Direct thermal imaging

Details Direct thermal imaging Direct thermal imaging

1995-04-25

1997-10-28

Tran, Huan H.

Incremental printing of symbolic information

Thermal marking apparatus or processes

With recording medium

347171, B41M 526

Patent

active

056821948

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to direct thermal imaging.


BACKGROUND OF THE INVENTION

In thermography two approaches are known:
1. Direct thermal formation of a visible image pattern by imagewise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
2. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an imagewise heated donor element onto a receptor element.
Thermal dye transfer printing is a recording method wherein a dye-donor element is used that is provided with a dye layer wherefrom dyed portions of incorporated dye are transferred onto a contacting receiver element by the application of heat in a pattern normally controlled by electronic information signals.
The optical density of transparencies produced by the thermal transfer procedure is rather low and in most of the commercial systems--in spite of the use of donor elements specially designed for printing transparencies--only reaches 1 to 1.2 (as measured by a Macbeth Quantalog Densitometer Type TD 102). However, for many application fields a considerably higher transmission density is asked for. For instance in the medical diagnostical field a maximal transmission density of at least 2.5 is desired.
High optical densities can be obtained using a recording material comprising on a support a heat-sensitive layer comprising a substantially light insensitive organic silver salt and a reducing agent. Such material can be image-wise heated using a thermal head causing a reaction between the reducing agent and the substantially light insensitive organic silver salt leading to the formation of metallic silver. The density level may be controlled by varying the amount of heat applied to the recording material. This is generally accomplished by controlling the number of heat pulses generated by a thermal head. An image having a grey scale is thus obtained.
As 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, N.Y. 10016 (1991), p. 498-499 in direct thermal printing signals are converted into electric pulses and then through a driver circuit selectively transferred to a thermal printhead. 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 thermal paper wherein the chemical reaction resulting in colour development takes place.
Heat-sensitive copying papers including a recording layer having a substantially light-insensitive organic silver salt and organic reducing agent in a thermoplastic binder such as polyvinyl acetate, polyvinyl butyral and cellulose nitrate (ref. Re 30,107 being reissue of U.S. Pat. No. 3,996,397) are less suited for use in thermographic recording operating with thermal printheads since these recording layers may stick to said printheads. Moreover, organic ingredients of the thermosensitive recording layer may exude by heat and may soil the thermal heads at an operating temperature in the range of 300.degree. to 400.degree. C. which are temperatures common in using thermal printheads. The undesirable transfer of said ingredients may be promoted by the pressure contact of the thermal head with the recording material, possible contact-pressures may be 200-500 g/cm.sup.2 to ensure a good transfer of heat. The heating time per picture element (pixel) may be less than 1.0 ms.
Because of its high density the obtained silver image is in principal suitable for use as a medical diagnostic image. However the following problems have been encountered. Uneveness of density occurs with the number of images that have been printed and damaging of the heat sensitive layer occurs. These problems can be overcome by the presence of a protective layer on the heat-sensitive

REFERENCES:
patent: 3996397 (1976-12-01), Laridon et al.
patent: 4643454 (1987-02-01), Ondis
patent: 5196861 (1993-03-01), Fisher, Sr.

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