Radiation imagery chemistry: process – composition – or product th – Thermographic process – Heat applied after imaging
Reexamination Certificate
1995-10-25
2002-11-12
Chea, Thorl (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Thermographic process
Heat applied after imaging
C430S510000, C430S511000, C430S576000, C430S577000, C430S619000, C430S944000, C430S945000
Reexamination Certificate
active
06479220
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to antihalation/acutance systems used in methods of imaging a photographic medium, in particular by exposure to a source of narrow band radiation.
BACKGROUND TO THE INVENTION
Poor resolution in imaging materials may result from unwanted exposure of areas immediately adjacent to intended image areas caused by the scattering of light at interfaces within the material. To overcome this problem, non-sensitizing absorbers are incorporated either within the media (an acutance dye) or in a separate layer (an antihalation layer).
In general, antihalation or acutance dyes are selected to have an absorption spectrum which matches that of the spectral response of the imaging media (see U.S. Pat. No. 4,581,325, EP-A-0 377 961 and U.S. Pat. No. 5,135,842). This is the case when a broad band source is used, for example, daylight, tungsten bulbs or xenon arcs, so that reflected or scattered light of any wavelength that would be capable of imaging the media may be absorbed.
In the case of narrow-band exposure devices, the situation is different. Devices such as lasers and light emitting diodes (LEDs) typically emit radiation over a very narrow wavelength range, for example a few nm, which is much narrower than the spectral response of typical photosensitive media, and it is inefficient, and may be counterproductive, to extend antihalation and acutance absorptions over the entire sensitivity range of the imaging media.
This is particularly relevant in the case of photothermographic materials, which do not have the benefit of a wet processing step to wash out or chemically bleach antihalation or acutance dyes once they have fulfilled their purpose. To avoid contamination of the final image, any absorber used for purposes of acutance or antihalation in photothermographic materials must either be substantially invisible to the naked eye (as is possible in the case of infrared sensitive materials), or must be bleachable under the thermal processing conditions. The “invisible” approach is described in U.S. Pat. No. 4,581,325 and EP-A-0 377 961, and the “bleachable” approach is disclosed in U.S. Pat. No. 5,135,842.
In practice, few dyes are genuinely invisible, and bleachable systems may still leave some residual stain, or require extended processing times to provide adequate bleaching, and so there is a need to minimise the quantity of dye used, consistent with acceptable image sharpness. Thus, for optimum image sharpness from exposure by narrow-band sources, such as lasers, acutance and antihalation dyes with intense, narrow absorption bands matching the output of the laser have been selected. These systems have not proved to be ideal, and resolution of the final image could be improved.
Non-sensitizing dyes are also used for the improvement of colour separation in multilayer photographic systems where the layers are sensitized to different wavelengths.
A typical colour negative film, for example, contains blue-, green-, and red-sensitive layers, with the blue-sensitive layer outermost and the red-sensitive layer nearest the base. Immediately below the blue-sensitive layer, there is usually provided a yellow filter which absorbs any blue light passing through the outermost layer, and transmits light of other wavelengths. This not only provides an antihalation effect for the outer layer, but also prevents unwanted exposure of the inner layers by blue light since photographic emulsions are inherently sensitive to blue light. Similarly, a magenta filter is positioned below the green layer to provide antihalation for that layer and to prevent unwanted exposure of the red layer.
False colour address materials typically comprise two or more photosensitive layers sensitized to different parts of the infrared (and/or visible) and are designed for exposure by two or more independently modulated sources such as lasers or LEDS. Cross-talk between layers is minimized by filters or by controlling the speed of each layer relative to the others (and adjusting the intensity of each source accordingly) or by a combination of these methods, see EP-A-0 479 167 and EP-A-0 502 508.
Dyes are selected for colour separation purposes to match as closely as possible the absorption spectrum of the dye to the spectral response of the layer immediately above it. It has been considered important to minimize absorption at longer wavelengths to avoid desensitizing lower layers.
BRIEF SUMMARY OF THE INVENTION
In a first aspect of the invention, there is provided a method of imaging a photothermographic material comprising exposing said material comprising a photothermographic medium to a source of narrow band radiation, said material comprising one or more non-sensitising acutance or antihalation dyes associated with said medium providing an absorption maximum (&lgr;max) within 10 nm of the wavelength of maximum output of the narrow band source and an optical density of at least 0.05 at a wavelength (&lgr;max+50) nm.
In a second aspect of the invention, there is provided a photothermographic material comprising at least one photothermographic medium associated with two or more non-sensitizing antihalation or acutance dyes for that photothermographic medium, said dyes having absorption maxima at different wavelengths such that the difference in wavelength between the maxima of longest and shortest wavelength is at least 20 nm, usually at least 30 nm, preferably at least 50 nm, the wavelength of maximum absorbance of at least one of said dyes being greater than the wavelength of maximum sensitivity of said photothermographic medium.
The term non-sensitising dyes is used to describe those dyes which do not extend the spectral sensitivity of the medium because of its presence in the element.
DESCRIPTION OF PREFERRED EMBODIMENTS
The preferred exposure source is a laser diode with an output in the range 700-900 nm, more preferably 750-850 nm. For best results, the output of the source should be at or near the sensitivity maximum of the media (e.g. within 50 nm thereof).
The dyes must be capable of providing an acutance or antihalation effect towards the photothermographic medium, and hence may be present in the same layer as the photothermographic medium, in an adjacent layer, or in a non-adjacent layer, providing only transparent, non-light-sensitive layer(s) intervene. For example, the dyes and the photothermographic medium may be coated on opposite sides of a transparent base.
Preferably, the media are of the dry silver type comprising a light-insensitive organic silver salt oxidising agent in reactive association with a light-sensitive silver halide catalyst and a reducing agent.
The wavelength of maximum sensitivity of the media may lie anywhere in the visible or near infrared region of the spectrum, but is preferably >600 nm, more preferably >700 nm, and most preferably around 800 nm.
The photothermographic media of the invention have at least two antihalation or acutance dyes which contribute significant absorbance (at least 0.05, preferably at least 0.1 OD) at wavelengths substantially in excess (e.g., by 50 nm or more) of the wavelength of intended exposure. This extra absorbance is found to provide significant improvements in image sharpness.
It is believed that part of the radiation absorbed by the sensitizing dye(s), acutance dye(s) and antihalation dye(s) in the media is re-emitted at longer wavelengths in the form of luminescence (i.e., fluorescence or phosphorescence). Dry silver materials may be particularly prone to luminescence because a considerable proportion of the sensitizing dye remains “free” (i.e., is not adsorbed on the surface of the silver halide grains), and hence is more likely to fluoresce.
Typical photothermographic (and especially IR-sensitized photothermographic) media exhibit a fairly flat spectral response (i.e., although maximum sensitivity is at W nm, there will be significant sensitivity at W+50 or even W+100 nm), and so the luminescence is capable of generating “secondary” image density, some of which will be in areas adjacent to
Grieve Duncan McL A.
Parkinson Michael L.
Chea Thorl
Eastman Kodak Company
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