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Dot overlap correction for raster output scanners using dual wavelengths

IP.com Disclosure Number: IPCOM000237511D
Publication Date: 2014-Jun-19
Document File: 9 page(s) / 445K

Publishing Venue

The IP.com Prior Art Database

Abstract

Xerographic printers use laser devices to create an image on a charged photoreceptor. This idea proposes using two or more different laser beams with different wavelengths in a multi-laser ROS (raster output scanner) for xerographic printing systems in order to compensate for xerographic development differences between isolated/low area coverage image content and solid area/high area coverage image content. For example, one beam (at wavelength "A") is used to image low area coverage image content, and another beam (at wavelength "B") is used to image high area coverage content. The wavelengths "A" and "B" are chosen such that the corresponding latent images on the photoreceptor, whether high or low area coverage, have very similar voltage distributions (because of the spectral sensitivity variations of the photoreceptor to the different wavelengths), so that the content is xerographically rendered similarly.

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Dot overlap correction for raster output scanners using dual wavelengths

Xerographic printers use laser devices to create an image on a charged photoreceptor.  This idea proposes using two or more different laser beams with different wavelengths in a multi-laser ROS (raster output scanner) for xerographic printing systems in order to compensate for xerographic development differences between isolated/low area coverage image content and solid area/high area coverage image content. For example, one beam (at wavelength "A") is used to image low area coverage image content, and another beam (at wavelength "B") is used to image high area coverage content. The wavelengths "A" and "B" are chosen such that the corresponding latent images on the photoreceptor, whether high or low area coverage, have very similar voltage distributions (because of the spectral sensitivity variations of the photoreceptor to the different wavelengths), so that the content is xerographically rendered similarly.

The intensity on a single spot produced by the raster output scanner (ROS) on the photoreceptor may just reach the development threshold. However, in a high area coverage image patch, the intensity profiles of the dots overlap and add up to overcome the development threshold.

Single isolated dot (left) high density patch (centre) lower density patch (right) Development threshold dashed line

If the power of a single dot is just at the development threshold, the power on high area coverage image patches would be much higher than that of the single dot and cause toner to pile up and a non-linear relationship between area coverage and development. In an example, a series of dots with a Gaussian profile of 10 µm FWHM have been added with a spacing of 5 µm to deliver an intensity about 4 times higher than that of a single dot (center of figure). However, if the separation is stretched to 10 µm, the intensity is only just above the single dot level.

If the overall power on the high area coverage image patches is adjusted, then the power of a single dot would also reduce below the development threshold. This relationship is fixed due to the spot shape and the spot spacing, assuming a fixed spot power.  The effect of spot size on the relationship between area coverage and resulting L* is shown below illustrating the effect.

This proposal addresses the effectiveness of the ROS proposing a method of controlling its effect on the photoreceptor for each spot hence reducing the development differential between single and multiple spots.

The diagram above shows a typical photoreceptor sensitivity against excitation wavelength. On this curve two typical wavelengths would be 780nm for the small spots and 580nm for the high area coverage, giving a reduction of approximately 2:1 in the development threshold differential.

A ROS with two beams working at different wavelengths is illustrated below. In this case, one of the beams will be tuned to the peak of the photoreceptor sensiti...