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Colour Bars - the devil is in the detail

Have you ever wondered what those colour patches on printed packaging are used for? These multicoloured strips have raised in prominence recently in our supermarkets and the world of FMCG (Fast Moving Consumer Goods). They seem to be slowly migrating from the hidden depths of carton flaps and on onto more visible areas, becoming increasingly hard to ignore.

So, what are these mysterious colour bars? Well, one could say they are a small yet mighty hero in colour critical print. Colour bars are a complexity in themselves; not all colour bars have the same function and they are placed on packs and labels for a variety of reasons.

Colour Bars Explained

Let us start with the basics:

This strip shows the 4 process colours of Cyan, Magenta, Yellow and Black (CMYK) printed as ‘solids’, i.e. 100% coverage, as well as 50%. These are basic patches that help the printer to make sure the ink is within tolerance for colour on the substrate that is being used. It also provides information about the ink ‘weight’ which can be interpreted as ink film thickness – ink is normally laid down on the substrate approximately 1-2 microns thick for lithography, 2-3 microns thick for flexography. The thicker the layer the more vibrant the colour.

This second strip show additional tints as well as solid colours. In this case the tints are printed at 25%, 50% and 75% coverage and this provides some more detailed information for the printer. Printing tints provides information about TVI (or Dot Gain) which is one of the most important variables to be controlled in any printing process. Generating TVI curves is necessary for the correct generation of printing plates for a press. Providing three tint values helps to generate points on the curves whilst software interpolates the intermediate points.

In each of the two methods above, CMY and K are printed separately, and they are used by printers for Process Control - ensuring a process is predictable, stable, and consistent.

However, for mixed colour images, such as the one shown here, a representation of the blended process colours is missing in these basic strips. It is therefore impossible to understand or measure what is happening in colour critical areas of these mixed colour images.

Whilst process control is useful for an operator, to ensure that the press is not deviating too far from an acceptable setting, there is no information here to provide an understanding of how images will look and whether the piece of print conforms to a Colour Standard (ISO12647, G7, etc).

It is probably worth considering what a printer must contend with at this point. Printing is not an exact manufacturing process like, for instance, making automotive components on a Computer Numerical Control (CNC) machine. Whilst modern presses now incorporate an abundance of computer technology, printing still involves the laying down of ink on a substrate and the many variables involved in that process – ink and chemistry, mechanical wear and tear of the press, different press operators, as well as the climate – variations in heat and humidity for instance. So, unlike a CNC machine repeatability on a printing press can be hard to control.

Here at Mellow Colour we provide the tools to help the printer to take control of these variables. In fact, the more tools the printer has to keep his ‘finger on the pulse’ of his press the better. Process control is just one of those tools and helps to indicate if the printing press or the consumables being used are behaving as expected.

However, process control is not the whole story. There are other tools that can help printers to stay on top of the process and take control of those variables.

This third strip, shown above is the PrintSpec strip from Mellow Colour. This strip contains all the elements used by printers for Process Control plus the following:

Three and two colour solid Overprints printed as Black, Red, Green and Violet patches.

These important patches show how well inks are printed on top of each other, hence the term overprints.

There are also three patches of grey made from C, M & Y and these provide a great deal of information about visual appearance.

The human visual system is extremely critical of slight colour shifts in near neutral and neutral colours - therefore we like to use 3-colour greys to manage this important aspect of printing.

Consequentially if the press is printing to an agreed Colour Standard and these 3-colour greys are within tolerance, it is safe to say that colour appearance will be acceptable.

The primary purpose of this more complex strip is to provide an understanding of how well a piece of print conforms to a pre-agreed Colour Standard. Measurements taken from the strip enables advanced software, such as Mellow Colour’s PrintSpec to provide a score for conformance to the agreed Colour Standard. The software uses complex algorithms to analyse conformance and print run variability and rate it with a score. This score can be reported back to the customer to reassure them that their pack, label, book, magazine or brochure will look acceptable and consistent in store.

One Strip, many benefits.

Providing a conformance report to the brand is very important – it is standard practice in other industries where quality assurance reports are the norm.

Interestingly, the information generated from this strip can also provide a huge benefit to the printer who can use it to diagnose issues on their press. Such issues can be as simple as an ink being laid down too thickly, to indicating a mechanical issue on one of the press units. Indeed, this print quality data can form the basis of a Print Quality Management System within a printing company and it is the Mellow Colour approach to use this as the basis for a Continuous Improvement approach to production, reducing waste and time in the manufacturing process.

In fact, a well implemented Print Quality Management system can reap many awards for brand and printer alike. Backed up by industry accreditation such as ISO12647, G7 or another pre-agreed Colour Standards, it can help to form strong partnerships between the two parties. Effectively implemented, a Print Quality Management program can also greatly reduce the need for press passes and couriering physical proofs and approval sheets thus removing cost from the supply chain.

So those mysterious strips, patches and blocks of colour on packaging and print are there for a variety of very important reasons. In their simplest format they are an important part of the manufacturing process. The more complex strips are the ones that have come to prominence in recent years and they are there not only to help to reassure the brand that their packaging meets an agreed Colour Standard, but as the basis of a Print Quality Management system for the brand right across their range of their printed products. They can also help the printers to monitor their production processes - just like holding a doctor’s stethoscope to the press - and be a driver for continuous improvement.


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