Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
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. Formation of a visible image pattern by transfer of a coloured species from an imagewise heated donor element onto a receptor element.
A survey of "direct thermal" imaging methods is given in the book "Imaging Systems" by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press--London and New York (1976), Chapter VII under the heading "7.1 Thermography". Thermography is concerned with materials which are not photosensitive, but are heat sensitive. Imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material.
According to a direct thermal embodiment operating by physical change, a recording material is used which contains a coloured support or support coated with a coloured layer which itself is overcoated with an opaque white light reflecting layer that can fuse to a clear, transparent state wherein the coloured support is no longer masked. Physical thermographic systems operating with such kind of recording material are described on pages 136 and 137 of the above mentioned book of Kurt I. Jacobson et al.
Yet 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.
It has been suggested to use a thermoreducable silver source in combination with a reducing agent in a direct thermal film in order to increase the optical density in transmission of a printed image (see EP-A-537.795). Although continuous tones can be obtained by said printing method, the gradation produced by said printing method is too high resulting in only a few intermediate density levels. Fluctuations in the heat transfer from the heat source to the printing material result in a density difference of the final image. Thus, it is extremely difficult to obtain images having a uniform density profile. A direct thermal printing method moreover has the disadvantage that in the non-image places the co-reactants always remains unchanged, impairing the shelf-life and preservability.
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 or incorporated dye is transferred onto a contacting receiving element by the application of heat in a pattern normally controlled by electronic information signals.
In European Patent Application No. 94200612.3, a thermal imaging process is provided using
(i) a donor element comprising on a support a donor layer containing a binder and a thermotransferable reducing agent capable of reducing a silver source to metallic silver upon heating and (ii) a receiving element comprising on a support a receiving layer comprising a silver source capable of being reduced by means of heat in the presence of a reducing agent, said thermal imaging process comprising the steps of
bringing said donor layer of said donor element into face to face relationship with said receiving layer of said receiving element, PA1 image-wise heating a thus obtained assemblage by means of a thermal head, thereby causing image-wise transfer of an amount of said thermotransferable reducing agent to said receiving element in accordance with the amount of heat supplied by said thermal head and PA1 separating said donor element from said receiving element. PA1 bringing said donor layer of said donor element into face to face relationship with said receiving layer of said receiving element, PA1 image-wise heating a thus obtained assemblage preferably by means of a thermal head, thereby causing image-wise transfer of an amount of said thermotransferable reducing agents to said receiving element in accordance with the amount of heat supplied and PA1 separating said donor element from said receiving element.
This printing method is further referred to as `reducing agent transfer printing` or `RTP`.
However, the stability of the donor element in said European Patent Application has been found to be poor. More particularly, the reducing agent tends to crystallize in the donor layer. As a result of this crystallization during storage, transfer of reducing agent is seen during printing, even on places where no heat has been applied by the thermal head. This leads to a printing fog in the final image. This problem is especially seen when a high amount of reducing agent is used in the donor layer. This high concentration is necessary to obtain high optical densities of the final printed image (above 2.0-2.5).
Moreover, the neutral hue of the grey scale of the printed image is dependent on the choice of a specific reducing agent. It is extremely difficult to find reducing agents that yield a neutral grey-tone (e.g. for medical diagnostics). It has also been found that the reducing agent shows the disadvantage after transfer that the oxidation product of the reducing agent tends to crystallize in the receiving element, giving rise to `white dust` at the surface of the print after storage.