Perfectly diffuse light is a fundamental assumption in the Kubelka-Munk (KM) model. This assumption is, however, seldom fulfilled by real media. In this work, we build a bridge between a weakly absorbing medium with an imperfectly diffuse light distribution and the corresponding KM model relying on a perfectly diffuse light distribution. We multiply the apparent K and S phenomenal coefficients deduced from a learning set medium by a scaling factor expressing the ratio of light diffuseness between the target medium and the learning set medium. For the target medium, thanks to this diffuseness scaling factor, theoretically predicted reflection and transmission spectra agree with the corresponding measured spectra. The illumination geometry, the optical properties and the thickness of the medium have an impact on the light diffuseness and therefore on the proposed diffuseness scaling factor.

Toner transfer efficiency in electrophotography and the resulting image quality are influenced by the resistivity of the substrate. Resistivity measurements are, however, often made using electric fields that are lower than those used in the transfer of toner in electrophotography. The dependence of surface and volume resistivity on electric field strength, including fields similar to those utilized in the toner transfer of electrophotographic printers, has been studied. Resistivities of paper samples with differences in grammage, filler content, and calendering were evaluated utilizing electrodes with a geometry in accordance with the ASTM D257 (volume resistivity) and raker-type electrodes (surface resistivity) applying electric fields of between 10² and 10⁵ V/cm. The surface resistivity and especially the volume resistivity of paper were found to be strongly dependent on the electric field strength, the characteristics of this dependence being influenced by paper properties. The study of the field dependence further indicated that the Poole-Frenkel type of hopping drift of ions could be applied to conduction in paper, although the electric field dependence was overlapped by paper compression effects. Shottky's model was considered also, but it seems that the role of contact effects is small.

Digital imaging continues its expansion into various applications. Spectral images are becoming more popular as one field of digital imaging. At the same time, the economic and security aspects in the utilization of images are finding increased emphasis. In this study, the authors apply a watermarking method for spectral images, whereby embedding is based on the three-dimensional wavelet transform. The authors study the influence of illumination to the watermarked images. The authors also define how to estimate an unknown illuminant. Experiments were performed on a dataset of 13 spectral images. These experiments indicate that the illumination in spectral images should be established before watermarking, owing to the properties of the illuminants. Using the proposed watermarking method, the embedded watermark is also robust towards lossy compression. Guidelines for the parameter selection for watermarking with the proposed approach are given.

Accurate recovery of spectral reflectances is important for color reproduction under a variety of illuminations, and several models have been proposed to recover them. To evaluate the quality (Q₁) of an image acquisition system aimed at recovery of spectral reflectances, we proposed an evaluation model based on Wiener estimation and showed that mean square errors between the recovered and measured spectral reflectances as a function of Q₁ agree quite well with the prediction from the model, and that estimation of the noise variance of the image acquisition system is essential to the evaluation model. In this paper, the evaluation model was applied to two different reflectance recovery methods, and it is confirmed that the proposed model can be applied to different methods.

This paper presents a learning-based principal component analysis technique for accurate representation of spectral color in low and high resolution spectral images. Three learning techniques, LLE, ISOMAP, and regressive principal component analysis (PCA), are studied for this purpose. The basic concepts for the regressive PCA technique, which is computationally efficient and represents a combination of standard PCA and regression, are examined. To utilize dimensionality reduction techniques such as LLE and ISOMAP as parametric mapping procedures, the methods must be modified by combining them with a regression approach which provides data mapping from a low-dimensional space to the input space. The LLE, ISOMAP, and regressive PCA learning techniques are compared with standard PCA using low-resolution spectral images. We show that the LLE and ISOMAP approaches are computationally demanding and are not well suited to high resolution image analysis. Regressive and standard PCA are then used in a test with high resolution spectral images. The comparative study based on the S-CIELAB ΔE and RMSE employs regressive PCA measures to illustrate accurate color representation.

With the current trend of digital convergence in mobile phones, mobile manufacturers are researching how to develop a mobile beam projector to cope with the limitations of a small screen size and to offer a better feeling of movement while watching movies or satellite broadcasting. However, mobile beam projectors may project an image on arbitrary surfaces, such as colored walls and papers, not only on a white screen as is mainly used in an office environment. Thus, a color correction method for the projected image is proposed to achieve good image quality irrespective of the surface color. Initially, luminance values of the original image in the YCbCr color space are changed to prevent unnatural luminance reproduction of the projected image, depending on the contrast and luminance of the surface image captured with a mobile camera. Next, the chromaticity values for the captured surface and white screen images are calculated using a ratio of the sum of three RGB values compared to one another. Then, their chromaticity ratios are multiplied by the converted image through an inverse YCbCr matrix to reduce the influence of modulating the color tone of the projected image due to spatially different reflectances on the surface. By projecting the compensation image on a texture pattern or single color surface, the image quality of the projected image can be improved, compared to that of the projected image on a white screen.

A charge control mechanism of charge control agents (CCAs) has been proposed in the present investigation that assumes an appreciable temperature increase at the "toner/carrier" interface due to triboelectrification. A further assumption is that the CCA is present on the surface of both toner and carrier. Because of local heating, the electrical conductivity of CCAs is increased remarkably to give a conductive channel, through which the carrier flow occurs effectively to charge the toner. These two assumptions have experimentally been verified. Especially, local heating of up to ∼100 °C has been confirmed by using a pigment marker that changes its color from black to red. Around this temperature, the electrical conductivity of CCAs is also found to increase significantly by one to three orders of magnitude as compared with that at room temperature.

In dry toner electrophotography, image fixing by roller fusing technology is affected by the process parameters, and by toner and paper properties. In this type of contact fusing, the fusing energy is applied in the form of pressure and conductive heat as a function of the dwell time determined by the process speed and fusing nip width. The present study was designed to provide a deeper understanding of the behavior of paper in a modified fusing nip with controlled speed and temperature. The effects of moisture content and coating color were examined as well, and some quality factors of toner images were correlated to the thermal behavior and bulk properties of the paper samples used in the experiment. In relation to electrophotography, the objective was to explore the role of paper in the fusing stage, and to show how different paper grades contribute to image quality at different fusing speeds and temperatures, which were the main fusing parameters controlled in the experimental setting. The results show that the thermal behavior of paper is strongly related to the paper mass expressed by grammage, and the thermal behavior is less sensitive to speed with increased paper density. Gloss and toner adhesion are affected by the thermal properties of paper. The experimental design, involving a wide range of fusing variables, confirmed the need to understand on the mechanism of toner fusing and the role of toner-paper thermal interactions more deeply. The thermally controlled fusing system used in this work was found to be useful in characterizing the influence of moisture content and coating color on the thermal behavior of paper and the quality of image fixing.