Incremental printing of symbolic information – Thermal marking apparatus or processes
Reexamination Certificate
2001-12-18
2003-07-22
Nguyen, Lamson (Department: 2861)
Incremental printing of symbolic information
Thermal marking apparatus or processes
C347S183000, C347S188000, C347S191000
Reexamination Certificate
active
06597385
ABSTRACT:
DESCRIPTION
1. Field of the Invention
The present invention relates to a device operable for applying thermal energy to a recording medium, the device comprising a thermal head having energisable heating elements which are individually addressable. In particular, the recording medium is a thermographic material, and the head relates to thermal imaging, generally called thermography.
2. Background of the Invention
Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy. Thermography is concerned with materials which are not photosensitive, but are sensitive to heat or thermosensitive and wherein imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material, by a chemical or a physical process which changes the optical density.
Most of the direct thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
In direct thermal printing, the heating of the thermographic recording material may be originating from image signals which are converted to electric pulses and then through a driver circuit selectively transferred to a thermal print head. The thermal print head consists of microscopic heat resistor elements, which convert the electrical energy into heat via the Joule effect. The electric pulses thus converted into thermal signals manifest themselves as heat transferred to the surface of the thermographic material, e.g. paper, wherein the chemical reaction resulting in colour development takes place. This principle is described in “Handbook of Imaging Materials” (edited by Arthur S. Diamond—Diamond Research Corporation—Ventura, Calif., printed by Marcel Dekker, Inc. 270 Madison Avenue, New York, ed. 1991, p. 498-499).
A particular interesting direct thermal imaging element uses an organic silver salt in combination with a reducing agent. An image can be obtained with such a material because under influence of heat the silver salt is developed to metallic silver.
Referring to
FIG. 1
, there is shown a global principle schema of a thermal printing apparatus
10
that can be used in accordance with the present invention (known from e.g. EP 0 724 964, in the name of Agfa-Gevaert). This apparatus is capable of printing lines of pixels is (or picture elements) on a thermographic recording material m, comprising thermal imaging elements or (shortly) imaging elements, often symbolised by the letters Ie. As an imaging element Ie is part of a thermographic recording material m, both are indicated in the present specification by a common reference number
5
. The thermographic recording material m comprises on a support a thermosensitive layer, and generally is in the form of a sheet. The imaging element
5
is mounted on a rotatable platen or drum
6
, driven by a drive mechanism (not shown) which continuously advances (see arrow Y representing a so-called slow-scan direction) the drum
6
and the imaging element
5
past a stationary thermal print head
20
. This head
20
presses the imaging element
5
against the drum
6
and receives the output of the driver circuits (not shown in
FIG. 1
for the sake of greater clarity). The thermal print head
20
normally includes a plurality of heating elements equal in number to the number of pixels in the image data present in a line memory. The image wise heating of the heating element is performed on a line by line basis (along a so-called fast-scan direction X which generally is perpendicular to the slow-scan direction Y), the “line” may be horizontal or vertical depending on the configuration of the printer, with the heating resistors geometrically juxtaposed each along another and with gradual construction of the output density.
Each of these resistors is capable of being energised by heating pulses, the energy of which is controlled in accordance with the required density of the corresponding picture element. As the image input data have a higher value, the output energy increases and so the optical density of the hardcopy image
7
on the imaging element
5
. On the contrary, lower density image data cause the heating energy to be decreased, giving a lighter picture
7
.
In the present invention, the activation of the heating elements is preferably executed pulse wise and preferably by digital electronics. Some steps up to activation of said heating elements are illustrated in
FIGS. 1 and 4
. First, input image data
16
are applied to a processing unit
18
. After processing and parallel to serial conversion (not shown) of the digital image signals, a stream of serial data of bits is shifted (via serial input line
21
) into a shift register
25
, thus representing the next line of data that is to be printed. Thereafter, under control of a latch enabling line
23
, these bits are supplied in parallel to the associated inputs of a latch register
26
. Once the bits of data from the shift register
25
are stored in the latch register
26
, another line of bits can be sequentially clocked (see ref. nr.
22
) into said shift register
25
. A strobe signal
24
controls AND-gates
27
and feeds the data from latching register
26
to drivers
28
, which are connected to heating elements
29
. These drivers
28
(e.g. transistors) are selectively turned on by a control signal in order to let a current flow through their associated heating elements
29
.
The recording head
20
is controlled so as to produce in each pixel the density value corresponding with the processed digital image signal value. In this way a thermal hard-copy
7
of the electrical image data is recorded. By varying the heat applied by each heating element to the carrier, a variable density image pixel is formed. A control algorithm must determine for every heating element the amount of energy which must be dissipated. In practice, the controller algorithm must deal with a variety of real-world problems:
Changing characteristics of the film media give different pixel sizes for the same nib (or heating element) energy, e.g. some examples:
a different humidity in the emulsion layer, making its thermal capacity different,
a different chemical composition of the image forming components.
Environmental characteristics like temperature and humidity may change:
a temperature rise of the environment must be taken into account as the image forming temperature will not rise and is given by the chemical composition of the emulsion layer,
humidity again changes the thermal capacity of the emulsion, producing different temperature rises when applying the same amount of energy.
The thermal process itself produces an excessive amount of heat which is not absorbed by the image forming media. This excessive heat is absorbed by a heat sink, but nevertheless, gives rise to temperature gradients internally in the head, giving offset temperatures in the nibs and between the several nibs. E.g. when the image forming process must have an accuracy of 1° C. in the image forming media, an increased offset temperature of 5° C. in the heat generating element must be taken into account when calculating the power to be applied to that element.
The heat generating elements are in the ideal case fully thermally isolated from each other. In practice, this is never the case and cross-talk exists between the several nibs. This cross-talk can be localised on several levels:
heat transfer between the several nibs in the thermal head structure itself,
heat transfer in the emulsion and film layer itself,
pixels are not printed one aside the other, but partly do overlap on the print media, mechanically mixing heat from one pixel with the other.
The electrical excitation of the nibs is mostly not on an isolated base. This means that not every nib resistor has its own electrical voltage supply which can be driven independent of all the other nibs. In general, some drive signals for driving the nibs are common to each other, this with the purpose of having reduced
AGFA-Gevaert
Feggins K.
Hoffman, Warnick & D'Alessandro
Merecki John A.
Nguyen Lamson
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