A method is described for calibrating a trichromatic halftone printing system in CIELAB colorimetric coordinates. The calibration method involves the use of three analytic equations rather than a LUT and interpolations. Calibration is done empirically by sending to the printing system a set of dot fraction vectors, [c m y]. CIELAB [L*a*b*] vectors are measured for each printed sample, and a statistical and algebraic analysis then leads to three algebraic equations for converting [L*a*b*] vectors to [c m y] vectors. The quality and efficiency of this calibration technique are discussed in comparison to the more common LUT technique with interpolations. Calibration quality was measured both as the average and the maximum value of CIELAB ΔE between the printed and the calculated [L*a*b*] vectors. The potential utility of the algebraic method of calibration is discussed in terms of the tradeoff between colorimetric accuracy, computational speed, and the ease of performing the calibration.

This article proposes a new method of natural color reproduction by adaptive color conversion of an image with highlights under arbitrary illuminants. This method uses a digitized RGB value of a color image under various light sources. Each pixel of the color image is represented as a 3-dimensional vector with red, green, and blue light-value components. The unknown light source used in the capturing process of an image is then estimated as the mean vector of the spatial means of the red, green, and blue image components. In contrast, standard light sources, such as D65 or D50, that appear white to the human eye, are described as a unit vector in RGB color space if color constancy is assumed. The proposed vector transformation can thus be defined as a vector rotation, through which the white point of an estimated light source is transformed to one of standard light. Next, a 3 × 3 homogeneous transformation matrix relative to the equivalent axis is produced and the input color of each pixel is transformed into a new RGB color space by using this matrix as a color filter. However, if there are highlights in the input image, the initial method proposed can cause some distortion due to the over-transformation of highlights. Accordingly, to solve this problem this article proposes a modified transformation function where the initial transformation matrix is corrected by regression based on identifying a highlight mapping without distortion. Thereafter, the correctly transformed red, green, and blue color values of each pixel in the new space are non-linearly mapped into integer values between 0 and 255 for a 24-bit natural color display and stored in digital media. The proposed method can be used in the color conversion of an image under arbitrary illuminants to one under daylight through improving the effect of the incident illumination and thereby enhancing the image quality.

Estimating the color of the illuminant under which an image of a scene was captured is a central part of solving the color constancy problem: that is, of deriving an illuminant independent representation of the reflectance's in a scene. In this article we develop a general correlation framework for solving the illuminant estimation problem. Within this framework estimation is posed as a correlation of the colors in an image with the colors that can occur under each of a set of possible lights. Rather than attempting to recover a single estimate of the illuminant as many previous authors have done, we, in the first instance, recover a correlation measure for each possible illuminant. We then select an estimate of the scene illuminant based on these correlations. The work presented here follows from previously published work.¹ In this article we extend that work by showing that the correlation framework is rich enough to allow many existing algorithms to be expressed within it. In particular the grey-world, max-RGB, gamut mapping, and Maloney–Wandell algorithms, perhaps the algorithms most widely cited in the literature, are presented in this correlation framework. This work together with work published elsewhere² shows that almost all published algorithms based on a Mondriaan world can be formulated in the framework presented here.

A new type of color mapping method named ‘categorical color mapping’ is developed for gamut mapping. Categorical color mapping provides an optimal mapping point while maintaining a relative color-categorical relationship between a source space and a destination space. Existing techniques in the field of color science and engineering have been developed based on matching criteria. However, there has been no matching criterion for design of a gamut mapping algorithm due to difference in device gamut between a source space and a destination space. Therefore, gamut mapping has remained a critical issue in comparison with the existing techniques. In categorical color mapping, a new matching criterion named ‘categorical color matching’ is introduced for implementing a matching criterion into the design of gamut mapping. Because categorical color matching is based on a color naming experiment, categorical color mapping is intended to include the perceptual information of the human visual system. This is another factor that differentiates the proposed method from conventional techniques in addition to the implementation of the matching criterion. This article describes the theory of categorical color mapping and its effectiveness in comparison with conventional gamut mapping techniques.

This article is linked to the previous article, “Categorical color mapping using color-categorical weighting method Part I: Theory” (Ref. 1) and shows its performance for gamut mapping through psychophysical experimentation. This method maps an original color to a reproduced color by weighting constraints mapping so as to match their color names between the original color and the reproduced color. A degree of the weighting is derived from a categorical color naming experimentation. Therefore, categorical color mapping incorporates an observer's response in calculating an optimal mapping point. The algorithm of categorical color mapping has seventeen constraints. Nine of them are given as the geometrical centers of gravity of the color categories. The others are positioned on the gamut surface. In the former case there was colorimetric matching pair-wise between a source space, and a destination space and the latter was given by categorical lightness mapping with constant hue. Categorical color mapping was evaluated as the superior method among tested methods in paired-comparison experiments. GCUSP and clipping method were applied as conventional techniques to compare to categorical color mapping. From the viewpoint of categorical color mapping, performance of GCUSP can be explained. In comparison with these conventional methods, hue linearity correction in CIELAB space by categorical color mapping was confirmed.

This article proposes Gamma-Compression Gamut Mapping Algorithm (GMA) based on the concept of Image-to-Device. Considering the relations of color gamut boundaries between the devices (such as printer, monitor) and image source, the proposed method adjusts the image color distributions, fitting to the output device gamut. The proposed image-to-device GMA works to preserve its gradation and chroma with minimum losses depending on the color distributions in the segmented hue-leaves. The following three typical mapping methods are compared and discussed: (1) Device-to-Device Gamma-Compression GMA, (2) Image-to-Device Gamma-Compression GMA; (3) Clipping GMA. Two kinds of color spaces, CIEL*a*b* and CIECAM97s-JCh, are used for testing these GMAs. The psychophysical experiments in CIECAM97s-JCh space resulted in better rendition than did CIEL*a*b* space. In natural color imaging applications, the proposed Image-to-Device GMA is superior to conventional Deviceto-Device GMA, and is a better way to maintain its gradation and higher chroma.

One common artifact in color printing is the moiré caused by the superposition of the different color separations. This problem is well known and a common approach to minimize the moiré is the use of rotated halftone dots of precise frequency and angle for the different separations. In low resolution devices, the precise relationship between the different separations is commonly hard to satisfy and compromises have to be made. This paper describes a method to vary the undercolor removal (UCR) and gray component replacement (GCR) schemes employed as a function of the predicted inter-separation moiré in order to allow for a better trade-off in the halftone design.

Nanosize silica, titania and alumina metal oxide particles are common xerographic toner additives for control of flow, charge, development and transfer. Typically the oxides are rendered hydrophobic by reaction of the surface hydroxyl groups with organosilanes. This treatment leads to improved flow and charging properties but also typically gives unacceptably long admix charging times. It is shown that admix times on alumina and titania can be dramatically reduced by adsorption of amines such as triethylamine. The most acidic hydroxyl groups of alumina and titania undergo a proton transfer to form a salt with the triethylamine: it is this species that gives rise to improved admix. In effect, the formation of the quaternary salt on the surface mimics the performance of traditional ionic charge control agents. Alternatively, the same decrease on admix can be achieved by incorporating covalently bound sulfate species on the oxide surface. The sulfate groups increase the ionic character through an enhancement of the Bronsted acidity of the oxide. Only a relatively few hydroxyl groups need to be converted to an ionic form (either by amine addition or sulfate incorporation) to dramatically improve admix. A majority of the hydroxyl groups remain unaffected. These remaining groups can then be reacted with an organosilane to yield an oxide with increased negative charge and improved flow. The decrease in admix time with amine pretreatment is independent of the effects of subsequent hydrophobic treatments. In this case, the shorter admix times generated by the quaternary amine salt is retained.

The forces needed to remove monodisperse spherical toner particles from an organic photoconductor were determined using electrostatic detachment for a series of particles having diameters between 2 μm and 12 μm. It was found that the removal force varied linearly with particle radius, as predicted by JKR (Johnson, Kendall and Roberts, Ref. 19). This result is inconsistent with the predictions of models that assume that the detachment forces are dominated by either a uniform charge distribution over the surface of the particle or localized charged patches. Moreover, reasonable works of adhesion are obtained if one assumes that the removal forces are dominated by surface, rather than electrostatic, forces. These results seem to suggest that, for spherical toner particles in this size range, adhesion may be dominated by van der Waals interactions.