The purpose of this work is to develop a method for the accurate reproduction of the spectral colors captured by digital camera. The spectral colors being the purest color in any hue, are difficult to reproduce without distortion on digital devices. In this paper, we attempt to achieve accurate hue reproduction of the spectral colors by focusing on two steps of color correction: the capture of the spectral colors and the color characterization of digital camera. Hence it determines the relationship among the spectral color wavelength, the RGB color space of the digital camera device and the CIEXYZ color space. This study also provides a basis for further studies related to the color spectral reproduction on digital devices. In this paper, methods such as wavelength calibration of the spectral colors and digital camera characterization were utilized. The spectrum was obtained through the grating spectroscopy system. A photo of a clear and reliable primary spectrum was taken by adjusting the relative parameters of the digital camera, from which the RGB values of color spectrum was extracted in 1040 equally-divided locations. Calculated using grating equation and measured by the spectrophotometer, two wavelength values were obtained from each location. The polynomial fitting method for the camera characterization was used to achieve color correction. After wavelength calibration, the maximum error between the two sets of wavelengths is 4.38nm. According to the polynomial fitting method, the average color difference of test samples is 3.76. This has satisfied the application needs of the spectral colors in digital devices such as display and transmission.
In order to study the property and performance of LED as RGB primary color light sources on color mixture in visual psychophysical experiments, and to find out the difference between LED light source and traditional light source, a visual color matching experiment system based on LED light sources as RGB primary colors has been built. By simulating traditional experiment of metameric color matching in CIE 1931 RGB color system, it can be used for visual color matching experiments to obtain a set of the spectral tristimulus values which we often call color-matching functions (CMFs). This system consists of three parts: a monochromatic light part using blazed grating, a light mixing part where the summation of 3 LED illuminations are to be visually matched with a monochromatic illumination, and a visual observation part. The three narrow band LEDs used have dominant wavelengths of 640 nm (red), 522 nm (green) and 458 nm (blue) respectively and their intensities can be controlled independently. After the calibration of wavelength and luminance of LED sources with a spectrophotometer, a series of visual color matching experiments have been carried out by 5 observers. The results are compared with those from CIE 1931 RGB color system, and have been used to compute an average locus for the spectral colors in the color triangle, with white at the center. It has been shown that the use of LED is feasible and has the advantages of easy control, good stability and low cost.
Color measurement and control of printing has been an important issue in computer vision technology . In the past,
people have used density meter and spectrophotometer to measure the color of printing product. For the color
management of 4 color press, by these kind meters, people can measure the color data from color bar printed at the side
of sheet, then do ink key presetting. This way have wide application in printing field. However, it can not be used in the
case that is to measure the color of spot color printing and printing pattern directly. With the development of
multispectral image acquisition, it makes possible to measure the color of printing pattern in any area of the pattern by
CCD camera than can acquire the whole image of sheet in high resolution. This essay give a way to measure the color of
printing by multispectral camera in the process of printing. A 12 channel spectral camera with high intensity white LED
illumination that have driven by a motor, scans the printing sheet. Then we can get the image, this image can include
color and printing quality information of each pixel, LAB value and CMYK value of each pixel can be got by
reconstructing the reflectance spectra of printing image. By this data processing, we can measure the color of spot color
printing and control it. Through the spot test in the printing plant, the results show this way can get not only the color bar
density value but also ROI color value. By the value, we can do ink key presetting, that makes it true to control the spot
color automatically in high precision.
The Computed-Tomography Imaging Interferometer (CTII) is a novel imaging spectrometer, which combines the
advantages of the conventional Fourier Transform Imaging Spectrometer (FTIS) and the ordinary
Computed-Tomography Imaging Spectrometer (CTIS). CTII obtains multi-angle projection interferograms by rotating
Dove prism placed in the collimating light beams. The image reconstruction is carried out by using
computed-tomography reconstruction algorithm named Radon transform. However, in experiments, images
reconstructed from the raw projection-interferogram sequences, are badly distorted. To solve this problem, we find when
Dove prism is rotated, its rotation center is not coincident to the optic axis of CTII. Therefore, the raw
projection-interferogram sequences have a few deviations to the ideal sequences. And then we find the deviations follow
certain law, so it is possible to rectify the raw projection-interferogram sequences. In this paper, two methods, the Linear
rectification method and the Cosine rectification method are proposed. The Linear rectification method uses an image
processing method, gets the dither value at each rotation angle, and rectifies the raw images. The Cosine rectification
method supposes the dither follows cosine change; the detail is presented in this paper. Finally, the reconstruction images
are presented. The reconstruction results show these two rectification methods are feasible and effective.
Theoretically speaking, image quality assessment algorithms are design to evaluate all kind of distorted images. However,
as the JPEG compressed image dramatically distinguished from images with white noise, every distortion type has its
own characteristic. A new method is proposed based on the characteristic of JPEG compressed image, witch correlated
well with the subjective result.
In cross-media color image reproduction, gamut mapping is needed due to gamut difference among different media. The first step of gamut mapping should be the determination of gamut boundaries of each medium involved, no matter what kind of mapping algorithm is to be used. It may be expected that an analytical expression for a boundary is preferable to a set of discrete data, since it would make the determination of the intersection point between a boundary and a "mapping line" easier and faster. This paper describes LCD display gamut boundary surfaces with a form of Zernike polynomial. In CIE1976L*a*b* color space, each color point on the boundary can be expressed as L*=L*(a*,b*) and every boundary can be expanded into a series of Zernike polynomials with appropriate coefficients. These coefficients can be obtained with sufficient experiment data of boundary points and existing algorithms. Experiments have been executed for a LCD display with(R,G,B) as its input. The 6 boundaries in RGB space would be formed respectively by (0,G,B),(R,0,B),(R,G,0),(255,G,B),(R,255,B) and (R,G,255) where each of R,G,B varies from 0 to 255. Then 6 corresponding sets of Zernike coefficients are calculated, based on about half of the measured L*a*b*'s for each boundary. A comparison between original measured data and the data predicted by Zernike polynomials shows that, not only for the data that have been used to calculate the coefficients, but also for those not used, the differences are acceptably small even negligible with only a few exceptions.
Traditionally, the grade discrimination and classifying of bowlders (emeralds) are implemented by using methods based on people's experiences. In our previous works, a method based on NCS(Natural Color System) color system and sRGB color space conversion is employed for a coarse grade classification of emeralds. However, it is well known that the color match of two colors is not a true "match" unless their spectra are the same. Because metameric colors can not be differentiated by a three channel(RGB) camera, a multispectral camera(MSC) is used as image capturing device in this paper. It consists of a trichromatic digital camera and a set of wide-band filters. The spectra are obtained by measuring a series of natural bowlders(emeralds) samples. Principal component analysis(PCA) method is employed to get some spectral eigenvectors. During the fine classification, the color difference and RMS of spectrum difference between estimated and original spectra are used as criterion. It has been shown that 6 eigenvectors are enough to reconstruct reflection spectra of the testing samples.
An innovative Fourier Transform hyperspectral imaging system based on reflective optics is currently being studied. It can record both spatial images and spectral information of a sample instantaneously. Substantial properties of the sample can be elucidated from such images. Compared to classical Imaging Spectrometer using lenses and prisms, the significant characteristic of this system is that it only uses reflective mirrors and just one beam splitter. Such structure will help to largely avoid the limitation of spectral range and the refraction non-homogenize both of which affect the quality of imaging. Therefore, the noticeable advantages of this system are high signal-to-noise ratio, high spatial and spectral resolution, large spectral bandwidth, high throughput, non-chromatic aberration and very compact optical structure in which just one imaging system could applicable to a rather wide spectral bandwidth. This project includes both theoretical analysis and development of an experimental instrument. With the instrument, the images that contain one-dimensional spatial image and another dimensional interferogram are already collected. The data processing system could transform the interferogram of each scene to its spectral information. The typical experimental results are given in this paper.
In cross-media color image reproduction, gamut mapping is needed due to gamut difference among different media. In order to implement gamut mapping, gamut boundaries of each medium involved should be first determined. It may be expected that an analytical expression for a boundary is preferred than a set of discrete data , since it would take less storage space and make the determination of the intersection point between a boundary and a "mapping line" easier and faster. In this article, a form of Zernike polynomial expression is suggested to be used as the expression of gamut boundary surface. For instance, if C1E1976L*a*b* is adopted as the color space for gamut mapping, then each color(point) on the boundary can be expressed as L*=L*(a*,b) and the boundary can be expanded into a series of Zernike polynomials with an appropriate coefficient for each of which. These coefficients can be obtained with sufficient experimental data of boundary points and existing algorithms. Experiments have been executed for a color printer with(R,G,B) as its input. The 6 boundaries in RGB space would consist of (0,G,B),(R,0,B),(R,G,0),(255,G,B),(R,255,B) and (R,G,255) where each of R,G,B varies from 0 to 255. Then 6 corresponding sets of Zernike coefficients are calculated, based on about half of the measured L*a*b*'s for each boundary. A comparison between original measured data and the data predicted by Zernike polynomials shows that, not only for the data that have been used to calculate the coefficients, but also for those not used, the differences are acceptably small even negligible with only a few exceptions.