TG 4000 Series web inspection system with Automatic Color Monitoring
The only standard excepted worldwide today is the CIE color space
as set down in 1931 by the Commission Internationale de l’Eclairage
(CIE). The CIE standard color table represents a cross section
of this color space. However, the Yxy-units in this system do
not correspond to the way the human eye perceives color variations.
In 1976 the CIEL*a*b* transformation was passed and in this 3
dimensional color space you have a good correlation between measured
and perceived color differences. By mathematical transformation
of the CIE coordinates, the three-dimensional CIELAB color space
can be computed. This color space combines the advantages of mathematical
exactness with those of color locations that are perceived to
be equal in distance. This method is decisive in its practical
benefit in printing. Two aspects are worth mentioning first, it
serves to achieve an objective matching of color, independent
of light conditions and subjective perception of color. Secondly,
it serves pressmen an excellent tool for ensuring quality. Seeing
color is one thing- printing it is another. Choosing colors is
a subjective matter. Determining the tolerances for reproducing
the specific color in print calls for objective criteria. Only:
how is the printer supposed to communicate about color with a
customer so that both agree- and at the same time ensuring on
an exact definition of what they see? The language of densitomotery,
as generally used in modern-day control of printing processes
remains limited to the standard process inks in printing. It is
a fact that ink density measuring carries a disadvantage: it does
not evaluate color in the way the human eye does: it only allows
conclusions as to the ink film thickness. By definition a densitometer
measures thickness of a layer of ink. Again, the correlation between
density and perceived color is very poor, because it is based
on unrealistic requirements. Furthermore the correlation between
deviations measured with a densitometer and visually perceived
color deviations is very poor. The filters used in a densitometer
do not correspond to the spectral sensitivity of the human eye.
To make the evaluation of color variations independent of subjective
evaluation and changes in illumination you have to use Colorimetry.
For color matching to be objective and in accordance with perception,
spectral color measuring is a prerequisite. Just as a fingerprint
belongs unmistakably to a particular person, each color is physically
characterized by its finite place in the spectral wavelength.
The conversion of this spectral wavelength into a defined color
location in the CIELAB color space makes possible, with the help
of colormetrics, an objective comparison of colors that also corresponds
to an individual sensation of color. In the CIELAB system, differences
in color are expressed as Delta E units for color location differences.
Comparing colors consists of three steps:
1. a light source illuminating the sample and the standard
2. the sample to be evaluated and the standard against which it
3. a means of detecting the light from the material being examined.
MEGASCAN AUTOMATIC COLOR MONITORING (TG 4000 SERIES)
The reference image and the sample images are illuminated by xenon
strobe lights as the images pass under the camera. Each camera
must be calibrated for specific sensitivity to the three color
spectrums red, green, and blue as well as the white point. The
camera and related hardware produces an image in the RGB color
space. The software then transforms the image to the CIELAB color
space. During processing, the RGB to CIE XYZ step of transformation
uses the hardware calibration data to provide gamma correction
to the image. The CIE XYZ to CIELAB transformation adjusts for
the specific illumination used. The CMC difference is calculated
from the CIELAB values of the reference and sample images. The
CMC difference that will trigger an alarm condition is adjustable
by the user, as the perceptually visual color differences allowed
may vary. The output will be in Delta E units.
1. Measuring in a CIELAB color space means that colors are measured
as they are perceived by the consumer. It is not useful to work
with density, RGB, HIS or other units when deviations measured
do not correspond to human perception.
2. The printer is alarmed immediately to shifts in color and has
time to correct the printing process before variations get critical.
Waste is reduced. The finished product is more homogenous and
consistent. Can include w quality reports based on continuous
3. The system has better sensitivity than the human eye in most
cases and removes human error, subjectivity, and fatigue from
the equation when used on a fast running press.
4. The system is not disturbed by changes in the illumination.
It constantly and automatically monitors the printed colors where
as ambient light might have a negative impact on human perception.
MegaScan Automatic Color Monitoring Summary
Color Alarm measures the amount of change that a color has made
since the operator entered it into the system. It can generate
logs (time, date, etc.) useful for measuring performance. The
monitoring software uses the CIELab coordinate system and the
CMC color difference equations to measure the changes. The operator
sets up the ink to give the correct color, enters these into the
system. During a program scan sequence the system automatically
monitors the colors for changes against the pre-set standards.
The system is capable of monitoring 8 different banks of color
at 8 different locations on the web. In other words, during a
single program scan sequence the TruColor TG 4000 Series can automatically
monitor up to 64 different colors. The operator in terms of Delta
E sets tolerances values. This is the same value used by desktop
colorimeters commonly found in print shops today. A Delta E of
1 is the borderline tolerance that the human eye is able to detect
a color shift/change. The alarm function has a range of Delta
E values from 0 - 10. The alarm output presents a graph to the
operator to show the trending of color change. When an out of
tolerance condition occurs, a flash warning is given on the touch-screen
control monitor, the camera stops it program scan sequence at
the color location and a highlight box outlines the color that
has shifted on the display monitor. If the system has an audible/visual
alarm beacon there is no flash warning on the control monitor.
However, the camera stops its program scan sequence (updates at
the location of the out of tolerance color) and the color is highlighted
on the display monitor.