Psychophysics study and reduction of banding artifact in secondary colors using an embedded implementation of closed -loop control on multiple motion systems in an electrophotographic process

Mu-Chih Chen, Purdue University

Abstract

Banding is a common print artifact, which shows as quasi-periodic contrast, hue, and saturation variations. Motion quality of moving components in a laser printer, such as speed fluctuation of photoconductor drums and laser beam vibrations, have been shown to be a major contribution factor to the occurrence of banding. The human visual system's contrast sensitivity threshold is often used as the detection threshold model for banding perception. On color electrophotographic (EP) platforms, due to multiple primary color layers being superimposed together, banding patterns in these different layers may interact and be perceptible. As a result, color banding appears as quasi-periodic hue or lightness gratings. In the literature, work has been done in studying human visual perception of hue variations in isoluminance conditions. However, in a color electrophotographic process, when multiple primary color layers superimpose, they may not necessarily be isoluminant. In addition, because banding is mainly caused by motion imperfections, to reduce the perception of banding, it is important to relate human visual response to banding to motion control variables. Hardcopy and softcopy psychophysics experiments were conducted to study perception of color banding characteristics. To use a printer to generate hardcopy psychophysics stimuli, an embedded closed-loop control system was implemented on the printer. The closed-loop control system effectively reduces the amplitude of the printer's intrinsic banding at multiple frequencies and allows extrinsic banding to be injected by controlling multiple motion systems. With the closed-loop control system, banding with frequencies that are Will integer and fraction multiples of the sampling frequencies can be reduced. The challenge of the implementation is to achieve performance while minimizing cost. The integration and design of the closed-loop control system is described. It was found that when banding in each of the primary colors is reduced to just below the detection thresholds, there is no guarantee that there will be no perceivable banding in secondary colors. To reduce banding in secondary colors, a tighter requirement for process control specification is needed than the simple reduction of banding in primary colors to just below individual perception thresholds. The hardcopy and softcopy experiments produced the same results when the same level of noise was included in the stimuli. The effect of the speed fluctuation of the photoconductor drums on the variations in the International Commission on Illumination's (CIE) L*a*b* is identified for different printed background levels. This mapping between the variations in the CIE L*a*b* and the drum speed fluctuation can convert the color banding detection thresholds obtained from softcopy psychophysics experiments to the process control specification requirement.

Degree

Ph.D.

Advisors

Chiu, Purdue University.

Subject Area

Mechanical engineering

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