Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1997-05-01
2001-02-06
Lee, Thomas D. (Department: 2724)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S451000, C358S296000, C382S254000, C382S266000
Reexamination Certificate
active
06185003
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of reproducing images on the basis of digital image data containing binary values of basic pixels (as defined infra) arranged in a raster, with an electrophotographic printing unit comprising at least one electrically chargeable photo-conductive medium, a selectively activatable light source for image-wise discharge of the said medium and a developing unit for developing the said medium with toner powder, wherein control signals for activating the said light source are generated on the basis of the digital image data, which control signals are adapted to compensate for properties of the printing unit.
The invention also relates to apparatus for application of the said method.
BACKGROUND OF THE INVENTION
Methods of reproducing digital binary images (also termed “half-tone images”) are generally known. A charged photo-conductive medium is exposed pixel-wise and thus discharged by way of a light source, e.g., an array of LEDs which is disposed transversely to the direction of transport of the photo-conductive medium. The result is a charge image formed of charged and discharged areas. The photo-conductive medium is then passed along a developing unit, usually a rotating magnetic roller with a developing powder. In these conditions the charge image is developed with toner.
There are two alternative systems with regard to exposure. In a first system, charged areas on the photo-conductive medium are developed while the discharged areas remain free. A system of this kind is termed a “white-writer”. In the other system the converse is the case. This is achieved by applying, between the developing unit and the photo-conductive medium, an electrical voltage difference equal to the surface potential of charged areas of the photo- conductive medium. A system of this kind is termed a “black-writer”.
In electrophotographic printers, the light source is usually such that it exposes a somewhat larger area than corresponds to the dimensions of a pixel. The reason for this is to prevent unexposed areas from remaining between the pixels. In the case of white- writers the result of this is that the areas developed with toner powder become smaller than intended, and this becomes particularly visible as a narrowing of lines and partial or complete disappearance of fine details. In black-writers, on the other hand, the areas developed with toner powder become somewhat larger, so that narrow lines are conspicuously widened and, for example, small letter characters merge.
Another effect that influences the edges of developed image portions is that in many developing systems, particularly those using developing powders of relatively high resistivity, electrical edge fields occur which cause enhanced development of toner at and just outside the edge of areas for development. In the case of white-writers, this electrical edge effect counteracts the said exposure effect and partly cancels it out, but in black-writing systems the two effects work in the same direction, so that action is required particularly in the case of the latter systems.
An obvious remedy to counteract extension of the exposed area would be to make the area exposed per pixel smaller. This can be done, for example, by reducing the intensity of the light source. The intensity of the light spot projected on to the photo- conductive medium for a pixel decreases from the center, frequently approximately like a Gaussian curve. A reduction of the intensity is therefore first perceptible at the edges of the light spot because a larger proportion of the descending flanks falls below the sensitivity threshold of the photo-conductive medium.
A disadvantage of this remedy, however, is that the discharge is locally no longer complete in larger exposed areas. In black-writers, the remaining residues of the charge obstruct the development so that the optical density of a print becomes unacceptably low in larger image areas. In white-writers, the remaining charge residues cause a (slight) development of toner powder in areas which should have remained white.
A second remedy to counteract the described undesired extension of the exposed area comprises shifting the time when the light source is switched on when the latter enters and leaves respectively an area for exposure during its relative movement with respect to the photo-conductive medium, i.e., variable timing for the light source. In the case of a white-writer, a method of this kind is described in U.S. Pat. No. 4,387,983. In this way the edge of an exposed and hence discharged area can be so shifted on the photo-conductive medium that the boundary of the developed area is situated exactly at the required location. A disadvantage of this solution is that it requires intervention in the timing of the light source control. Since high-frequency signals are involved here, specialized expensive components must be used for implementation. Moreover, in the case of LED arrays particularly, the control circuits associated with the commercially available products are often not suitable for carrying out small timing changes applicable solely to portions of the array. LED arrays in which this is possible are very expensive.
SUMMARY OF THE INVENTION
The object of the invention is to provide a solution to the above problems while retaining fixed timing for the light source.
To this end, according to the invention, two sub-pixels are generated for each basic pixel of the digital image data, namely a first sub-pixel always corresponding in value to the value of the associated basic pixel and a second sub-pixel for which the value is determined in dependence on the value of the associated basic pixel and on the values of basic pixels in a predetermined area around the associated basic pixel. The light source is activated with control signals generated in accordance with each of the generated sub-pixels.
Thus according to the invention, each pixel is divided up into two sub-pixels which are exposed independently of one another by the light source. The first sub-pixel is exposed in accordance with the digital image data applicable to the pixel of which it forms a part. The same applies to the second sub-pixel unless (in the direct surroundings, i.e., in the said predetermined area) there is a transition between an area for exposure and one which is not to be exposed. In the latter case, any exposure of the second sub-pixel is switched off so that then there is only the exposure of the associated first sub-pixel. The second sub-pixel is exposed if the digital image data specify the exposure of the basic pixel and the values of basic pixels in the predetermined area around the associated basic pixel indicate that in that area there is no transition between an image portion for exposure and one which is not to be exposed. In this way, the light energy at edges and small structures is so reduced that there is no extension of an exposed area.
These and other objects of the present invention are achieved by providing an apparatus for reproducing an image, comprising: a first generator for generating digital image data in the form of binary values of basic pixels arranged in a raster; an electrophotographic printing unit for reproducing an image on an image support on the basis of processed digital image data, the printing unit having at least one electrically chargeable photo-conductive medium, a selectively activatable light source for image-wise discharge of the said medium and a developing unit for developing discharged portions of said medium with toner powder; a processing unit; and a control unit. The processing unit receives digital image data from the first generator, generates a first sub-pixel and a second sub-pixel for each basic pixel, gives the first sub-pixel a value equal to the value of the associated basic pixel, and gives the second sub-pixel a value dependent upon the values of the basic pixels in a predetermined area around the associated basic pixel. The control unit delivers control signals that selectively activate the light source in acco
Lee Thomas D.
Oce'-Nederland, B.V.
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