Thermal transfer printing method

A thermal transfer printing method uses an ink donor medium sheet (12) having solid ink layers (11) whose optical reflection density is smaller than a maximum value of the optical reflection density of picture elements to be printed, and comprises the first step of printing the picture elements by moving a plain paper (4) as a record medium and the ink sheet (12) in a forward direction (FW) so that the solid ink layers are transferred onto the plain paper (4), the second step of moving the plain paper (4) in a backward direction (BW) and the third step of transferring the solid ink layers (11) in an overlapping manner onto the picture elements printed in the first step, by moving the plain paper (4) and the ink sheet (12) again in the forward direction (FW). In the first and third steps, the solid ink layers (11) are completely transferred onto the plain paper (4). The gradation of the picture elements is determined by the number of transfer times. Therefore, each picture element in the gradation can be printed in an extremely stable manner with good reproducibility.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a thermal transfer printing method in 
which an ink sheet including thermally transferable solid ink layers 
formed on a base film is used and the above described solid ink layers are 
heated and transferred onto a record medium so that a picture to be 
printed is decomposed into a plurality of picture elements to perform 
printing. 
2. Description of the Prior Art 
A thermal transfer printing method, as well as an ink jet printing method, 
has become noticed as one of the simple methods for printing on plain 
paper and for color printing, and recently has begun to be practically 
applied. As the thermal transfer method, two methods are known: one is a 
"melting transfer method" in which solid ink layers provided on a base 
film are heated and melted to be attached and printed onto plain paper, 
and the other is a "sublimation transfer method" in which subliming dyes 
included in ink layers are sublimated or evaporated. 
FIG. 1 is a schematic view showing a basic structure of a conventional 
melting transfer printing apparatus. A solid ink layer 2 having thermal 
transferability is provided on a base film 1 so that an ink sheet 3 is 
formed. The base film 1 is made of condenser paper, for example. The solid 
ink layer 2 is made of wax or organic resin binder and coloring agents. A 
thermal head 5 is provided on the side of the base film 1 of the ink sheet 
3. A rubber roller 7 is provided on the side of the solid ink layer 2 of 
the ink sheet 3. Between the solid ink layer 2 and the rubber roller 7, 
plain paper 4 is provided. The thermal head 5 includes a plurality of 
heating resistors 6 in a dotted configuration, for example. When the 
heating resistors 6 of the thermal head 5 are supplied with electric 
current and heated, the solid ink layer 2 is heated and melted through the 
base film 1 of the ink sheet 3 and a melted portion 2A is transferred onto 
the plain paper 4. If either the magnitude (W/dot) of the voltage applied 
to the heating resistors 6 or the pulse width is changed, in principle the 
amount of the solid ink layer 2A to be transferred changes and it seems 
possible to perform gradation printing. 
FIG. 2 is a graph showing a relation between the power applied to the 
thermal head and the optical reflection density OD of a printed picture 
element. In this case, the thermal head is one for use in facsimile and 
the size of the heating resistor is 80.times.200 .mu.m.sup.2. Referring to 
FIG. 2, the circles indicate averages of the values measured ten times and 
each bar indicates a range of scatter of the measured values. The pulse 
width of the heating current is 2 msec. As is clear from the graph, the 
amount of scatter in measured values of the optical reflection density OD 
is extremely large when a picture element of an intermediate optical 
density is printed using a conventional ink sheet. As a result, using of 
the ink sheet which serves for melting transfer operation contains a 
disadvantage that a picture including picture elements whose optical 
densities are changed, that is, a gradation picture cannot be stably 
printed. 
FIG. 3 is a schematic view showing a structure of a conventional ink sheet 
whereby three-value picture element optical densities can be obtained. A 
base film 1 is coated with a black solid ink layer 8 having a high melting 
point, which is further coated with a gray solid ink layer 9 having a low 
melting point. Thus, an ink sheet 10 for three-value printing is formed. 
FIG. 4 is a conception view showing a state in which solid ink layers are 
transferred from the FIG. 3 ink sheet onto the plain paper 4. FIG. 4A 
shows a state in which only the gray solid ink layer 9 is transferred onto 
the plain paper 4. This transfer in FIG. 4A can be performed by selecting 
a temperature at which the gray solid ink layer 9 is melted but the black 
solid ink layer 8 is not melted. FIG. 4B shows a state in which both the 
black solid ink layer 8 and the gray solid ink layer 9 are transferred 
onto the plain paper 4. The transfer in this case can be performed by 
raising the temperature to a point at which the black solid ink layer 8 
and the gray solid ink layer 9 are both melted. Thus, using the ink sheet 
10 for three-value printing shown in FIG. 3, three-value optical densities 
for picture elements, that is, two optical densities shown in FIGS. 4A and 
4B and the optical density in case where no transfer is made, can be 
obtained. 
In theory, an ink sheet of more than three values can be formed if a large 
number of solid ink layers having different melting points are provided in 
an overlapping manner on the base film 1, but practically three values are 
the maximum limit for this kind of ink sheet. This is because on the low 
temperature side, a picture to be printed cannot be well fixed to the 
plain paper, and on the high temperature side, the heat resisting property 
of the thermal head is limited. In an ink sheet for three-value printing 
as described above, variation of the ambient temperature or variation of 
the heating temperature of a heating material due to repetition of 
printing operation, etc. increases instability such as blackening of a 
picture element to be colored gray. In addition, since two solid ink 
layers are superimposed on the base film, there is a disadvantage in that 
it is difficult to manufacture an ink sheet. 
As a result, it is desired to provide a thermal transfer printing method 
capable of printing the elements in the gradation; stably and with 
excellent reproducibility. 
SUMMARY OF THE INVENTION 
The present invention is, in brief, a thermal transfer printing method in 
which at least one ink sheet including at least one thermally transferable 
solid ink layer formed on a base film is used and the above described 
solid ink layer is heated and transferred onto a record medium so that 
picture elements of a picture to be printed are printed on the record 
medium, the optical reflection density of said solid ink layer being 
smaller than a maximum value of the optical reflection density of said 
picture elements. A thermal transfer printing method of the present 
invention comprises at least three steps: a first step of printing at 
least one picture element out of said picture elements by moving said 
record medium and said ink sheet in a forward direction so that said solid 
ink layer is transferred onto said record medium, a second step of 
returning said medium in a state before said first step, and a third step 
of transferring said solid ink layer in an overlapping manner onto the 
picture elements printed in said first step, by moving again said medium 
and said ink sheet in the forward direction after said second step. 
In accordance with the present invention, in the above described first and 
third steps, the solid ink layer on the ink sheet is entirely transferred 
onto the transferring medium. The gradation of picture elements is 
determined by the number of transfer times. Contrary to a conventional 
method, in order to represent the gradation, it is unnecessary to transfer 
the picture elements with an intermediate optical density by controlling 
the voltage applied to the thermal head, nor to use a plurality of solid 
ink layers having different melting points. Therefore, in accordance with 
the present invention, it is made sure that all the picture elements 
included in the gradation can be printed in an extremely stable manner 
with excellent reproducibility. 
Therefore, a primary object of the present invention is to provide a 
thermal transfer printing method capable of printing stably and with 
excellent reproducibility all the picture elements included in the 
gradation. 
A principal advantage of the present invention resides in that all the 
picture elements included in the gradation can be printed in an extremely 
stable manner with excellent reproducibility though an ink sheet inferior 
in gradation representation is used. 
Another advantage of the present invention is that a thermal transfer 
printing method in accordance with the present invention can be applied to 
various printing apparatus such as a printer, facsimile and the like. 
These objects and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 5 is a basic principle view showing a printing method in accordance 
with the present invention. Solid ink layers 11 having a low optical 
density are formed on a base film 1. The optical reflection density of the 
solid ink layers is approximately 1/n (n.gtoreq.2) of a final picture 
density to be obtained. In the following, description is made assuming 
n=4. A thermal head 5 is provided on the side of the base film 1 of the 
ink sheet 11, and plain paper 4 is provided on the other side. 
Printing of a gradation picture in accordance with the present invention 
will be made in the following manner. By moving the ink sheet 11 and the 
plain paper 4 in a forward direction FW, a first solid ink layer 11A is 
transferred to the plain paper 4 at the position of the thermal head 5. 
After that, the plain paper 4 returns in a backward direction BW. Then, 
the ink sheet 11 and the plain paper 4 are moved again in the forward 
direction FW, whereby a second solid ink layer 11B is transferred. In the 
same manner, a third solid ink layer 11C and a fourth solid ink layer 11D 
are transferred, and thus printing operation is completed. In this 
example, the optical reflection density OD of the solid ink layers 11 with 
low optical density is set to 0.4 in order that a maximum saturation 
density 1.6 can be obtained by four times of transfer. 
FIG. 6 is a schematic view showing an apparatus in which a printing method 
in accordance with the present invention is embodied. An ink sheet 12 of 
low optical density is wound in the forward direction FW by means of a 
motor 14 for ink sheet. A motor 13 for rubber roller makes a rubber roller 
7 rotate in the forward direction and in the backward direction, so that 
the plain paper 4 is moved in the forward direction FW and in the backward 
direction BW. Under the rubber roller 7, a thermal head 5 is provided. 
This apparatus is controlled by a microcomputer 15. The microcomputer 15 
comprises a central processing unit 151, a random access memory 152, a 
read only memory 153 and an input/output unit 154, which are connected 
with each other. The input/output unit 154 is connected with the motors 13 
and 14 and the thermal head 5. Furthermore, a picture data source 16 is 
connected to the input/output unit 154. The motors 13 and 14 are, for 
example, pulse motors by which a rotational angle is controlled according 
to the number of pulses sent from the microcomputer 15 so as to determine 
the positions of the plain paper 4 and the ink sheet 12. The thermal head 
5 contains heating resistors 6 (not shown) disposed in line in a dotted 
manner, for example. 
Now, the operation of the apparatus shown in FIG. 6 will be roughly 
described in the following. FIG. 7 is a flow chart showing the outline of 
the operation of the apparatus shown in FIG. 6. In the step S1, picture 
data to be printed is entered from the picture data source 16 into the 
microcomputer 15 and stored in the random access memory 152. In the step 
S2, an amount of data corresponding to one line is read out from the 
random access memory 152 and in the step S3, the amount of picture data 
corresponding to one line is printed on the plain paper 4. In the step S4, 
paper moving corresponding to one line is carried out. In the step S5, it 
is determined according to the number of lines and other data memorized in 
the microcomputer 15 whether it comes to an end of the picture. If it 
comes to the end, the program proceeds to the step S6, and if not, returns 
to the step S2. In the step S6, it is determined whether transfer 
operation for P times is completed or not, and if it is completed, the 
program terminates, and if not, the program proceeds to the step S7. In 
the step S7, the plain paper 4 is reset in the initial position and the 
program returns to the step S2. Thus, transfer operation for P times is 
performed. 
FIG. 8 is a graph showing a relation between the optical reflection density 
OD obtained by the printing method shown in FIG. 5 and the transfer times 
P. As is clear from the graph, printing is performed with five gradations 
in all based on a white ground and optical reflection densities 0.4, 0.8, 
1.2 and 1.6. In the above described printing method, each of the solid ink 
layers 11 of low optical density is completely transferred from the ink 
sheet 12 of low optical density for each time of transfer operation and 
transfer is never controlled with an intermediate amount, which differs 
from a conventional method. Accordingly, the gradation is determined by 
the number of transfer times and the printing density of each gradation is 
extremely stable with excellent reproducibility. 
FIG. 9 shows picture elements composed of a group of a plurality of dots. 
In order to further increase the number of gradations, a picture element 
may be comprised of a dot matrix, so as to change the number of dots 
included in the dot matrix as well as the number of transfer times. FIG. 9 
shows a case in which the dot matrix is a 2.times.2 dot matrix and 
transfer is performed two times in an overlapping manner. In FIG. 9, the 
reference character D1 indicates a dot to which transfer is applied for 
one time and the reference character D2 indicates a dot to which transfer 
is applied twice in an overlapping manner. In this example nine gradations 
in all including a white level can be represented. 
Next, description will be made of a case in which a thermal transfer 
printing method in accordance with the present invention is applied to the 
printing of a color picture. FIG. 10 is a schematic view showing an 
apparatus in which a color thermal transfer printing method is embodied. 
The following description is given principally for the purpose of 
clarifying the different points from the apparatus shown in FIG. 6. In an 
ink sheet 17 for color printing, a plurality of solid ink layers of 
different colors are individually formed in the divided regions on the 
base film. In this example, an ink sheet 17 for color printing comprises a 
yellow color solid ink layer, a magenta color solid ink layer and a cyanic 
color solid ink layer. Under the ink sheet 17, a color sensor 18 is 
provided. The color sensor 18 is connected to the microcomputer 15. 
The operation of the apparatus shown in FIG. 10 will be described in the 
following. FIG. 11 is a flow chart showing briefly the operation of the 
apparatus shown in FIG. 10. In the step S11, picture data of three primary 
colors (yellow, magenta and cyanic color) is entered from the picture data 
source 16 into the microcomputer 15 so as to be stored. In the step S12, 
the yellow portion out of the ink sheet is set to a predetermined transfer 
position. This set state is detected by the color sensor 18. In the steps 
S13 to S18, the same operation as in the steps S2 to S7 in FIG. 7 is 
performed, and the description thereof is omitted. In the step S17, when 
it is determined that transfer for P times is completed, the program 
proceeds to the step S19. In the step S19, the magenta portion out of the 
ink sheet is set to a predetermined transfer position, and the set state 
is detected by the color sensor 18. The operation in the step S20 is the 
same as the operation in the steps S13 to S18. In the step S21, the cyanic 
color portion of the ink sheet is set to a predetermined transfer 
position, and this set state is detected by the color sensor 18. The 
operation in the step S22 is the same as that in the steps S13 to S18. 
Thus, a colored picture with a plurality of gradations is printed. 
FIG. 12 shows picture elements each of which comprises a group of a 
plurality of dots of different colors. In this example, a yellow picture 
element arrangement , a cyanic color picture element arrangement 
and a magenta picture element arrangement have respectively one to 
four gradations. If transfer is twice performed in an overlapping manner 
similarly to the case of FIG. 9, eight gradations can be obtained. The 
picture element arrangement is obtained when the three colors as 
arranged in the picture element arrangements to are transferred in 
an overlapping manner. As is understood from the drawings, the color dots 
in the low optical density region are made not to be overlapped with each 
other. On the contrary, in the picture element arrangement , the color 
dots in the low optical density region are overlapped with each other. As 
clear from the comparison of the picture element arrangements and , 
the number of dots in the low optical density region for representing the 
same gradation is larger in , and accordingly the picture element 
arrangement has an advantage that the resolution is less deteriorated. 
In the above described embodiment, only a method using an ink sheet coated 
simply with solid ink layers of low optical density was described. 
However, if ink layers of low optical density and ink layers of high 
optical density of a different color are separately formed on a common ink 
sheet and only the color of the low optical density ink layers is made to 
have gradation, several limited colors can be printed with a decreased 
number of transfer times. 
According to a method as described above in which transfer is made for a 
plurality of times using an ink sheet of low optical density, there might 
be a few problems that because of an increased number of transfer times, 
the printing period becomes longer and the area of an ink sheet to be used 
is made larger. For the purpose of dissolving such problem, another 
embodiment in which a larger number of gradations can be represented with 
limited transfer times will be described in the following. This method is 
referred to as a bit-plane multiplex printing method. 
FIG. 13 shows an ink sheet 22 for multiplex printing having solid ink 
layers of three kinds with different optical reflection densities. Solid 
ink layers 19 to 21 of N kinds, three in this case, having different 
optical reflection densities are individually formed on the divided 
regions of the base film 1. The optical reflection density of the first 
solid ink layer 19 is DA, the optical reflection density of the second 
solid ink layer 20 is DB and the optical reflection density of the third 
solid ink layer 21 is DC. Assuming that the density DA of the solid ink 
layer 19 is D.sub.O, the densities DA, DB and DC of the respective solid 
ink layers are made to satisfy almost exactly the following equation: 
##EQU1## 
In order to sufficiently make use of the characteristics of the present 
invention described hereinafter, the relation of the equation (1) must be 
established almost exactly. As an example, D.sub.O is set to 0.25 as a 
value of optical reflection density. In consequence, the densities DA, DB 
and DC are respectively 0.25, 0.5 and 1.0. 
FIG. 14 shows an typical example of decomposition of an original picture to 
bit-plate pictures. In this case, the original picture OP has eight 
gradations in all O, D.sub.O, . . . , 7D.sub.O, for example. The picture 
elements PE1, PE2, . . . , PE8 in the original picture OP have 
respectively optical reflection densities O, 1D.sub.O, . . . , 7D.sub.O. 
According to a conventional method in which solid ink layers of a single 
optical density are overlapped, it is necessary to perform printing 
operation seven times in order to print such an original picture OP. In 
addition, according to a conventional method using solid ink layers having 
different melting points, it is necessary to perform transfer operation in 
the printing conditions of seven types. In this embodiment, contrary to 
the above mentioned conventional methods, the original picture OP can be 
printed with a decreased number of printing times as described below. 
First, the original picture OP is decomposed into three pictures BP1, BP2 
and BP3. The picture BP1 having an optical reflection density D.sub.0 is a 
two-value picture of "1" (transfer) or "0" (no transfer). Similarly, the 
pictures BP2 and BP3 respectively have optical reflection densities 
2D.sub.0 and 4D.sub.0 and are two-value pictures. The above described 
pictures BP1 to BP3 are referred to hereinafter as bit-plane pictures. The 
bit-plane pictures shown in FIG. 14 have combinations of picture elements 
corresponding to the original picture OP. For example, optical reflection 
densities 3D.sub.0 and 7D.sub.0 can be obtained by the following equation: 
##EQU2## 
Thus, with a combination of three bit-plane pictures, picture element 
densities of 2.sup.3 =8 gradations can be obtained. More particularly, by 
transferring the bit plane pictures of three kinds of optical density, a 
picture having eight density levels can be printed. 
Accordingly, to the above described method, in general following relation 
is established: 
EQU MS=2.sup.N (3) 
where N indicates the number of optical reflection densities of the solid 
ink layers and MS indicates the maximum number of picture element 
densities obtained by this method. 
In such a case, in order that the picture printed by this method may have 
the best quality, it is preferred to fulfill the following conditions: 
(1) Density levels of the picture elements of the N sheets of bit-plane 
pictures can be obtained stably and a relation of D.sub.0 .times.2.sup.N 
is maintained between the respective density levels. 
(2) The sum of the bit-plane pictures becomes a sum of picture element 
densities as obtained by the equation (2). 
The first condition can be fulfilled in accordance with this embodiment, 
when an ink sheet for multiplex printing including a plurality of solid 
ink layers with different optical densities as shown in FIG. 13 is used 
and the state in which the solid ink layers are completely transferred is 
set to "1" and the state in which no transfer is made is set to "0". In 
such a method, contrary to a conventional heat sensitive printing method 
or a conventional thermal transfer printing method shown in FIGS. 3 and 4, 
printing in an intermediate state is not needed, and accordingly the 
picture element densities are extremely stable. Thus, the above described 
first condition can be satisfied. 
The second condition can be fulfilled, in the embodiment, when the optical 
reflection densities obtained by repetition of transfer are given as a sum 
of the bit-plane pictures. This is because the transfer condition of each 
bit-plane picture can be maintained the same. As a method that can satisfy 
simultaneously the above described first and second conditions, at 
present, there does not seems to be any other method than the method of 
this embodiment using ink sheets of the same kind or several kinds having 
solid ink layers of different optical densities. 
FIG. 15 shows a model of a state in which picture elements having 
eight-value optical reflection densities are printed. In this example, the 
ink sheet 22 for multiplex printing shown in FIG. 13 is used and the 
bit-place pictures BP1 to BP3 shown in FIG. 14 are overlapped in order. 
More particularly, the first to third solid ink layers 19 to 21 having 
different optical densities in accordance with the picture element 
densities of a picture to be represented are transferred in an overlapping 
manner. As is clear from the drawing, with a combination of solid ink 
layers of three kinds, picture element densities of 2.sup.3 =8 gradations 
are provided. An apparatus in which the above described method is applied 
is similar to the apparatus shown in FIGS. 6 and 10. 
In accordance with the above described method, in principle, the gradation 
representation range of a picture element per se can be remarkably 
enlarged, if a large number of different solid ink layers are used. More 
particularly, when solid ink layers of N kinds are used and the optical 
reflection density D.sub.M of the respective layers satisfied the 
following equation, picture element densities of 2.sup.M gradations can be 
obtained. 
EQU D.sub.M =2.sup.M-1 .multidot.D.sub.0 (4) 
where M is an integer number satisfying the condition 1.ltoreq.M.ltoreq.N 
and D.sub.0 is the minimum value of the optical reflection densities of 
the above described solid ink layers of N kinds. For example, if solid ink 
layers of eight kinds are used, picture element densities of 256 
gradations can be obtained. Accordingly, in this embodiment, transfer 
operation has only to be performed eight times and the period for printing 
a picture can be remarkably reduced, while in accordance with a 
conventional method, transfer in different 255 conditions has to be made 
in order to obtain picture element densities of 256 gradations. 
A set value D.sub.M of each density level of an ink sheet constituting a 
bit-plane picture must satisfy the above described equation (4) in order 
that the density levels of the respective picture elements of a picture to 
be obtained may be provided at an equal interval. However, if the density 
levels are not provided at an equal interval, a desired gradation picture 
can be printed with a small number of transfer times, as described below. 
More particularly, if in a plurality of optical densities of the ink 
sheet, there exists a relation in which a certain density cannot be 
obtained by a combination of other plural number of densities, the number 
of gradations S becomes maximum with respect to the number of printing 
times P. As result, by printing operation for P times, a picture having 
picture element densities of S=2.sup.P gradations at maximum can be 
obtained. However, if there are solid ink layers of K kinds having the 
same density out of the solid ink layers of N kinds, the number of 
gradations S to be obtained is represented by the following equation and 
the number of picture element densities to be obtained is decreased. 
##EQU3## 
Nevertheless, as far as the relation S&gt;N is maintained, there is a merit 
that printing can be made with a smaller number of transfer times than the 
number of gradations S. In addition, in this case, it is not necessarily 
needed to use an ink sheet including a plurality of solid ink layers of 
different densities provided on the same base film as shown in FIG. 13. 
Separate ink sheets of different densities may be used. 
On the other hand, most preferably, the density levels of the respective 
solid ink layers satisfy the condition of the equation (4), but in 
practice the optical density of the overlapped solid ink layers sometimes 
does not coincide with the sum of all the optical densities, and 
accordingly, even if its value may be changed to the extent that the 
following equation is fulfilled, a gradation picture can be formed. 
EQU D.sub.M =(2+.alpha.).sup.M-1 .multidot.D.sub.0 (6) 
where .alpha. is a number satisfying the condition 
-0.5.ltoreq..alpha..ltoreq.+0.5. 
In the above description, a monochromatic example was given. However, 
multiple colors can be easily applied by using an ink sheet having a 
plurality of solid ink layers of different colors and different optical 
reflection densities. FIG. 16 shows an ink sheet 23 for color multiplex 
printing having a plurality of solid ink layers of different colors and 
different optical reflection densities. This ink sheet 23 includes solid 
ink layers of three primary colors, which are yellow, cyanic color and 
magenta. These colors have respectively optical reflection densities 
D.sub.0 and 2D.sub.0. The following description will be made of the 
operation in case where color multiplex printing is performed using such 
an ink sheet. FIG. 17 is a flow chart showing the color multiplex printing 
operation. FIG. 18 is a flow chart showing the operation of one portion in 
FIG. 17. In the step S21, picture data is entered from the above mentioned 
picture data source 16 into the microcomputer 15 and stored. In the step 
S22, a picture of yellow of the first optical density is printed. In the 
step S23, a yellow picture of the second optical density is printed. Then, 
similar operation is repeated in the subsequent steps. In the step S24, a 
yellow picture of the N-th optical density is printed. Operation in the 
steps S25 to S30 is similarly to the above described operation. Internal 
operation in each of the steps S22 to S30 is shown in FIG. 18. More 
particularly, in the step S41, an ink sheet of a certain color of a 
certain optical density is set. In the step S42, an amount of data 
corresponding to one line is read out from the microcomputer, and in the 
step S43, the data is printed on the plain paper. In the step S44, the 
paper is moved by an amount corresponding to one line. In the step S45, it 
is determined whether it comes to the end of the picture, and if it comes 
to the end, the program proceeds to the step S46 in which the plain paper 
is reset to the initial position. If it does not come to the end, the 
program proceeds to the step S42. 
In the foregoing, some embodiments of the present invention were described 
in detail. Although in the above description, only an example in which the 
optical density of a picture element per se is changed is given, a 
pseudo-gradation reproducing method such as a Dither method or a density 
pattern method or the like may be simultaneously used for the purpose of 
increasing the gradation reproducing ability. In addition, the combination 
of colors may not be limited to the combination of yellow, cyan and 
magenta. A combination of orange, green and blue or the like may be 
applied. As an ink material, not only material of wax type but also 
organic resin binder may be used. As coloring agents, either pigments or 
dyes may be used. The ink sheet is provided preferably with solid ink 
layers of thermal melting property on the base film, but it may be 
provided with solid ink layers having thermal sublimation property. The 
base film is preferably a condenser paper or a plastic film, but is not 
limited to the above described material as far as it is a thermally 
transferable material. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.