Kenro Ohsawa

Color image reproduction based on multispectral and multiprimary imaging: experimental evaluation

Multispectral imaging is significant technology for the acquisition and display of accurate color information. Natural color reproduction under arbitrary illumination becomes possible using spectral information of both image and illumination light. In addition, multiprimary color display, i.e., using more than three primary colors, has been also developed for the reproduction of expanded color gamut, and for discounting observer metamerism. In this paper, we present the concept for the multispectral data interchange for natural color reproduction, and the experimental results using 16-band multispectral camera and 6-primary color display. In the experiment, the accuracy of color reproduction is evaluated in CIE (Delta) Ea*b* for both image capture and display systems. The average and maximum (Delta) Ea*b* = 1.0 and 2.1 in 16-band mutispectral camera system, using Macbeth 24 color patches. In the six-primary color projection display, average and maximum (Delta) Ea*b* = 1.3 and 2.7 with 30 test colors inside the display gamut. Moreover, the color reproduction results with different spectral distributions but same CIE tristimulus value are visually compared, and it is confirmed that the 6-primary display gives improved agreement between the original and reproduced colors.

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.

Multiprimary display optimized for CIE1931 and CIE1964 color matching functions

A novel color reproduction method that is independent of the viewing angle is proposed. In this method two sets of tristimulus values corresponding to the CIE 1931 and the CIE 1964 color matching functions are reproduced using a six- primary display. The digital counts of the six-primary display to reproduce a given set of six-stimulus values can be uniquely determined by using a 6 by 6 matrix which is calculated from the two color matching functions and the primary spectra of the six-primary display. In this paper we report the result of the simulation that examines the color reproduction accuracy using this method when a spectrum of the object is given. Comparing the color reproduction accuracy achieved by the 6-primary display using our method with that by a current RGB display it is verified that the proposed method can improve the accuracy.

Natural color reproduction system for telemedicine and its application to digital camera

For a reliable diagnosis via visual telecommunication systems, it is very important to reproduce patient images with correct color. But it is not certain by conventional color imaging systems especially when the illumination of observing site is different from that of the image capturing site. We present a natural color reproduction system for telemedicine. In this system, the color of a patient under observing illumination is calculated from multispectral image. For accurate color reproduction, the spectral reflectance is calculated using principal components of human skin in the process. The color image is displayed on a calibrated display device. In the experiment, it is confirmed that the system realizes good accuracy in the color reproduction of human skin from 10 band multispectral image. And for simplification of the system we also adapted this process to commercial digital RGB camera.

Spectrum-based color reproduction system for motion picture

The architecture of color reproduction system based on spectral information is addressed. This system aims to realize the accurate color reproduction under arbitrary illuminants and color matching functions and anticipates the ultimate reproduction of spectral reflectance and spectrum of objects. This system is like an expanded version of that proposed by the International Color Consortium (ICC). One of the features of this system is the usage of the spectral space for the profile connection space alternative to the colorimetric space. It retains compatibility with the indigenous color management functions of the current system. The proposed system is further expanded to construct the spectrum-based color reproduction system for motion picture. Six-band HDTV camera and six-primary projection display system, which is realized in our laboratory as experimental spectrum-based color reproduction system for motion picture, is briefly introduced.