Published: 19 September 2018 | No comments yet
Baldwin Vision Systems DeltaCam
Colour management is critical for pleasing the customer by achieving the agreed colour targets set for a job. Sean Smyth looks at how it can be a struggle for flexo printers to keep everyone happy by the very nature of the process being used.
An annoyingly recurring feature about printing in my experience is that it is impossible to break the laws of physics. And following the pronouncements of Isaac Newton and his counterparts back in the day there are lots of physical laws on optics and light. It is annoying because customers usually want more – brighter, cleaner, more colourfast and critically it has to match the particular colour standard associated with the brand. This may not always be possible in the flexo universe, hence the widespread use of spot colours.
Humans see colour through certain light sensitive cells at the back of the eye (well, actually it converts light incident on the retina into nerve impulses that the brain interprets as seeing). A typical human eye contains 6-7 million cone cells and around 120 million rod cells. These light sensitive photoreceptor rod (colorimetric) and cone (intensity) cells only detect red, green and blue light. All of the colours and brightness we see results from the brain interpreting this mass of input. Printing works by the ink absorbing part of the overall light, removing some of the red/blue/green light which is seen as the particular colour. Down to the pigments
Process colour printing uses cyan, magenta and yellow in various mixes to represent all of the colours the eye can see. Each of these pigments selectively absorbs light, magenta absorbs green, cyan absorbs red and yellow takes our blue. Add black to provide some modelling and dirtiness, not to mention the benefit of printing text, and you should be able to reproduce all colours the eye can see, the light reflected from the surface. However, this is where those pesky laws of physics come in. The pigments used are not colour metrically pure, so they do not fully absorb each portion of the light. So process colour printing can only reproduce a limited portion of what we can see, it is the role of the flexo converter to make the best of the possibilities. We have a small colour gamut that we want to represent a much wider gamut with, and this is where colour management comes in.
Colour management identifies the achievable colour gamut that a particular combination of press, inks and substrates can print, and then remaps the whole visible colour space into this. Humans have extremely good colour difference mechanisms, but lousy colour memory. They can see small differences, and the role of colour measurement is to squeeze the whole range of colour into the available colour space, so that the relative colour differences remain visible in the reproduction. However, sometimes this is not what is required and colour management can be set up to maintain the relative colorimetric values of in-gamut colours, at the expense of out-of-gamut ones that are heavily clipped, to avoid changing brand colours in packs.
Take the right steps
Implementing colour management requires several steps to be carried out. Probably the most important is commitment by management to allocate resources (press time, plates and materials) to set up the system, and subsequently to refresh. There are many readily available tools for colour management, a spectrophotometer, ideally with a plotter or scanner to read test patterns, and control software are required. Then there is a series of tasks following an increasingly well-proven methodology. This involves calibration of all parts of the pre-press including soft and physical proofs, characterisation through fingerprinting the standard printing conditions. Under standard conditions print a test forme containing a range of colour values (different combinations of the process colours, from 0% to 100% with multiple overprints). These colour patches will cover the range of colours that can be printed, from no ink to overprinted solids. They are measured using a spectrophotometer and software maps the record colour values to the dot percentages, resulting in the colour profile. The profile is a tool that allows software to convert any measured colour into a combination of the process colours.
BST’s iPQ-Spectral product for colour measurement
There are lots of providers selling components and systems, including output measurement and reporting tools while independent experienced practioners are widely available. The Esko Color Engine is widely used in flexo, with Pantone and X-Rite sister companies in the Danaher Group providing a very powerful set of modules, while it is compatible with most other providers as well.
In flexo a key component is the management of spot colours and conversion into process colours, including extended gamut fixed palette printing. Conversion is a key step in flexo quality, how native supplied files are transformed through a series of mathematical algorithms is important. Most systems use an ICC methodology, working in an RGB colour space that is converted into cyan, magenta, yellow and black, or a wider version with additional channels. A key test when choosing a system is to examine how transformations are carried out, by using some known images and comparing the final output on press, to see which provides the most accurate colour fidelity. An important issue to look at when doing a colour transform on CMYK files, usually supplied as a standard format is how the black channel is handled. As well as pure colour black may be critical in holding modelling information in the image and in pure RGB transforms this structure may be degraded. As colour transforms are not reversible there will always be an element of information degradation during a transform. So, a simple test of a system is to go from one standard to another and then do a further transform back to the first. Then compare the resulting file with the original side-by-side.
Usually colour management refers to controlling the artwork files, proofs and monitoring the press output with a spectrophotometer on-press. So ORIS Flex Pack from CGS provides an interface to Roland DG inkjet printers for pack proofs and prototypes with high levels of colour accuracy and cost savings against wet proofs.
Close the loop
Colour management can also operate as a closed loop control system on a flexo press. Users measure and report on jobs as they are printed, with certification available to prove consistency to clients. Press control equipment provider Baldwin has set up a new Vision Systems Segment within the company, after acquiring QuadTech and PC Industries, offering new colour and inspection technology.
Karl Fritchen, president of Baldwin Vision Systems, commented, ‘By combining engineering talent and expertise, with the infusion of more financial resources we are poised to make a significant impact on printers’ bottom lines. Our aim is to simplify workflows and help printers easily manage increasingly complex requirements and their ability to connect data and workflows to automate their processes.’ ColorTrack is colour management system designed for flexo converters, Baldwin terms it ‘A highly adaptive colour expert in a box’. It works with hand-held and inline spectral measurement configurations, the software facilitating real-time, accurate colour control, including spot colours when used. The software enables press operators to make colour corrections without ink reformulation, by suggesting adjustments such as ink strength or anilox changes.
Baldwin claims its in-line colour measurement system, DeltaCam, makes on press spectral measurement affordable. It helps converters to reduce time and waste, while maintaining colour throughout the job. It can be used in conjunction with other inspection systems, providing full inspection on all products. It adds colour control to the usual defect detection, such as missing elements, marks and ink spots, register variation, content issues and streaks.
The benefits of managing colour are two-fold; boosting customer satisfaction by providing high quality printed output that matches proofs and is consistent across and between runs, and; boosting operational efficiency in process colour printing in an optimised production system to minimise set-up times and waste (that can also provide customer cost reductions in some of the benefit is passed on). Ink optimisation can yield significant reduction in the amount of cyan, yellow and black to hit the same colours, reducing ink spend and making print control simpler.
Implementing a system takes a little time and there are costs, but it is not hard. It yields significant productivity boosts and happier customers. So, what are you waiting for?