Doubling the Color Gamut Volume of Ink Jet Prints using Simple Post-Processing

We describe a novel coating process for preserving and enhancing color images from ink jet prints. It involves stabilizing the printed image, followed by selective chemical interaction with the inks. The coating provides liquid, gas and UV light protection. It also changes the way light interacts with the original colorants. Untrained viewers (e.g. ordinary consumers) report the coated images appear more photographic than the original ink jet image. They prefer these images to the originals by a wide margin and report that they would pay up to $1.00 per print for them1. They describe the colors as more luminous and vibrant. To better understand the reaction of these people we have conducted a small study of the spectral properties of the treated images using standard color calibration prints as test samples. Measurements of the treated samples and their comparisons with untreated samples show the method’s ability to enlarge the gamut’s volume up to at least the double size of the initial color gamut volume. Introduction Ink jet printing is widely used for creating low cost, photolike or photo-realistic prints. Enormous improvements in the quality of these printers have been achieved in nearly every new generation of ink jet printers. In general, these improvements require use of specialized inks, a wider variety of inks and the use of specialized substrates [1, 2, 3, 4, a.o.]. Our approach is somewhat different. It derives from several years of informal, private studies and experimentation to improve the quality of ink jet prints. We have focused on using the lowest cost and simplest inks and substrates available2. We believe our studies apply to a broad range of printing processes, beyond Ink Jet. The concept generally relates to a novel coating process intended to enhance the lifetime and color image properties of printed images. Our process involves stabilizing the printed image, followed by a selective chemical interaction with the inks. The coating provides liquid, gas and UV light protection. The structure of the coating results in enhanced illumination of the printed image and a resulting change in the appearance of the image. Treatment Process Although a variety of methods can be used [5], the fundamental approach is to transfer some or all of the colorants embedded in the substrate of the printed image into a transparent structured coating. 1Informal reactions from VC bankers and patent agents. 2Our** exploration began in 1996 with the purchase of a low cost, older HP Ink Jet printer. To maintain continuity and stability within these studies, it served as the print engine for all subsequent work. The treatment process described is performed after the image has been printed. A transparent structural polymer layer is applied to the print. It serves a variety of purposes. It is semipermeable, porous and sponge like. Its structure is used as a conduit through which a second polymeric material is applied to the print. The two are selected so that they do not chemically interact. The second polymer (reflow agent) interacts with the inks in the printed image. The objective is to transfer some or all of the image into the transparent structured covering layers. The first (matrix) layer provides structure to the coating, limiting the transport of inks. This confines the spread of ink. Another objective is to facilitate Gaussian diffusion of colorant to both soften the defined edge of ink pixels and to allow some limited mixing of ink. The objective is to create colorant clouds within the transparent over layer. The intertwined dual-polymer structure solidifies resulting in a coating that imparts protective properties to and extends the lifetime of the image. The porosity of the first coating provides reservoirs into which colorants diffuse and mix. Doing so changes the optical properties of the image resulting in a dramatic enlargement of the color gamut. Moving the colorants into the transparent layer modifies the illumination of the inks used to create the image. Those colorants that are transferred into the structured over-layer interact with light via reflection (as is normal for a printed image) and via translucence: light reflected from the substrate passes multiple times through the colorant clouds increasing its luminosity (like a stain-glass window). The images analyzed in this paper use selective ink interaction. This formulation uses a reflow agent that interacts and transports only the CMY colorants (not black ink). We choose to do so to retain sharp, highly detailed image. In a sense we are merely mimicking methods used for encoding television images. Television images are encoded and broadcast such that the luminance of the image is more highly sampled and has a higher transmission bandwidth than does the color portion of the image. Separation of color and luminance in broadcast television reflects both historical and psycho-visual factors. The different sampling rates reflect variations in the retinal mosaic and the various roles played by the rods and cones. Our approach merely incorporates these insights into a process for treating printed subject matter. Experimental and Gamut Volume Comparisons In order to evaluate the gamut expansion performance of the introduced treatment, three samples of the ECI 2002 target [6] were printed using a HP670C printer in its draft quality, economical ink consuming mode. Our choice of printer was predicated on this printers relatively low resolution (300 dpi) and the simplicity of its inks. The ECI targets were printed on A4 sheets 350 400 450 500 550 600 650 700 750 0 0.5 1 1.5 2 − lo g 1 0 ( R (λ ))