Thermal transfer recorder

In a color thermal transfer recorder, L is set to a distance from a printing position to first and second sensors for detecting each of marks, q is set to a pitch between the marks, N is set to an integer equal to 2 or more, S is set to an arbitrary value from zero to the pitch q, and .delta. is set to an arbitrary value from 0 to 4 mm. The values L, q, N, S and .delta. are set such that the following formula L=N.times.q+S+.delta. is satisfied. The values L, q, N, S and .delta. may be set such that the following formula L=N.times.q+S-.delta. is satisfied. In this recorder, various kinds of marks are formed on an ink sheet and are detected by the first and second sensors so that a conveying operation of the ink sheet is controlled. In accordance with this structure of the recorder, the distance from the printing position to the first and second sensors is increased so that a degree of freedom in design of the recorder is increased.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates to a color thermal transfer recorder in which 
many heating elements in a thermal head are selectively heated and a color 
image is formed on a sheet of image receiving paper while an ink sheet and 
the image receiving paper sheet are supported between the thermal head and 
a platen and the ink sheet is conveyed at a speed lower than that of the 
image receiving paper sheet. 
2. Description of the Related Art 
In a well-known color thermal transfer recorder, an image is generally 
printed on a sheet of image receiving paper while an ink sheet is conveyed 
at a speed lower than that of the image receiving paper sheet. In such a 
recorder, a using amount of the ink sheet can be reduced so that running 
cost of the recorder can be reduced. 
For example, the color thermal transfer recorder of this kind is shown in 
Japanese Patent Application Laying Open (KOKAI) No. 61-242869. In this 
patent application, a plurality of ink layers are arranged in the 
longitudinal direction of an ink sheet and have colors different from each 
other. The plural ink layers are set to one set of color groups. The ink 
sheet is used by repeatedly forming the color groups in the longitudinal 
direction. When an image is printed, this ink sheet is conveyed in its 
longitudinal direction and ink of each of the ink layers of each of the 
color groups formed in the ink sheet is transferred onto a sheet of image 
receiving paper. Thus, a color image is recorded on the image receiving 
paper sheet. 
In this case, the ink sheet has a leading color mark, a heading mark and a 
terminal end mark. The leading color mark is formed to detect that a 
leading end region of each of the color groups reaches a printing position 
between a heating element of a thermal head and a platen. The heading mark 
is formed to detect that a leading end region of an ink layer of each of 
colors except for a leading ink layer of each of the color groups reaches 
the printing position. The terminal end mark is formed to detect a 
terminal end of the ink sheet. These marks are detected by a detecting 
means constructed by first and second sensors so that conveyance of the 
ink sheet is controlled. 
The above first and second sensors are generally arranged in predetermined 
positions in the vicinity of the printing position such that each of the 
marks can be reliably detected. 
As described later, such arranging positions of the first and second 
sensors are determined in accordance with a pitch between the ink layers 
in a conveying direction of the ink sheet. It is generally considered that 
the positions of the first and second sensors must be set to approach the 
pointing position as this pitch is reduced. 
As mentioned above, the ink sheet is conveyed at a speed lower than that of 
the image receiving paper sheet in the color thermal transfer recorder. In 
this recorder, a conveying amount of the ink sheet is smaller than that of 
the image receiving paper sheet. Accordingly, the pitch between the ink 
layers is reduced in comparison with a recorder in which the ink sheet and 
the image receiving paper sheet are conveyed at an equal speed. Since the 
pitch between the ink layers is small, the first and second sensors must 
be generally arranged very near the printing position for the above 
reasons. 
However, various kinds of constructional elements such as the thermal head, 
the platen, etc. are arranged in a region near the printing position. 
Accordingly, it is not easy to arrange the first and second sensors in 
such a region. Therefore, a degree of freedom in design of the recorder is 
greatly restricted. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a color 
thermal transfer recorder in which first and second sensors can be 
arranged in positions greatly separated from a printing position without 
any problems. 
The above object can be achieved by a color thermal transfer recorder in 
which a plurality of ink layers having colors different from each other 
are arranged in a longitudinal direction of an ink sheet and are set to 
one set of color groups and the ink sheet is used by repeatedly forming 
the color groups in the longitudinal direction; 
many heating elements of a thermal head are selectively heated while the 
ink sheet and a sheet of image receiving paper are supported between the 
thermal head and a platen and the ink sheet is conveyed in the 
longitudinal direction thereof at a speed lower than that of the image 
receiving paper sheet; 
a color image is formed on the image receiving paper sheet by transferring 
ink of each of the ink layers of each of the color groups formed in the 
ink sheet onto the image receiving paper sheet; 
the ink sheet has: 
a leading color mark for detecting that a leading end region of each of the 
color groups reaches a printing position between the heating elements and 
the platen; 
a heading mark for detecting that a leading end region of each of the color 
ink layers except for a reading ink layer of each of the color groups 
reaches the printing position; and 
a terminal end mark for detecting a terminal end of the ink sheet; 
each of the marks is detected by detecting means constructed by first and 
second sensors; 
the first and second sensors are arranged on a downstream side of the ink 
sheet in a conveying direction thereof from the printing position; and 
when L is set to a distance from the printing position to each of the first 
and second sensors, q is set to a pitch between the marks, N is set to an 
integer equal to 2 or more, S is set to an arbitrary value from zero to 
the pitch q, and .delta. is set to an arbitrary value from 0 to 4 mm, the 
values L, q, N, S and .delta. are set such that the following formula 
EQU L=N.times.q+S+.delta. 
is satisfied. 
The above object can be also achieved by a color thermal transfer recorder 
in which a plurality of ink layers having colors different from each other 
are arranged in a longitudinal direction of an ink sheet and are set to 
one set of color groups and-the ink sheet is used by repeatedly forming 
the color groups in the longitudinal direction; 
many heating elements of a thermal head are selectively heated while the 
ink sheet and a sheet of image receiving paper are supported between the 
thermal head and a platen and the ink sheet is conveyed in the 
longitudinal direction thereof at a speed lower than that of the image 
receiving paper sheet; 
a color image is formed on the image receiving paper sheet by transferring 
ink of each of the ink layers of each of the color groups formed in the 
ink sheet onto the image receiving paper sheet; 
the ink sheet has: 
a leading color mark for detecting that a leading end region of each of the 
color groups reaches a printing position between the heating elements and 
the platen; 
a heading mark for detecting that a leading end region of each of the color 
ink layers except for a leading ink layer of each of the color groups 
reaches the printing position; and 
a terminal end mark for detecting a terminal end of the ink sheet; 
each of the marks is detected by detecting means constructed by first and 
second sensors; 
the first and second sensors are arranged on an upstream side of the ink 
sheet in a conveying direction thereof from the printing position; and 
when L is set to a distance from the printing position to each of the first 
and second sensors, q is set to a pitch between the marks, N is set an 
integer equal to 2 or more, S is set to an arbitrary value from zero to 
the pitch q, and .delta. is set to an arbitrary value from 0 to 4 mm, the 
values L, q, N, S and .delta. are set such that the following formula 
EQU L=N.times.q+S-.delta. 
is satisfied. 
In each of the above color thermal transfer recorders, the detecting means 
preferably has at least one auxiliary sensor in addition to the first and 
second sensors to detect that a leading end region of an ink layer of any 
color reaches the printing position. Further, an auxiliary mark detected 
by the auxiliary sensor is preferably formed on the ink sheet. 
In each of the-above color thermal transfer recorders, a distance from the 
printing position to the auxiliary sensor is desirably set to be equal to 
or greater than a distance from the printing position to the first and 
second sensors. 
Each of the above color thermal transfer recorders is preferably 
constructed such that the leading end region of each of the color groups 
is judged to reach the printing position, 
the leading end region of each of the color ink layers except for the 
leading ink layer of each of the color groups is judged to reach the 
printing position, or 
the terminal end of the ink sheet is judged according to whether one sensor 
detects one mark and another sensor then detects another mark after a 
predetermined time. 
In accordance with the above color thermal transfer recorders, the first 
and second sensors can be arranged in positions greatly separated from the 
printing position without any problems. 
Further objects and advantages of the present invention will be apparent 
from the following description of the preferred embodiments of the present 
invention as illustrated in the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of a color thermal transfer recorder in the 
present invention will next be described in detail with reference to the 
accompanying drawings. 
FIG. 1 is an explanatory view schematically showing a color thermal 
transfer recorder in accordance with one embodiment of the present 
invention. FIG. 2 is a plan view showing one example of an ink sheet used 
in this recorder. 
Many ink layers are arranged in a longitudinal direction of the ink sheet 1 
shown in FIG. 2. The ink layers are coated and formed on a base sheet. In 
this case, plural ink layers arranged in the longitudinal direction of the 
ink sheet 1 have colors different from each other and constitute one set 
of color groups. Such color groups are numerously and repeatedly formed in 
the longitudinal direction of the ink sheet 1. 
FIG. 2 shows two color groups G.sub.1 and G.sub.2. Each of the color groups 
G.sub.1 and G.sub.2 has a yellow ink layer shown by Y.sub.1 and Y.sub.2, a 
magenta ink layer shown by M.sub.1 and M.sub.2, a cyan ink layer shown by 
C.sub.1 and C.sub.2, and a black ink layer shown by K.sub.1 and K.sub.2. 
The four color ink layers composed of the yellow, magenta, cyan and black 
ink layers in each of the color groups G.sub.1 and G.sub.2 are arranged in 
the same color order in the longitudinal direction of the ink sheet 1. 
Such color groups are numerously arranged in the longitudinal direction of 
the ink sheet 1. FIG. 2 shows two color groups G.sub.1 and G.sub.2 among 
these color groups. In the example shown in FIG. 2, the color group 
G.sub.2 is set to a final color group of the ink sheet 1. 
Each of the color groups may be constructed by yellow, magenta and cyan ink 
layers without using the black ink layers K.sub.1 and K.sub.2. 
As shown in FIG. 1, the above ink sheet 1 and a sheet 2 of image receiving 
paper are supported between a platen 3 and a thermal head 4. In this 
supporting state, the ink sheet 1 is conveyed in the direction of an arrow 
A. In contrast to this, the image receiving paper sheet 2 are wound around 
a portion of the platen 3 and is conveyed in the direction of an arrow B 
by rotating the platen 3 in the counterclockwise direction. At this time, 
the ink sheet 1 is conveyed at a speed lower than that of the image 
receiving paper sheet 2 in a longitudinal direction of the ink sheet 1. 
When the conveying speed of the ink sheet 1 is set to v.sub.1 and the 
conveying speed of the image receiving paper sheet 2 is set to v.sub.2, 
the ink sheet 1 and the image receiving paper sheet 2 are conveyed in the 
relation of v.sub.1 =v.sub.2 /n (n&gt;1). 
As mentioned above, when the ink sheet 1 and the image receiving paper 
sheet 2 are conveyed, many heating elements 5 in the thermal head 4 are 
selectively heated in accordance with a picture signal. Ink of each of the 
ink layers of each of the color groups formed in the ink sheet 1 is melted 
or sublimated by heat of each of the heating elements 5 and is transferred 
onto the image receiving paper sheet 2. Thus, a color image is formed on 
the image receiving paper sheet 2. The ink is printed on the image 
receiving paper sheet 2 in a position of each of the heating elements 5 of 
the thermal head 4. This position of each of the heating elements 5 is set 
to a printing position X. 
When the ink is generally melted as mentioned above, this ink is 
constructed by pigments. In contrast to this, when the ink is sublimated, 
this ink is constructed by dyes. 
As shown in FIG. 2, various kinds of marks are formed every color group in 
an end region of the ink sheet 1 in a width direction thereof so as to 
control a conveying operation of the ink sheet 1 as described later. For 
example, in the case of the color group G.sub.2, a leading color mark 6 
and heading marks 7, 8, 9 are formed on the ink sheet 1 in association 
with this color group G.sub.2. These marks are similarly formed with 
respect to each of the other color groups. 
The leading color mark 6 in each of the color groups is formed to detect 
that a leading end region of each of the color groups reaches the printing 
position X between a heating element 5 and the platen 3. The heading marks 
7, 8 and 9 in each of the color groups are formed to detect that a leading 
end region of each of the color ink layers except for a leading ink layer 
in each of the color groups reaches the printing position X. In this 
embodiment, this leading ink layer is set to the yellow ink layer. 
In this embodiment, the color group G.sub.2 of the ink sheet 1 is set to a 
final color group for finally performing a printing operation. This color 
group G.sub.2 has terminal end marks 10 and 11 for detecting a terminal 
end of the ink sheet 1. Such terminal end marks 10 and 11 are not formed 
in the color groups except for the final color group such as G.sub.2. 
The recorder shown in FIGS. 1 and 2 has a detecting means constructed by a 
first sensor S.sub.a and a second sensor S.sub.b for detecting each of the 
above-mentioned marks. In this embodiment, these sensors S.sub.a and 
S.sub.b are fixedly arranged on a downstream side of the ink sheet in a 
conveying direction thereof from the printing position X. 
Each of the first sensor S.sub.a and the second sensor S.sub.b is 
illustrated as one example in FIGS. 1 and 2 and is constructed by a 
photosensor of a reflecting type. As shown in FIG. 1, reflecting plates 12 
and 13 are fixedly arranged in positions opposite to the respective 
sensors S.sub.a and S.sub.b in a state in which the ink sheet 1 is located 
between the reflecting plates 12, 13 and the sensors S.sub.a and S.sub.b. 
An entire face of the ink sheet 1 shown in FIG. 2 except for regions of the 
above-mentioned marks is coated with ink. Light is transmitted through 
only an uncoated portion of each of the marks in which no ink sheet is 
coated with ink. Accordingly, when each of the marks reaches a position 
below each of the sensors S.sub.a and S.sub.b, light emitted from a light 
emitting element of each of the sensors S.sub.a and S.sub.b is transmitted 
through each of the marks and is reflected on each of the reflecting 
plates 12 and 13. The reflected light is again transmitted through each of 
the marks and is incident to a light receiving element of each of the 
sensors S.sub.a and S.sub.b so that each of the marks is detected. 
The relative constructions of the marks and the sensors will next be 
clearly described while the above printing operation and various kinds of 
operations relative to the printing operation are explained in detail. 
For example, a color image is formed on the image receiving paper sheet 2 
by each of the ink layers Y.sub.2, M.sub.2, C.sub.2 and K.sub.2 in the 
final color group shown by G.sub.2 in FIG. 2. As shown in FIG. 1, the 
image receiving paper sheet 2 is first conveyed toward the printing 
position X from an unillustrated paper supplying portion in the direction 
of an arrow B. An operation of the platen 3 is stopped when a leading end 
2a of the image receiving paper sheet 2 has reached the printing position 
X or a starting position shown in FIG. 1 and slightly passing through the 
printing position X. Thus, the image receiving paper sheet 2 is stopped in 
this position. 
In contrast to this, as shown in FIGS. 2 and 3, the leading color mark 6 of 
the ink sheet 1 is detected by the first sensor S.sub.a and is stopped in 
a position shown in each of FIGS. 2 and 3 by a detecting signal of the 
first sensor S.sub.a. At this time, a leading end region of the color 
group G.sub.2 is located in the printing position X. In this embodiment, a 
most leading end 14Y of this color group G.sub.2 is located in the 
printing position X. This most leading end 14Y is also equal to a most 
leading end of the yellow ink layer Y.sub.2. Thus, when the first sensor 
S.sub.a detects the leading color mark 6, it is detected that the leading 
end region of the color group G.sub.2 has reached the printing position X. 
When the ink sheet 1 and the image receiving paper sheet 2 are conveyed as 
mentioned above, the thermal head 4 is escaped upward from a position 
thereof shown in FIG. 1. 
When the first sensor S.sub.a detects the leading color mark 6 as mentioned 
above, the thermal head 4 is towered to the position shown in FIG. 1. 
Then, the ink sheet 1 and the image receiving paper sheet 2 are supported 
by the thermal head 4 and the platen 3 therebetween. 
When such a preparing operation is completely performed, the platen 3 is 
rotated in the counterclockwise direction in FIG. 1 so that the image 
receiving paper sheet 2 and the ink sheet 1 are respectively conveyed in 
the directions of arrows B and A. At this time, as mentioned above, ink of 
a leading ink layer in the color group G.sub.2 is transferred onto the 
image receiving paper sheet 2. In this embodiment, ink of the yellow ink 
layer Y.sub.2 is transferred onto the image receiving paper sheet 2. Thus, 
a yellow image is formed on the image receiving paper sheet 2. 
Thus, the printing operation with respect to the yellow ink layer Y.sub.2 
is performed. After this printing operation is completely performed, the 
thermal head 4 is again raised and the platen 3 is rotated in the 
clockwise direction. Thus, the image receiving paper sheet 2 is conveyed 
in a direction opposite to the arrow B. The platen 3 is stopped in a 
position in which the leading end 2a of the image receiving paper sheet 2 
is returned to the starting position shown in FIG. 1. The image receiving 
paper sheet 2 is stopped in this starting position. 
When the heading mark 7 of the ink sheet 1 conveyed in the direction of the 
arrow A is detected by the second sensor S.sub.b, the ink sheet 1 is 
stopped by a detecting signal of this second sensor S.sub.b. At this time, 
a leading end region of a second ink layer in the color group G.sub.2 has 
reached the printing position X. In this embodiment, a most leading end 
14M of the magenta ink layer M.sub.2 has reached the printing position X. 
Similar to the printing operation with respect to the yellow ink layer 
Y.sub.2, the printing operation with respect to the magenta ink layer 
M.sub.2 is next performed. Thus, a magenta image is formed on the image 
receiving paper sheet 2 in a state in which the yellow and magenta images 
overlap each other. 
When the printing operation with respect to the magenta ink layer is 
completely performed, the thermal head 4 is escaped upward as in the above 
case and the image receiving paper sheet 2 is returned to the starting 
position. Then, the heading mark 8 for the next cyan ink layer C.sub.2 is 
detected by the second sensor S.sub.b. The ink sheet 1 is stopped by a 
detecting signal of this second sensor S.sub.b. At this time, a leading 
end region of the cyan ink layer C.sub.2 has reached the printing position 
X. In this embodiment, a most leading end 14C of the cyan-ink layer 
C.sub.2 has reached the printing position X. Subsequently, the printing 
operation with respect to the cyan ink layer C.sub.2 is performed. 
Similar to the above case, when this printing operation is completely 
performed, the image receiving paper sheet 2 is returned to the starting 
position. Then, the heading mark 9 for the black ink layer K.sub.2 is 
detected by the second sensor S.sub.b. At this time, a leading end region 
of the black ink layer K.sub.2 has reached the printing position X. In 
this embodiment, a most leading end 14K of the black ink layer K.sub.2 has 
reached the printing position X. The ink sheet 1 is stopped in this state. 
Subsequently, the printing operation with respect to the black ink layer 
K.sub.2 is performed. Thus, a color image composed of plural ink colors is 
formed on the image receiving paper sheet 2. 
When the printing operation with respect to the final black ink layer 
K.sub.2 is completely performed, the thermal head 4 is raised and the 
image receiving paper sheet 2 forming the color image thereon is conveyed 
in the direction of the arrow B. The image receiving paper sheet 2 is then 
discharged to an unillustrated paper discharging section. 
When no color group is equal to the final color group G.sub.2, a leading 
color mark of the next color group is detected by the first sensor S.sub.a 
and the ink sheet 1 is stopped. Subsequently, a series of the 
above-mentioned operations is performed. Thus, a color image is formed on 
the next image receiving paper sheet. As shown in this embodiment, when 
the printing operation with respect to the final color group G.sub.2 is 
performed, the terminal end marks 10 and 11 are detected by the first and 
second sensors S.sub.a and S.sub.b after the printing operation with 
respect to the final black ink layer K.sub.2 has completely performed. A 
display section additionally arranged in a recorder body then displays 
that the ink sheet 1 should be replaced with a new one. For example, the 
display section displays these contents on the basis of detecting signals 
of the first and second sensors S.sub.a and S.sub.b by commands from an 
unillustrated controller including a central processing unit (CPU). An 
operator replaces the ink sheet with another in accordance with this 
display. 
In the above example, the leading color mark 6 is formed on one end side of 
the ink sheet 1 in a width direction thereof. This leading color mark 6 is 
detected by the first sensor S.sub.a. Each of the heading marks 7, 8 and 9 
is formed on the other end side of the ink sheet 1 in the width direction. 
Each of the heading marks 7, 8 and 9 is detected by the second sensor 
S.sub.b. Each of the two terminal end marks 10 and 11 is formed on each of 
the end sides of the ink sheet 1 in the width direction. The terminal end 
marks 10 and 11 are detected by the first and second sensors S.sub.a and 
S.sub.b. FIG. 4 shows operating states of the sensors S.sub.a and S.sub.b 
at this time. The controller judges each of these marks by differences 
between combinations of detecting states of the sensors S.sub.a and 
S.sub.b. However, in this example, no controller can judge whether each of 
the three heading marks 7, 8 and 9 is a mark for an ink layer of any 
color. 
The above combinations of the detecting states can be suitably changed. For 
example, the leading color mark may be constructed by two marks and these 
two marks may be detected by the two sensors S.sub.a and S.sub.b. Further, 
the terminal end marks may be constructed by one mark and this one mark 
may be detected-by one of the sensors. 
In the embodiments shown in FIGS. 1 to 4, it is judged that each of the 
marks has reached each of the sensors S.sub.a and S.sub.b when each of the 
sensors S.sub.a and S.sub.b detects a leading end edge of each of the 
marks 6 to 11 on an upstream side of the ink sheet in the conveying 
direction thereof. For example, this leading end edge is set to an edge 
designated by reference numeral 106 in FIG. 2 in the case of the mark 6. 
As shown in FIG. 4, a mark is detected by rise of a detecting pulse of 
each of the sensors in the above so-called front edge detection. In this 
example, the leading end edge of each of the marks is in conformity with 
each of the most leading ends 14Y, 14M, 14C and 14K of the ink layers. 
The marks 6 to 11 and the first and second sensors S.sub.a and S.sub.b 
function as mentioned above. 
In the following description, reference numeral L is set to a distance from 
the printing position X to each of the sensors S.sub.a and S.sub.b. 
Reference numeral q is set to a pitch of each of the marks. In this case, 
when each of the marks is detected by each of the sensors S.sub.a and 
S.sub.b as mentioned above, a most leading end of each of the color groups 
or a most leading end of each of the ink layers in each of the color 
groups can reach the printing position X and a terminal end of the ink 
sheet 1 can be detected as mentioned above by setting the values L and q 
such that the following first formula is satisfied. 
EQU L=N.times.q (N is an integer equal to one or more) 
The pitch q is equal to a pitch between the ink layers in the conveying 
direction of the ink sheet 1. 
FIG. 17 is an explanatory view similar to FIG. 3 when N=1. As can be seen 
from FIG. 17, a predetermined color image can be obtained and a terminal 
end of the ink sheet 1 can be detected by forming each of the marks in a 
position shown in FIG. 17 while a conveying operation of the ink sheet 1 
is controlled as mentioned above even when N=1. 
However, in the recorder of the present invention, the printing operation 
is performed while the ink sheet 1 is conveyed at a speed lower than that 
of the image receiving paper sheet 2. Therefore, a pitch of each of the 
ink layers is small so that the pitch q between the marks is very small. 
Accordingly, when N=1 is set, L=q is formed so that this distance L is 
also very small. Therefore, each of the sensors S.sub.a and S.sub.b must 
be located very near the printing position X. 
However, as shown in FIG. 1, constructional elements such as the platen 3, 
the thermal head 4, etc. are arranged in the vicinity of the printing 
position X. Accordingly, it is not easy to arrange each of the sensors 
S.sub.a and S.sub.b in the vicinity of the printing position X. Thus, when 
N =1 is set, a degree of freedom in design of the recorder is greatly 
restricted. 
Therefore, in the present invention, N in the above first formula is first 
set to an integer equal to 2 or more. Each of FIGS. 2 and 3 shows an 
example in which N is set to 2. If N is set to an integer equal to 2 or 
more, the conveying operation of the ink sheet 1 can be correctly 
controlled. Further, each of the sensors S.sub.a and S.sub.b can be 
greatly separated from the printing position X so that the degree of 
freedom in design of the recorder can be greatly increased. In the 
examples of FIGS. 1 to 3, each of the sensors S.sub.a and S.sub.b is 
arranged at the distance L twice the pitch q. Accordingly, no sensors 
interfere with the other constructional elements so that the recorder can 
be easily designed. 
In the embodiments shown in FIGS. 2 to 4, when each of the sensors S.sub.a 
and S.sub.b detects each of the marks in each of the color groups, the 
most leading end of each of the color groups and the most leading end of 
each of the ink layers in each of the color groups are located just in the 
printing position X. However, when each of the marks is detected by each 
of the sensors S.sub.a and S.sub.b, each of the most leading ends 14Y, 
14M, 14C and 14K may be located on a downstream side of the ink sheet in 
the conveying direction thereof by a slight distance .delta. from the 
printing position X. 
In an example shown in FIG. 5, when the leading color mark 6 is detected by 
the first sensor s.sub.a, the most leading end 14Y of the color group 
G.sub.2 is stopped in a state in which this most leading end 14Y is 
shifted on a side of the sensors S.sub.a and S.sub.b by the distance 
.delta. from the printing position X. In this case, when each of the marks 
is detected by each of the sensors S.sub.a and S.sub.b, a leading end 
region of each of the ink layers can reach the printing position X, or a 
terminal end of the ink sheet 1 can be detected by setting values L and q 
such that the following second formula is satisfied. 
EQU L=N.times.q+.delta. (N is an integer equal to 2 or more) 
Value L is set to a distance from the printing position X to each of the 
sensors S.sub.a and S.sub.b. Value q is set to a pitch between the marks. 
In the example shown in FIG. 5, the above leading end region of each of 
the ink layers is provided by an ink sheet portion on an upstream side of 
each of the most leading ends 14Y, 14M, 14C and 14K by the distance 
.delta.. In this case, each of the first and second sensors S.sub.a and 
S.sub.b can be also greatly separated from the printing position X by 
setting N to an integer equal to 2 or more. FIG. 5 shows an example in 
which N is set to 2. 
In this example, no ink layer portion shown by .delta. is used to perform 
the printing operation. Accordingly, when the value of .delta. is 
excessively large, utilization efficiency of the ink sheet is reduced. 
Therefore, in the present invention, .delta. is set to an arbitrary value 
from 0 to 4 mm (0.ltoreq..delta.&lt;4). .delta. is set to zero in the example 
shown in each of FIGS. 2 and 3. 
As mentioned above, utilization efficiency of the ink sheet can be improved 
if .delta. is set to zero. In contrast to this, if .delta. is set to be 
greater than zero, it is possible to prevent colors of a completed color 
image from being mixed with each other. In FIG. 5, for example, when 
.delta. is set to zero and the same construction as FIG. 3 is used and the 
printing operation with respect to the yellow ink layer Y.sub.2 is started 
in the printing position X, there is a fear that ink of the black ink 
layer K.sub.1 adjacent to the yellow ink layer on its left-hand side is 
also transferred onto the image receiving paper sheet 2. In contrast to 
this, when .delta. is set to be greater than zero, occurrence of such a 
problem can be prevented. Accordingly, it is preferable to set .delta. to 
be a larger value. However, when .delta. is set to be excessively large, 
utilization efficiency of the ink sheet 1 is reduced as mentioned above. 
Therefore, in the present invention, .delta. is set to be ranged from 0 to 
4 min. 
In each of the above embodiments, the ink layers are continuously formed 
without forming clearances therebetween. However, FIG. 6 shows an example 
of an ink sheet 1 in which clearances P are formed between ink layers and 
N is set to 2. Similar to the embodiments shown in FIGS. 1 to 5, the 
above-mentioned second formula can be also satisfied in the example of 
FIG. 6. In FIG. 6, .delta. can be also set to an arbitrary value ranged 
from 0 to 4 mm. In this example, the clearances P are formed between the 
ink layers even when .delta. is set to zero. Accordingly, it is possible 
to prevent colors of a color image from being mixed with each other. 
In a general recorder of this kind, a mark is commonly formed in a region 
of a boundary portion of each of the ink layers. In each of the above 
embodiments, a leading end edge of each of the marks in a detected 
position thereof is set to be in conformity with a most leading end of 
each of the ink layers in accordance with such a general structure. 
However, in the present invention, as mentioned above, each of the marks 
can be arranged at a predetermined pitch q in any position of the ink 
sheet 1 in the longitudinal direction thereof. 
FIG. 7 is an explanatory view showing one example of such an arrangement. 
Reference numerals designating constructional portions in FIG. 7 
correspond to those in FIGS. 1 to 6. In FIG. 7, a leading end edge of each 
of marks 6 to 11 in a detected position thereof is not set to be in 
conformity with a most leading end of each of ink layers, but is separated 
from this most leading end by a distance S. 
In such a construction, similar to the above embodiments, when each of the 
marks is detected by each of sensors S.sub.a and S.sub.b, a leading end 
region of each of the ink layers can reach a printing position X, or a 
terminal end of the ink sheet 1 can be detected by setting values L, N, q, 
S and .delta. such that the following third formula is satisfied. 
EQU L=N.times.q+S+.delta. 
Values L, N, q and S in this third formula are equal to those in the 
above-mentioned second formula. Namely, N is set to an integer equal to 2 
or more. In the example of FIG. 7, N is set to 2. Value q is set to a 
pitch between the ink layers as a pitch between the marks. .delta. is set 
to be ranged from 0 to 4 mm. As mentioned above, S is set to a distance 
between each of the marks and a most leading end of each of the ink 
layers. Accordingly, S can be set to an arbitrary value from zero to the 
pitch q. 
The above second formula is provided in a special case of the third formula 
in which S is set to zero. In the embodiments of FIGS. 2 to 5, the 
clearances P between the ink layers shown in FIG. 7 are set to zero. In 
the embodiments of FIGS. 2 and 3, P and .delta. are set to zero. 
The above third formula is a formula showing a general structure of the 
present invention provided when the first and second sensors S.sub.a and 
S.sub.b are arranged in positions on a downstream side of the ink sheet in 
a conveying direction thereof from the printing position. The sensors 
S.sub.a and S.sub.b can be greatly separated from the printing position by 
setting N in the third formula to an integer equal to 2 or more. 
In the above embodiments, the two sensors S.sub.a and S.sub.b are arranged 
on the downstream side of the ink sheet in the conveying direction thereof 
from the printing position X. However, the two sensors S.sub.a and S.sub.b 
can be arranged on an upstream side of the ink sheet in the conveying 
direction thereof from the printing position X. 
Each of FIGS. 8 and 9 is an explanatory view showing one example of such an 
arrangement. Reference numerals in FIGS. 8 and 9 correspond to those in 
each of the above-mentioned embodiments. In a color thermal transfer 
recorder shown in each of FIGS. 8 and 9, similar to each of the above 
embodiments, a predetermined color image is obtained while the conveying 
operation of an ink sheet 1 is controlled. As clearly seen from FIGS. 8 
and 9, when each of marks is detected by each of sensors S.sub.a and 
S.sub.b to obtain this predetermined color image, a leading end region of 
each of ink layers can reach a printing position, or a terminal end of the 
ink sheet 1 can be detected by setting values L, N, q, S and .delta. such 
that the following fourth formula is satisfied. 
EQU L=N.times.q+S-.delta. 
In this fourth formula, L is set to a distance from the printing position X 
to each of the sensors S.sub.a and S.sub.b. Value q is set to a pitch 
between the ink layers as a pitch between the marks. N is set to an 
integer equal to 2 or more. In FIGS. 8 and 9, N is set to 2. .delta. is 
set to an arbitrary value from 0 to 4 min. S is set to a distance from a 
detected position of each of the marks to a most leading end of each of 
the ink layers. In this example, a leading end edge of each of the marks 
is arranged in the detected position thereof. S is an arbitrary value from 
zero to the pitch q. In each of FIGS. 8 and 9, each of clearances P 
between the ink layers can be set to a suitable value including zero. 
Thus, the same recording operation as each of the above embodiments can be 
performed in the embodiments shown in FIGS. 8 and 9. 
As can be seen from the above explanation, when the sensors S.sub.a and 
S.sub.b are arranged on a downstream side of the ink sheet in a conveying 
direction thereof from the printing position X, the above values L, q, S, 
etc. are determined such that the above third formula is satisfied. In 
contrast to this, when the sensors S.sub.a and S.sub.b are arranged on an 
upstream side of the ink sheet in the conveying direction from the 
printing position X, the above values L, q, S, etc. are determined such 
that the above fourth formula is satisfied. 
In the color thermal transfer recorder of this kind, a color image can be 
obtained as mentioned above. If a monochromatic image can be also obtained 
in addition to the color image, it is possible to increase uses of the 
recorder. FIG. 10 is an explanatory view showing one example of a color 
thermal transfer recorder having a function for providing such 
monochromatic and color images. 
In this example, similar to the embodiment shown in FIG. 2, first and 
second sensors S.sub.a and S.sub.b are arranged in positions separated 
from a printing position X by a distance L on the downstream side of an 
ink sheet in a conveying direction thereof. Further, another auxiliary 
sensor S.sub.c is fixedly arranged in a position separated from the 
printing position X in comparison with the first and second sensors 
S.sub.a and S.sub.b. This sensor S.sub.c is also constructed by a 
photosensor of a reflecting type. An unillustrated reflecting plate 
similar to each of the reflecting plates 12 and 13 shown in FIG. 1 is 
arranged in a position opposite to this sensor S.sub.c. 
Each of marks is formed on the ink sheet 1 and is arranged as follows. 
Namely, a leading color mark is composed of two marks 6a and 6b located in 
both end portions of the ink sheet 1 in a width direction thereof. One 
auxiliary mark 6c is further formed on the downstream side of the ink 
sheet in the conveying direction thereof in comparison with the leading 
color mark. The auxiliary mark 6c is added to the leading color mark. The 
ink sheet 1 also has two heading marks 7a and 7b for a magenta ink layer 
M.sub.2 and one heading mark 8a for a cyan ink layer C.sub.2. The ink 
sheet 1 further has a heading mark 9a for a black ink layer K.sub.2 and 
another auxiliary mark 9b located on the downstream side of the ink sheet 
in the conveying direction thereof in comparison with the heading mark 9a. 
The auxiliary mark 9b is added to the heading mark 9a. A terminal mark is 
constructed by one mark 11a. 
Similar to the above embodiments, marks 6a, 6b, 7a, 7b, 8a, 9a and 11a 
except for the auxiliary marks 6c and 9b are arranged at the pitch q. The 
distance L from the printing position X to the first and second sensors 
S.sub.a and S.sub.b is also set such that the above third formula is 
satisfied. In the example shown in FIG. 10, .delta. and S are set to zero 
in the third formula and N as an integer equal to 2 or more is set to 2. 
In FIG. 10, clearances P between the ink layers are not formed, but may be 
formed. 
In this example, when the two marks 6a and 6b are detected by each of the 
first and second sensors S.sub.a and S.sub.b and the auxiliary mark 6c is 
detected by the auxiliary sensor S.sub.c, a leading end region of a color 
group G.sub.2 is stopped in the printing position X. In this example, this 
leading end region is set to a most leading end 14Y of the color group 
G.sub.2. From this stopping state, similar to the above embodiments, a 
printing operation is performed with respect to each of color ink layers 
Y.sub.2, M.sub.2, C.sub.2 and K.sub.2. Thus, similar to the above 
embodiments, a color image is formed on an image receiving paper sheet. 
Namely, as shown in FIG. 11, when three sensors S.sub.a, S.sub.b and 
S.sub.c respectively detect three marks 6a, 6b and 6c, a controller judges 
that the leading end region of the color group G.sub.2 has reached the 
printing position X. When the first and second sensors S.sub.a and S.sub.b 
respectively detect two marks 7a and 7b, the controller judges that a 
leading end region of the magenta ink layer M.sub.2 has reached the 
printing position X. Similarly, when the first sensor S.sub.a detects a 
mark 8a, the controller judges that a leading end region of the cyan ink 
layer C.sub.2 has reached the printing position X. Further, when the 
sensors S.sub.a and S.sub.c respectively detect marks 9a and 9b, the 
controller judges that a leading end region of the black ink layer K.sub.2 
has reached the printing position X. When the second sensor S.sub.b 
detects a mark 11a, the controller judges that this detected mark shows a 
terminal end of the ink sheet 1. Then, the recorder displays that the ink 
sheet should be replaced with another. 
Thus, detecting combinations of the marks detected by the three sensors 
S.sub.a, S.sub.b and S.sub.c are different from each other. Therefore, it 
is possible to judge whether the leading end region of an ink layer of any 
color has reached the printing position X. Accordingly, as mentioned 
above, a color image can be obtained, and a monochromatic image can be 
also obtained. 
For example, when an image of only magenta color is obtained, the image is 
formed on a sheet of image receiving paper by only the magenta ink layer 
M.sub.2 from a detecting state in which the two marks 7a and 7b are 
detected by the first and second sensors S.sub.a and S.sub.b. An image of 
only cyan color is obtained if the printing operation is started from a 
detecting state in which the mark 8a is detected by the first sensor 
S.sub.a, and the image is formed on the paper sheet by only the cyan ink 
layer C.sub.2. Thus, a monochromatic image of any color can be obtained 
and an overlapping image of two or three colors can be also obtained. 
As mentioned above, in this embodiment, the recorder detects whether or not 
the leading end region of an ink layer of any color has reached the 
printing position X. Such a detecting operation is performed by using a 
detecting means having the first and second sensors S.sub.a and S.sub.b 
and at least one auxiliary sensor. This auxiliary sensor is set to S.sub.c 
in the example of FIG. 10. The auxiliary marks 6c and 9b detected by the 
auxiliary sensor S.sub.c are formed on the ink sheet 1. In this case, a 
pitch q of each of marks except for the auxiliary marks 6c and 9b, and a 
distance L from the printing position X to the first and second sensors 
S.sub.a and S.sub.b are also set such that the above third formula is 
satisfied. Accordingly, it is possible to arrange the sensors S.sub.a and 
S.sub.b such that these sensors are greatly separated from the printing 
position X. 
In an embodiment shown in FIG. 12, first, second and third sensors S.sub.a, 
S.sub.b and S.sub.c are arranged on the upstream side of an ink sheet in a 
conveying direction thereof from a printing position X. Similar to the 
embodiments shown in FIGS. 10 and 11, each of marks 6a, 6b, 6c, 7a, 7b, 
8a, 9a, 9b and 11a is detected by each of the sensors S.sub.a, S.sub.b and 
S.sub.c. Further, in the embodiment shown in FIG. 12, the above-mentioned 
fourth formula is satisfied, and .delta. and S are set to zero and N as an 
integer equal to 2 or more is set to 2. In accordance with this 
construction, it is possible to perform the same recording operation as 
the embodiment shown in FIG. 10. Detecting states of the marks detected by 
the sensors S.sub.a, S.sub.b and S.sub.c are similar to those in FIG. 11. 
For example, in the embodiment shown in FIG. 10, after the ink sheet 1 is 
conveyed in the direction of an arrow A from a conveying state shown in 
FIG. 10, there is a fear of an error in operation of the recorder since no 
detecting timing shown in FIG. 11 can be obtained by detecting the mark 6b 
by the auxiliary sensor S.sub.c before the marks 7a and 7b are 
respectively detected by the first sensor S.sub.a and the second sensor 
S.sub.b. Similarly, in the embodiment shown in FIG. 12, there is also a 
fear of an error in operation of the recorder caused by detecting the mark 
7b by the auxiliary sensor S.sub.c before the marks 7a and 7b are 
respectively detected by the first and second sensors S.sub.a and S.sub.b. 
There is further a fear of an error in operation of the recorder caused by 
detecting the mark 11a by the auxiliary sensor S.sub.c before this mark 
11a is detected by the second sensor S.sub.b . 
For example, the following measures are taken to prevent such errors. 
(1) In detection of a mark using each of the first and second sensors 
S.sub.a and S.sub.b, no controller judges that this detected mark is a 
mark until the ink sheet 1 has moved by e.g., a distance corresponding to 
one pitch of an ink layer or a distance slightly shorter than this one 
pitch since each of the first and second sensors S.sub.a and S.sub.b 
detected this mark. 
(2) The controller is constructed such that no unnecessary mark is judged 
as a mark when each of the sensors S.sub.a, S.sub.b and S.sub.c detects 
the unnecessary mark. For example, when only the auxiliary sensor S.sub.c 
detects a mark, no controller judges that this detected mark is a mark. 
Otherwise, only when the auxiliary sensor S.sub.c and the first sensor 
S.sub.a detect a mark, the controller judges that this detected mark is a 
mark to be detected. 
The above recording operation of the recorder can be also performed even 
when the auxiliary sensor S.sub.c in each of the embodiments shown in FIG. 
10 and 12 is arranged in a position close to the printing position X in 
comparison with the first and second sensors S.sub.a and S.sub.b. However, 
in such an arrangement, since the auxiliary sensor S.sub.c approaches the 
printing position X, it might be meaningless to arrange the first and 
second sensors S.sub.a and S.sub.b such that these sensors are greatly 
separated from the printing position X. 
Therefore, in the embodiments shown in FIGS. 10 and 12, a distance L.sub.1 
from the printing position X to the auxiliary sensor S.sub.c is set to be 
greater than the distance L from the printing position X to the first and 
second sensors S.sub.a and S.sub.b so that all the sensors S.sub.a to 
S.sub.c are greatly separated from the printing position X. Such an 
arrangement prevents a degree of freedom in design of the recorder from 
being reduced. The distances L.sub.1 and L may be set to be equal to each 
other. In this case, the auxiliary sensor S.sub.c is arranged in a 
position shifted from the first and second sensors S.sub.a and S.sub.b in 
a width direction of the ink sheet 1. 
In the above embodiments, each of marks is detected by each of the sensors 
at a leading end edge thereof. However, each of the marks may be detected 
by each of the sensors at a rear end edge thereof. FIGS. 13 and 14 show 
embodiments corresponding to those of FIGS. 7 and 9 when a rear edge 
detecting system for detecting a rear end edge of a mark is used. The 
construction of a color thermal transfer recorder in each of these 
embodiments is similar to that in each of the above embodiments and an 
explanation of this construction is therefore omitted in the following 
description. 
The two sensors S.sub.a and S.sub.b are used in each of the embodiments 
shown in FIGS. 1 to 9 and FIGS. 13 and 14. In these embodiments, when 
these sensors S.sub.a and S.sub.b respectively detect the terminal marks 
10 and 11, the controller judges that each of the terminal marks is a 
terminal end of the ink sheet. An output of the controller is then 
transmitted to a display section and this display section displays that 
the ink sheet should be replaced with another. At this time, there is a 
fear of an error in operation of the recorder when there are slight errors 
in arranging position of each of the sensors S.sub.a and S.sub.b and 
forming position of each of the marks. 
In FIG. 15, the first and second sensors S.sub.a and S.sub.b are operated 
by terminal marks 10 and 11 and detect the terminal marks 10 and 11 by 
rising portions S.sub.1 and S.sub.2 of detecting pulses of these sensors. 
However, when the above errors are caused, a slight time difference 
.DELTA.t is caused between rise times of the sensors S.sub.a and S.sub.b. 
When such a time difference .DELTA.t is caused and it is judged that each 
of the marks 10 and 11 has simultaneously reached each of the sensors 
S.sub.a and S.sub.b at the rise times of the sensors S.sub.a and S.sub.b, 
the recorder judges that the first sensor S.sub.a has detected the leading 
color mark 6 although the first sensor S.sub.a actually detected the 
terminal mark 10. Accordingly, an error in operation of the recorder is 
caused. Such an error is similarly caused in each of the embodiments shown 
in FIGS. 10 to 12. 
Therefore, in the embodiment shown in FIG. 15, for example, when the first 
sensor S.sub.a detects the mark 10 by its rising pulse portion S.sub.1 and 
the second sensor S.sub.b then detects another mark 11 after a 
predetermined time of T seconds, the controller judges that the mark 11 is 
a terminal end of the ink sheet 1. Then, the controller performs a 
predetermined control operation based on this judgment. In this case, 
T&gt;.DELTA.t is set. In accordance with such a construction, each of the 
marks 10 and 11 can be correctly judged even when both the sensors S.sub.a 
and S.sub.b detect the marks 10 and 11 with the time difference .DELTA.t. 
When the first sensor S.sub.a detects a certain mark and no second sensor 
S.sub.b then detects this mark after T seconds, the controller judges that 
the first sensor S.sub.a has detected the leading color mark 6. Thus, a 
predetermined operation is next performed. 
FIG. 16 shows an example of rear edge detection in which both the sensors 
S.sub.a and S.sub.b detect a mark by falling pulse portions S.sub.3 and 
S.sub.4. In this case, for example, when the first sensor S.sub.a detects 
a certain mark by the falling pulse portion S.sub.3 and the second sensor 
S.sub.b then detects another mark by the falling pulse portion S.sub.4 
after T seconds, the controller judges that these sensors S.sub.a and 
S.sub.b have detected the terminal end marks 10 and 11. In contrast to 
this, when no second sensor S.sub.b detects the above another mark after 
the T seconds, the controller judges that the first sensor S.sub.a has 
detected the leading color mark 6. Thus, there is no fear of an error in 
detection even when the time difference .DELTA.t is caused for a falling 
period of each of the sensors S.sub.a and S.sub.b. In this case, 
T&gt;.DELTA.t is set. 
The above operations explained with reference to FIGS. 15 and 16 are 
similarly performed when the sensors S.sub.a and S.sub.b detect another 
mark and the sensors Sa, S.sub.b and S.sub.c shown in FIGS. 10 to 12 
detect each of marks. After a predetermined time has passed since one 
sensor detected a mark, it is judged whether another sensor detects 
another mark or not. From this judgment, it is judged whether a leading 
end region of each of color groups reaches a printing position. It is also 
judged whether a leading end region of each of color ink layers except for 
a leading ink layer of each of the color groups reaches the printing 
position. Further, it is judged whether a detected mark is a terminal end 
of the ink sheet or not. 
In each of the above embodiments, each of the marks is formed in an 
uncoated portion of the ink sheet 1 which is not coated with ink. However, 
each of the marks may be formed by forming a notch or hole in the ink 
sheet 1. In this structure, 100% of light is transmitted through each of 
the marks so that occurrence of an error in operation of a sensor can be 
prevented. Each of the marks can be also formed by coating the ink sheet 
with a high reflectivity portion such as a white portion for reflecting 
light from the sensor. In this structure, the reflecting plates 12 and 13 
can be omitted. A shape of each of the marks can be suitably set. For 
example, each of the marks may be formed in a square shape, a rhombic 
shape, a circular shape, etc. in addition to the rectangular shape shown 
in the above embodiments. 
For example, each of the marks can be arranged in a clearance between ink 
layers, etc. When each of the marks is formed in each of end portions of 
the ink sheet in a width direction thereof as in the above-mentioned 
embodiments, it is not necessary to form a large clearance for arranging 
each of the marks between the ink layers. Accordingly, it is possible to 
reduce an entire length of the ink sheet. Further, it is possible to 
prevent an uncoated region of ink from being formed by each of the marks 
between the ink layers. Accordingly, occurrence of wrinkles in the ink 
sheet can be prevented when the ink sheet is wound around a winding 
member. 
In the above embodiments, an entire ink sheet portion except for the marks 
is coated with ink. Accordingly, the occurrence of wrinkles in the ink 
sheet can be also prevented when the ink sheet is wound around the winding 
member. The ink sheet can be formed such that each of the marks is formed 
by coating an outside region of each of the ink layers in the width 
direction of the ink sheet with ink and no sheet portion except for each 
of the marks in this outside region is coated with ink. In this case, an 
uncoated portion of ink having a large area is formed in both end portions 
of the ink sheet in the width direction thereof. Accordingly, when this 
ink sheet is wound, this uncoated portion of ink is formed with wrinkles. 
However, such problems are not caused in the above-mentioned embodiments 
of the present invention. 
In accordance with first and second structures of the present invention, 
first and second sensors for detecting each of marks of an ink sheet can 
be arranged such that these sensors are greatly separated from a printing 
position. Accordingly, a degree of freedom in design of a color thermal 
transfer recorder can be increased without any problems. 
In accordance with a third structure of the present invention, in addition 
to the above effects, a monochromatic image can be also formed on the ink 
sheet so that uses of the color thermal transfer recorder can be 
increased. 
In accordance with a fourth structure of the present invention, an 
auxiliary sensor can be also arranged such that the auxiliary sensor is 
greatly separated from the printing position. Accordingly, the degree of 
freedom in design of the recorder can be increased. 
In accordance with a fifth structure of the present invention, an error in 
detection of each of the sensors can be prevented. 
Many widely different embodiments of the present invention may be 
constructed without departing from the spirit and scope of the present 
invention. It should be understood that the present invention is not 
limited to the specific embodiments described in the specification, except 
as defined in the appended claims.