Method of printing on a transparency sheet

A printer having a paper printing mode and a transparency printing mode detects a leading edge portion of an image-receiving sheet. Based on detection of an opaque leading edge portion of a transparent sheet, the paper printing mode is disabled and a sheet size having boundaries is selected. The printer deposits ink on the transparent sheet within the boundaries of the selected sheet size.

BACKGROUND OF THE INVENTION 
This invention relates to a transparency sheet, particularly a transparency 
sheet for use in an ink jet printer. 
U.S. patent application Ser. No. 07/715,063 filed Jun. 12, 1991 
(hereinafter referred to as "the prior application") discloses an ink jet 
printer for forming an image on an image-receiving sheet. The disclosure 
of the prior application is hereby incorporated by reference herein. 
The ink jet printer described in the prior application comprises a drum for 
supporting the image-receiving sheet, which is typically of paper, as ink 
drops are deposited on the sheet to form the desired image. A stepper 
motor is connected drivingly to the drum for rotating the drum, and an ink 
jet head is mounted on a carriage that extends parallel to the axis of the 
drum. The carriage is movable longitudinally of the drum, and as the 
carriage moves, the ink jet head is energized to deposit ink drops on the 
sheet. 
In order to load the printer with a sheet, the drum is rotated to a load 
position and a clamp at the periphery of the drum is opened. The sheet is 
fed substantially tangentially towards the clamp, and the clamp is closed 
and thereby grips the leading edge portion of the sheet. The drum is 
rotated through about 70.degree. from the load position to a print start 
position, in which the clamp is close to the path of the ink jet head. 
During printing, the carriage is driven axially of the drum while the ink 
jet head scans the sheet and ejects ink drops onto the sheet, and the drum 
is rotated stepwise at the end of each scan of the ink jet head. At the 
end of printing, the drum is rotated in the same direction until the 
trailing edge of the sheet is at an exit position, and the drum is then 
rotated in the reverse direction, introducing the trailing edge of the 
sheet into an exit path. The clamp is opened and releases the leading edge 
portion of the sheet and the sheet is discharged from the printer. 
In a practical implementation of the printer described in the prior 
application, the printer includes two reflective sensors for sensing the 
presence of a paper sheet on the drum. Each sensor includes a light 
emitter positioned to direct a light beam towards the drum and a light 
detector for receiving reflected light. The surface of the drum is highly 
reflective and reflects light in a specular fashion, whereas paper 
reflects light in a diffuse manner. Therefore, when paper is interposed 
between a sensor and the surface of the drum, the sensor provides an 
output signal having a lower level than when there is no paper present. It 
is essential to proper operation of the sensors that the surface of the 
drum remain highly reflective, and therefore it is important that no ink 
be applied to the drum. Further, if ink is applied to the drum there is an 
increased possibility of sheet misfeeds, since the surface of the drum 
then has different mechanical properties from when it is clean. 
One of the sensors (the Y-axis sensor) is located stationarily between the 
load position and the print start position. When the drum rotates from the 
load position to the print start position, the output of the Y-axis sensor 
indicates whether the sheet has been successfully loaded. If the sheet has 
been successfully loaded, the output of the Y-axis sensor allows further 
operation to take place. If the output signal of the Y-axis sensor 
indicates that the sheet has not been successfully loaded, the printing 
operation aborts and another attempt is made to load a sheet. Further, 
towards the end of the printing operation the Y-axis sensor detects the 
trailing edge of the sheet and its output ensures that printing ceases 
before the trailing edge reaches the printing position. 
The other sensor (the X-axis sensor) is mounted on the traversing carriage 
with the ink jet head. When the drum has rotated to the start print 
position, but before printing takes place, the carriage is driven 
lengthwise of the drum and the X-axis sensor scans the drum. The output 
signal of the X-axis sensor during this probe scan confirms that there is 
an image-receiving sheet at the printing position and also provides 
information regarding the location of the edges of the sheet along the 
axis of the drum. This information is used to ensure that ink is applied 
only between the two edges of the sheet that extend longitudinally with 
respect to the path of movement of the sheet. The X-axis sensor continues 
to sense the longitudinal edges of the sheet throughout the printing 
operation so that if, for example, the sheet becomes narrower from the 
leading edge towards the trailing edge, the image will be progressively 
cropped to ensure that no ink is applied to the drum. 
It is frequently desirable to form an image on a transparency sheet to 
allow the image to be displayed using an overhead projector. Transparency 
sheets for overhead projectors are typically made of polyester film. A 
transparency sheet cannot normally be printed successfully in an ink jet 
printer simply by feeding the transparency sheet into the printer instead 
of a paper sheet. For example, if a transparency sheet were introduced 
into the printer described in the prior application without making changes 
to accommodate the difference between the optical properties of a 
transparency sheet and those of a paper sheet, the Y-axis sensor would not 
detect the presence of the sheet and accordingly the printer would not 
recognize that the sheet had been loaded and printing could not take 
place. In addition, the mechanical properties of a transparency sheet made 
of polyester film are quite different from those of paper of the kind 
normally used in an ink jet printer, particularly with respect to the 
coefficient of friction, and this can cause difficulties in handling a 
transparency sheet in an ink jet printer. 
SUMMARY OF THE INVENTION 
According to a first aspect of the invention there is provided a 
transparency sheet comprising a substrate of transparent material, said 
substrate having an image-receiving portion and a leading edge portion 
joined to the image-receiving portion, and the sheet further comprising a 
layer of opaque material adhering to the substrate over said leading edge 
portion. 
According to a second aspect of the invention there is provided a 
transparency sheet comprising a substrate of transparent material, said 
substrate having an image-receiving portion and an edge portion joined to 
the image-receiving portion along an easily torn line, and a layer of 
material attached to the edge portion, said layer having frictional 
properties substantially similar to those of paper of the kind used in an 
ink jet printer. 
According to a third aspect of the invention there is provided a method of 
operating a printer having a drum for supporting an image-receiving sheet 
and a print head movable axially of the drum for depositing ink on the 
sheet, the printer having a paper printing mode in which it deposits ink 
on the sheet within an area extending substantially from a detected 
leading edge to a detected trailing edge, but not beyond, said method 
comprising storing information relating to at least two sheet sizes, 
providing a transparency sheet comprising a transparent image-receiving 
portion and an opaque leading edge portion joined to the image-receiving 
portion, delivering the transparency sheet to the drum and clamping the 
leading edge portion of the sheet to the drum, optically detecting the 
leading edge portion of the sheet, disabling the paper printing mode of 
the printer, selecting one of said sheet sizes based on the optical 
detection of the leading edge portion, and depositing ink on the sheet 
within the boundaries of the selected sheet size.

DETAILED DESCRIPTION 
Referring to FIGS. 1-3, the illustrated printer comprises a drum 2 having a 
mantle 6 with a cylindrical external surface. The drum is mounted in a 
frame 8 to rotate about the central axis of the external surface of the 
mantle and is driven to rotate by means of a stepper motor 10 (FIG. 3). 
The manner in which the mantle is supported and the manner by which drive 
is imparted to the drum 2 are described in detail in the prior 
application. The motor 10 operates in response to a motor driver 12 (FIG. 
3), and a motor position counter (not shown) is incremented each time a 
pulse is applied to the stepper motor to rotate the drum 2 in the 
counterclockwise direction shown in FIG. 1 and is decremented each time a 
pulse is applied to the motor to rotate the drum 2 in the clockwise 
direction. Accordingly, the count accumulated in the motor position 
counter is representative of the instantaneous angular position of the 
drum 2. 
As described in the prior application, the mantle of the drum 2 is formed 
with slots through which stem elements 18 of a clamp 22 extend. The stem 
elements 18 extend substantially radially relative to the peripheral 
surface of the mantle. The clamp 22 also includes a gripping portion 26 
that is connected to the stem elements and projects substantially at right 
angles from the stem elements in the clockwise direction seen in FIG. 1. 
The clamp 22 is spring biased towards the closed position, in which the 
gripping portion 26 engages the peripheral surface of the mantle 6, and is 
displaceable to an open position by means of a clamp opening mechanism 
(not shown). The gripping portion 26 has a slot (not shown) that is 
equidistant from the two ends of the drum 2, for a purpose that will 
become apparent as this description proceeds. The structure of the clamp 
22 and the mechanism for opening the clamp are described in greater detail 
in the prior application. 
When a sheet 28 is to be loaded into the printer, pulses are applied to the 
stepper motor 10 to rotate the drum to bring the clamp 22 to the position 
shown in FIG. 1. This position is referred to herein as the 0.degree. 
position, and other angular positions of the drum about its central axis 
are referred to by their angular displacement from the 0.degree. position 
in the counterclockwise direction seen in FIG. 1. 
The printer further comprises entry guides 34, 36 defining an entry path 
for loading a sheet 28 into the printer. The entry guides 34, 36 lead to a 
nip 40 defined between an idler roller 44 and a driven roller 46. When a 
sheet 28 is to be loaded into the printer, its leading edge is introduced 
between the entry guides 34, 36, either manually or through use of an 
automatic picker, and the rollers 44, 46 advance the sheet to bring its 
leading edge towards the clamp 22. The minimum length of sheet 28 that can 
be loaded into the printer is at least as great as the distance between 
the nip 40 and the 0.degree. position, since the rollers 44, 46 must have 
control of the sheet 28 in order to deliver its leading edge portion to 
the clamp. 
At about the 0.degree. position is a clamp guide 50, which is generally 
U-shaped and is pivotally mounted to the printer frame 8 at the free end 
of one limb 52, while its other limb 54 presents a planar guide surface 56 
towards the leading edge of a sheet 28 leaving the nip 40. When the clamp 
22 is in the 0.degree. position and is open, the gripping edge 26 of the 
clamp 22 engages a notch between the base of the clamp guide 50 and the 
lower end of the limb 54, so that the clamp 22 guide pivots away from the 
peripheral surface of the mantle and the surface 56 of the limb 54 directs 
a sheet 28 received from the rollers 44 and 46 to the clamp 22. When the 
clamp 22 is closed and the drum 2 rotates in the counterclockwise 
direction, the clamp guide 50 pivots in the clockwise direction and the 
lower end of the limb 54 serves to retain the sheet 28 in contact with the 
mantle. 
At about the 20.degree. position a Y-axis sensor 58 is supported by a rail 
60 for emitting light toward the exterior surface of the mantle 6 or a 
sheet 28 thereon and collecting reflected light. The sensor 58 provides an 
output signal representative of the power at which it collects light. The 
Y-axis sensor 58 is equidistant from the two ends of the drum 2, so that 
when the drum 2 rotates the clamp 22 passes the 20.degree. position, the 
slot in the gripping portion 26 passes under the Y-axis sensor 58. 
Therefore, the gripping portion 26 does not obscure the mantle or a sheet 
28 thereon from the Y-axis sensor 58. 
At about the 75.degree. position is a pair of guide rails 64. These guide 
rails 64 are supported by the frame 8 and serve to support and guide 
movement of a carriage 70 that carries both an ink jet head assembly 74 
and an X-axis sensor 76. The carriage 70 is coupled drivingly to a 
servomotor 80 (FIG. 3), which drives the carriage 70 reciprocatingly along 
the guide rails 64 in response to a command provided to a motor driver 82. 
A clock strip 90 extends parallel to the drum 2 adjacent the path of the 
carriage 70, and an encoder 94 carried by the carriage 70 interacts with 
the clock strip 90 and generates pulses as the carriage 70 moves along the 
guide rails 64. A carriage position counter (not shown) is incremented 
each time a pulse is provided by the encoder when the carriage 70 is 
moving in one direction and is decremented each time a pulse is provided 
by the encoder when the carriage 70 is moving in the opposite direction. 
The count accumulated in the carriage position counter is therefore 
representative of the instantaneous position of the carriage 70 along the 
axis of the drum. 
The ink jet head assembly includes an array 84 of ink jet heads and 
associated ink reservoirs and is connected by means not shown to an ink 
jet drive amplifier 86. When the ink jet drive amplifier 86 is in 
operation, the ink jet heads eject drops of ink toward a printing zone P 
at the periphery of the drum. 
The ink jet drive amplifier 86 and the motor drivers 12, 82 operate under 
control of a state machine 88 (FIG. 3). The state machine 88 has a paper 
printing mode and a transparency printing mode, both of which will be 
described in further detail below. 
In operation, the printer receives information defining an image and stores 
this information in a memory 92. When the sheet 28 that is loaded into the 
printer is a sheet of paper, the Y-axis sensor 58 detects the leading edge 
of the sheet 28 when the drum 2 is rotated to advance the clamp 22 
counterclockwise from the 0.degree. position and confirms that the sheet 
has been loaded. In response to the output of the Y-axis sensor, the state 
machine enters the paper printing mode. 
The drum rotates to bring the clamp 22 to the 70.degree. position and comes 
to a halt. The carriage 70 traverses the drum 2 once in each direction and 
during this probe scan the X-axis sensor 76 detects the position of each 
longitudinal edge of the sheet 28 along the axis of the drum 2. The state 
machine 88 then enters a printing mode, in which the drum 2 is rotated 
stepwise and between steps the print head scans the drum 2 and the ink jet 
heads 84 deposit ink on the paper. The 70.degree. position is such that 
the ink jet heads 84 are able to deposit ink up to about 0.5 cm from the 
leading edge of the sheet 28, and the X-axis sensor 76 controls 
energization of the ink jet heads 84 so that they can print up to about 
0.5 cm from each longitudinal edge of the sheet 28. The Y-axis sensor 88 
continues to detect the sheet 28 until the trailing edge of the sheet 28 
passes under the Y-axis sensor 88. The state machine 88 stops the printing 
operation when the drum 2 has rotated through a predetermined angle after 
the Y-axis sensor 58 detects the trailing edge of the sheet 28 such that 
the ink jet head is able to deposit ink up to about 0.5 cm from the 
trailing edge of the sheet 28. 
When printing is complete, the drum 2 rotates through a further angle of 
about 165.degree., and thereby positions the trailing edge of the sheet 28 
above an exit path defined between exit guides 96, 98. The drum then 
rotates in the clockwise direction and introduces the trailing edge of the 
sheet into the exit path. 
The exit guides 96, 98 of FIG. 1 feed the trailing edge towards a pair of 
press rolls 102, 104 defining a nip 108. At least one of the press rolls 
102, 104 is driven, and as they feed the sheet 28 through the nip 108, 
drops of thermal wax ink deposited on the sheet 28 are compressed. The 
rolls 102, 104 feed the sheet 28 between stripper guides 112, 114, which 
direct the sheet 28 to a narrow gap defined between an output guide 118 
and a selectively driven exit roller 120. Operation of the exit roller 120 
deposits the sheet 28 in a collection tray 124. 
FIGS. 4 and 5 illustrate a transparency sheet 128 that comprises a 
substrate of polyester material having a leading edge strip 130 that is 
about 1.2 cm long and a trailing edge strip 134 that is also about 1.2 cm 
long. The two strips 130, 134 extend along the two longer edges of the 
main body 132 of the sheet 128 and are detachable from the main body of 
the sheet along lines of perforation 136, 138 leaving a secondary sheet of 
a standard size, for example so-called letter size or international A4 
size. The leading edge strip has a coating 140 of white ink on one side 
and has contrasting arrows imprinted on the coating 140 to indicate the 
direction of feed into the printer. Paper tape 142 is bonded to the 
trailing edge strip on the same side as the ink coating 140. The sheet is 
preferably provided with a coating 140 of finely divided silica on its 
printed side. 
When the transparency sheet 128 is loaded into the printer, the leading 
edge strip is received in the clamp 22, and when the drum 2 rotates the 
clamp past the Y-axis sensor, the Y-axis sensor detects that a sheet has 
been successfully loaded. However, the leading edge strip obscures the 
reflective surface of the drum 2 over a rotational interval that is very 
much smaller than the interval corresponding to the minimum length of 
sheet that can be loaded, and the state machine 88 interprets this as 
indicating that the sheet 128 that has been loaded is a transparency 
sheet. The state machine therefore enters the transparency printing mode. 
When the clamp 22 reaches the 70.degree. position and the carriage 70 first 
traverses the drum 2, the X-axis sensor's 76 probe scan allows information 
regarding the X-axis dimension of the leading edge strip to be obtained 
from the carriage position counter. The state machine 88 includes a table 
containing data defining several standard sizes of sheet 128. For example, 
the stored data might contain information defining boundaries for standard 
letter size (8.5 inches by 11 inches, or 21.6 cm by 27.9 cm) and 
international A4 size (21.0 cm by 29.7 cm). In these two cases, the 
leading edge strips are, respectively, 27.9 cm long and 29.7 cm long. The 
main body of the transparency sheet is either 21.6 cm by 27.9 cm or 21.0 
cm by 29.7 cm. Thus, the overall dimension of the sheet 128 perpendicular 
to the leading edge strip is 24.0 cm for letter size and 23.4 cm for 
international A4 size. In the transparency printing mode, the output 
signal provided by the X-axis sensor 76 during the probe scan allows the 
X-dimension of the leading edge strip to be determined, and the state 
machine 88 uses this information to determine whether the length of the 
leading edge strip corresponds to letter size or international A4 size. 
Further, the information provided by the X-axis sensor 76 allows the 
location of the transparency sheet 128 along the X-axis to be determined. 
As mentioned previously, the printer prints to within about 0.5 cm of the 
leading edge of the sheet 128 in the paper printing mode. Since the 
clamping portion 26 of the clamp 22 has a dimension of about 0.3 cm about 
the periphery of the drum, this implies that printing takes place to 
within about 0.2 cm of the clamping portion 26. In the transparency 
printing mode, if printing took place within 0.2 cm of the clamping 
portion 26, ink would be deposited on the leading edge strip. 
When the probe scan has been completed, the drum 2 is further rotated in 
the counterclockwise direction to bring the main body of the transparency 
sheet 128 into the printing zone, and printing is carried out in the usual 
way by scanning the ink jet head over the sheet 128 and selectively 
energizing the ink jet head. The coating of silica on the transparency 
sheet 128 improves the ink-receiving qualities of the sheet 128 as 
compared with uncoated polyester film. 
During printing in the transparency priority mode, the X-axis sensor 78 is 
not able to detect the edges of the sheet 128 and therefore the ends of 
the printing scan are not controlled dynamically by the output of the 
X-axis sensor 76, but rather by the output provided during the probe scan. 
Printing continues until just before the trailing edge strip enters the 
printing zone, this being determined on the basis of the size of the main 
body of the transparency sheet as stored in the memory. When printing is 
complete, the drum 2 is further rotated in the counterclockwise direction 
until it reaches the angular position at which the trailing edge strip is 
just above the exit path. The drum 2 then rotates in the clockwise 
direction, feeding the trailing edge strip into the exit guide towards the 
nip 108. The coefficient of friction between the trailing edge strip and 
the upper pressure roll is considerably higher than that between polyester 
film and the pressure roll, and this results in improved feeding of the 
sheet 128 into the nip and subsequent ejection of the sheet 128 into the 
collection tray. Further, the coating of silica provides roughness or 
tooth for improved feeding of the sheet 128. 
It will be appreciated that the invention is not restricted to the 
particular embodiment that has been described and that variations may be 
made therein without departing from the scope of the invention as defined 
in the appended claims and equivalents thereof.