The Amber Project: A Survey of Methods and Inks for the Reproduction of the Color of Translucent Objects

Unlike regular pigments based on selective light absorption, the so-called “effect pigments″ are based on the phenomena of structural color, or selective reflectance. Structural color has appealing aesthetic qualities, such as angle-dependent hue, and is able to produce lightfast colors. When used as a pigment, however, the gamut of the print is more limited, the color is difficult to measure, and therefore color management and preprint process become challenging. The aim of this paper is to compare the behavior of effect pigments in the processes of lithographic and screen printing with standard pigments used in so-called process inks, and to analyze their optical properties when used on their own or in combination with absorption pigments. An image of amber beads was printed as screen prints and lithographs. Three sets of inks were used: Set one: Standard process inks in the colors cyan, magenta, yellow and black (CMYK); set two: RGB inks formulated with Merck Spectraval™ pearlescent pigments which allow additive red, green, blue printing on a black substrate; and set three: golden inks formulated with pigments from the Merck Iriodin™ and Pyrisma™ effect pigment range. The image was printed on white and black paper. The optical appearance was assessed visually, and spectra and color coordinates were measured.

A precise method of color space conversion in the digital printing process based on PSO-DBN

Neural networks have been widely used in color space conversion in the digital printing process. The shallow neural network easily obtains the local optimal solution when establishing multi-dimensional nonlinear mapping. In this paper, an improved high-precision deep belief network (DBN) algorithm is proposed to achieve the color space conversion from CMYK to L*a*b*. First, the PANTONE TCX color card is used as sample data, in which the CMYK value of the color block is used as input and the L*a*b* value is used as output; then, the conversion model from CMYK to L*a*b* color space is established by using DBN. To obtain better weight and threshold, DBN is optimized by a particle swarm optimization algorithm. Experimental results show that the proposed method has the highest conversion accuracy compared with Back Propagation Neural Network, Generalized Regression Neural Network, and traditional DBN color space conversion methods. It can also adapt to the actual production demand of color management in digital printing.

Statistical evaluation of a Color Managed Digital Printing Workflow (CMDPW) consistency [4th C of CMW]

The purpose of this applied research was to determine the Color Managed Digital Printing Workflow (CMDPW) consistency (4th C of the color management) over a period of time [100 days, (N = 100)]. The quality of digital color printing is determined by these influential factors: screening method applied, type of printing process, calibration method, device profile, ink (dry-toner or liquid-toner), printer resolution and the substrate (paper). For this research, only the color printing attributes such as the overall average color deviation [ACD, ΔE (2000)] and the solid ink density (SID) were analyzed to examine the CMDPW process consistency in a day-to- day digital printing operation. These are the color attributes which are monitored and managed for quality accuracy during the printing. Printed colors of the random sample size (n = 80) were measured against the GRACoL2013 standards to derive the colorimetric/densitometric values. Reference colorimetric values used in the analysis were the threshold deviations (acceptable color deviations) as outlined in the ISO12647-7 standards (GRACoL2013). A control charts analysis was applied for further determining the process (CMDPW) SID and ACD variation. The data collected were run through multiple software applications (MS-Excel/SPSS/Minitab) to apply various statistical methods. Analyzed data from the experiment revealed that the printed colorimetric values were in match (aligned) with the GRACoL 2013 (reference/target). Since the SID values of CMYK colors were in control throughout the process, this enabled the CMDPW to produce consistent acceptable color deviation (Average Printed ΔE (2000) = 2.978; SD = 0.437; Acceptable Threshold color deviation is ΔE (2000) ≤ 3.00).

Impact of a Transparent Coating on the Reflectance of a Line Halftone Pattern

Many prints are coated to increase their resistance or to enhance their appearance. Applying a smooth transparent layer on a print darkens and saturates its color, an easily observable effect which can be predicted in order to obtain better color management of coated surfaces and ink saving. A model was thus developed which describes the reflectance of a single-ink line halftone in optical contact with a transparent smooth coating. It is based on the peculiar way light diffuses inside the coating layer, a phenomenon called the “halo effect”. The model was compared to two experiments conducted at different scales where line halftones were coated with different coating thicknesses. The experiments enabled us to identify and measure the darkening effect caused by a coating layer, and validated the model.

DIGITAL COLOR REPRODUCTION AND DOCUMENTATION OF OIL PAINTING USING IMAGE PROCESSING

Digital photography is a method easily employed to capture images. Mobile devices such as smartphones and tablets as well as dedicated cameras are used to capture images. Furthermore, as hardware continues to improve, the resolution of images in 100 MP and 8K videos has recently been increased, and the quality of images enhanced through various types of image processing software. However, because the focus of photography is to record a moment activity, recording the orginal color of subject has been pushed back from priority. Digital photographs of cultural heritage are mainly captured for aesthetic appreciation and database construction. Those images for the purpose of the latter should record the exact color and shape of the subject. However, the theoretical approach of this is difficult and relatively complex procedures are able to create images that differ from the original color. Thus, it means falling in value that culture heritage data of important for color conservation. Various attempts have been made throughout the industry to capture the original colors of the subject accurately. The color management system (CMS) is a typical method of such. In this study, digital color reproduction and record studies were conducted on representative oil paintings of modern times in Korea. Accordingly, the light source’s color index and color temperature were measured. Furthermore, a color recording environment was established and digital images were acquired. Subsequently, it was able to record the original color through CMS-based image processing. It is believed that this study will enable one to understand the production techniques of works through the color record of oil paintings and help one to monitor conservation conditions such as discoloration.