Model-Based Calibration Approach to Improve Tone Consistency for Color Electrophotography

Abstract

This paper aims to improve tone consistency for color electrophotography (EP). Color accuracy is crucial for printing. Calibrations are performed in EP printers to maintain consistent tone reproduction curves (TRCs). The calibration performance directly impacts the quality of color rendering. In this study, a model-based calibration approach is proposed to stabilize the TRCs. Regression models are developed to predict the primary color TRCs based on operating conditions and/or on-board sensor measurements. During a calibration, TRCs are predicted by the regression models, and the deviation is compensated by adjusting EP control parameters and modifying the tone correction mapping. The effectiveness of the proposed approach is validated using an off-the-shelf in-line four color EP printer under various environmental conditions and consumable factors. The experimental results demonstrate a 63.3% reduction in root-mean-squared error over the existing calibration method. VC 2011 Society for Imaging Science and Technology. [DOI: 10.2352/J.ImagingSci.Technol.2011.55.6.060505] INTRODUCTION The color reproduction quality of an electrophotographic (EP) printing system can be affected by changes in operating conditions. A tone reproduction curve (TRC) is commonly used to characterize the color that is reproduced on the printed media for each primary color. A TRC is obtained by measuring the reproduced color on the output media at a set of predetermined halftone levels. A consistent TRC implies that the reproduced color does not change under different operating conditions. Ensuring the consistency of the TRCs is the first step to ensure consistent color reproduction. Calibrations are performed in an EP system to improve TRC consistency. For an EP engine, tone reproduction on a given media is sensitive to variations in temperature, humidity, air pressure, organic photoconductor (OPC) drum age, throughputs, and other environmental factors and operating conditions. During a calibration, a number of primary color patches are printed on either the internal transfer media or the final output media at a set of predetermined halftone levels. Sensors embedded in the printer measure the printed patches. EP control parameters and the tone correction mapping are adjusted based on these on-board sensor measurements to ensure consistent primary color reproduction. Researchers have proposed approaches to improve tone reproduction consistency. Li and Dianat proposed a calibration routine to minimize least-squared photoconductor TRC errors under various operating conditions using a static linear model between the EP parameters and the sensor values with the presence of disturbances and uncertainty in the system description. Staelin et al. developed machine learning models to improve color reproduction consistency for an HP Indigo digital press. Sim et al. used time-sequential sampling to address the difficulty in generating reliable measurements that span the color gamut of a color EP engine. Totsuka et al. proposed a tone curve Received Sep. 27, 2010; accepted for publication Aug. 8, 2011; published online Dec. 16, 2011 1062-3701/2011/55(6)/060505/9/$20.00. J. Imaging Sci. Technol. Nov./Dec. 2011 060505-1 Journal of Imaging Science and TechnologyV 55(6): 060505-1–060505-9, 2011. VC Society for Imaging Science and Technology 2011