Thermal recording apparatus

The improved thermal recording apparatus using a thermal head comprises an image processing unit which receives image data from an image supply source and which performs sharpness compensation and other image processing jobs on the received image data to construct data for the image to be recorded thermally and thermistor for measuring a temperature of the thermal head, wherein the image processing unit changes the coefficient of sharpness compensation in accordance with the temperature of the thermal head. The thermal recording apparatus can sufficiently reduce the drop in image sharpness due to an increased temperature of the thermal head and, optionally, the drop in image sharpness due either to an increased recording speed or to a reduced gamma value of the thermal material or to both factors to ensure the consistent production of unblurred, high-quality images.

BACKGROUND OF THE INVENTION 
This invention relates to the art of thermal recording apparatus using a 
thermal head. 
Thermal recording materials comprising a thermal recording layer on a 
substrate such as a film, which are hereunder referred to as thermal 
materials, are commonly used to record the images produced in diagnosis by 
ultrasonic scanning. 
This recording method, commonly referred to as thermal image recording, 
eliminates the need for wet processing and offers several advantages 
including convenience in handling. Hence, the use of the thermal image 
recording system is not limited to small-scale applications such as 
diagnosis by ultrasonic scanning and an extension to those areas of 
medical diagnoses such as CT, MRI and X-ray photography where large and 
high-quality images are required is under review. 
As is well known, thermal image recording involves the use of a thermal 
head having a glaze in which heat-generating elements for heating the 
thermal recording layer of a thermal material to record an image are 
arranged in one direction and, with the glaze a little pressed against the 
thermal material (thermal recording layer), the thermal material is 
relatively moved in the auxiliary scanning direction perpendicular to the 
main scanning direction in which the glaze extends, as the respective 
heat-generating elements of the glaze are heated imagewise by energy 
application to heat the thermal recording layer, thereby accomplishing 
image reproduction. 
Not only the thermal recording apparatus but also various other image 
recording apparatus including laser printers and plate-making apparatus 
are adapted to perform sharpness compensation in order to produce 
high-quality, clear and well modulated images by means of edge enhancement 
for improved image sharpness. In practice, however, the sharpness of 
recorded images is affected by various factors which, in the case of 
thermal recording, include the temperature of the thermal head, the 
recording speed (auxiliary scanning transport speed) and the gamma value 
of the thermal material used. Stated specifically, the image sharpness 
deteriorates with the increasing temperature of the thermal head 
(heat-generating elements) and with the increasing recording speed (the 
speed of movement of the thermal head relative to the thermal material) 
but with the decreasing gamma value of the thermal material and the 
recorded image will look blurred if these phenomena occur. More 
specifically, as for the recording speed, the image sharpness in the 
auxiliary scanning direction deteriorates with the increasing recording 
speed, and the image sharpness in the main scanning direction deteriorates 
with the decreasing recording speed, and the recording image will look 
blurred, if there phenomena occur. Among others, the increase in the 
temperature of the thermal head is most influential in the sharpness of 
the recorded image. 
The reduced sharpness of the recorded image will lead to the deterioration 
of the quality of finished images and can be a serious problem in 
applications that require the recording of high-quality images. In the 
above-stated medical applications, images of particularly high quality are 
required and the reduction in sharpness is an obstacle to the viewing of 
the correct image, potentially leading to a wrong diagnosis. 
SUMMARY OF THE INVENTION 
The present invention has been accomplished under these circumstances and 
has as an object providing a thermal recording apparatus with which the 
drop in image sharpness due to an increased temperature of the thermal 
head and, optionally, the drop in image sharpness due either to an 
increased recording speed or to a reduced gamma value of the thermal 
material or to both factors can also be sufficiently reduced to ensure the 
consistent production of high-quality and well modulated images. 
To achieve the above object, the invention provides a thermal recording 
apparatus comprising: 
an image processing unit which receives image data from an image supply 
source and which performs sharpness compensation and other image 
processing jobs on the received image data to construct data for the image 
to be recorded thermally; 
a thermal head having a glaze of which heat-generating elements are 
arranged in one direction, and are heated in accordance with said data for 
the image to be recorded thermally to perform image recording on a thermal 
recording material; 
means for moving the thermal recording material relative to the thermal 
head in a direction perpendicular to the direction in which said 
heat-generating elements are arranged, with the thermal recording material 
being kept in contact with said glaze; and 
means for measuring a temperature of the thermal head, 
wherein said image processing unit includes means for changing the 
coefficient of sharpness compensation in accordance with the temperature 
of the thermal head. 
It is preferred that the thermal recording apparatus of the invention 
additionally has at least one means selected from the group consisting of 
means for detecting the recording speed, means for detecting the gamma 
value of the thermal recording material and means for setting the gamma 
value of the thermal recording material and said image processing unit has 
means for changing the coefficient of sharpness compensation in 
consideration of not only the temperature of the thermal head but also at 
least one of the recording speed and the gamma value of the thermal 
recording material. 
It is also preferred that said means for changing the coefficient of 
sharpness compensation changes separately the coefficient of sharpness 
compensation in the direction in which said heat-generating elements are 
arranged, and the coefficient of sharpness compensation in the direction 
in which said thermal recording material is moved.

DETAILED DESCRIPTION OF THE INVENTION 
The thermal recording apparatus of the invention will now be described in 
detail with reference to the preferred embodiments shown in the 
accompanying drawings. 
FIG. 1 shows schematically a thermal recording apparatus of the invention. 
The thermal recording apparatus generally indicated by 10 in FIG. 1 and 
which is hereunder simply referred to as a "recording apparatus" performs 
thermal image recording on thermal recording materials of a given size, 
say, B4 (namely, thermal recording materials in the form of cut sheets, 
which are hereunder referred to as "thermal materials A"). The apparatus 
comprises a loading section 14 where a magazine 24 containing thermal 
materials A is loaded, a feed/transport section 16, a recording section 20 
performing thermal image recording on thermal materials A by means of the 
thermal head 66, and an ejecting section 22. In addition, as shown in FIG. 
3b, the thermal head 66 in the recording section 20 is connected to an 
image processing unit 80, an image memory 82 and a recording control unit 
84, and the image processing unit 80 in turn is connected to a corrected 
data storage unit 86. 
In the thus constructed recording apparatus 10, the feed/transport section 
16 transports the thermal material A to the recording section 20, where 
the thermal material A against which the thermal head 66 is pressed is 
transported in the auxiliary scanning direction perpendicular to the main 
scanning direction in which the glaze extends (normal to the paper of FIG. 
1 and indicated by arrow c in FIG. 3b) and in the meantime, the individual 
heat-generating elements are actuated imagewise to perform thermal image 
recording on the thermal material A. 
The thermal materials A comprise respectively a substrate of film such as a 
transparent polyethylene terephthalate (PET) film, paper and the like 
which is overlaid with a thermal recording layer. 
Typically, such thermal materials A are stacked in a specified number, say, 
100 to form a bundle, which is either wrapped in a bag or bound with a 
band to provide a package. As shown, the specified number of thermal 
materials A bundled together with the thermal recording layer side facing 
down are accommodated in the magazine 24 of the recording apparatus 10, 
and they are taken out of the magazine 24 one by one to be used for 
thermal image recording. 
The magazine 24 is a case having a cover 26 which can be freely opened. The 
magazine 24 which contains the thermal materials A is loaded in the 
loading section 14 of the recording apparatus 10. 
The loading section 14 has an inlet 30 formed in the housing 28 of the 
recording apparatus 10, a guide plate 32, guide rolls 34 and a stop member 
36; the magazine 24 is inserted into the recording apparatus 10, via the 
inlet 30, cover 26 first; thereafter, the magazine 24 as it is guided by 
the guide plate 32 and the guide rolls 34 is pushed until it contacts the 
stop member 36, whereupon it is loaded at a specified position in the 
recording apparatus 10. 
The feed/transport section 16 includes a sucker 40 for grabbing the thermal 
material A by application of suction, transport means 42, a transport 
guide 44 and a regulating roller pair 52 located in the outlet of the 
transport guide 44. The thermal materials A are taken out of the magazine 
24 in the loading section 14 and transported to the recording section 20. 
The transport means 42 is composed of a transport roller 46, a pulley 47a 
coaxial with the roller 46, a pulley 47b coupled to a rotating drive 
source, a tension pulley 47c, an endless belt 48 stretched between the 
three pulleys 47a, 47b and 47c, and a nip roller 50 that is to be pressed 
onto the transport roller 46. The forward end of the thermal material A 
which has been sheet-fed by means of the sucker 40 is pinched between the 
transport roller 46 and the nip roller 50 such that the material A is 
transported downstream. 
When a signal for the start of recording is issued, the cover 26 is opened 
by the OPEN/CLOSE mechanism (not shown) in the recording apparatus 10. 
Then, the sheet feeding mechanism using the sucker 40 picks up one sheet 
of thermal material A from the magazine 24 and feeds the forward end of 
the sheet to the transport means 42 (to the nip between rollers 46 and 
50). When the thermal material A has been pinched between the transport 
roller 46 and the nip roller 50, the sucker 40 releases the material, and 
the thus fed thermal material A is supplied by the transport means 42 into 
the regulating roller pair 52 as it is guided by the transport guide 44. 
When the thermal material A to be used in recording has been completely 
ejected from the magazine 24, the OPEN/CLOSE mechanism closes the cover 
26. 
The distance between the transport means 42 and the regulating roller pair 
52 which is defined by the transport guide 44 is set to be somewhat 
shorter than the length of the thermal material A in the direction of its 
transport. The advancing end of the thermal material A first reaches the 
regulating roller pair 52 by the transport means 42. The regulating roller 
pair 52 is normally at rest. The advancing end of the thermal material A 
stops here and is subjected to positioning. 
When the advancing end of the thermal material A reaches the regulating 
roller pair 52, the temperature of the thermal head 66 (glaze 66a) is 
checked and if it is at a specified level, the regulating roller pair 52 
start to transport the thermal material A, which is transported to the 
recording section 20. 
FIG. 2 shows schematically the recording section 20. 
The recording section 20 has the thermal head 66, a platen roller 60, a 
cleaning roller pair 56, a guide 58, a fan 76 for cooling the thermal head 
66 (see FIG. 1) and a guide 62. The thermal head 66 is capable of thermal 
recording at a recording (pixel) density of, say, about 300 dpi. The head 
comprises a body 66b having the glaze 66a in which the heat-generating 
elements performing thermal recording on the thermal material A are 
arranged in one direction, that is in the main scanning direction 
(perpendicular to the paper of FIGS. 1, 2 and parallel to the direction of 
arrow c in FIG. 3), and a heat sink 66c fixed to the body 66b. The thermal 
head 66 is supported on a support member 68 that can pivot about a fulcrum 
68a either in the direction of arrow a or in the reverse direction. 
The platen roller 60 rotates at a specified image recording speed while 
holding the thermal material A in a specified position, and transports the 
thermal material A in the auxiliary scanning direction perpendicular to 
the main scanning direction (direction of arrow b in FIG. 2). 
The cleaning roller pair 56 consists of an adhesive rubber roller 56a made 
of an elastic material and a nonadhesive roller 56b. The adhesive rubber 
roller 56a picks up dirt and other foreign matter that has been deposited 
on the thermal recording layer in the thermal material A, thereby 
preventing the dirt from being deposited on the glaze 66a or otherwise 
adversely affecting the image recording operation. 
Before the thermal material A is transported to the recording section 20, 
the support member 68 in the illustrated recording apparatus 10 has 
pivoted to an UP position (in the direction opposite to the direction of 
arrow a) so that the thermal head 66 (or glaze 66a) is not in contact with 
the platen roller 60. 
When the transport of the thermal material A by the regulating roller pair 
52 starts, said material is subsequently pinched between the cleaning 
rollers 56 and transported as it is guided by the guide 58. When the 
advancing end of the thermal material A has reached a RECORD START 
position corresponding to a position directly beneath the glaze 66a as 
illustrated in FIG. 2, the support member 68 pivots in the direction of 
arrow a and the thermal material A becomes pinched between the glaze 66a 
on the thermal head 66 and the platen roller 60. The glaze 66a is pressed 
onto the recording layer while the thermal material A is transported in 
the auxiliary scanning direction b by means of the platen roller 60, the 
regulating roller pair 52 and the transport roller pair 63 as it is held 
in a specified position. 
During this transport, the individual heat-generating elements on the glaze 
66a are actuated imagewise to perform thermal image recording on the 
thermal material A. 
The recording apparatus 10 of the invention performs thermal image 
recording with a coefficient of sharpness compensation that is altered in 
accordance with the temperature of the thermal head 66 and other relevant 
parameters in the manner described below. 
FIG. 3a is a schematic perspective view of the thermal head 66 and FIG. 3b 
is a block diagram of the system for controlling the recording with the 
thermal head 66. 
As FIG. 3b shows, the system for controlling the recording with the thermal 
head 66 is essentially composed of the image processing unit 80, image 
memory 82 and the recording control unit 84. The image processing unit 80 
in turn is connected to the corrected data storage unit 86 for storing 
weighting functions or tables for the coefficient of sharpness 
compensation. 
Image data from an image (data) supply source R such as CT or MRI is sent 
to the image processing unit 80. 
The image processing unit 80 is the combination of various kinds of image 
processing circuits and memories. It receives the image data from the 
image supply source R and performs specified image processing jobs, such 
as sharpness compensation for edge enhancement, tone correction for 
producing an appropriate image in accordance with the gamma value and 
other characteristics of the thermal material A, temperature compensation 
for adjusting the energy of heat generation in accordance with the 
temperature of heat-generating elements, shading compensation for 
correcting the uneven density caused by the shape variability and other 
factors of the glaze 66a on the thermal head 66, resistance compensation 
for correcting the difference between the resistances of individual 
heat-generating elements, and black ratio compensation for ensuring that 
image data representing the same density will yield a color of the same 
density in spite of the variation in the drop of supply voltage to the 
thermal head due to the change in the pattern to be recorded. If 
necessary, the image processing unit 80 may perform formating (i.e., 
enlargement or reduction and frame assignment), whereupon the data for the 
image to be thermally recorded by means of the thermal head 66 is 
delivered as an output to the image memory 82. 
As FIG. 3b shows, the fins of the heat sink 66c in the illustrated thermal 
head 66 have a cutout 66d formed at five sites of the area corresponding 
to the glaze 66a and thermistors 88 are provided at the base of the heat 
sink 66c at those sites. Each thermistor 88 detects the temperature of the 
glaze 66a by measuring the temperature of the base of the heat sink 66c; 
in the illustrated case, the temperature of the glaze 66a is detected in 
the five locations (i.e., the temperature of the heat-generating element 
in each of those locations is measured). 
The results of temperature detection with thermistors 88 are sent to the 
image processing unit 80, which determines the temperatures of the 
individual heat-generating elements by a suitable method such as linear 
interpolation and performs the aforementioned temperature compensation on 
the basis of the thus determined temperatures. 
It should be noted that in the recording apparatus 10 of the invention, not 
only is the temperature compensation effected but also the coefficient of 
sharpness compensation is corrected in accordance with the results of 
measurement of the temperature of the glaze 66a (or the heat-generating 
elements). 
In a preferred embodiment of the recording apparatus 10, the image 
processing unit 80 may optionally be supplied with signals S for the 
recording speed, namely, the speed of transport by the platen roller 60 
and the data on the gamma value of the thermal material A such that in 
addition to the temperature of the glaze 66a, the recording speed and/or 
the gamma value of the thermal material A is also taken into consideration 
for correcting the coefficient of sharpness compensation and preferably 
correcting the coefficient of sharpness compensation in the main scanning 
direction and/or in the auxiliary scanning direction in accordance with 
the recording speed, and the necessary sharpness compensation is performed 
using the thus corrected coefficients. 
It is preferable to correct separate coefficients in the main and auxiliary 
scanning directions, in accordance with the recording speed, the 
temperature of the glaze 66a and the gamma value of the thermal material 
A, since the decrease in image sharpness is different in the main and 
auxiliary scanning directions. 
While sharpness compensation can be performed by various known methods, a 
description of an exemplary procedure follows. 
Assume that an image signal is dividable into n.times.n pixel signals 
S.sub.ij (i=1, 2, . . . , n; j=1, 2, . . . , n). Further assume that for 
each pixel signal S.sub.ij, a pixel is written on a pixel line i at a jth 
position in the direction in which the glaze 66a extends. The first step 
of sharpness compensation is to convert the pixel signal S.sub.ij into a 
first unsharpness signal U.sup.1.sub.ij which is an electrically blurred 
image signal. 
The first unsharpness signal U.sup.1.sub.ij is obtained by averaging the 
pixel signal S.sub.ij and the surrounding pixel signals as follows: 
##EQU1## 
where M is the mask size, or the number of pixels used to construct the 
first unsharpness signal U.sup.1.sub.ij, and L is defined as (M-1)/2. 
Then, the first unsharpness signal U.sup.1.sub.ij is further averaged to 
calculate a second unsharpness signal U.sup.2.sub.ij. The second 
unsharpness signal U.sup.2.sub.ij is calculated by the following equation: 
##EQU2## 
The difference between the first unsharpness signal U.sup.1.sub.ij and the 
second unsharpness signal U.sup.2.sub.ij is multiplied by the coefficient 
of sharpness compensation K and added to the first unsharpness signal 
U.sup.1.sub.ij (see the following equation 3) to produce a sharpness 
compensated pixel signal S.sub.ij' 
EQU S.sub.ij' =U.sup.1.sub.ij +K.multidot.(U.sup.1.sub.ij -U.sup.2.sub.ij)(3) 
As already mentioned, the sharpness of a thermally recorded image is 
affected by the temperature of the thermal head 66 (or the heat-generating 
elements), the recording speed and the gamma value of the thermal material 
A. Specifically the sharpness of the recorded image decreases with the 
increasing temperature of the thermal head 66 and with the increasing 
recording speed but with the decreasing gamma value of the thermal 
material A. 
To deal with this situation, the invention performs sharpness compensation 
by altering the relevant coefficient K in accordance with the temperatures 
of heat-generating elements that have been determined on the basis of the 
results of temperature detection with thermistors 88. In a preferred 
embodiment of the illustrated recording apparatus 10, in addition to the 
temperatures of the heat-generating elements, the recording speed and/or 
the gamma value of the thermal material A are taken into consideration in 
altering the coefficient K. Most preferably, effects of recording speed on 
the sharpness compensation in the main scanning direction and/or in the 
auxiliary scanning direction are also taken into consideration in altering 
the coefficient K to thereby perform the intended sharpness compensation. 
As already mentioned, the image processing unit 80 is connected to the 
corrected data storage unit 86 for storing weighting functions or tables 
for the coefficient K. 
The corrected data storage unit 86 stores four kinds of weighting function 
(or corresponding tables). The first is used to calculate the weight 
.alpha. for obtaining the coefficient K corrected for the temperature of 
the thermal head 66 (or heat-generating elements) as shown in FIG. 4a. The 
second is used to calculate the weight .beta..sub.1 for obtaining the 
coefficient K in the main scanning direction, corrected for the recording 
speed as shown in FIG. 4b, the third is used to calculate the weight 
.beta..sub.2 for obtaining the coefficient K in the auxiliary scanning 
direction, corrected for the recording speed as shown in FIG. 4c. The 
fourth is used to calculate the weight .delta. for obtaining the 
coefficient K corrected for the gamma value of the thermal material A as 
shown in FIG. 4d. 
On the basis of the detected temperatures of heat-generating elements, the 
image processing unit 80 in the process of sharpness compensation 
calculates (or reads out) the weight .alpha. using the function (or table) 
that is stored in the corrected data storage unit 86 and which is shown in 
FIG. 4a and multiplies a predetermined reference value of the coefficient 
K by the weight .alpha. to calculate the applicable value of K (=reference 
value.times..alpha.). The image processing unit 80 then performs sharpness 
compensation using the thus calculated coefficient K. 
The recording apparatus 10 operates basically at a constant recording 
speed. Generally, the lower the recording speed, the higher the quality of 
the image produced and, conversely, rapid image recording can be realized 
by increasing the recording speed. If the recording apparatus is capable 
of operating in three modes, i.e., a normal-speed mode, a high-speed mode 
for performing rapid image recording, and a high-image quality mode in 
which low-speed recording is performed with a view to producing high image 
quality, the sharpness of the recorded image will deteriorate as the 
recording speed is increased. 
In this case, the weighting functions (curves) associated with the 
recording speed which are respectively distinct in the main and auxiliary 
scanning directions may be previously stored in the corrected data storage 
unit 86, as shown in FIGS. 4b and 4c. A filtering may be effected for the 
main and auxiliary scanning directions using the separate coefficients of 
sharpness compensation. 
That is, the image processing unit 80 employs the functions stored in the 
corrected data storage unit 86 which are shown in FIGS. 4a, 4b and 4c, and 
calculates not only the temperature-associated weight .alpha., but also at 
least one of the weights .beta..sub.1 and .beta..sub.2 respectively in the 
main and auxiliary scanning directions in accordance with the recording 
speed, multiplies separately the reference value of the coefficient K by 
the weights .alpha. and .beta..sub.1 or .beta..sub.2 to calculate 
separately the applicable values of K in the main and auxiliary scanning 
directions (K=reference value.times..alpha..times..beta..sub.1, and 
reference value.times..alpha..times..beta..sub.2), and performs separately 
the desired sharpness compensation in the main and auxiliary scanning 
directions using the calculated coefficients K. 
It is preferred to adjust separately the coefficients K of sharpness 
compensation in accordance with the recording speed, using both of the 
weights .beta..sub.1 and .beta..sub.2 in the main and auxiliary scanning 
directions, but it is also possible to perform adjustment only using the 
one having a greater effect, or to use a weight previously calculated in 
consideration of the two weights. 
Filtering of filter size 3 is described below as an example of the 
sharpness compensation effected using the coefficients of sharpness 
compensation separately calculated in the main and auxiliary scanning 
directions. 
Assume that the respective coefficients of sharpness compensation in the 
main and auxiliary scanning directions for the image data of the pixels 
having number M in the main scanning direction, and pixels having number N 
in the auxiliary scanning direction are K.sub.m and K.sub.s, respectively. 
Further assume that the image data before compensation and after 
compensation are respectively D.sub.m,n and D.sub.m,n. In consideration of 
the pixel (m,n) (m=1-M, n=1-M), the sharpness compensation is effected in 
the main scanning direction, by the following equations: 
EQU K.sub.-1 =-K.sub.m /2, K.sub.0 =1+K.sub.m, K.sub.1 =-K.sub.m /2 
Then, the sharpness compensation is effected in the auxiliary direction, by 
the following equations: 
EQU K.sub.-1 =-K.sub.s /2, K.sub.0 =1+K.sub.s, K.sub.1 =-K.sub.s /2 
Thus, the sharpness compensation can be effected in a separate strength in 
the main and auxiliary scanning directions. 
The gamma (.gamma.) value of the thermal material A varies with humidity 
and the environment in which the recording apparatus 10 is installed and 
the image sharpness deteriorates with the decreasing gamma value. 
Therefore, if the gamma value of the thermal material varies, the image 
processing unit 80 employs two of the functions stored in the corrected 
data storage unit 86 which are shown in FIGS. 4a and 4d and calculates not 
only the temperature-associated weight .alpha. but also the weight .delta. 
associated with the gamma value of the thermal material, multiplies the 
reference value of the coefficient K by the weights .alpha. and .delta. to 
calculate the applicable value of K (=reference 
value.times..alpha..times..delta.), and performs the desired sharpness 
compensation using the calculated coefficient K. 
If the recording speed and the gamma value of the thermal material are both 
expected to vary, the image processing unit 80 multiplies the reference 
value of K by all of the weights .alpha., .beta..sub.1 or .beta..sub.2 and 
.delta. to calculate the applicable values of K which are different in the 
main and auxiliary scanning directions (K=reference 
value.times..alpha..times..beta..sub.1 .times..delta., and reference 
value.times..alpha..times..beta..sub.2 .times..delta.) and performs the 
desired sharpness compensation using the calculated coefficients K. 
Thus, the recording apparatus 10 of the invention ensures that thermally 
recorded images of high quality that are free from any blur and which have 
satisfactory sharpness can be produced in a consistent manner irrespective 
of the temperature of the thermal head 66, and in a preferred case, even 
without regard to the recording speed and the gamma value of the thermal 
material A. 
For effective operation of the recording apparatus 10, the weighting 
functions or tables for providing the weights .alpha., .beta..sub.1, 
.beta..sub.2 and .delta. may be determined as appropriate for various 
factors including the characteristics of the thermal head 66 used, the 
characteristics of the thermal material A and the design specifications of 
the apparatus (e.g. heating and cooling efficiencies). If the adjusted 
coefficient of sharpness compensation exceeds a threshold value, either an 
undershoot or an overshoot will occur. To avoid this difficulty, the 
threshold value may be directly applied in sharpness compensation if it is 
exceeded by the adjusted coefficient K or, alternatively, the weighting 
functions or tables are preferably set in such a way that they will not 
exceed the associated threshold values. 
When calculating the applicable values of the coefficient K by multiplying 
the reference value by the weight .beta..sub.1 or .beta..sub.2 associated 
with the recording speed, the method for detecting the recording speed is 
not limited in any particular way. If, as already noted above, the image 
recording apparatus 10 has three different operating modes, high-speed 
mode, normal-speed mode and high-quality image recording mode, the image 
processing unit 80 may detect the recording speed in accordance with the 
mode selected by the operator, or the recording speed may be entered by 
the operator, or, the speed of transport with the platen roller 60 may be 
detected by suitable means such as detection of pulses. 
If the applicable value of the coefficient K is to be calculated by 
multiplying the reference value by the weight .delta. associated with the 
gamma value of the thermal material A, the latter may be entered by the 
operator, or alternatively, the image processing unit 80 may be so adapted 
that it calculates the gamma value of the thermal material A while it is 
effecting tone correction. 
After the sharpness compensation, the image processing unit 80 performs 
other specified image processing jobs such as tone correction, temperature 
compensation, shading correction, resistance compensation and black ratio 
correction. Upon optional formating, image data is produced in association 
with the thermal recording to be done with the thermal head 66. These 
image data are delivered from the image processing unit 80 to be stored in 
the image memory 82. 
The recording control unit 84 reads the stored image data sequentially out 
of the image memory 82 line by line in the direction in which the glaze 
66a extends. The control unit 84 then supplies the thermal head 66 with a 
recording signal representing each of the thusly read image data (and 
represented by the duration of time for which voltage is applied 
imagewise). 
The individual image recording dots on the thermal head 66 generate heat in 
accordance with the received recording signal and, as already described 
above, thermal image recording is performed on the thermal material A as 
it is transported in the direction of arrow b by such means of transport 
as the platen roller 60. 
After the end of thermal image recording, the thermal material A, as it is 
guided by the guide 62, is transported by the platen roller 60 and a 
transport roller pair 63 to be ejected into a tray 72 in the ejecting 
section 22. The tray 72 projects exterior to the recording apparatus 10 
via the outlet 74 formed in the housing 28 and the thermal material A 
carrying the recorded image is ejected via the outlet 74 for takeout by 
the operator. 
On the foregoing pages, the thermal recording apparatus of the invention 
has been described in detail but the present invention is in no way 
limited to the stated embodiments and various improvements and 
modifications can of course be made without departing from the spirit and 
scope of the invention. 
As described above in detail, the thermal recording apparatus of the 
invention is capable of reducing the drop in image sharpness due to an 
increased temperature of the thermal head and, preferably, it can also 
prevent the drop in image sharpness in the main and auxiliary scanning 
directions due to the increase in the recording speed and/or the decrease 
in the gamma value of the thermal material. The apparatus thusly has the 
advantage of producing unblurred, high-quality images in a consistent 
manner.