Tetra-chroma Display: Color Accuracy between Soft-proofing and Hardcopy

This paper introduces tetra-chroma display unit that has, in contrast to the contemporary trichromatic displays, four primary colors. Extra fourth cyan light allows better reproduction not only region of cyan colors but also, thanks to advanced position of green primary light, orange and yellow colors. These both regions are printable through modern inkjet printers. Colorimetric driving is discussed in detail and also reproduction on the basis of spectral matching isn’t omitted. An aim is to give for graphic designer or photographer solution that make easier preparation of their art work from the color accuracy point of view. Tetra-chroma display unit offers high quality soft-proofing. 1 Color gamuts of displays and printers Position of contemporary three display primaries (in terms of chromaticity diagram) is given by compromise to cover with the given reproduction triangle the area of the most frequently occurred colors. As for green primary light in contemporary reproductive triangle, in contrast to the blue and red primary, is the most compromisingly located to achieve satisfactory reproduction of yellows as well as cyans. Due to this compromise, cyans and orange-yellows that are achievable in printing are not reproducible by the contemporary trichromatic display systems. This leads into poor function of soft-proofing – simulation of printed colors on the monitor [1]. On Fig. 1 below, there can be seen color gamuts of Eizo CG222W display and Epson Stylus PRO 7900 printer & UltraChrome HDR inks (configuration with Photo K) & Fomei REAL Velvet paper. REAL Velvet paper meets conditions on proofing paper type 1/2 according to ISO12647-2 standard, hence comparison of REAL Velvet color gamut with ISO Coated v2 gamut is reasonable to mark, too (see Fig. 1 below). As can be seen, although marked monitor profile has expanded gamut (today trend: moving from sRGB to Adobe RGB gamut), there are still regions of colors, that are printable but are not reproducible on monitor. Figure 1: Projection of color gamuts into the ab plane of CIE Lab color space (on the left) and CIE xy chromaticity diagram (on the right) The reproduction of really all existing colors is not desirable in every task since very often given imaging system cannot generate (or transmit) such color gamut and then statistical redundancy of the signals driving the display increases and gamut of reproduction is not exploited. By looking at contemporary reproduction triangle in CIE xy or CIE u'v' chromaticity diagram, it is evident, that cardinal significance has addition of the fourth reproduction light into area of cyans. Thanks to this cyan reproductive light, the green reproductive light would be shifted towards the spectrum locus similarly as in the case of NTSC standard. It means, that the choice of the new cyan primary light carries an advantageous position for green primary. The contemporary sRGB reproductive triangle and the new proposed RGCB tetragon is shown on Fig. 2 below. The position of R and B primary is in the case of RGCB the same as in sRGB specification (see Tab. 1). The new position of G primary is xG = 0.26, yG = 0.7, and new C primary has chromaticity coordinates as follows: xC = 0.05, yC = 0.6. The primary lights were not chosen too spectral to be achievable through different technologies. From the spatial resolution point of view, the addition of the fourth primary light still ensures the reach of the desired resolution not only for TV but also for graphic, fine art and scientific application. Contemporary possibilities of graphic cards and given displays allow trouble-free adding of the fourth light – the graphic cards offer a sufficient capacity and displays have so small image pixels that one picture element, which today consists of three pixels, can be created by four pixels without loss of quality in spatial resolution. 0 0.2 0.4 0.6 0.8 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 x y TRIchroma (sRGB) TETRAchroma (RGCB)