The following is a brief description of some of the measuring methods in use and their benefits, from the manufacturers’ point of view.

Andreas Dau, Head of Product Management, Research & Development, EAE:
The EAE loop measuring system uses CMYK-based colour measurement that measures both in the visible  light spectrum as well as additionally in the IR range. The colours cyan, magenta and y are measured in the visible light spectrum. But this spectrum is no longer sufficient to measure black, as a printed black or grey area can be produced either from a chromatic or achromatic composition. In both cases, a spectophotometer would display the same L*a*b*values, but it would be impossible to distinguish between them for measurement purposes. In order to be able to exactly determine the composition (wholly black ink or produced from a combination of CMY inks) of a black or grey balance, in the CMYK process, as used by the EAE loop system, measuring is done additionally in the IR range because it is only possible to detect pure black with certainty in the IR range.

In other measuring processes, e.g. based on RGB measurements, cameras are used. But these cameras cover only a very small part of the printed web. Accordingly, no colour measurement is carried out on the rest of the web where there are no  RGB cameras. When the measurement is done using a CMYK measuring head, the latter continually records across the entire web, so that all image parts are covered. The  advantage of the EAE process is it applies a continuous, extensive control across the complete page.   

Another major advantage of the EAE CMYK colour measurement process ist hat it makes the control of the water very simple, as the sensor that is used is able, for example, to recognise toning in the colour cyan on black text. This information is missing in the RGB or L*a*b* colour space.

The CMYK colour measurement process also differs from other measuring processes in that it works also in cases where no TIFF data with the necessary target data are available.   

If L*a*b* values are required for quality management and reporting purposes, e.g. to give evidence of the ability to produce quality to advertisers, the EAE system makes them available.   

Peter Leidig, Managing Director, Grafikontrol GmbH (German subsidiary of  Grafikontrol S.p.A., Italy)

The colour measuring process used by the Grafikontrol Colorscan system, according to  Peter Leidig, “works with a line-scan camera that is capable of recording pixel by pixel and thus composing the image. Colour LEDs are used for lighting in order to produce the corresponding spectrums. With Colorscan it is possible to obtain both densitometrical values and  L*a*b* values.“ The measurements can be done either by using corresponding print marks or without marks, depending on the operator’s preference. This makes the system suitable for use both in the newspaper and semi-commercial areas. If measuring marks are used, the minimum height of the mark is 2 mm.

Yvonne Smeekens, Marketing Assistant, Q.I. Press Controls BV 

The IDS scanner measures colours by using proprietary CCD sensors with mathematical Neural Neugebauer algorithms using ICC profiles and digital print to split colours in 32 different channels for real spectral colour measurements. These 32 channels creates colour values far within an accuracy of ∆E1 which is a demand to comply to ISO standards. The camera is the only one in the industry using LED D50 light, polarizer’s and ideal black packing as required for ISO standards. The use of polarizer’s allows measuring wet ink with the same values as dry ink. This provides the flexibility of reading the different colours due to the drying process of ink anywhere in the web lead. The scanner reads from a free-running web whereby the web should be within a range of + and - 4 mm from the ideal focus distance. It is evident that the scanner must be mounted exactly perpendicular to the web. The benefit of being flexible to measure ink anywhere in the web lead combined with the capability of the camera’s to measure from an unsupported web creates multiple advantages. The installation does not require additional mechanical hardware on press which prevents extra cost, installation time, maintenance cost, pollution in the print and waste due to extended web lengths. The light passing through the paper cannot be reflected by a supporting idler and therefore not disturb the measurement. Measuring from an unsupported web requires less camera cleaning since there are no air flow circulations with ink mist as appearing around idlers. The LED light ensures a long life span and an easy to remove Ink Mist Shield (IMS) simplifies cleaning.

The scanner moves automatically from side to side across the width of the web and measures the entire web. During this movement measurements are constantly taken and values passed on to the control system. Measuring an entire web of 1200 mm by 1600 mm on a press producing 40,000 copies per hour takes approximately 10 seconds. This time may vary depending on production speed, sheet size and image.

Using CCD sensor technology allows high resolution measurements. The IDS system measures with 200 DPI which allows to measure colour in small black text.

IDS is fully integrated, No operator intervention! IDS does not require additional training on how to load jobs or correct on colour and damp. Therefore IDS is simpler to use and less errors are made by the operator.

Vince Balistrieri, General Manager-Commercial/Newspaper, Director of Engineering, QuadTech, Inc

QuadTech’s Color Control System and Web Inspection System with AccuCam uses pre-press image files to create L*a*b* target aim point values. Its six-channel spectral sensor measures the printed web and calculates the L*a*b* values of the entire image, then brings the printed image to the specified L*a*b* target values and automatically maintains the color values throughout the production run. AccuCam’s six-channel spectral sensor provides accurate CIELAB measurements of each pixel from the print production. Next, by comparing these measurements against a target colour for that pixel, a colour error (ΔL*, Δa*, and Δb*, not a ΔE) is determined for each pixel. Given the ink coverage at each pixel, the system knows how the colour of each pixel will change with a change in inking levels. The system decides upon the inking level that comes closest to matching colour over all the pixels. Image-based colour control will generally require a compromise, but with AccuCam, the tradeoffs are weighed against each other based on how the eye perceives the colour difference.

Benefits of Image-Based Color Control using six-channel sensor:
- 100% of image evaluated
- Provides accurate L*a*b* values for better print-to-prepress file target image match
- Color move is best compromise over entire page, every pixel gets a vote#
- CIELAB space is preferable to CMYK space or RGB space for making trade-offs, since it corresponds to the way that we see color

RGB sensors can be used for image-based color control, but ultimately the RGB measurements must be converted to CIELAB values. This is a challenge and QuadTech as well as several others have written white papers on the subject of RGB to CIELAB conversons. The research has shown that accurate CIELAB values cannot be obtained from RGB sensors. The less accurate the CIELAB measurement values are the more they compromise the print-to-prepress file match. In addition, RGB sensors are not well suited to separate black from overprinted colors.

An RGB + infrared sensor would also be an option for image-based color control. The advantage of infrared is that it has the ability to separate out black from the other colors. This solution still relies on a conversion from RGB to CIELAB and assumes that the ultimate goal is to reach a specified amount of each ink. If that is the goal, then you clearly must determine the amount of each ink that is present. However, the preferred goal is to match the correct CIELAB value. To control to CIELAB value aims, you must measure CIELAB, and must measure it accurately.

The colour control systems of 3TControl and DCOS are both based on densitometry.

Sergio Muñoz, Head of Business Development, 3TControl

IPA uses micro-marks for each ink zone that are very small, practically invisible. The colour control is based on densitometry. “We measure ink density and control the press using the same and proven techniques that operators do manually. The automated solution is based on easy, proven and traditional methods that all printers conceptually would accept. Consequently, we deliver the highest level of transparency on the process opening the door to continuous improvement,” says Sergio Muñoz, Head of Business Development at 3TControl.

Mattias Andersson, Managing Director, DCOS

The CRC4 camera measures the density on micro-marks that can hardly be detected by the naked eye: One set of marks is placed in each ink key position. In each mark you have two dots with a diameter of 0,22mm. According to Mattias Andersson, Managing Director of DCOS, densitometric measurement has some benefits compared to colour measurement: “Perhaps the most important factor is that it is a less costly technology, it is a stable and proven technology that does not require advanced calibration procedures. Colour measuring systems normally require yearly calibration which easily gets overseen, and this is also very costly. 

DCOS works mainly on the newspaper side (coldset and semi commercial printing), and when we took our decision for this technology there were several color measuring system available (especially on heatset) but it was a known fact that it was very hard to find a reasonable payback time on a large newspaper press at the price level where these system are. With the densitometric technology and a multifunctional scanner (density, register, fan-out, dot gain & print defects) it creates a very attractive business case." And he adds: "Most of our customers like to follow IFRA standard and our system has a measure accuracy of +-0,05D”.

More information on closed-loop colour control systems (status 2011):