Abstract
The halftone screen with fine vertical gratings is often used to modulate image in the printing technology. When the halftone screen is disturbed by the jitter in horizontal direction we observe the lattice fringe (called banding) in the horizontal direction. Surprisingly, any frequency component does not exist in the spatial frequency domain of the banding. Also, the banding is detected even when the spatial frequency of the halftone screen exceeds the frequency limit of the visual sensitivity.
In this study we measured the spatial frequency properties of the halftone screen and the jitter in order to see the banding. The circular stimuli were of the vertical sinusoidal gratings with horizontal sinusoidal jitters which made on the paper with an ink-jet printer. The viewing distance was 2.5 m. The stimulus size was 6.9 degree of diameter. We varied the spatial frequency of gratings and jitters around the spatial resolution limit of the visual system as experimental parameters. The observer's task was to judge whether he saw the banding pattern in each stimulus with different jitter amplitude.
The results showed that the banding was observed for the jitter amplitude of lower than 10 arc-sec in the grating with spatial frequency of less than 60 cpd. This means that the jitter amplitude threshold was in the level of hyper acuity. Also a banding pattern was observed even at the grating frequency of higher than the resolution limit.
These results suggest that a nonlinear process, e.g., adaptive sampling or nonlinear function, may exist somewhere between the photoreceptor mosaic and a higher level of the visual system to yield the banding pattern.