Printer calibration using a tone reproduction curve and requiring no measuring equipment

A method for calibrating a printer determines a tone reproduction curve for the printer. A plurality of bands of different accurate gray level standards and a corresponding number of smoothly increasing sequences of gray level specifications for the printer are printed adjacent to one another. Adjacent bands and sequences are compared, and a point along each band where the band appears equal to the adjacent gray level specification sequence is determined. The points are interpolated and the interpolated values are used to form a tone reproduction curve for the printer. The tone reproduction curve, representing the correspondence between the gray level specification of the printer and the actual gray levels produced by the printer, can then be used to calibrate the printer. The calibrating method can also be used for color printers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to calibration of a printer and, more 
particularly, to calibration of a printer using a tone reproduction curve, 
the calibration requiring no measuring equipment. 
2. Description of the Related Art 
Electronic printers are generally capable of producing gray tones, as well 
as standard black and white tones in order to produce gray tones. The 
printers accept gray level specifications as input and produce 
corresponding gray areas on a printed page. This production of the 
corresponding gray areas is often performed by a halftoning operation 
wherein a fine pattern of black and white dots is printed, the dots 
appearing as varying gray tones in accordance with the number of black and 
white dots used when viewed from a distance. The allowed gray level 
specifications generally vary over some finite range such as 0 to 1 or 0 
to 255. The colors at the extremes (i.e., 0 and 1 or 0 and 255) are white 
and black, numbers between the extremes yielding intermediate gray tones. 
However, while the gray level specifications may vary linearly, a linear 
change in gray levels typically does not result. Instead, there may be a 
particular threshold before lighter toned grays become visible. Similarly, 
darker toned grays may prematurely appear black. 
In order to accurately produce a desired gray level, one must know the 
correspondence between the gray level specification of the printer and the 
actual gray level produced by the printer. A tone reproduction curve (TRC) 
provides an illustration of the corresponding relationship. If the TRC is 
known, one can compensate for the nonlinearities of the printer. 
The TRC, which is dependent upon the stability of the printing device, may 
change frequently, or the halftone used to produce gray levels may be 
altered. Thus, to accurately assess the corresponding relationship, the 
TRC must be measured frequently. 
In order to determine the TRC, a sample set of gray level specifications 
can be printed and the gray levels actually produced by the device can be 
measured by a device such as a densitometer. The measured points can then 
be fitted to a curve, this curve being the TRC. 
This method of determining the TRC, however, requires special equipment 
such as the densitometer to measure the gray level. Further, a skilled 
operator is desired to accurately assess the results of use of the 
densitometer. The special equipment and skilled operator are not always 
available for use with every printer. 
Accordingly, it would be advantageous to enable determination of the TRC 
without requiring the use of special equipment and/or a skilled operator. 
U.S. Pat. No. 4,941,057 to Lehmbeck et al. discloses a digital image 
proofing process. The processing arrangement allows a number of image 
processing techniques to be tested on a single sample sheet at a single 
time. A sample segment of the image may be selected for testing, and 
successively repeated in a sample output for the number of image 
processing techniques to be tested. The proof pages are printed on the 
printer, with the output of the proof page demonstrating the system 
operation with the desired image processing technique. The reference 
provides no disclosure of calibration of a printer. 
U.S. Pat. No. 3,489,849 to Hedger discloses a facsimile test pattern 
generator. The method consists of a test set for testing the operation of 
a system adapted to reproduce and to record a facsimile in response to 
receipt signals. The method of the reference also provides no disclosure 
of a calibration method for a printer. 
SUMMARY OF THE INVENTION 
The present invention provides a method for calibrating a printer using a 
tone reproduction curve for the printer. A plurality of bands of different 
accurate gray level standards and a corresponding number of smoothly 
increasing sequences of gray level specifications for the printer are 
printed adjacent to one another. Adjacent bands and sequences are 
compared, and a point along each band where the band appears equal to the 
adjacent gray level specification is determined. The points are 
graphically depicted and connected to form the tone reproduction curve for 
the printer. The tone reproduction curve represents the correspondence 
between the gray level specification of the printer and the actual gray 
levels produced by the printer. Thus, the tone reproduction curve can then 
be used to calibrate the printer, the calibration being performed without 
the use of any special measuring equipment. The calibrating method of the 
present invention can also be used for color printers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Gray levels can be produced in a variety of manners on a printer. For 
example, gray levels can be produced by printing a fine pattern of black 
and white dots (halftoning), printing a coarse pattern of black-and-white 
checks, printing a coarse pattern of black-and-white stripes, etc. 
Printed coarse patterns such as black-and-white stripes and black-and-white 
checks can be highly visible when viewed from a normal viewing distance 
and thus be unacceptable for normal printing. However, when viewed from a 
greater viewing distance, such coarse patterns can yield a very accurate 
gray level since the coarse patterns avoid small isolated black or white 
areas and because the coarse patterns have a low perimeter to area ratio. 
Thus, if a coarse pattern could be found which accurately produces a 
desired gray level, then one could compare the grays normally produced by 
the printer against the accurate coarse pattern gray level standard to 
find a point on the tone reproduction curve for the printer. This is the 
theory of the present invention. 
As illustrated in FIG. 1, such a comparison can be made by printing 
smoothly increasing sequences 14, 18, 22, 26, 30, 34, 38, 42, 46, 50 and 
54 of gray level specifications next to bands 12, 16, 20, 24, 28, 32, 36, 
40, 44, 48 and 52 of the coarse pattern gray level standard (e.g., striped 
standard). The sequences and bands are printed such that one smoothly 
increasing sequence of the gray level specifications is adjacent to one 
band of the gray level standards. 
When viewing FIG. 1 from a distance, the patterns appear gray in color. The 
varying gray level in the sequences 14, 18, 22, 26, 30, 34, 38, 42, 46, 50 
and 54 appears to progress from lighter than the corresponding standards 
12, 16, 20, 24, 28, 32, 36, 40, 44, 48, and 52 to darker than the 
standards. For each adjacent pair (e.g., pair 24 and 26), a point where 
the grays appear equal can be determined. Such a determination can be made 
subjectively (e.g., visually) or objectively. The position of the 
crossover point provides an indication of which gray level specification 
on the printer provides the same gray as the coarse pattern standard. 
FIG. 1 illustrates the technique of the present invention used for the 
calibration of the black color separation of a printer. For the particular 
printer, it was found that vertical stripes of at least two pixels in 
width produce accurate gray levels. FIG. 1 illustrates the test chart 
comparing the halftone gray level specifications against striped patterns 
for 1.0, 0.9, 0.75, 0.67, 0.6, 0.5, 0.4, 0.33, 0.25, 0.1 and 0 gray 
levels. From the test chart, for each of the gray levels, a point can be 
determined where the grays for each adjacent pair of gray level 
specifications and coarse pattern standards appear equal. One can then use 
well known interpolation techniques to find approximate values for the TRC 
at gray levels which lie between those measured. 
As illustrated in FIG. 2, a graph of the luminance for the printer (i.e., 
the TRC), the points for each of the gray levels can be graphically 
depicted. These points are graphically depicted as the encircled points. 
FIG. 2 also illustrates the luminance for the printer as measured by a 
Gretag colorimeter. As illustrated in the Figure, the luminance as 
measured by the crossover points of the present invention very closely 
approximates the luminance measured by a Gretag colorimeter. Accordingly, 
the present invention enables determination of the accurate TRC with out 
requiring the use of any special equipment. Further, by connecting the 
crossover points, the TRC for the particular printer can be determined. 
While the invention discloses adjacent printing of a plurality of bands of 
gray level standards and a corresponding number of smoothly increasing 
sequences of gray level specifications, alternative manners of providing 
the standards and specifications can be performed. For example, templates 
can be provided of the gray level standards, and the templates can be 
positioned adjacent to the gray level specifications. Accordingly, the 
gray level standards would not have to be repeatedly printed. 
While this invention has been described in conjunction with a specific 
embodiment thereof, it is evident that many alternatives, modifications 
and variations will be apparent to those skilled in the art. For example, 
although the invention has been discussed with respect to black and gray 
calibration, the technique of the present invention can also be used to 
find the TRC's for other color separations. One can, for example, print 
and compare patterns with cyan ink instead of black ink. The crossover 
point determined by the present invention would then indicate which cyan 
level index is needed to produce a particular amount of cyan coverage. 
Accordingly, the preferred embodiment of the invention as set forth herein 
is intended to be illustrative, not limiting. Various changes may be made 
without departing from the spirit and scope of the invention as defined in 
the following claims.