Density calibration chart

Details Density calibration chart Density calibration chart

2000-09-11

2004-06-15

Williams, Kimberly (Department: 2626)

Facsimile and static presentation processing

Natural color facsimile

Measuring, testing, and calibrating

C358S001900, C358S406000, C358S001200

Reexamination Certificate

active

06750993

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a method of constructing density calibration charts.
Density calibration is performed in order to correct the differences in sensitivity that exist between recording materials or printers. Even if one wants to print a density of 40%, the result is not always the same due to manufacturing errors inherent in recording materials and printers. Printing on a certain kind of recording material may give a density of 38% rather than 40% whereas printing on another kind of recording material may give a thicker density of 43%. The same applies to printers. A certain kind of printer may give a density of 38% rather than 40% but another kind of printer may give a thicker density of 43%.
Density calibration is necessary to make pre-press correction of such differences in sensitivity between recording materials or printers. The procedure of density calibration is as follows.
(1) First, with the printer (or recording material) of interest being initialized, print a density calibration chart (or a density test pattern which will be described later with reference to
FIG. 5
) on paper.
(2) Then, measure the color density on the printed density calibration chart either visually or with a densitometer
90
(see FIG.
2
).
(3) The next step is correcting any color or density deviation on the basis of the result of color densitometry. To this end, correction data for printing in slightly darker or lighter densities is supplied to an editing device
10
(personal computer), particularly in a color density calibration
1
D table in a data memory
107
(see FIG.
3
), as well as into a color density calibration
1
D table
3043
in a color printer
30
(see FIG.
4
).
These three steps complete the process of density calibration.
(4) Image data is subsequently input to the editing device
10
, which performs density conversion on the input image data with reference to the internal density calibration
1
D table and the resulting correct data is sent to the printer (i.e., color printer)
30
for presswork.
If a printer which tends to print in darker shades receives data for printing a certain color at a density of 40%, the steps (1)-(4) allow for printing in a slightly reduced engine gradation, eventually yielding a print having the same density as the target. The same is true with a recording material.
The color printer
30
shown in
FIG. 2
consists of a look-up table computing unit (hereunder referred to as “LUT”)
31
, a head driver
32
, a head
33
, a controller (CPU)
34
, a motor driver
35
and a motor
36
. LUT
31
generates strobe pulse widths (&mgr;s) corresponding to gradation values of 0-255 and is driven with the head driver
32
to impart a quantity of energy associated with the head
33
. Details of the editing device
10
are shown in FIG.
3
.
Connected to a system bus
106
in the editing device
10
are CPU
101
, program memory
102
, RAM
103
, keyboard or mouse
104
, display
105
, data memory
107
and input/output interface circuit
108
. Connected to the input/output interface circuit
108
are color scanner
99
and color printer
30
. Programs as for percent dot area conversion and printer conditions correction computation are stored in the program memory
102
. Image data, CMYK data, chart data and other kinds of data are stored in the RAM
103
. Stored in the data memory
107
are a density calibration
1
D (one-dimensional) table
1071
and a gray correction
1
D table
1072
. The density calibration
1
D table
1071
contains inverse functions corresponding to a density calibration chart
80
shown in FIG.
2
. In practice, data coming from the color scanner
99
passes through the input/output interface circuit
108
and the system bus
106
to enter the RAM
103
, where it is corrected with the stored CMYK data before being output to the color printer
30
.
FIG. 4
shows the software configuration in the color printer
30
.
As shown, the color printer
30
includes a color correction computing section
301
, a data outputting section
302
, a synthesis computing section
303
and a memory
304
. The memory
304
has the following ready for use: printing conditions correction data
3041
, standard colors transformation data
3042
, density calibration
1
D table
3043
and gray correction
1
D table
3044
; each of these data and tables consists of more than one unit. The data in the density calibration
1
D table
3043
is input to the synthesis computing section
303
, which is also supplied with the other elements to be corrected, i.e., printing conditions correction data
3041
, standard colors transformation table
3042
and the data in the gray correction
1
D table
3044
, and the values obtained by computations for synthesis are sent to the color correction computing section
301
, thereby constructing a composite look-up table
3011
. The CMYK data coming from the editing device
10
is processed by computation for color correction which involves only one step of conversion with the look-up table
3011
and the result is output to the data outputting section
302
.
FIG. 5
shows a conventional density calibration chart. As shown, the chart consists of four columns, Y, M, C and K, of squares that have been printed out and which are arranged stepwise from zero to a hundred percent in terms of density that corresponds to a designated percent dot; such squares are hereunder referred to as “color patches”. In the density calibration chart shown in
FIG. 5
, the color patches for a maximum density (100%) are at the top of the columns and those for a minimum density (0%) are just below them in order to clearly mark off the density range. The intervening tones are arranged by density.
The arrangement of patches by density in a single column has one serious problem. If a denser patch which is upstream in the column is printed, the thermal head warms up and if it is immediately used to print the subsequent patch, the latter has a higher density than the target value, thus making it impossible to produce the desired calibration chart.
With a view to solving this problem, a calibration chart of the type shown in
FIG. 6
has been developed. The obvious difference this chart and the one shown in
FIG. 5
is that a patch in a denser area that has been printed is physically spaced from the next patch by a distance corresponding to the period over which the head cools down. The thermal head to which high energy has been applied to print the higher density patch cools down during that period and has returned to the initial temperature when the next patch is to be printed. The cool-down period is not necessary for printing patches of lower densities since no high energy is applied to the thermal head. In
FIG. 6
, the cool-down period is provided between patches having ID No. 1 (100% dot density) to ID No. 10 (58% dot density) but not between patches having ID No. 11 (43.6% dot density) to ID No. 19 (0% dot density).
In
FIG. 6
, the physical space corresponding to the cool-down period is held constant between patches of higher densities but if desired, it may be reduced with decreasing density.
Nevertheless, the provision of such cool-down periods unduly prolongs the density calibration chart, making it impossible to increase the number of patches that can be incorporated in the chart.
SUMMARY OF THE INVENTION
An object, therefore, of the present invention is to provide a density calibration chart that ensures adequate cool-down periods for the thermal head and which yet has no need to increase the space between patches.
This object can be attained by the density calibration chart according to the first aspect of the invention comprising color patches on which densities from the shadow to the highlight are printed in decreasing order, wherein that said printed color patches are arranged in a check pattern.
Preferably, the color patches are arranged on a plurality of columns for a color, and color patches of sequent density level are arranged on different columns.
The invention also provides a method of correcti

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