Field of the invention
[0001] The present invention relates to a method and an apparatus for thermal recording.
Background of the invention
[0002] Thermal imaging or thermography is a recording process wherein images are generated
by the use of imagewise modulated thermal energy. 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. A
particular interesting direct thermal imaging element uses an organic silver salt
in combination with a reducing agent. Such combination may be imaged by a suitable
heat source such as e.g. a thermal head, a laser etc.
[0003] A black and white image can be obtained with such a material because under the influence
of heat the silver ions are developed to metallic silver. However, it appears to be
difficult to obtain a neutral black tone image. Furthermore, it appears to be difficult
to obtain a sufficiently high density as required in certain applications (e.g. in
graphical applications).
[0004] Thermal recording information on a thermographic material by means of so-called "flying
spot scanning" is well-known from the prior art.
[0005] The thermal recording can be carried out with the aid of different types of recording
devices, e.g. a flat bed type recording (see Fig. 1), a capstan type recording device
(see Fig. 2), an internal drum type ITD recording device (see Fig. 3) or an external
drum type XTD recording device (see Figs. 4, 5 and 6). An extensive description of
such recording devices can be found e.g. in EP 0 734 148 and in US 5,932,394 (both
in the name of Agfa-Gevaert), so that in the present description any explicit and
extensive replication is superfluous.
[0006] EP-A 0 485 148 discloses an image recording apparatus for recording an image by application
of light beam to a photosensitive member, comprising: a photosensitive member; light
source means for emitting first and second beams, one of the first and second beams
bearing image information; and scanning means for scanning said photosensitive member
with the first and second beams with a time interval so that they are overlapped on
said photosensitive material.
[0007] EP-A 0 842 782 discloses a method of thermally recording a gradation image on a thermosensitive
recording material (S) having a photothermal converting agent for converting light
energy into thermal energy to develop a color at a density depending on the thermal
energy, comprising the steps of: applying a laser beam (L) having a level of light
energy depending on a gradation of an image to be recorded on the thermosensitive
recording medium (S); and scanning the thermosensitive recording medium (S) with the
laser beam (L) at a speed of at least 5 m/s.
[0008] EP-A 1 104 699 discloses a method for recording an image on a thermographic material
(m) comprising the steps of: providing a thermographic material having a thermal imaging
element (le), a transparent thermal head (TH) having energisable heating elements
(Hi), and a radiation beam (L), activating heating elements of said thermal head and
imagewise and scanwise exposing said imaging element by means of said radiation beam,
such that the total energy resulting from said thermal head and from said radiation
beam has a level corresponding to a gradation of the image to be recorded on said
imaging element, wherein said imagewise and scanwise exposing is carried out by passing
said radiation beam through transparent parts of said thermal head.
[0009] US 5,932,394 discloses a method for generating on a lithographic printing plate a
screened reproduction of a contone image, comprising the steps of: (1) transporting
a thermosensitive imaging element through an exposure area, the imaging element having
thereon at least one scan line including a plurality of microdots, at least one microdot
being an effective microdot;(2) scanwise exposing said thermosensitive imaging element
according to screened data representative for tones of a contone image with a set
of radiation beams as said thermosensitive imaging element is transported through
said exposure area, at least one of said radiation beams being an effective radiation
beam, at any given moment during said exposure at least two radiation beams of said
set of radiation beam impinge on different microdots of a scanline on said imaging
element, so that by completion of the exposure step each effective microdot of said
scanline has been impinged by all effective beams of said set, wherein said thermosensitive
imaging element includes an image forming layer on a hydrophilic surface of a lithographic
base, said image forming layer comprising hydrophobic thermoplastic polymer particles
and a compound capable of converting light into heat, said compound being present
in one of said image forming layer and a layer adjacent thereto.
[0010] Thermal recording according to the prior art by means of a flying spot laser on a
thermographic material generally only gives a sufficient density if the energy radiated
by the laser beam is so high that unwanted side-effects occur (e.g. burning, shrinkage
and irregular expansion). If one diminishes the energy in order to eliminate such
side effects, the output density is unacceptably low.
[0011] This problem in particular applies to graphical applications often requiring optical
densities greater than 3.0 or 4.0 or even 5.0 D. In addition, high spatial resolutions
as e.g. higher than 600 or even 1200 dpi are often required or small line-widths e.g.
smaller than 40 or even 20 µm or fine pixel-sizes e.g. finer than 40 or even 20 µm.
Aspects of the invention
[0012] It is an aspect of the present invention to provide an apparatus for thermal recording
which is capable of yielding images with improved tone neutrality.
[0013] It is a further aspect of the present information to provide a method for recording
information, which is capable of yielding images with improved tone neutrality.
[0014] Further aspects and advantages of the invention will become apparent from the description
hereinafter.
Summary of the invention
[0015] Aspects of the present invention are realized by an apparatus for thermal recording
an image in a substantially light-insensitive thermographic material m having a burning
temperature T
b, the substantially light-insensitive thermographic material m comprising a thermosensitive
element having a conversion temperature T
c, a support, and at least one light-to-heat conversion agent, comprises a means for
generating a radiation beam 20 including wavelengths λ absorbed by the light-to-heat
conversion agent and an optical means of scanning a line 40 of the substantially light-insensitive
thermographic material m with the radiation beam 20 at different positions thereon
along a scanning direction at each point of time in a scanning cycle.
[0016] Aspects of the present invention are also realized by a method for recording information,
comprising the steps of: providing an apparatus for thermal recording 1, the above-mentioned
substantially light-insensitive thermographic material m (5); generating a radiation
beam 20 including wavelengths λ absorbed by the light-to-heat conversion agent and
being modulated in accordance with the information to be recorded; scanning a line
40 of the substantially light-insensitive thermographic material m a first time with
the radiation beam, thereby heating the line of the substantially light-insensitive
thermographic material m to a first predetermined temperature T
1 being above the conversion temperature T
c and below the burning temperature T
b of the substantially light-insensitive thermographic material m; re-scanning the
same line of the substantially light-insensitive thermographic material m a plurality
of times n
s with the same radiation beam being identically modulated in accordance with the information
to be recorded.
[0017] Aspects of the present invention are also realized by the use of the above-mentioned
method in laser thermography.
[0018] Further advantages and embodiments of the present invention will become apparent
from the following description and drawings.
Detailed description of the invention
Brief description of the drawings
[0019]
Fig. 1 schematically shows a flat bed type recording device suitable for use in a
method according to the present invention.
Fig. 2 shows a capstan type recording device suitable for use in a method according
to the present invention.
Fig. 3 schematically shows an internal drum type recording device suitable for use
in a method according to the present invention.
Fig. 4 schematically shows an external drum type recording device suitable for use
in a method according to the present invention.
Fig. 5 shows a preferred embodiment of a laser thermographic apparatus suitable for
use in a method according to the present invention.
Fig. 6 is a pictorial view of another thermographic system XTD suitable for use according
to the present invention;
Fig. 7 principally shows the evolution over time of the temperature reached in a thermographic
material while applying a plurality of scannings according to the present invention.
Fig. 8.1 shows three consecutive scanning lines on a thermographic material passing
an external drum.
Fig. 8.2 shows an enlarged detail of a printed line have a line-width bl.
Fig. 9 shows a relation between a control voltage to an acousto-optical modulator
and the percentage of transmitted laser power.
Fig. 10 shows an output density on a thermographic material, in function of (i) a
pixel-distance dY to the central axis of a printed line, (ii) a preheating temperature TP and (iii) a number of sweeps ns.
Fig. 11 shows a practical evolution over time of the temperature Tm in the thermosensitive element if an information is recorded in one single sweep.
Fig. 12 shows a practical evolution over time of the temperature reached in the thermosensitive
element if an information is recorded by applying a plurality of scannings according
to the present invention.
Fig. 13 shows a three-dimensional distribution of the available intensity (or power)
of a Gaussian laser beam.
Fig. 14.1 shows the geometrical spread of the temperature Tm reached in a thermographic material when scanned according to a preferred embodiment
of the invention.
Fig. 14.2 shows the geometrical spread of the temperature Tm reached in a substantially light-insensitive thermographic material when scanned
according to another preferred embodiment of the invention.
Fig. 15 shows the efficiency η of a laser system when different line-thicknesses are
applied.
Fig. 16 shows the efficiency η of a laser system when different spatial resolutions
are applied.
Fig. 17 shows the configuration of a substantially light-insensitive thermographic
material suitable for application within the present invention.
Fig. 18.1 and 18.2 respectively shows a thermographic system incorporating a first
and a second position of the thermosensitive element with respect to a holding means.
Parts list
[0020]
- 1
- thermal printing system
- 5
- substantially light-insensitive thermographic material m
- 10
- moving mirror (e.g. polygon)
- 12
- radiation detecting element
- 14
- holding means (e.g. flat)
- 15
- drum
- 17
- hardcopy print
- 18
- drive system for drum
- 19
- laser-diode-array
- 20
- writing radiation beam
- 21
- radiation source
- 22
- filter
- 23
- spin motor
- 24
- lens
- 25
- reference radiation beam
- 26
- first mirror
- 27
- second mirror
- 28
- modulator
- 29
- concave lens
- 31
- control of drum (temperature, speed)
- 32
- power supply (polygon, modulator)
- 33
- speed control of polygon
- 34
- control of radiation source (incl. cooling)
- 35
- control of video signal
- 40
- line
- 41
- line-length B
- 42
- BOL
- 43
- EOL
- 44
- line-width bl
- 46
- material width Wm
- 50
- temperature evolution over time
- 51
- heating curve
- 52
- cooling curve
- 55
- ambient temperature Ta
- 56
- temperature T2
- 57
- conversion temperature Tc
- 58
- temperature T1
- 59
- burning temperature Tb
- 61
- three-dimensional distribution of a Gaussian beam-intensity
- 62
- two-dimensional distribution of temperature Tm1
- 63
- two-dimensional distribution of temperature Tm2
- 65
- support
- 66
- substrate
- 67
- thermosensitive element
- 68
- protective layer
- 69
- backing layer
- 80
- supply magazine
- 81
- capstan
- 82
- tension roller
- 84
- take-up system
- 102
- supply magazine
- 104
- belt
- 105
- tension roller
- 107
- sheet of thermographic material
- 108
- roller
- 109
- roller
- 110
- controller
- 113
- ventilator
- 116
- sheet exit
- 117
- keyboard
- 118
- laser source
- 119
- modulator
- 120
- first objective
- 121
- polygon mirror
- 122
- second objective
- 123
- sheet input
- 124
- sheet feeder
- 125
- imaging and processing unit/recording unit
- X
- fast-scan-direction
- Y
- slow-scan-direction
Terms and definitions
[0021] By the term "laser thermography" is meant an art of direct thermography comprising
a uniform preheating step not by any laser and an imagewise exposing step by means
of a laser (see e.g. EP-A 1 104 699).
[0022] The term "thermography" for the purposes of the present application is concerned
with materials which are not directly photosensitive, but are sensitive to heat or
thermosensitive and wherein a visible change in a thermosensitive imaging material
is brought about by the application of sufficient imagewise applied heat to bring
about a change in optical density. This image-wise applied heat can be applied by
a heat source in the direct vicinity of the thermosensitive material or it can be
realized in the thermosensitive material as a result of the absorption of image-wise
applied light by the presence in the thermosensitive material of at least one light-to-heat
conversion agent.
[0023] The term "thermographic material" (or more completely worded as a 'thermographic
recording material', hereinafter indicated by symbol m) comprises a thermosensitive
element or direct thermal imaging element being substantially light-insensitive, and
a support.
[0024] The term light-insensitive means that light is not directly involved in the image-forming
process, but does not exclude light being indirectly involved such as in the case
of light absorption by at least one light-to-heat conversion agent.
[0025] The term substantially light-insensitive means not intentionally light sensitive.
[0026] The terms "main-scan-speed v
x" or "processing speed" are used interchangeably, as well as the terms "slow-scan
speed v
y" or "transportation speed". The processing direction X and the transportation direction
Y are indicated in many drawings (see Figs. 1, 3, 4, 5 and 8.1).
[0027] If, e.g. for commercial reasons, a line-time t
l and a resolution (e.g. dpi) are known, the corresponding slow-scan speeds v
y can be calculated using the expression:
Here we assume that the resolution is equal in both directions X and Y, so that symbolically
[0028] The "sweep-time" t
s (in s) of a flying spot laser system is the time between the beginning of the scanning
of one line 40 of pixels (BOL
j) and the beginning of the scanning of the same line of pixels (BOL
j+1). Reference is made to Fig. 8.1, showing three consecutive scanning lines on a thermographic
material passing through an internal (stationary) drum ITD, or mounted on e.g. an
external (rotating) drum XTD.
[0029] If n
f represents the number of faces and n
p the number of revolutions of the polygon mirror (per second), it applies that
For example, some experiments were carried out at n
f = 8 and n
p = 1875 (rpm), which results in a t
s of about 4 ms/sweep. Other experiments with the same rotating mirror were carried
out at and n
p = 750 (rpm), which results in a t
s of about 10 ms/sweep. Still other experiments with the same rotating mirror were
carried out at and n
p = 500 (rpm), which results in a t
s of about 15 ms/sweep.
[0030] The "total line-time t
l" of a flying spot laser system is the time between the beginning of the printing
of one line of pixels and the beginning of the printing of the next line of pixels
in the printer transport direction Y (often called "slow-scan or sub-scan direction";
and clearly differentiated from a so-called "fast-scan or main-scan direction X").
[0031] Since n
s represents the number of sweeps, it follows that
Equations 3 and 4 have been used for calculating characteristic values for the next
table, in preparation of a practical experiment with same the polygon mirror (n
f = 8) rotating at various speeds (see n
p = 205 to 2500 rpm).
|
|
ns corresponding to a |
np (rpm) |
ts (ms) |
tl of 225 ms |
tl of 630 ms |
tl of 1260 ms |
|
30.00 |
7.5 |
21 |
42 |
250 |
15.00 |
15.0 |
42 |
84 |
500 |
10.00 |
22.5 |
63 |
126 |
750 |
7.50 |
30.0 |
84 |
168 |
1000 |
6.00 |
37.5 |
105 |
210 |
1250 |
5.00 |
45.0 |
126 |
252 |
1500 |
4.29 |
52.5 |
147 |
294 |
1750 |
3.75 |
60.0 |
168 |
336 |
2000 |
3.33 |
67.5 |
189 |
378 |
2250 |
3.00 |
75.0 |
210 |
420 |
The term "line-width b
l " may be self-speaking and is shown (having ref. nr. 44) in Fig. 8.2. Following equation
applies:
In a later section relating to comparative experiments, the physical origin of a
line-width b
l will be explained in reference to fig. 14.1 showing the geometrical spread 62 of
the temperature T
m reached in a first thermographic material when scanned according to a preferred embodiment,
and to fig. 14.2 showing the geometrical spread 63 of the temperature T
m reached in a second substantially light-insensitive thermographic material m
2 when scanned with a second preferred embodiment.
[0032] The "spatial resolution" means the precision (or separation) with which a picture
is reproduced, measured in number of lines that can be distinguished in a picture
e.g. expressed in lines/mm, or in dots per inch (dpi). The highest resolution which
can be attained by a thermographic system, is here symbolised by dpi
upp.
[0033] The "pixel-writing time t
p " (expressed in s) means the time needed for writing one pixel. Following mathematical
relation between pixel-writing time t
p (expressed in s), spatial resolution (expressed in dots per inch DPI) and speed v
x (expressed in m/s) applies:
The term "efficiency η of radiation beam " is defined in relation to a geometrical
spread of the available intensity (or power) of the radiation beam (e.g. a Gaussian
laser beam as shown in Fig. 13) and comprises the ratio of the (quasi-) total area
of such intensity curve to the area of the intensity curve as restricted to temperatures
of the substantially light-insensitive thermographic material m being higher than
conversion temperature T
c. These area can be easily calculated by means of a definite integral calculus.
[0034] An "original" is any hard-copy or soft-copy containing information as an image in
the form of variations in optical density, transmission, or opacity. Each original
is composed of a number of picture elements, so-called "pixels". Further, in the present
application, the terms pixel and dot are regarded as equivalent. Furthermore, according
to the present invention, the terms pixel and dot may relate to an input image (known
as original) as well as to an output image (in soft-copy or in hard-copy, e.g. known
as print).
[0035] In the present application, a "pixel output D
o" or shortly an "output D
o" comprises a quantification of a pixel printed on a thermographic material, the quantification
possibly relating to characteristics as density (symbolised by D), size, etc.
[0036] Some more specific terms will be explained in the following sections.
Thermographic material
[0037] The substantially light-insensitive thermographic material m having a burning temperature
T
b, used in the present invention, comprises a thermosensitive element having a conversion
temperature T
c, a support and at least one light-to-heat conversion agent. The substantially light-insensitive
thermographic material m may be opaque or transparent. The thickness of the thermosensitive
element is generally in the range of about 7 to 25 µm (e.g. 20 µm) and the thickness
of the support is generally in the range of about 60 to 180 µm (e.g. 175 µm). Suitable
support materials include poly(ethylene terephthalate).
[0038] The substantially light-insensitive thermographic material m may further comprise
a subbing or substrate layer 66 with a typical thickness of about 0,1 to 1 µm (e.g.
0.2 µm) and/or a protective layer 68 with a typical thickness of about 2 to 6 µm (e.g.
4 µm) on the same side of the support as the thermosensitive element (for numbering
see Fig. 17). Optionally, on the other side of the support 65 a backing layer 69 may
be provided containing an antistatic and/or a matting agent (or roughening agent,
or spacing agent, terms that often are used as synonyms) to prevent sticking and/or
to aid transport of the substantially light-insensitive thermographic material m.
Further details about the configuration of such substantially light-insensitive thermographic
material m are disclosed in EP 0 692 733.
[0039] The light-to-heat conversion agents are preferably transparent to visible light and
are to be found in the thermosensitive element and/or in an adjacent layer thereto
as a solid particle dispersion, a solution or part as solid particles and part as
a solution therein. Suitable light-to-heat conversion agents include infrared absorbing
dye and absorbers. The light-to-heat conversion agents are preferably homogeneously
distributed together or separately in the thermosensitive element, a constituent layer
of the thermosensitive element and/or an adjacent layer to the thermosensitive element.
[0040] The thermosensitive element contains the ingredients necessary for bringing about
the image-forming reaction. The element may comprise a layer system in which the ingredients
necessary for bringing about the image-forming reaction may be dispersed in different
layers, with the proviso that the ingredients active in the image-forming reaction
are in reactive association with one another i.e. during the thermal development process
one type of active ingredient must be present in such a way that it can diffuse to
the other types of active ingredients so that the image-forming reaction can occur.
[0041] Any type of thermosensitive material with different image-forming reactions can be
used in the present invention. A preferred thermographic material for use in the present
invention is the so-called "laser induced dye transfer LIDT", which is described in
US 5,804,355. A preferred image-forming reaction is the reaction of one or more substantially
light-insensitive organic silver salts with one or more reducing agents, the reducing
agents being present in such a way that they are able to diffuse to the particles
of substantially light-insensitive organic silver salt so that reduction to silver
can occur.
[0042] Preferred substantially light-insensitive organic silver salts for use in the substantially
light-insensitive thermographic material used in the present invention are substantially
light-insensitive silver salts of an organic carboxylic acid, with substantially light-insensitive
silver salts of a fatty acid, such as silver behenate, being particularly preferred.
[0043] The so-called "conversion temperature or threshold T
c " is defined as being the minimum temperature of the substantially light-insensitive
thermographic material m necessary during a certain time range to bring about an image-forming
reaction, so as to form visually perceptible image.
[0044] If the temperature of the substantially light-insensitive thermographic material
increases above T
c, the recording density increases further, but generally non-linearly. A substantially
light-insensitive thermographic material used according to the present invention generally
has a T
c between 75 and 120°C, more specifically between 80 and 110°C.
[0045] The "burning temperature T
b" of a substantially light-insensitive thermographic material m is the lowest temperature
at which any burning might occur, irrespectively in which layer it might happen (e.g.
in a support 65, in a substrate layer 66, in a thermosensitive element 67, in a protective
layer 68, or/and in a backing layer 69, see Fig. 17 for the numbering).
Apparatus for thermal recording of an image in a substantially light-insensitive thermographic
material
[0046] Aspects of the present invention are realized by an apparatus for thermal recording
an image in a substantially light-insensitive thermographic material m having a burning
temperature T
b, the substantially light-insensitive thermographic material m comprising a thermosensitive
element having a conversion temperature T
c , a support, and at least one light-to-heat conversion agent, comprises a means for
generating a radiation beam 20 including wavelengths λ absorbed by the light-to-heat
conversion agent and an optical means of scanning a line 40 of the substantially light-insensitive
thermographic material m with the radiation beam 20 at different positions thereon
along a scanning direction at each point of time in a scanning cycle.
[0047] According to a first embodiment of the apparatus, according to the present invention,
the radiation beam 20 is capable of being modulated in accordance with the information
to be recorded.
[0048] According to a second embodiment of the apparatus, according to the present invention,
the optical scanning means is capable of heating the line of the substantially light-insensitive
thermographic recording material m to a first predetermined temperature T
1 being above being above the conversion temperature T
c and below the burning temperature T
b of the substantially light-insensitive thermographic material m.
[0049] According to a third embodiment of the apparatus, according to the present invention,
the apparatus further comprises a means of cooling the line 40 of the substantially
light-insensitive thermographic material m to a second predetermined temperature T
2 being below the conversion temperature T
c.
[0050] According to a fourth embodiment of the apparatus, according to the present invention,
the apparatus further comprises a means of re-scanning the line of the substantially
light-insensitive thermographic material m a plurality of times n
s with the radiation beam being identically modulated in accordance with the information
to be recorded.
[0051] Fig. 6 is a pictorial view of an apparatus for thermal recording according to the
present invention.
[0052] According to a fifth embodiment of the apparatus, according to the present invention,
the thermographic material is mountable on a holding means 14 (which might be a flat
bed), e.g. on an external drum 15.
[0053] According to a sixth embodiment of the apparatus, according to the present invention,
the thermographic material is mountable on a holding means 14, for example a drum,
capable of heating the substantially light-insensitive thermographic material to a
preheating temperature T
p below a conversion temperature T
c of the substantially light-insensitive thermographic material.
[0054] According to a seventh embodiment of the apparatus, according to the present invention,
the means of generating a radiation beam 20 is a laser beam.
[0055] According to an eighth embodiment of the apparatus, according to the present invention,
the means of generating a radiation beam 20 is a coherent light source (11) comprising
a semiconductor- or diode-laser (optionally fibre coupled), a diode-pumped laser (as
a neodymium-laser), or an ytterbium fibre laser.
[0056] According to a ninth embodiment of the apparatus, according to the present invention,
the means of generating a radiation beam 20 is an infrared or near-infrared laser
beam i.e. with emission in the wavelength range λ = 700-1500 nm. Suitable lasers include
a Nd-YAG-laser (neodymium-yttrium-aluminium-garnet; 1064 nm) or a Nd-YLF-laser (neodymium-yttrium-lanthanum-fluoride;
1053 nm). Typical suitable laser diodes emit e.g. at 830 nm or at 860-870 nm.
[0057] When a laser scans over the thermographic material, the temperature on the recorded
pixels rises and an image-forming process occurs in the thermosensitive element, e.g.
reduction of a substantially light-insensitive silver salt of the thermographic material,
and a perceptible image appears. After writing a first line, a motor (not shown in
drawing Fig. 6) transports the drum one step.
[0058] According to a tenth embodiment of the apparatus, according to the present invention,
the means of generating a radiation beam 20 is a laser beam (e.g. A YAG-doped ytterbium-laser
Yb-YAG emitting a beam of 1030 nm with 20 W power in continuous wave; e.g. type 'DisKlaser'
available from the company NANOLASE) which is modulated by a modulator 28, e.g. an
acoustic modulator, which can be activated or deactivated.
[0059] Fig. 6 shows the laser beam 20 being deflected by a first mirror 26, passing through
a modulator 28, e.g. an acoustic modulator, which can be activated or deactivated.
When it is activated the laser beam goes to a second mirror 27 and may pass through
two lenses to adjust the (vertical) beam-diameter an then comes to moving mirror 10,
e.g. a polygon with eight faces. This polygon turns the beam via a fθ objective 29
to a torroidal lens (not explicitly shown) which focuses the beam on the substantially
light-insensitive thermographic material.
[0060] According to an eleventh embodiment of the apparatus, according to the present invention,
the optical scanning means comprises a light deflecting means for deflecting the laser
beam to scan the substantially light-insensitive thermographic material m with the
deflected laser beam, such as a polygon mirror. The radiation beam scans faster or
slower over the substantially light-insensitive thermographic material m, depending
upon the speed of the movable components in the optical scanning means, such as a
polygon mirror.
[0061] According to a twelfth embodiment of the apparatus, according to the present invention,
the apparatus further includes a further heating means.
[0062] According to a thirteenth embodiment of the apparatus, according to the present invention,
the apparatus further includes a further heating means comprising an external drum,
such as shown in Fig. 4 and Fig. 8.1 and disclosed in US 5,932,394.
[0063] Fig. 4 schematically shows an external drum type recording device having a so called
"imaging array" (e.g. a laser-array). In such embodiment, a carriage carrying an array
19 of e.g. laser-diodes, has to move (or to sweep) at least two times from one side
(e.g. BOL) of the drum 15 to the other side (e.g. EOL) of the drum. Although this
seems to need a longer line-time (because of the mechanical movements of the carriage),
it has to be emphasised that such an array preferably scans the substantially light-insensitive
thermographic material m (5) with at least two laser beams at a same time (sometimes
called "comb-wise"), thus gaining (because of the electro-optical simultaneity) in
line-time.
[0064] According to a fourteenth embodiment of the apparatus, according to the present invention,
the apparatus further includes a further heating means comprising a transparent thermal
head (which is not separately shown in Fig. 5), as disclosed in EP-A 1 104 699.
[0065] Fig. 5 shows a preferred embodiment of a laser thermographic apparatus suitable for
use in a method according to the present invention. In Fig. 5, ref. 5 is the thermal
imaging element, 17 a hardcopy print, 20 is a laser beam, 102 a supply magazine, 104
a belt, 105 a tension roller, 108 a roller, 109 a roller, 110 a controller, 113 a
ventilator, 116 imaged and processed sheets, 117 a keyboard, 118 a laser source, 119
a modulator, 120 a first objective, 121 a polygon mirror, 122 a second objective,
123 blank sheets to be imaged, 124 a sheet feeder, 125 an imaging and processing unit,
126 a pressure roller. A full description of a laser thermographic printer can be
found in DE-A 196 36 253.
[0066] According to a fifteenth embodiment of the apparatus, according to the present invention,
the apparatus includes controllable parameters comprising 1) specifications of the
substantially light-insensitive thermographic material m and the light-to-heat conversion
agent, 2) temperature T
p of the drum, 3) position of the thermosensitive element with respect to the drum,
4) power of a laser, 5) input of a modulator 6) transportation speed v
y of the substantially light-insensitive thermographic material m, 7) speed np of a
rotating optical means, 8) number n
s of sweeps during one line-time t
l.
[0067] According to a sixteenth embodiment of the apparatus, according to the present invention,
the apparatus excludes a transparent thermal head.
Method for recording information
[0068] Aspects of the present invention are realized by a method for recording information
(e.g. imagedata and barcodes), comprising the steps of: providing an apparatus for
thermal recording 1, a substantially light-insensitive thermographic material m (5),
the thermographic material having a burning temperature T
b (e.g. about 300°C), and comprising a thermosensitive element having a conversion
temperature T
c (e.g. ranging between 80°C and 110°C, according to the specific type of thermographic
material), a support, and at least one light-to-heat conversion agent; generating
a radiation beam 20 including wavelengths λ absorbed by the light-to-heat conversion
agent and being modulated in accordance with the information to be recorded (i.e.
image-wise); scanning a line (40 in Fig. 8.1) of the substantially light-insensitive
thermographic material m a first time with the radiation beam, thereby heating the
line of the substantially light-insensitive thermographic material m to a first predetermined
temperature T
1 being above the conversion temperature T
c and below the burning temperature T
b of the substantially light-insensitive thermographic material m; re-scanning the
same line of the substantially light-insensitive thermographic material m a plurality
of times n
s with the radiation beam being identically modulated in accordance with the information
to be recorded.
[0069] Fig. 7 shows the evolution over time of the temperature attained in a thermographic
material while applying a plurality of scannings according to the present invention).
[0070] In Figs. 11 and 12 several experiments are shown in which the first predetermined
temperature T
1 was about 200°C.
[0071] According to a first embodiment of the method, according to the present invention,
the method further comprises cooling the line 40 of the substantially light-insensitive
thermographic material m to a second predetermined temperature T
2 being below the conversion temperature T
c, with non-forced cooling, i.e. natural, physical decay of the temperature over time,
being preferred. Examples of forced cooling is cooling with a blower.
[0072] In general, the second predetermined temperature T
2 is between the conversion temperature T
c and the ambient temperature T
a. In a preferred embodiment, the second predetermined temperature T
2 is nearly at ambient temperature T
a. In another preferred embodiment, wherein a substantially light-insensitive thermographic
material m is in contact with a holding means 14 (e.g. being flat as shown in Fig.
1, or e.g. being cylindrical as shown by a drum 15 in Figs. 3-6, and 8.1) the second
predetermined temperature T
2 is at the so-called preheating temperature T
p , so that T
2 = T
p (see Fig. 7). In those embodiments wherein the holding means 14 or the drum 15 is
not preheated, the temperature T
p is ambient temperature T
a (so that T
2 = T
p = T
a).
[0073] According to a second embodiment of the method, according to the present invention,
the method further comprises the removal of the substantially light-insensitive thermographic
material m from the apparatus for thermal recording 1, thereby delivering a hard-copy
print (indicated by ref. nr. 17 in Fig. 5) of the information.
[0074] According to a third embodiment of the method, according to the present invention,
an upper limit of spatial resolution (dpi
upp) is controlled by determining a main-scan-speed v
y in relation to the first predetermined temperature T
1.
[0075] For example, if, for a given substantially light-insensitive thermographic material
m and for a given predetermined temperature T
1, it is desired to increase a spatial resolution in a hard-copy print 17 up to a required
value dpi
upp, the main-scan-speed v
x might be increased.
[0076] The speed of the radiation beam over the substantially light-insensitive thermographic
material increases with increasing speed of the rotating polygon. By virtue of the
normally non-square distribution of the intensity of the laser beam (see Fig. 13),
only a part of the thermographic material irradiated attains a temperature higher
than the conversion temperature T
c (see Figs. 14.1 and 14.2). Hence, at a higher main-scan-speed v
x smaller lines will be recorded. If the slow-scan-speed v
y is also increased correspondingly, a higher spatial resolution is attained, i.e.
dpi
upp.
[0077] Since at a higher main-scan-speed v
x a decreased efficiency η of the laser system is observed (see e.g. Fig. 15, to be
explained below), it may be necessary to increase the number of sweeps n
s in order to obtain a density which is sufficiently high.
[0078] According to a fourth embodiment of the method, according to the present invention,
the method further comprises a step of controlling a spatial resolution (dpi) of the
hardcopy print 17 by choosing the first temperature T
1 substantially higher than T
c.
[0079] In certain circumstances, the first temperature T
1 is relatively close to the T
c (as shown in Fig. 14.1 for a thermographic material scanned at a rather high main-scan-speed
v
x), which results in thinner lines be obtained.
[0080] In other preferred circumstances, the first temperature T
1 is relatively far away from the T
c (as shown in Fig. 14.2 for a same thermographic material scanned at a rather low
main-scan-speed v
x), which results in thicker lines being obtained.
[0081] If it is desired to increase the spatial resolution (dpi) in a hard-copy print 17,
e.g. up to the upper limit dpi
upp for a given substantially light-insensitive thermographic material m and for a given
main-scan-speed v
x, the first temperature T
1 should be decreased.
[0082] Figs. 14.1 and 14.2 also illustrate another embodiment of the present invention.
For a same upper limit of the temperature T
m, the spatial resolution (dpi) of the hardcopy print 17 can be controlled by selecting
the type of thermographic material, especially with respect to the conversion temperature
T
c e.g. if an apparatus were to comprise two or more film cassettes comprising at least
two kinds of thermographic materials, say m
1 and m
2 having respective conversion temperatures T
c1 and T
c2.
[0083] In general, Fig. 14.1 shows the geometrical spread 62 of the temperature T
m reached in a thermographic material when scanned according to a preferred embodiment,
and Fig. 14.2 shows the geometrical spread 63 of the temperature T
m reached in a thermographic material when scanned according to a second preferred
embodiment.
[0084] More in detail, from one point of view, Fig. 14.1 shows the geometrical spread of
the temperature T
m reached in a thermographic material when scanned with a high speed laser beam; and
Fig. 14.2 shows the geometrical spread of the temperature T
m reached in a thermographic material when scanned with a low speed laser beam. From
another point of view, Fig. 14.1 shows the geometrical spread of the temperature T
m reached in a first substantially light-insensitive thermographic material m
1 when scanned with a laser beam; and Fig. 14.2 shows the geometrical spread of the
temperature T
m reached in a second substantially light-insensitive thermographic material m
2 when scanned with a same laser beam.
[0085] It may be quite clear that in a method according to the present invention the burning
temperature T
b is not to be exceeded (see Figs. 7, 11 and 12).
[0086] Fig. 11 shows the actual evolution over time of the temperature T
m in the thermosensitive element if an information is recorded in one single sweep
and Fig. 12 shows the actual evolution over time of the temperature T
m attained in the thermosensitive element if an information is recorded by applying
a plurality of scannings according to the method of the present invention. Applying
a plurality of scannings eliminates unwanted side-effects such as deformation, colouring
and burning.
[0087] According to a fifth embodiment of the method, according to the present invention,
the plurality of times n
s comprises at least two times (n
s ≥ 2; see also Figs. 7, 10 and 12).
[0088] According to a sixth embodiment of the method, according to the present invention,
the plurality of times n
s is defined such that a desired pixel output (D
o) is achieved.
[0089] In certain circumstances, the first temperature T
1 is relatively close to T
c (as shown in Fig. 14.1), which results in thinner lines being attained such that
more sweeps have to be performed in order to attain a sufficient density in the output
print 17 (especially in the mid of the line width 44, see Figs. 8.2, 10, 14.1 and
14.2).
[0090] In other preferred circumstances, the first temperature T
1 is relatively distant to the T
c (as shown in Fig. 14.2), thereby obtaining thicker lines and generally requiring
less sweeps.
[0091] According to a seventh embodiment of the method, according to the present invention,
an upper limit of spatial resolution (dpi
upp) is controlled by determining an energy radiated by the radiation beam in relation
to a main-scan-speed v
x.
[0092] The laser output is required to produce a sufficient energy to enable a desired density
to be obtained with the substantially light-insensitive thermographic material m.
When a laser scans over the thermographic material, the temperature on the recorded
pixels rises, the imaging-forming reaction occurs and a perceptible image appears.
After writing a first line, a motor (not shown in drawing Fig. 6) transports the drum
one step.
[0093] According to an eighth embodiment of the method, according to the present invention,
the method further comprises a step of defining a position (wherein the scanning of
the substantially light-insensitive thermographic material m is carried out) of the
thermosensitive element with respect to a holding means 14 or a drum 15. We refer
to Fig. 18.1 and 18.2 respectively showing a thermographic system incorporating a
first and a second position (REPL versus RPEL) of the thermosensitive element with
respect to a holding means 14, or a drum 15.
[0094] According to a ninth embodiment of the method, according to the present invention,
the method further comprises a step of further heating (also called "background heating
or preheating") the substantially light-insensitive thermographic material m to a
preheating temperature T
p before and/or during scanning thereof with the radiation beam (see Figs. 5 and 7).
[0095] Fig. 17 (not shown to scale) shows a cross-section of a configuration of a substantially
light-insensitive thermographic material m suitable for application within the present
invention.
[0096] According to a tenth embodiment of the method, according to the present invention,
the substantially light-insensitive thermographic material m comprises a thermosensitive
element consisting of at least one layer, the thermosensitive element comprising a
substantially light-insensitive organic silver salt and a reducing agent therefor
in thermal relationship therewith, the reducing agent being in a layer of said thermosensitive
element containing said substantially light-insensitive organic silver salt and/or
in an adjacent layer of the thermosensitive element such that the reducing agent is
present such that it is in thermal working relationship with said substantially light-insensitive
organic silver salt.
[0097] According to an eleventh embodiment of the method, according to the present invention,
the method further comprises a step of further heating the substantially light-insensitive
thermographic material m with a transparent thermal head.
[0098] Furthermore, in addition to Gaussian and non-Gaussian beam intensities, it may be
advantageous to shape the writing spot such that it becomes a "top-hat" writing spot.
This may be carried out e.g. by so-called diffractive optical elements (DOE).
[0099] According to a twelfth embodiment of the method, according to the present invention,
the substantially light-insensitive thermographic material excludes an image-forming
layer on a hydrophilic surface.
Industrial application
[0100] The apparatus for thermal recording an image, according to the present invention,
is used for recording information in substantially light-insensitive thermographic
materials for medical and graphics applications.
EXAMPLES
[0101] All experiments were carried out on an XTD-embodiment as shown in Fig. 6. Practical
dimensions of this system (see also Fig. 8.1) included: the diameter D
d of drum 15 being 70 mm, the width of the drum 15 being 250 mm and the width W
m (46) of the thermographic material being 200 mm.
[0102] A preferred embodiment of the present invention was tested and evaluated extensively.
The controllable parameters mentioned above, are summarized in the following paragraph.
1) Thermographic specifications of the substantially light-insensitive thermographic
material m and of the IR-absorber (e.g. spectral bandwidth and sensitivity) were selected
from a matrix of available values.
2) The temperature Tp of the drum 15 was controlled in a range between 30°C and 150°C, more preferably
between 50°C and 120°C, and set most typically at discrete values of 70, 75, 80, 85,
90 and 100°C.
3) As regards the position of the thermographic material 5 with respect to the drum
15, the influences of two possibilities (mentioned as REPL versus RPEL in Figs. 18.1
and 18.2) were explored.
4) The radiation source 21 was a YAG doped Yb -laser having a wavelength λ of 1030
nm. An available power of 20 Watt (in continuous wave mode) resulted in about 9 Watt
impacting on the thermographic material 5. Sometimes lower values for the power have
been chosen by reducing the power supply (e.g. a control current of 45 A corresponded
to a power of 20 W).
5) The input Vc,m of a modulator 28, more specifically the voltage supply to an acousto-optic-modulator
AOM (in particular, an AOM as e.g. type 1110AF_AIFO_2 supplied by CRYSTAL TECHNOLOGY
CORPORATION, was generally set at 1 Volt, which gave an output Po,m of about 93 % (see also Fig. 9 showing a relation between a control voltage Vc,m to an acousto-optical modulator and the percentage of transmitted laser power Po,m).
6) The transportation or slow-scan-speed vy of the substantially light-insensitive thermographic material m ranged between 0.35
and 4.5 mm/s. Particularly tested speeds included 0.35, 0.42, 0.52, 0.70, 1.05, 1.25
and 2.00 mm/s.
7) The speed np of the rotating optical means (e.g. a mirror or a polygon) ranged between 250 and
3500 rpm. Particularly tested speeds included 444, 500, 750 and 1875 rpm.
8) The number ns of sweeps (during a line-time tl) ns ranged from 1 time to 400 times. Particularly tested values included 3, 6, 12, 18,
24, 30, 42, 50, 63, 100, 200 and 400 sweeps.
[0103] It should be noted that the line-time t
l can be derived from the sweep-time t
s (cf. n
p and equation 4)and from the number n
s of sweeps. The t
l values tested included 20, 30, 40, 50, 60, ... 225, 630, to 1260 ms.
[0104] Extensive experimentation was carried out during the test programme leading to the
present invention. For sake of brevity, two sets of experiments are described below
in detail to illustrate the invention more clearly.
[0105] Fig. 10 records the results of the first set of experiments and shows an output density
(e.g. ranging up to 4.5 D) for a substantially light-insensitive thermographic material
m, as a function of:
(i) a pixel-distance dY (e.g. ranging from -50 µm to + 50 µm) to the central axis cL of a printed line 40 (see also Fig. 8.2),
(ii) a background heating or preheating temperature Tp (e.g. 90 °C or 100 °C), and
(iii) a number of sweeps ns (e.g. ranging from 3 times to 30 times.
[0106] From these experiments it can be concluded that:
i) more sweeps result in a higher density; more sweeps result in a broader line-width;
ii) for a background temperature Tp = 90 °C, at least 30 sweeps are necessary in order to attain an acceptable density;
iii) for a background temperature Tp = 100 °C, at least 12 sweeps are necessary in order to attain an acceptable density;
iv) a higher background temperature makes it possible to record faster, but the resolution
of the output image decreases; and
v) by recording according to the present invention, a density D > 4.0 is attainable
without losing tone neutrality.
[0107] Figs. 15 and 16 record the results of the second set of experiments, which confirmed:
(i) that a higher speed of revolution of the polygon normally resulted in a smaller
line-width and hence in a higher spatial resolution, but also (ii) that a higher speed
of rotation of the polygon resulted in a lower efficiency η of the thermographic system.
Given a particular system and a particular substantially light-insensitive thermographic
material m, our tests concerning spatial resolution resulted in Fig. 15 showing the
efficiency η of the laser system when different line-thicknesses were applied, and
in Fig. 16 showing the efficiency η of the laser system when different spatial resolutions
were applied.
[0108] For ensure that the term "efficiency η of radiation beam" is well understood, Fig.
14.1 is referred to in which the geometrical spread of the temperature attained in
a first substantially light-insensitive thermographic material m
1 when scanned with a Gaussian laser beam, and to Fig. 14.2 showing the geometrical
spread of the temperature reached in a second substantially light-insensitive thermographic
material m
2 when scanned with a same Gaussian laser beam. It may be noted that according to the
present invention, high spatial resolutions e.g. higher than 600 or even 1200 dpi
or small line-widths e.g. smaller than 40 or even 20 µm are attained.
[0109] Having described in detail preferred embodiments of the current invention, it will
now be apparent to those skilled in the art that numerous modifications can be made
therein without departing from the scope of the invention as defined in the appending
claims.