Bernhard Hill

Comparative analysis of the quantization of color spaces on the basis of the CIELAB color-difference formula

This article discusses the CIELAB color spave within the limits of optimal colors including the complete volume of object colors. A graphical representation of this color space is composed of planes of constant lightness L* with an net of lines parallel to the a* and b* axes. This uniform net is projected onto a number of other color spaces (CIE XYZ, tristimulus RGB, predistorted RGB, and YCC color space) to demonstrate and study the structure of color differences in these spaces on the basis of CIELAB color difference formulas. Two formulas are considered: the CIE 1976 formula *** and the newer CiE 1994 formula ***. The various color spaces considered are uniformly quantized and the grid of quantized points is transformed into CIELAB colordinates to study the distribution of color differences due to basic quantization steps and to spacify the areas of the colors with the highest sensitivity to color discrimination. From a threshold value for the maximum color difference among neighboring quantized points searched for in each color space, concepts for the quantization of the color spaces are drived. The results are compared to quantization concepts based on average values of quantization errors published in previous work. In addition to color spaces bounded by the optimal colors, the studies are also applied to device-dependent color spaces limited by the range of a positive RGB cube or by the gamut of colors of practical print processes (thermal dye sublimation, chromalin, and match print). For all the color spaces, estimation of the number of distinguishable colors are given on the basis of a threshold value for the color difference perception of *** = 1 and *** = 1.

Color capture, color management, and the problem of metamerism: does multispectral imaging offer the solution?

Color management systems are being introduced worldwide to improve the color quality of digital image capture and device independent electronic color image reproduction. To be able to supply device independent color data at interfaces in imaging systems, device dependent color correction is required. The paper discusses concepts envisaged for color correction in image capturing devices with respect to fundamental requirements on color analysis. The common image capturing technology is based on the use of three color channels. Main points of the discussion are the shortcomings of this technology to analyze metameric colors correctly and the question if this will be an essential point for future imaging technology. Further parts of the paper cover the alternative multispectral technology. Multispectral cameras delivering the complete spectrum of color stimuli of each pixel of an image are available in the laboratory. This technology offers s solution to the problem of metameric color analysis and offers flexibility to match different illuminants as well, yet, the amount of additional effort is large. The paper summarizes studies and ideas on multispectral color technology and on how this technology might be introduced in future imaging and color management systems.

Multispectral color technology: a way toward high-definition color image scanning and encoding

Multispectral color image scanning and the transport of relevant spectral information up to output devices offer essential advantages compared to conventional three channel color reproduction. This is particularly true in open image communication systems which have to consider many different color reproduction technologies. The first part of the paper covers fundamental problems of three channel color image scanning. Afterwards, the basic ideas of a multispectral reproduction technology are presented to solve the problems. The multispectral scanner as a main part of the system is addressed in the next chapter. A major problem of multispectral technology is the encoding of the large amount of spectral data delivered by the scanner. A second important point with respect to the application of multispectral technology is compatibility with existing color systems using tristimulus color values. The paper present concepts for compatible and efficient spectral data encoding and discusses aspects of optimization strategies. The state of the art of multispectral technology and an outlook are given finally.

Optimization of total multispectral imaging systems: best spectral match versus least observer metamerism

The paper deals with total multispectral imaging systems assembled from a multispectral camera or scanner for capturing color images or natural scenes and a multichannel color image display for image synthesis. The aim of the multispectral technology is to reproduce an approximation to the spectral color stimuli of an original image in order to reduce color mismatches between original and reproduction for every human observer and any illuminant considered. Two system aspects are discussed in more detail: the representation of spectral data by a set of multispectral values at the interface between camera and display and the method of controlling the channels of the multichannel display to reproduce colors at least errors for every human observer. An expansion of spectral stimuli functions into a series of basis functions with weighting coefficients called multispectral values is used for the data representation at the interface. The set of basis functions is either optimized with respect to best fitting the original color stimuli or to produce least observer metamerism. It is shown that observer matched basis functions lead to smaller color errors. Best results for the control of the multichannel display are achieved with an iterative process using stochastic pattern generation. It is shown that maximum color errors for a large test set of spectral color stimuli on one side and 24 different human observers on the other are below 1,6 CIE-(Delta) E94 units if the experimental spectral characteristics of a 6-channel display are considered.

Multiprimary display: discounting observer metamerism

Multispectral imaging has been proposed to overcome the shortcomings of conventional three channel imaging. This technique uses spectral data to describe the color of each pixel, allowing for a significant increase in color accuracy. While most publications on multispectral imaging deal with different aspects of image acquisition, the output of multispectral images becomes increasingly important. This paper describes a way to use a six-primary display as a multispectral output device. Additionally to a significant increase in device gamut, the use of more than three primaries introduces additional degrees of freedom for displaying a given color. This allows to account for observer metamerism by displaying the color in such a way that the color differences are minimal for every observer. An optimization algorithm was derived that calculates optimal control values for the six channels of the projector. One important aspect of such a method is that the control values need to be constrained, because for each channel of the projector the maximum output is limited and it is also impossible to create a negative output. Using a linear programming technique such a method was found. The methods performance was evaluated using simulation. The methods dependency on the definition of the white point is discussed. Finally, it is shown that mean errors of approx. 0.5 (Delta) Eab can be achieved.

LED-illuminated CCD slide scanner with a new concept for color processing

Accurate color reproduction of slide images on a display is a complicated procedure. It requires the characterization and control of several imaging processes and the application of appropriate color-appearance models to account for the differences in viewing conditions on a projection screen in the dark surround on the one hand and on a display in dim surround on the other hand. This paper reviews the development and evaluation of a new concept of scanning of color slides. The concept results in a three dimensional look-up table to achieve a high image quality. This table is calculated for all possible film colors under a variety of viewing conditions and considers the gamut mapping of slide colors and the transformation to RGB signals to drive a display.

Biometrische Messung der Pupillenreaktion

Die Messung der Pupillenweite und -reaktion (Pupillometrie) auf Lichtreize wird bereits seit langer Zeit zu diagnostischen Zwecken benutzt, um Aussagen uber Vitalfunktionen zu tatigen, z.B. in Schlaflaboren zur Beurteilung der Schlafrigkeit. Ziel dieser Arbeit war die Entwicklung und Realisierung eines Versuchsaufbaus zur standardisierten und reproduzierbaren Messung des zeitlichen Verlaufs des Pupillendurchmessers in Abhangigkeit von definierten Lichtreizen. Anhand des Graphen wurden mehrere charakteristische Parameter bestimmt. Aufgrund einer hohen Bildrate von bis zu 560 Bildern pro Sekunde und einer geringen Beleuchtungsstarke von ca. 25 Lux wird mit dem Messaufbau eine hohe Genauigkeit erreicht. Ein Zusammenhang zwischen Alkoholkonsum und einer veranderten Reaktion der Pupille konnte nachgewiesen werden.