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Printhead Arrangement for High Resolution Printing of Grey Scale Images

IP.com Disclosure Number: IPCOM000109413D
Original Publication Date: 1992-Aug-01
Included in the Prior Art Database: 2005-Mar-24
Document File: 2 page(s) / 116K

Publishing Venue

IBM

Related People

Dove, DB: AUTHOR [+2]

Abstract

The reproduction of images containing a range of shades of grey or density of color by a high contrast printing method, such as an offset press or electronic printers using electrophotographic, magnetographic, ionographic or thermal transfer techonologies, is carried out by two primary methods. In the offset press, the levels of grey are emulated by an array of dots of equal optical density but of varying size. Thus, the reproduction of 100 grey levels requires the possibility of printing dots of 100 different sizes. The dots are commonly placed in a dense array (80-150 or more lines of dots per inch) so that individual dots are scarcely visible.

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Printhead Arrangement for High Resolution Printing of Grey Scale Images

       The reproduction of images containing a range of shades
of grey or density of color by a high contrast printing method, such
as an offset press or electronic printers using electrophotographic,
magnetographic, ionographic or thermal transfer techonologies, is
carried out by two primary methods.  In the offset press, the levels
of grey are emulated by an array of dots of equal optical density but
of varying size.  Thus, the reproduction of 100 grey levels requires
the possibility of printing dots of 100 different sizes.  The dots
are commonly placed in a dense array (80-150 or more lines of dots
per inch) so that individual dots are scarcely visible.  The
electronic printer, however, typically reproduces dots of only one
size and in this case, grey levels are reproduced by grouping
together dots into so-called tiles or super-pels.  Varying
grey-levels are attained by printing more or fewer dots within the
tile, and the number of grey levels is equal to the number of dots
within the tile.  As an example, a printer having an addressability
of 300 dots/inch could simulate a 100 line screen with a tile of 3 by
3 dots.  In this case, only 9 levels of grey could be achieved, by
placing 1, 2, 3, ..., 9 dots within the tile.  The resolution of the
printer is greatly reduced in general since pictures are built up
from tiles, not dots.

      In this article a method is described whereby a large range of
grey levels can be attained by printing dots of two sizes with a tile
of a few dots only.  The key is that the dot's sizes are chosen in
a particular ratio described here to maximize the number of grey
levels that may be printed with minimal loss in resolution.

      Figs. 1A and 1B show two possible printhead arrangements to
allow the printing of the two different dot sizes.  In this example,
a light shutter array using liquid crystal or other electro-optic
material consists of either a single or a dual line of light shutters
where each pixel location a...