A thermal backprinting apparatus 10 has a print head 18 that prints characters on the reverse side of sheet 12. The sheet is driven past a print head 18 to skip every other line and print head 18 is energized to print every other pixel and form a quarter-tone image.

BACKGROUND 
The present invention relates to thermal printers, and, in particular, to 
thermal printers for printing quarter-tone images on the reverse side of 
image-bearing media. 
In certain printing equipment, such as silver-halide printers, or 
electro-photographic printers, or thermal printers, large numbers of 
images can be produced over time. Especially in large volume printing 
applications, it becomes important to identify individual prints with such 
information as the owner of the print or the condition that existed to 
create the print or the source of the image. In these printers, such 
information is printed on the back, or non-image bearing, reverse surface 
of the prints. This is especially true in photographic style images, that 
cover the entire obverse surface of the print. In these photographic style 
prints, it is undesirable to have the labelling information on the front 
surface or attached as a tag. 
In photographic systems, several techniques have been used to create labels 
on the back of the prints. In the Kodak 3510 printer, for instance, an 
impact printing system is used to record information on the back of 
prints. These impact printers typically have a ribbon that carries dye 
disposed in a vegetable oil. The impact of the hammers transfers some of 
the ink to the back of the print. One disadvantage of this method is that 
the ink can smear in the presence of water and solvents. Another 
disadvantage of this method is that high impact forces can cause marks on 
the silver halide emulsion. 
Recently, improvements have been made in thermal printing systems that have 
made the use of thermal printers attractive in marking the backs of 
prints. See, for example, the thermal backprinting apparatus shown in U.S. 
Pat. No. 4,629,312. This technology uses a thermal print head, which 
consists of a number of thermally resistive elements, to transfer 
compounds from a carrier web. In one embodiment, the marking compound is a 
wax compound. These compounds tend to smear and have poor durability. More 
recently, compounds have been formulated of resins such as polyethylene or 
polyolefins. These polymers have exhibited improved wear, solvent and 
scratch resistance. 
Using a thermal marking system to label the backs of prints from thermal, 
electrophotographic and silver-halide printers presents several problems. 
The thickness of the media receiving backprinting varies from 0.005 to 
0.007 inches. The image material is a compound structure of paper with two 
sided coatings of a low density polymer, such as polypropylene or 
poly-ethylene. As such, the media are generally translucent. The thermal 
transfer material is a resin laden with carbon black. A typical thermal 
printer uses a print head with a linear array of 512 heating elements to 
create a 7.times.9 pixel array for each character. The transfer material 
is engaged by the head, pressed against the back of a print, and the array 
is energized. After backprinting, the dark, backprinted text forms a 
shadow on the obverse image. Such a shadow is especially noticeable when 
the print was placed against a white background or held up to bright 
lights. Such shadows are highly objectionable and render thermal 
backprinting unsuitable for commercial use. 
SUMMARY 
The invention provides a method and apparatus for thermal backprinting that 
reduces backprint shadows. The invention provides a thermal backprinter 
capable of quarter-tone printing. In particular, the thermal print head is 
energized and the print sheet is moved so that one printed pixel is not 
adjacent any other printed pixel. Stated another way, each printed pixel 
is circumscribed by background pixels. More specifically, for a linear 
array of thermal pixel printing elements, every other pixel printing 
element is energized and every other line is printed. In one embodiment, 
every other pixel printing element is disabled, otherwise incapable of 
being heated or simply omitted with its space left empty. 
In an alternate embodiment comprising a matrix array of rows and columns of 
pixel printing elements, those elements are selectively energized so that 
any energized element is circumscribed by non-energized areas.

DETAILED DESCRIPTION 
Referring to FIG. 1, there is shown in schematic form a backprinting 
apparatus 10 which is adapted to print alpha-numeric images on an image 
bearing sheet 12. The image bearing sheet 12 has a front, image bearing 
surface disposed facing the surface of a platen drum 16. The back surface 
of sheet 12 faces a backprint carrier web 14 disposed between the image 
bearing sheet 12 and a print head 18. The platen drum 16 is mechanically 
coupled to a drive mechanism 15 that is operated under control of a 
controller 17. Drive mechanism 15 advances the drum 16 and the image 
bearing sheet 12 past the stationary print head 18 during a printing 
cycle. 
The print head 18 has a plurality of heating elements (See FIG. 2) which 
press the backprint web 14 against the back surface of the image bearing 
sheet 12. The backprint web 14 is driven from a supply roller 20 onto a 
takeup roller 22 by a drive mechanism 23 coupled to the takeup roller 22. 
The drive mechanisms 15 and 23 each include a stepper/gear motor which 
advances the backprint carrier web 14 and the image bearing sheet 12 
relative to the heating elements of the print head 18. As will be clear to 
those skilled in the art, the motors of the mechanisms 15 and 23 can be DC 
motors. The print head 18 is electrically coupled via line 19 to the 
controller 17. Controller 17 controls the operations of the drive 
mechanisms 23, 15 as well as the electrical energization of the heating 
elements of the print head 18. 
With reference to FIG. 2, selected thermal pixel resistive heating elements 
50 are schematically illustrated. Elements 50 are selectively energized by 
closing respective switching elements 60. Upon closure of the switch 
element 60 with a thermal pixel element 50, a voltage from a voltage 
source V.sub.s is disposed across pixel 50. Current flows through the 
thermal pixel 50 causing it to rise in temperature. The accompanying rise 
in temperature causes the transfer of material carried on the backprint 
web 14 from web 14 to the back surface of image bearing sheet 12. 
Controller 17 is operative to individually address one or more of the 
switch elements 60. In the preferred embodiment of the invention every 
other switching element 60 is addressed by the controller 17. In addition, 
controller 17 operates the drive mechanisms 23 and 15 to cause the platen 
16 to move and the print head 18 to be de-energized every other line so as 
to skip every other line in printing characters. 
With reference to FIGS. 1 and 2, the operation of apparatus 10 will be 
briefly described. Drive signals are provided to the drive mechanisms 15, 
23 from the controller 17. Controller 17 may be in the form of a 
microcontroller or microcomputer. Control signals from controller 17 cause 
the drum 16 to rotate and bring successive, contiguous areas of the image 
bearing sheet 12 beneath and opposite print head 18. A portion of the 
backprint web 14 is disposed between the print head 18 and the image 
bearing sheet 12. In a manner well known in the art, controller 17 
provides signals via control line 19 to the print head 18 for selectively 
energizing the printing elements 50-56. For examples of control techniques 
and apparatus including microcontrollers for controlling print heads, see, 
U.S. Pat. Nos. 4,621,271; 4,691,211, all assigned to the same assignee as 
this patent. 
By the selective energization of every other print element 50, and by the 
skipping of every other line of print, the present invention yields a 
quarter-tone image such as that shown in FIG. 4. The quarter-tone image of 
FIG. 4 is advantageous in that it contains enough information to render 
the letter "T" recognizable and at the same time contains few enough pixel 
elements so that shadows cast onto the front side of image bearing member 
12 are substantially reduced. An example of a prior art technique, shown 
in FIG. 3, has resulted in undesireable shadows cast upon the obverse side 
of image bearing member 12, particularly when the sheet 12 is disposed 
against a white background or is illuminated by light from its reverse 
side. It will be noted that in FIG. 4, each printed pixel is circumscribed 
by eight background pixels. In no event is a printed pixel adjacent any 
other printed pixel. 
With reference to FIG. 4, it can be seen that for example pixel 33 is 
surrounded by blank or background pixels. In addition, line 42 that 
contains pixel 33 is followed by a blank line 43. The next line bearing 
pixels is line 44, and so on. The result is that any printed pixel is 
always surrounded by blank pixels. Prints made using this method produced 
text that appears gray due to the fact that each individual pixel 33 is 
small thereby effectively producing a quarter-tone image. When the front 
images are viewed on top of a white background or with a strong light from 
the back, the text of the back has a significantly reduced shadow effect 
on the image bearing surface of sheet 12. Moreover, the resulting text 
printed on the back of sheet 12 is clearly legible and commercially 
acceptable. 
In the preferred embodiment of the invention, an edge type thermal print 
head 18 manufactured by ROHM, part number KT2002-CA, was used. This 
particular head 18 consists of 512 thermal pixel elements 50 indicated as 
resistors. The thermal pixel elements are pitched at eight elements per 
millimeter. Each thermal pixel element 50 can be independently fired by 
shifting in 512 data bits from controller 17 into a shift register 61. On 
a rising signal of a clock 20, the data stored in shift register 61 is 
transferred to latch 59. The platen 16 generally consists of a 0.719 inch 
diameter roller that is driven by a stepper/gear motor drive 15 having a 
stepping rate of 480 steps per revolution. The print media disposed on 
carrier web 14 was a typical thermal resin transfer material. In the 
preferred embodiment the material was a resin type R5 manufactured by 
Astro-Med Corporation. The stepper motor drive mechanism 15 and the print 
head 18 are controlled by controller 17 that receives a data string which 
is decoded into a firing sequence for the head 18. Depending upon the 
particular media used on web 14, between 12 and 18 volts are required for 
V.sub.s in order to transfer the media in 2.5 m sec time period. The media 
in the resin transfer is an all-or-nothing process, whereby the resin 
delaminates from the carrier web 14 and warm flows under pressure from the 
head 18 so that it bonds to the back surface of image bearing sheet 12. As 
mentioned above, the text string was so printed that for every pixel that 
might be printed a blank pixel was printed and for every printed line, the 
stepper motor drive mechanism 15 was advanced an additional two pulses 
without energizing the print head 18 so as to create a line of blank 
pixels. 
FIG. 5 illustrates in more detail a control circuit 42 for print head 18. 
The print head 18 is energized in response to signals that represent a 
line of data, e.g., row 42 of FIG. 4. These signals are stored as data in 
the memory of controller 17. As a numerical example, the print head 
assembly can be formed of 512 individual thermal pixels 50. One line at a 
time is printed. Each thermal pixel 50 is shown as a resistor. The first 
64 thermal pixels (0-63) are assigned to Group 1. The next 64 thermal 
pixels (64-127) are assigned to Group 2. Groups 3-8 are each assigned 64 
thermal pixels. Each thermal pixel is electrically connected to a constant 
voltage power supply (shown as V.sub.s in FIG. 2) and a NAND gate 60. When 
both inputs to a NAND gate 60 are high, the output of the NAND gate 60 is 
connected to ground and a current pulse is generated. One input to each 
NAND gate is from a group strobe signal and the other input is from a 
stage of a series of flip-flop latches 59 which contains 512 stages, one 
for each NAND gate. The latches are connected in parallel to the 512 
stages of a shift register 61. 
Current pulses are applied to a single thermal pixel 50. The pulse width is 
the time period a group strobe signal is on. After all the groups have 
been addressed one time the above process is repeated N-1 times. After 
data are latched in the latches 59, a new line of data are entered into 
the 512 shift register stages. This process of entering data in the shift 
register can take place while thermal pixels are being energized. The duty 
cycle of a thermal pixel is that time equal to the pulse width divided by 
the repetition period. 
In an operation, interface 68 under the control of the controller 17 
provides clock signals to the shift register 61. At the same time a binary 
test data signal is clocked into the stages of the shift register 61 until 
all 512 stages either contain a high "1" or a low "0" signal level or 
state. A latch signal causes the data in each of the shift register stages 
to be entered into the corresponding stages of the latches 59. A high 
signal level signal held on the output of its latch stage is connected to 
its corresponding NAND gate 60. The thermal pixels of each group may be 
simultaneously addressed (the group enable signal is high) in parallel, in 
sequence, or in a staggered manner e.g. all even numbered groups and then 
all odd numbered groups by providing a high group enable signal. The 
particular pattern chosen for addressing the groups will depend upon the 
amount of current required to effect a heat transfer of material from a 
web 14 and the heat disapating characteristic of the thermal pixel element 
50. 
In another embodiment of this invention, it is proposed that print head 18 
may be modified to permanently include blank pixels for every other pixel 
printing element. So, for example, in FIG. 2, every other thermal pixel 
element 50 would simply be omitted from the print head 18. Such omission 
is indicated in the FIG. 2 by the dashed outline surrounding print 
elements. This new head would have no further need for control circuitry 
to blank the pixels since the heating elements associated with such pixels 
would be missing. Thus, a printing apparatus 10 would load only half as 
much data, and, depend upon the physical head structure to provide the 
horizontal element of the quarter-toning process. The blank line control 
accomplished by controller 17 and drive mechanism 15 would still be 
required. The advantages of such a custom head would be a lower cost and 
lower demand on the printer control electronics by reducing over all data 
load rates. 
The preferred embodiment of the invention is the quarter tone printing 
application described above. However, in its broader aspects the invention 
contemplates other applications in which less than all of the contiguous 
pixels of a character are printed. Those skilled in the art understand 
that a given text or character can be scaled up in size to include a 
number of pixels. For example, the letter "T" of FIGS. 3 and 4 could be 
scaled to have a top leg of two or more rows of pixels. Likewise, the 
vertical leg could include two or more columns. With such a scaled font, 
one could print more than a quarter tone image but less than every pixel 
and still achieve a backprinted character with a reduced shadow. As such, 
the invention includes such scaled font applications where some but not 
all contiguous pixels are printed. See FIG. 6 for examples of such scaled 
applications with reduced shadows. Inasmuch as fonts can be selected from 
available computer software, e.g., Postscript, in order to practice the 
invention one skilled in the art could modify such programs to print only 
a predetermined fraction of possible pixels. In the preferred embodiment, 
one of nine pixels are printed. But the invention would work equally as 
well with slightly more or less a percentage of pixels, e.g., 10%-30%. 
Other percentages are selectable and will depend upon the scale of a font, 
the translucence of the image bearing sheet, the density of ink, and other 
factors well known in the art of printing thermal text on translucent 
material. 
Having thus described the preferred embodiments of the invention, those 
skilled in the art will appreciate that further modifications and changes 
may be made to the invention without departing from the spirit and scope 
of the following claims.