Spectral modeling of a six-color inkjet printer

After customizing an Epson Stylus Photo 1200 by adding a continuous-feed ink system and a cyan, magenta, yellow, black, orange and green ink set, a series of research tasks were carried out to build a full spectral model of the printers output. First, various forward printer models were tested using the fifteen two color combinations of the printer. Yule- Nielsen-spectral-Neugebauer (YNSN) was selected as the forward model and its accuracy tested throughout the colorant space. It was found to be highly accurate, performing as well as a more complex local, cellular version. Next, the performance of nonlinear optimization-routine algorithms were evaluated for their ability to efficiently invert the YNSN model. A quasi-Newton based algorithm designed by Davidon, Fletcher and Powell (DFP) was found to give the best performance when combined with starting values produced from the non-negative least squares fit of single-constant Kubelka- Munk. The accuracy of the inverse model was tested and different optimization objective functions were evaluated. A multistage objective function based on minimizing spectral RMS error and then colorimetric error was found to give highly accurate matches with low metameric potential. Finally, the relationship between the number of printing inks and the ability to eliminate metamerism was explored

Practical spectral capture systems for museum imaging

Spectral imaging systems for capturing museum artwork have not moved beyond the research laboratory. System components and their selection criteria necessary to assemble a complete system are explored in this paper categorically. The final system is practical if it meets the criteria of speed cost and accuracy necessary to act as a direct replacement for the traditional RGB based digital imaging systems in museum photo studios. It is hoped that the advantages of spectral imaging will be widely recognized and adopted once a practical system is available

Evaluation of a modified sinar 54M digital camera at the National Gallery of Art, Washington DC during April, 2005

On April 26 – 27 2005, the RIT team brought their prototype Sinar 54M camera system to the National Gallery of Art’s Imaging Department. The team included Lawrence Taplin, Mitchell Rosen, and Roy Berns. RIT brought a spectroradiometer to measure the daylight fluorescent viewing illumination and if time allowed, NGA’s CRT display. They also brought a Macintosh G5 to control the camera and for image processing. NGA is using a production Sinar 54M. This enabled a direct comparison between NGA’s digital imaging workflow and the RIT workflow. Both cameras used the same lighting, Broncolor pulsed Xenon, lighting geometry, and photography studio. A set of test targets was imaged by each camera system. In addition, Matisse’s Pot of Geraniums, previously imaged by RIT using their 31-channel spectral camera,1-3 was also imaged. The digital masters from each camera system were converted to an output profile for an Epson Ultrachrome inkjet printer. One-to-one prints were made for the Matisse painting and compared with the actual painting under the daylight fluorescent viewing environment. The RIT prototype camera and the print comparison were evaluated by NGA staff from the imaging, conservation, and finance departments

Clinical Skin Microbiology RazaAlyHowardMaibachCharles CThomas1978SpringfieldIllinois133$14.50, 50 illustrations

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Comparative study of spectral reflectance estimation based on broad-band imaging systems

We have been practicing spectral color estimation for museum artwork imaging and spectral estimation. We have had success using both narrow-band imaging based on a liquid crystal tunable filter (LCTF) and various broad-band imaging approaches using the same monochromatic digital camera system. Details about our spectral color imaging system description, imaging procedures and the performance of spectral estimation methods used can be found in our previous technical reports.1,2 In previous reports we focused in methods of reconstruction from narrow-band images using LCTF, while we only reported preliminary analyses of reconstruction from wide-band images using six glass filtered images and a red-green-blue filter combined with and without a light-blue Wratten filter. There are practical advantages of using commercially available RGB cameras with this method if such a broad-band image acquisition system has sufficient estimation accuracy. We previously captured two sets of six broad-band images obtained by glass filters mounted in a wheel with glass filters, with and without extra absorption filter.1 In this report, we expand the analyses of spectral estimation using wide-band images by switching the red filter with a long-red filter in order to test the concept of using long-red, green and blue channels of the camera combined with and without lightblue absorption filter. The performance of this new configuration is compared to the imaging using all six filters of the filter wheel, as well as the configuration using six channels derived from red-green-blue filters without and with absorption filter

Comparison of the accuracy of various transformations from multi-band images to reflectance spectra

This report provides a comparative study of the spectral and colorimetric accuracy of various transformations from multi-band digital signals to spectral reflectance. The multiband channels were obtained by multi-channel visible-spectral imaging (MVSI) using a monochrome CCD and two different filtering systems. In the first system we used a liquid-crystal tunable filter (LCTF) capturing 31 narrow-band channels. We also used a filter wheel with a set of 6 glass filters imaging with and without an extra Wratten absorption filter giving a total of 12 channels. Four different mathematical methods were tested to derive reflectance spectra from digital signals: pseudo-inverse, eigenvector analysis, modified-discrete sine transformation (MDST) and non-negative least squares (NNLS). We also considered two different approaches to sampling the digital signals; in one approach we averaged the digital counts

Using the Matrix R method for spectral image archives

Conventional color digital cameras can only produce three-channel images so they are limited when high-quality color reproduction is required. Alternatively, spectral imaging increases the number of channels and can retrieve spectral reflectance for each scene pixel. The major goal of spectral imaging is high spectral accuracy, while it may also be beneficial to achieve high colorimetric accuracy for a specific viewing condition. A new spectral reconstruction method, called the matrix R method, was developed to achieve both goals simultaneously. An experiment was performed to test this method. The experimental results have been very promising; average color difference for all targets evaluated was about 1.3 CIEDE2000 and 2.0% RMS. These results suggest that this new method is a promising method for building digital image databases for museums, archives and libraries

Pigment selection using Kubelka-Munk Turbid media theory and non-negative least square technique

This report describes a process of pigment selection for reconstructing the Gamblin Conservation Colors and various artist pigments dispersed in linseed oil. Single constant Kubelka–Munk (K-M) turbid media theory and a non-negative least square (NNLS) optimization technique were employed in this experiment. Eleven pigments were selected as representative of the 30-pigments Gamblin Conservation Colors. These were quinacridone red (PV 19), venetian red (PR 101), cadmium red medium (PR 108), cadmium yellow medium (PY 37), indian yellow (PY 83), chromium oxide green (PG 17), phthalocyanine green (PG 7), phthalocyanine blue (PB 15:2), cobalt blue (PB 28), titanium dioxide white (PW 6), and ivory black (PBK 9)

Exploring the color inconstancy of prints

The color inconstancy of prints is related to the ink spectral properties and the lookup table for multiink printing systems. In this paper, color inconstancy was investigated for several ink-jet printers based on their ink set and the default lookup tables. A virtual model for each printer was created to determine the range of color inconstancy that a specific ink set could achieve. The color inconstancy performance of each default lookup table was evaluated by evaluating the color inconstancy of a printed test target. The optimum combinations of three- and four-chromatic inks were investigated to minimize the color inconstancy and keep a relative large color gamut simultaneously. The results showed that the color inconstancy can be decreased significantly without compromising the reproduction colorimetric accuracy. Moreover the color inconstancy can be improved by appropriate ink design

Spectral imaging using a commercial colour-filter array digital camera

A multi-year research programme is underway to develop and deliver spectral-based digital cameras for imaging cultural heritage at the National Gallery of Art, Washington DC, and the Museum of Modern Art, New York. The cameras will be used for documentation, production imaging, and conservation science. Three approaches have undergone testing: a liquid-crystal tunable filter (LCTF) coupled with a monochrome camera, a six-position filter wheel containing absorption filters coupled with a monochrome camera, and a two-position filter slider containing absorption filters coupled with a colour-filter array (CFA) colour camera. The last approach is the most practical as it uses conventional digital photography methodologies and equipment and can easily be incorporated into existing museum workflows. A virtual camera model was created that predicted camera signals from incident radiation and was used to design a pair of absorption filters. The filters were fabricated and tested using a commercial CFA digital camera. Our first experiments have been very promising: Average accuracy was under 1 CIEDE2000 and about 1.5 per cent RMS for both calibration and verification data. This level of performance was superior to our other, more complex approaches