments. The data of rough estimation is then modulated by two window filters on frequency and spatial domain. The Luther-Ives condition and the smoothness condition are introduced to design the window, and help to achieve the optimal estimation of the system spectral sensitivity. Finally, the proposed method is verified by some comparison experiments. The results show that the estimated spectral sensitivity is basically consistent with the measured results of the monochromator experiments, the relative full-scale errors of the RGB three-channel is obviously lower than the Wiener filtering method and the Fourier band-limitedness method. The proposed method can estimate the spectral sensitivity of the trichromatic digital camera very well, which is of great significance for the colorimetric characterization and evaluation of imaging systems.

Assessment of Camouflage Effectiveness Based on Perceived Color Difference and Gradient Magnitude.

We propose a new model to assess the effectiveness of camouflage in terms of perceived color difference and gradient magnitude. The "image color similarity index" (ICSI) and gradient magnitude similarity deviation (GMSD) were employed to analyze color and texture differences, respectively, between background and camouflage images. Information entropy theory was used to calculate weights for each metric, yielding an overall camouflage effectiveness metric. During the analysis process, both spatial and color perceptions of the human visual system (HVS) were considered, to mimic real-world observations. Subjective tests were used to compare our proposed method with previous methods, and our results confirmed the validity of assessing camouflage effectiveness based on perceived color difference and gradient magnitude.

Large-aperture UV (250~400 nm) imaging spectrometer based on a solid Sagnac interferometer.

Developing an ultraviolet (UV) imaging spectrometer is challenging due to a low level of incident power of photon flux, large chromatic aberration, and relatively low quantum efficiency of imaging sensor in UV waveband. In this paper, a large-aperture UV (250~400 nm) Fourier transform imaging spectrometer is presented for close-range hyperspectral sensing with high spatial resolution and decent spectral resolution. An advanced design based on a modified solid Sagnac interferometer working in UV waveband of 250~400 nm is introduced to improve the interferometric stability. A large-aperture and a reflective-transmissive filtering system are used to increase the spectral purity of the incident UV radiation, and air-spaced achromatic doublets are designed to address the chromatic aberration. The finished spectrometer has a spatial resolution of 23.44 µm on the target plane, a wavelengths resolution of 1.59 nm at 250 nm, and can provide approximately 59 wavelength samples over the waveband of 250~400 nm. The proposed imaging spectrometer acquires a hyperspectral data cube through push-broom scanning in a few minutes. Examples of UV hyperspectral imaging are demonstrated with a sample of resolution test chart, and a cotton sample with vitamin C (VC) and vitamin B6 (VB6) traces. Based on the analysis of spectra, monochromatic images, and k-Means clustering results, it can be concluded that the spectrometer is capable of UV hyperspectral imaging with excellent spectral accuracy, spatial performance, compactness, and robustness. The potential applications of the proposed instrument include materials analysis and traces detection with UV spectral characteristics.

Spectral Reflectance Reconstruction with Nonlinear Composite Model of the Metameric Black.

Metamerism phenomenon is an important problem in spectral reflectance reconstruction and color reproduction. In this paper, a 3-primary color CCD camera is used to acquire spectral information in CIE standard illuminant D65 and a nonlinear composite model is established, including principal component analysis and neural network method (PCA-NET) to modify the Matrix R Method based on the Metameric Black theory. The standard Munsell color card is used in spectral reflectance reconstruction experiment and the results are evaluated and discussed. The experimental results verified that the PCA-NET algorithm can accurately fit the nonlinear relationship between the output signal of the camera and the principal component coefficients; and it can be used in the R matrix algorithm instead of the linear algorithm; the new method can serve as a promising technique for building a spectral image database whihc is better than the original Matrix R Method. In the fixed illumination environment, the mean RMS of the test set is 0.76 improved, and the mean STD of the test set is 0.85 improved, which can effectively improve the accuracy of spectral reflectance reconstruction. The modified matrix R method has the advantages of higher accuracy and easy implementation, and it can be used in the field of color reproduction and spectral reflectance reconstruction.

Model-based restoration of underwater spectral images captured with narrowband filters.

Underwater spectral imaging is a promising method for mapping, classification and health monitoring of coral reefs and seafloor inhabitants. However, the spectrum of light is distorted during the underwater imaging process due to wavelength-dependent attenuation by the water. This paper presents a model-based method that accurately restores brightness of underwater spectral images captured with narrowband filters. A model is built for narrowband underwater spectral imaging. The model structure is derived from physical principles, representing the absorption, scattering and refraction by water and the optical properties of narrowband filters, lenses and image sensors. The model coefficients are calibrated based on spectral images captured underwater and in air. With the imaging model available, energy loss due to water attenuation is restored for images captured at different underwater distances. An experimental setup is built and experiments are carried out to verify the proposed method. Underwater images captured within an underwater distance of 260 cm are restored and compared with those in air. Results show that the relative restoration error is 3.58% on average for the test images, thus proving the accuracy of the proposed method.

High resolution ultraviolet imaging spectrometer for latent image analysis.

In this work, we present a close-range ultraviolet imaging spectrometer with high spatial resolution, and reasonably high spectral resolution. As the transmissive optical components cause chromatic aberration in the ultraviolet (UV) spectral range, an all-reflective imaging scheme is introduced to promote the image quality. The proposed instrument consists of an oscillating mirror, a Cassegrain objective, a Michelson structure, an Offner relay, and a UV enhanced CCD. The finished spectrometer has a spatial resolution of 29.30µm on the target plane; the spectral scope covers both near and middle UV band; and can obtain approximately 100 wavelength samples over the range of 240~370nm. The control computer coordinates all the components of the instrument and enables capturing a series of images, which can be reconstructed into an interferogram datacube. The datacube can be converted into a spectrum datacube, which contains spectral information of each pixel with many wavelength samples. A spectral calibration is carried out by using a high pressure mercury discharge lamp. A test run demonstrated that this interferometric configuration can obtain high resolution spectrum datacube. The pattern recognition algorithm is introduced to analyze the datacube and distinguish the latent traces from the base materials. This design is particularly good at identifying the latent traces in the application field of forensic imaging.

Research on Ground-Based LWIR Hyperspectral Imaging Remote Gas Detection.

The new progress of ground-based long-wave infrared remote sensing is presented, which describes the windowing spatial and temporal modulation Fourier spectroscopy imaging in details. The prototype forms the interference fringes based on the corner-cube of spatial modulation of Michelson interferometer, using cooled long-wave infrared photovoltaic staring FPA (focal plane array) detector. The LWIR hyperspectral imaging is achieved by the process of collection, reorganization, correction, apodization, FFT etc. from data cube. Noise equivalent spectral radiance (NESR), which is the sensitivity index of CHIPED-1 LWIR hyperspectral imaging prototype, can reach 5.6 x 10⁻8 W · (cm⁻¹ · sr · cm²)⁻¹ at single sampling. The data is the same as commercial temporal modulation hyperspectral imaging spectrometer. It can prove the advantage of this technique. This technique still has space to be improved. For instance, spectral response range of CHIPED-1 LWIR hyperspectral imaging prototype can reach 11. 5 µm by testing the transmission curve of polypropylene film. In this article, choosing the results of outdoor high-rise and diethyl ether gas experiment as an example, the authors research on the detecting method of 2D distribution chemical gas VOC by infrared hyperspectral imaging. There is no observed diethyl ether gas from the infrared spectral slice of the same wave number in complicated background and low concentration. By doing the difference spectrum, the authors can see the space distribution of diethyl ether gas clearly. Hyperspectral imaging is used in the field of organic gas VOC infrared detection. Relative to wide band infrared imaging, it has some advantages. Such as, it has high sensitivity, the strong anti-interference ability, identify the variety, and so on.

Testing the performance of CIECAM02 model using two distinct glossinesses of color printing atlas.

Psychophysical experiments have been carried out in which the color appearance of high-gloss and low-gloss color printing atlases was assessed using a magnitude estimation method under three different light sources. A linearity fitting algorithm by the method of least squares was applied to evaluate the correlation between visual experiment data and predicted results through the CIECAM02 model of different glossinesses of color printing atlases. The research result has shown that there was a good correlation between mean visual results and predicted results for hue and lightness of two distinct glossinesses of the color printing atlas. The correlations of hue and lightness between visual experiment data and predicted results were the best and the second best, respectively, under the same light source. Colorfulness was the worst. Meanwhile, high-gloss and low-gloss color printing atlases have been estimated and compared in the viewing conditions of 0°/45°. The result has indicated that the hue of the color printing atlas could not be affected by gloss levels under three light sources. Colorfulness of the color printing atlas almost could not be affected by gloss levels under D65 and TL84 light sources. Lightness of the color printing atlas could be affected by gloss levels under a TL84 light source.

[A novel spatial modulation Fourier transform spectrometer with adjustable spectral resolution].

In the premise of fulfilling the application requirement, the adjustment of spectral resolution can improve efficiency of data acquisition, data processing and data saving. So, by adjusting the spectral resolution, the performance of spectrometer can be improved, and its application range can be extended. To avoid the problems of the fixed spectral resolution of classical Fourier transform spectrometer, a novel type of spatial modulation Fourier transform spectrometer with adjustable spectral resolution is proposed in this paper. The principle of the novel spectrometer and its interferometer is described. The general expressions of the optical path difference and the lateral shear are induced by a ray tracing procedure. The equivalent model of the novel interferometer is analyzed. Meanwhile, the principle of the adjustment of spectral resolution is analyzed. The result shows that the novel spectrometer has the merits of adjustable spectral resolution, high stability, easy assemblage and adjustment etc. This theoretical study will provide the theoretical basis for the design of the spectrometer with adjustable spectral resolution and expand the application range of Fourier transform spectrometer.

[Correlation of visual spectral response with aging and its effect on color discrimination].

The variation in normal observers' color perception and discrimination will occur with the aging effect. The authors carried out a psychophysical experiment with the method of binocular observational technique. Three observers at the age of 21, 32 and 58 were organized to match the 9 printed color samples on EIZO monitor with the repetition of 7 times. Groups of data for cross-media color matching with different age were established to investigate the response of photoreceptor cells. The results indicated that with the aging effect the chroma of the matched color will decrease obviously and the spectral response of the blue channel will decrease for blue and purple colors. The results validate the prediction of CIEPO06 model to some extent and provide some theoretical basis and experimental supports for the application of color reproduction and the color evaluation of the products related to age.

[Spectrum analysis and measurment research on visual density of the national standard densitometer].

Visual density is an important index for the estimation of photographic film and visual photo. The development of biomedical image and macromolecule image needs high measurement uncertainty of visual density in spectrum characteristic. The ISO standard of visual density measurement conditions was modified in 2001. A series of equipments designed for densitometer enhance the spectral matching between the national standard densitometer and the ISO standard function. And a new measurement formula is given based on theory analysis. According to the measurement data and uncertainty analysis, the combined uncertainties are 0.001 1 (0 < D < 4.0) and 0.003 1 (4.0 < or = D < 6.0).

[High precision all-reflection Fourier transform imaging spectrometer spectral calibration using homogeneous broadening of the wave number model].

All-reflection Fourier transform imaging spectrometer (ARFTIS) is a novel imaging spectrometer. The specialty is not only high spectrum resolution, but also wide band and non-chromatism. It is good for remote sensing field of wide band imaging. Single spectrum calibration, average calibration and weighted average calibration are three common calibration methods. However, they all are limited. Because they cannot meet the demand on both convenience and high precision. In the present paper, the authors propose a novel model for spectrum calibration. It can work in high precision with single spectrum calibration. At the same time, the method is steady, and the average error is less than 5% with multi-bands calibration. It provides a convenient way for the non-professional calibration situation and outer simply calibration work.

[Spectral reconstruction using algebraic reconstruction technology for Fourier transform computed tomography imaging spectrometer].

In the present paper, the authors will introduce our research on spectral reconstruction of Fourier transform computed tomography imaging spectrometer by means of the algebraic reconstruction technology. A simulation experiment was carried out to demonstrate the algorithm. The spatial similarities and spectral similarities were evaluated using the normalized correlation coefficient. The performance of ART was evaluated when the quantity of projection is 45. In that case, filter back projection can't work well. Actual spectral slices were reconstructed by using ART in the last part of this paper.