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

Expanding Dimensionality in Cinema Color: Impacting Observer Metamerism through Multiprimary Display

Television and cinema display are both trending towards greater ranges and saturation of reproduced colors made possible by near-monochromatic RGB illumination technologies. Through current broadcast and digital cinema standards work, system designs employing laser light sources, narrow-band LED, quantum dots and others are being actively endorsed in promotion of Wide Color Gamut (WCG). Despite artistic benefits brought to creative content producers, spectrally selective excitations of naturally different human color response functions exacerbate variability of observer experience. An exaggerated variation in color-sensing is explicitly counter to the exhaustive controls and calibrations employed in modern motion picture pipelines. Further, singular standard observer summaries of human color vision such as found in the CIE’s 1931 and 1964 color matching functions and used extensively in motion picture color management are deficient in recognizing expected human vision variability. Many researchers have confirmed the magnitude of observer metamerism in color matching in both uniform colors and imagery but few have shown explicit color management with an aim of minimized difference in observer perception variability. This research shows that not only can observer metamerism influences be quantitatively predicted and confirmed psychophysically but that intentionally engineered multiprimary displays employing more than three primaries can offer increased color gamut with drastically improved consistency of experience. To this end, a seven-channel prototype display has been constructed based on observer metamerism models and color difference indices derived from the latest color vision demographic research. This display has been further proven in forced-choice paired comparison tests to deliver superior color matching to reference stimuli versus both contemporary standard RGB cinema projection and recently ratified standard laser projection across a large population of color-normal observers

Prototype software for colorant formulation using Gamblin conservation colors

When selecting pigments from a large set for restorative inpainting, it can often be challenging to create a mixture that will provide an exact match to the original artwork under a range of viewing and illumination conditions. In this research, a prototype computer program was developed that will aid the user by providing a color match and paint recipe that exhibits minimal metamerism when compared to the original artwork. The Gamblin Conservation Colors, a set of 43 colorants specially formulated for inpainting, were characterized in terms of their optical properties, absorption and scattering, according to Kubelka-Munk turbid media theory. Formulations were made using traditional spectrophotometric measurements and image-based measurements. The multispectral imaging system consisted of a trichromatic CFA camera coupled with two absorption filters; spectral reflectance data for each pixel location was estimated with a transformation based on calibration target images. Three targets were used for testing formulation accuracy: a target consisting of mixtures of Gamblin Conservation Colors, and two oil paintings. Pigment selection was reasonably successful, and good predictions resulted from both measurement techniques, but for more complex tasks such as pigment identification, a more rigorous colorant characterization approach may be needed. Predictions from image-based measurements were generally less accurate, and improvements in the camera model would likely remedy this. It is expected that this software will be of assistance to conservators by simplifying the process of selecting from a large set of available pigments, as well as reducing the possibility of damage to painted surfaces in cases where direct measurements are impractical. The open source nature of the software provides the opportunity for changes and addition of features in the future

Correcting cross-media instrument metamerism for reflectance estimation in multispectral imaging

Author name used in this publication: John H. Xin2010-2011 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

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

Spectral printing of paintings using a seven-color digital press

The human visual system is trichromatic and therefore reduces higher dimensional spectral data to three dimensions. Two stimuli with different spectral power curve shapes can result in the same cone response and therefore match each other. Color reproduction systems take advantage of this effect and match color by creating the same cone response as the original but with different colorants. ICC color management transforms all colors into a three-dimensional reference color space, which is independent from any input or output devices. This concept works well for a single defined observer and illumination conditions, but in practice, it is not possible to control viewing conditions leading to severe color mismatches, particularly for paintings. Paintings pose unique challenges because of the large variety of available colorants resulting in a very large color gamut and considerable spectral variability. This research explored spectral color reproduction using a seven-color electrophotographic printing process, the HP Indigo 7000. Because of the restriction to seven inks from the 12 basic inks supplied with the press, the research identified both the optimal seven inks and a set of eight artist paints which can be spectrally reproduced. The set of inks was Cyan, Magenta, Yellow, Black, Reflex Blue, Violet and Orange. The eight paints were Cadmium Red Medium, Cadmium Orange, Cadmium Yellow Light, Dioxazine Purple, Phthalo Blue Green Shade, Ultramarine Blue, Quinacridone Crimson and Carbon Black. The selection was based on both theoretical and experimental analyses. The final testing was computational indicating the possibility of both spectral and colorimetric color reproduction of paintings

Spherical sampling methods for the calculation of metamer mismatch volumes

In this paper, we propose two methods of calculating theoretically maximal metamer mismatch volumes. Unlike prior art techniques, our methods do not make any assumptions on the shape of spectra on the boundary of the mismatch volumes. Both methods utilize a spherical sampling approach, but they calculate mismatch volumes in two different ways. The first method uses a linear programming optimization, while the second is a computational geometry approach based on half-space intersection. We show that under certain conditions the theoretically maximal metamer mismatch volume is significantly larger than the one approximated using a prior art method