Method for increasing thermal print quality

Details Method for increasing thermal print quality Method for increasing thermal print quality

2003-09-18

2004-09-28

Evanisko, Leslie J. (Department: 2854)

Incremental printing of symbolic information

Thermal marking apparatus or processes

Having driving circuitry for recording means

C400S120070, C400S120010, C347S183000

Reexamination Certificate

active

06798433

ABSTRACT:

BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to a printing method for increasing thermal printer quality, and more specifically, to a printing method for increasing thermal printer quality when printing a pixel at a gray level x on paper by a printer.
2. Description of the Prior Art
Photo printers are different from general printers. The major difference is that photo printers print images such as a photo pictures on paper with high picture quality. Please refer to FIG.
1
and FIG.
2
.
FIG. 1
is a diagram of a prior art photo printer
10
.
FIG. 2
is a simplified exploded view of the photo printer
10
shown in FIG.
1
. As shown in
FIG. 1
, the photo printer
10
has a ribbon
14
, a thermal print head
12
, a ribbon driver
18
, and a roller set
20
. The ribbon
14
has a plurality of sections, and each section is used for storing a different colored dye. The thermal print head
12
is fixed inside the photo printer
10
for heating the color dyes so that the color dyes are transferred onto a photo paper
16
. The ribbon driver
18
is used for moving the ribbon
14
back and forth so that the thermal print head
12
can transfer a specific colored dye stored on the ribbon
14
onto the corresponding photo paper
16
. The roller set
20
is used for holding the photo paper
16
and moving the photo paper
16
along a predetermined direction. Therefore, the fixed thermal print head
12
is capable of printing a color image on the photo paper
16
.
As shown in
FIG. 2
, the thermal print head
12
has a plurality of heaters
22
that are arranged linearly and spaced equally for heating the ribbon
14
. The colored dye stored on the ribbon
14
is heated, and is transferred onto the photo paper
16
. When the thermal print head
12
starts printing images, each heater
22
positioned on the thermal print head
12
heats the ribbon
14
so that a plurality of corresponding pixels X
1
will form a line image Y
1
. The photo paper
16
driven by the roller set
20
is then moved along the predetermined direction at a predetermined speed. Another line image Y
2
is then printed on the same photo paper
16
next to the line image Y
1
. Accordingly, a plurality of line images are printed on the same photo paper
16
to complete the printing operation.
As mentioned above, the total number of heaters
22
positioned on the thermal print head
12
determines the corresponding number of pixels X
1
of each line image printed on the photo paper
16
. Moreover, the color concentration, that is, the gray level of each pixel X
1
printed on the photo paper
16
is determined by the corresponding heater
22
, which has a specific duration of each heating operation, and the total number of heating cycles.
Please refer to FIG.
3
and FIG.
4
.
FIG. 3
is a diagram of gray levels and a corresponding driving signal
30
according to the photo printer
10
shown in FIG.
1
.
FIG. 4
is a diagram of a binary data sequence of the driving signal
30
shown in FIG.
3
. As shown in FIG.
3
and
FIG. 4
, before the thermal print head
12
of the photo printer
10
starts printing images onto the photo paper
16
, all of the heaters
22
positioned on the thermal print head
12
are activated for a predetermined period Tp. In this way, each heater
22
will approach a predetermined printing temperature before printing. The above-mentioned procedure is called a preheating operation. In addition, the driving signal having a pulse with a binary value “1” activates the corresponding heater
22
, and the driving signal corresponding to a binary value “0” deactivates the heater
22
. Next, the photo printer
10
continuously activates the same heater
22
according to the corresponding gray level of the pixel X
1
. In other words, each heater
22
positioned on the thermal print head
12
is activated repeatedly according to the desired gray level of the corresponding pixel. The overall heating operation of the heater
22
is represented by a driving signal
30
and its corresponding binary values. The duration Tu of a pulse
32
is the heating time unit for activating the heater
22
.
The heater
22
of the photo printer
10
can produce 256 (0~255) gray levels to print the corresponding pixel X
1
with an appropriate gray level. A gray level corresponding to a lightest color concentration is equal to 0, and a gray level corresponding to a darkest color concentration is equal to 255. In other words, when the pixel X
1
acquires a corresponding gray level equaling N, which is an integer between 0 and 255, the corresponding heater
22
is successively activated N times. Therefore, N pulses
32
of the driving signal
30
are repeatedly generated. That is, N binary “1” values are input to the heater
22
continuously. Please note that the photo paper
16
is printed one line at a time. Because each pixel X
1
positioned on the same line may have different gray levels, each heater
22
has to wait for 255 durations Tu so that the thermal print head
12
can then print the next line image. That is, a first heater
22
could finish printing a corresponding pixel X
1
with a smaller gray level within a short time. However, another heater
22
printing a corresponding pixel X
1
with a greater gray level may take a longer time. The actual heating durations are therefore centralized in the early period of the total heating duration. The more continuous printing durations, the more the heat accumulation. Additionally, the heat accumulation effect causes the system temperature to increase and the next printing gray level will stray from the predetermined gray level thereby affecting the printing quality. For example when the pixel X
1
acquires a corresponding gray level equaling 64, the corresponding heater
22
has to be successively activated 64 times. Therefore, 64 pulses
32
of the driving signal
30
are repeatedly generated. That is, 64 binary “1” values are input to the heater
22
continuously. In actuality, the heat accumulation effect usually makes the corresponding gray level greater than 64 and results in printing distortion.
SUMMARY OF INVENTION
It is therefore a primary objective of the present invention to provide a method of increasing thermal printer quality when printing a pixel at a gray level x on paper by a printer to solve the problems mentioned above.
Briefly summarized, a printing method is disclosed for printing a pixel at a gray level x on paper by a printer. The printer comprises a thermal print head, which comprises a heater for heating a ribbon to print pixels from a gray level 1 to m−1 on the paper. The method comprises: if x is not greater than a value n, heating the ribbon x times and evenly distributing the heating initiation times of the x times between the time point 0 and the time point (m*(x−1)
) for printing the pixel at the gray level x on paper. If x is greater than a value n, the ribbon is heated x times and the heating initiation times of the n times are evenly distributed between the time point 0 and the time point (m*(n−1)
) and the heating initiation times of the x−n times is evenly distributed after the heating initiation time points of the n times.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.


REFERENCES:
patent: 4704617 (1987-11-01), Sato et al.
patent: 4933686 (1990-06-01), Izumi et al.
patent: 2003/0043232 (2003-03-01), Fang et al.

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