Improving Productivity Through Quality


TruColor TG 4000 Series web inspection system with Automatic Color Monitoring Software

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 is compared
3. a means of detecting the light from the material being examined.


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 measurements.

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.