Spray-painting system with automatic color calibration

An automatic system for the spray painting of color images in which accurate color reproduction is achieved by a separate look-up table generated for each jet and stored in the computer memory. During the reproduction of an image, the computer uses the look-up tables to determine the pulse width of an air modulation stream to reproduce the precise color density called for by the input data. Because slight variations in the characteristics of two jets spraying the same color would produce objectionable lining in the image, a separate look-up table is provided for each jet. A color test image is generated for a selected jet by successively varying the width of the pulse modulation from the minimum to the maximum. For example, while the jet is making a single scan across the medium, the pulse width modulation of the air stream is varied in 256 separate steps to produce a test image that varies from the lightest to the darkest density of the particular color being sprayed. This pattern is scanned by a densitometer that delivers its output to the computer. The position of the densitometer along the color test image is correlated with the modulation pulse width that produced that particular color density and this information is stored in a look-up table for use during the generation of a reproduced image. A dark current is measured by the densitometer and subtracted from the calibration readings so that the calibration is independent of ambient illumination.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to automatic spray systems in which a number of 
spray heads traverse a medium to produce a color image in response to a 
coded input. In a particular system, an image to be copied is scanned 
optically and the signals from the scanner control the characteristics of 
each of the spray jets to accurately reproduce the image. Such spray 
systems are used in preparing outdoor advertising displays, for the 
reproduction of art work and for other applications. More particularly, 
the invention relates to an automatic spray system in which a test image 
formed under the control of a computer for each basic cole, is scanned by 
a densitometer and the information from the densitometer utilized for 
preparing a set of look-up tables that relate the spray characteristics of 
each jet spray head to reproduce the appropriate color density. 
2. Description of Related Art 
Many different kinds of automatic spray systems have been devised. U.S. 
Pat. No. 1,709,926 describes a system in which three separate jets, each 
spraying one of the primary colors, are individually controlled by 
electrically-operated valves to produce a blended color image. U.S. Pat. 
No. 3,553,371 describes a multi-jet spray system in which the flow of ink 
from each of the jets is controlled either by controlling the air pressure 
that produces the spray or by direct control of the ink supply. The medium 
to be sprayed is carried by a rotating drum and the jets are moved 
laterally across the face of the drum to produce a spiral pattern. U.S. 
Pat. No. 4,914,522 describes a multi-jet spray system in which the density 
of each color sprayed on the medium is controlled by pulse-width 
modulation of an air stream that atomizes the ink. These and many other 
systems for controlled spray painting of mediums have been devised. 
Throughout these developments, the accurate reproduction of colored images 
has remained a recurrent problem. In systems in which two sets of jets 
produce an interleaved scanning pattern, it is even more important to 
maintain precise control over the color density produced by each of the 
jets. 
The spray characteristics of the individual jets are quite different. 
Slight variations in the dimensions or shape of jet components influence 
the amount and format of the delivered spray. Moreover, the 
characteristics of a single jet may change during a period of extended 
use, for example, from the drying of ink on or adjacent to the jet. Any 
such variations cause a variation in the rate at which the ink is 
delivered to the medium and a change in the color density of the pattern. 
To reproduce a colored image with accuracy, it is necessary that each jet 
be individually controlled in such manner as to produce the prescribed 
color density. In systems in which two sets of heads scan alternate lines 
of the image, any variation between the operating characteristics of the 
two sets of heads will produce lining on the image. Thus, even if the 
adjustment is such as to provide acceptable color reproduction, the image 
may be impaired by the presence of lining in the image caused by the 
slightest variations in the spray characteristics of the two sets of 
heads. In an automatic spray system in which color density is controlled 
by pulse width modulation, it is necessary to have an absolute correlation 
between the pulse width and the resulting color density. 
SUMMARY OF THE INVENTION 
The present invention provides an automatic system in which a separate 
look-up table is generated for each jet and stored in the computer memory. 
During the reproduction of an image, the computer uses the look-up tables 
to determine the pulse modulation width necessary to reproduce the precise 
color density called for by the input data. Because slight variations in 
the characteristics of two jets spraying the same color would produce 
objectionable lining in the image, a separate look-up table is provided 
for each jet. 
A color test image is generated for a selected jet by successively varying 
the width of the pulse modulation from the minimum to the maximum. For 
example, while the jet, is making a single scan across the medium, the 
pulse width modification of the air stream is varied in 256 separate steps 
to produce a test image that varies from the lightest to the darkest 
density of the particular color being sprayed. This pattern is scanned by 
a densitometer that delivers its output to the computer. The position of 
the densitometer along the color test image is correlated with the 
modulation pulse width that produced that particular color density and 
this information is stored in a look-up table for use during the 
generation of a reproduced image. 
To compensate for variable ambient lighting conditions, a "dark" current 
measurement is made with no light reflected from the test pattern. This 
dark current is subtracted from the densitometer readings during the 
calibration run so that the calibration is independent of the particular 
ambient lighting conditions at the time of calibration.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown by FIGS. 1-3, a roll 2 of imaging material 4, which may be paper 
or vinyl or some other suitable sheet material, usually white, is 
supported by the frame 6 of the imaging system. The medium passes over an 
idler roller 8 (shown only in FIG. 1) around a rubber-covered drive roller 
12 and onto a take-up roller 14. 
The drive roller 12 is driven by a stepper motor 16 (FIG. 2) that 
incrementally advances the medium by successive scanning lines. A series 
of rollers 18, which press the medium 4 against the surface of the roller 
12, provide support for the roller 12 and prevent slippage between the 
roller and the medium 4. The take up roller 14 is driven by a suitable 
drive mechanism (not shown), of any of the well known mechanisms for 
applications of this kind, that transmits sufficient torque to maintain 
the medium 4 under appropriate tension. 
Two sets of ink jet spray heads, generally indicated at 20 and 22, are 
positioned adjacent the surface of the medium 4 where it passes over the 
drive roller 12. The heads 20 and 22 are supported by a carriage 24 that 
is slidably mounted on a rail structure 26 (FIG. 3) and is driven back and 
forth horizontally across the medium 4 by a motor drive 28 (FIG. 2) and a 
reversing drive cable 32. Ink for the spray heads is supplied from a 
remote storage center (not shown) that is connected by flexible supply 
tubes, diagrammatically illustrated at 34 in FIG. 3, to four small ink 
reservoirs 36A, 36B, 36C and 36D supported by the carriage 24. As used 
here and in the claims, "ink" means a color medium having either dyes or 
pigments as the coloring agent. Each of the four reservoirs carries one of 
four colors of ink, typically cyan, magenta, yellow and black, and is 
connected by the supply tubes 38A, 38B, 38C and 38D Go the ink spray 
heads. For example, as shown in FIGS. 2 and 3, the ink reservoir 36A, 
which carries the cyan ink, is connected by the tube 38A to the ink spray 
head 20A and to the ink spray head 22A. The other spray jets are connected 
by similar supply tubes to the reservoirs 36B, 36C and 36D. 
The general method of operation of the system is illustrated by FIG. 4 
which shows only two spray heads 20A and 22A. The other six spray heads 
operate in the same manner. A source 42 of constant pressure compressed 
air is connected to two modulation valves 44A and 44B. The valve 44A 
controls the flow of air to the head 20A by delivering short pulses of 
high-velocity air to the head. The air flows across an ink meniscus in the 
spray head 20A to draw ink from the ink reservoir 36A and atomize it onto 
the medium 4. The density of the color produced is varied by varying the 
duration of the air pulses delivered by the valve 44A which is controlled 
by a computer 46. This system of control by pulse width modulation of an 
air stream is described in U.S. Pat. No. 4,914,522. The air stream is 
modulated to reproduce an image being scanned by a conventional optical 
scanner 48 or to produce an image from previously recorded dam. The 
operation of the valve 44A is controlled in accordance with information 
from a set of color look-up tables 52 stored in the computer memory. The 
ink jet spray heads may, for example, be similar to those described in 
U.S. Pat. No. 4,914,522. 
To provide the basic data for the look-up tables 52, a single selected 
spray head is operated and is supplied with air that is pulse modulated in 
increasing pulse widths as the carriage 24 scans across the medium 4. For 
example, the pulse width may be varied in 256 separate steps so that the 
test pattern produced is a line of a single color extending across the 
medium 4 with increasing density. A number of identical scans are made in 
order to produce a test image of the desired vertical width. The result is 
a test image such as that illustrated at 54 in FIG. 5. 
To provide the calibration, a densitometer, illustrated diagrammatically at 
56, (FIGS. 4 and 5) is mounted on the carriage 24 and is connected to the 
computer 46. To perform the calibration, the densitometer is positioned at 
the point indicated at A in FIG. 5 and is then moved toward the right at a 
constant rate of speed. The densitometer passes first over a white section 
58 of the test image, in which no ink has been sprayed upon the medium, 
followed by an initial fiducial mark 62 of maximum density for the 
particular color being measured or some other dark color. This maximum 
density is produced by a series of spray pulses each of maximum duration 
with minimum time intervals between successive pulses. As the densitometer 
56 traverses the fiducial mark, the densitometer readings rise rapidly to 
a maximum value. Because the densitometer beam has a finite width, but 
narrower than the width of the fiducial mark 62, the density readings rise 
gradually to a peak value and then drop back to the original value as the 
densitometer traverses a second white strip indicated at 64. This 
information, in the form of digital numbers indicating the density 
reading, is fed into the computer 46 which then determines the physical 
position of the center of the initial fiducial mark 62. 
After passing over the initial fiducial mark, and the narrow white strip 
64, the densitometer passes over the test image 54 formed of 256 discrete 
steps of increasing color density. At the end of the test image, the 
densitometer traverses a narrow white strip 66 positioned between the last 
step, the darkest color density of the test image, and a terminal fiducial 
mark 68 followed by another white space 72. The mark 68 is similar to the 
initial mark 62 and is of the highest density and is of known width. The 
sudden rise in transmitted density readings indicates the terminal 
fiducial mark 68 and permits determination of the center-line of the mark 
68. To smooth minor variations in the readings, multiple scans of the test 
image may be made with identical parameters and the average of the 
readings used for each step of the test image. 
The foregoing procedure permits determination of the precise distance 
between the initial fiducial mark 62 and the terminal fiducial mark 68 
based on the known constant scanning speed of the carriage 24. Since all 
of the 256 color steps of the test image are of equal width, the density 
reading received by the computer can be correlated for each step of the 
test image 54. Because the modulation pulse widths of the air stream that 
produced each step of the test image are known, the color correction 
tables 52 can be constructed in which each reading of the densitometer is 
correlated precisely with the pulse modulation width required to produce 
that color density with the characteristics of the selected jet spray 
head. FIG. 6 shows a typical curve of the relationship between pulse width 
and the densitometer readings. 
To compensate for ambient light conditions and paper variability, a 
densitometer reading is taken at the beginning of each calibration program 
that measures the reflection from the medium while being exposed to 
surrounding ambient illumination. This paper density value is fed into the 
computer 46 where it is subtracted from the densitometer readings during 
the color calibration so that the color correction is independent of 
ambient lighting conditions. 
A separate calibration exactly like the one just described is performed for 
each of the eight jet spray heads resulting in a different look-up tables. 
When an image is scanned, the color elements indicated by the scanner 48 
are correlated by the computer 46 using the look-up tables 52 to produce 
the precise modulation pulse width required to reproduce the color of the 
image being scanned. 
For most applications, the test image should include at least 128 separate 
discrete color steps each forming one element of the look-up table. 
Preferably, the test image is formed of 256 separate discrete color steps. 
The densitometer used may be any of a number of commercially available 
densitometers and so is not described in detail here. Preferably, the 
densitometer measures the color density by reflected light and carries a 
pulsed light source so that the image, after correction for the dark 
current and ambient light, is always examined as under identical 
conditions of illumination. Typically, the densitometer makes a large 
number of individual measurements of each color sample and an integrated 
average of the individual measurements is transmitted to the computer to 
be used in creating the look-up tables. A stream of low pressure air may 
be maintained around the perimeter of the active area of the densitometer 
to prevent fogging of the densitometer lens or light source by the color 
mist from the spray heads.