Review and analysis of spectral characterization models and halftoning for multi-channel printing

There has been a long tendency to accurately communicate colour from outside world to a print. Every object has the ability to selectively absorb and reflect certain wavelengths of the incoming light. As the Human Visual System (HVS) is performing its function through three-chromatic nature of its sensors, we build our system and devices that perform its colour information acquisition and reproduction based on this fact. Most of the imaging devices we use today are still three to four colour (channel) processes. Reducing the dimensionality of the incoming light to three, we have established trichromatic colour management paradigm. The problem has always been that this reduced trichromatic information could be resolved of many of the spectral metamers, and this could occur not only when we change the illumination but also when we change the observer. Spectral information is therefore, the true and full information about an object’s ability to selectively attune the light. Spectral printer models, takes this physical information of its primaries and with linear addition, they derive the spectral output. The models rely heavily to assumptions made to the physical behaviour of the ink, paper and light which is less than obvious and easy to model. With this difficulty to model the physical behaviour come the advantages of using spectral system: reduced metamerism, higher accuracy, convenience in incorporation of the other physical phenomena (gloss, textured surface), possibility to convey true appearance of the scene, etc. Although spectral imaging has been used in many industries for many decades (e.g. medicine, security), the attention in graphic arts is received around two decades ago. However, even till today, there has not been a real application of the spectral reproduction system. The reason is that with advantages come the shortcomings: complexity and computational intensity. The current ICC architecture is still not able to incorporate such system due to its shortcomings (ICC White Paper, 2010). Therefore, spectral reproduction or spectral printing is jet to see the light on the greater scale. It is however, one of the most researched areas in printing industry for the last two decades, as the computational power increases and the number of the researches are working around it (Taplin, 2001, Urban and Rosen, 2008, Tzeng and Berns, 1998, Gerhardt and Hardeberg, 2006). Introduction of the multichannel printing has occurred within the conventional printing where the standard CMYK printing processes were enriched by two more channels where the gamut of the conventional printing was simply not enough to satisfy the demanding needs. These ink configurations are almost a standard in today ink jet printing, as it is the technology that provides a high flexibility needed to incorporate such system. Spectral printing has the interest in using these multichannel processes in the sense of better sampling of the input spectra and producing the spectrally matched output. The logic behind this is that more colours will mean more degrees of freedom in reproduction of the right colour. Classical example of this is that there is only one CMYK four colour combination, but multiple one exists if we have a seven channel system. Same holds for three and two colour combination. Printer models convert the input to the output by modelling the behaviour of the printing system and its colorants. Physical or spectral models are relying on the physical information of the primary colours and make their approximation based on the assumption linear mixing of the colorants on the substrate.