Wide-format Printing: Understanding colour management

by all | 22 February 2016 3:11 pm

By Jim Raffel
When professionals in the wide-format printing sector hear the term ‘colour management,’ many immediately think of the process of creating custom International Color Consortium (ICC) profiles. The entire colour management process involves much more than that, however, including many variables that need to be understood and controlled.

For most shops in the industry, the goals of colour management include matching colours between devices, achieving accurate spot colours, repeating the same job over time and establishing a sufficiently wide gamut to produce enough colours to satisfy customers’ requirements. These goals, which all relate to colour consistency, can be reached through the following three-step process:

1. Printer calibration.
2. Colour verification.
3. Process control.

With this process, it becomes easier to create customized ICC profiles that will continue to work well over time—but the ease of the process does not mean it is a simple one, as there are many concepts that need to be understood, proficiently managed and mastered.

ICC profiles
An ICC profile is like a digital ‘map,’ laying out how a given wide-format inkjet printer should place a combination of inks on a given substrate to produce colours to certain specifications. This is important because not all printers produce colour exactly the same way; those from different manufacturers use different ink and printhead technologies. And the variety of raster image processors (RIPs) and substrates available today can add to the complexity of achieving consistent, repeatable colours.

In many cases, ICC profiles are available from printer and substrate manufacturers, ready for loading into RIP software. These are referred to as ‘canned’ profiles.

There are several areas of concern with canned ICC profiles, however, especially in terms of ink restrictions. As temperatures and humidity levels differ widely from region to region, canned profiles are designed with ink restrictions low enough to work in any environmental conditions, but these may not be ideal for a given sign shop’s production department.

There may also be issues with regard to the ink and substrate used to develop a canned profile. A signmaker may be using the same substrate and printer, but a third-party ink. It is highly unlikely the printer manufacturer or even the substrate supplier would create a canned profile using that ink, so the results could be unpredictable. Even different inks of the same type can have distinct properties that will result in colour differences in the finished image, especially if those inks are from different suppliers.

The same is true of the media. A single, generic, canned profile may be set up specifically for glossy vinyl, for example, but there are hundreds of different glossy vinyls available from various suppliers. There is simply no way a single profile can take into account all of the varying surface characteristics of those materials.

Indeed, a single ink laid down exactly the same way on different media will not appear the same. The microscopic characteristics of each substrate’s surface will affect how that substrate accepts ink and, thus, its visual appearance.

For these reasons, the best option for ensuring a common visual appearance across various printing platforms is to develop customized profiles using a sign shop’s RIP software’s own colour management capabilities, coupled with a spectrophotometer for measuring colour targets. While this is not a simple process, it can become relatively easy and repeatable with a bit of training and practice. And the return on investment (ROI) will be high, not only because it will be easier to repeat jobs for ongoing clients with more predictable and consistent results, but also because if ‘colour drift’ does occur, the sign shop will have the in-house tools to fix any problems quickly, without having to bring in outside resources.

Eyesight and colour theory
Even colour management experts know their eyes can be fooled when it comes to matching. They can become tired after a long day, just like the rest of the body, so they will perceive colours differently in the evening than in the morning.

For that matter, several forms of colour-blindness will typically affect nine to 10 per cent of men (but almost no women). Other conditions that can affect how the eyes perceive colours include ambient lighting, the surrounding colours and the physical proximity of those other colours.

There are also important differences between how computer monitors display colours and how printers output colours. On a computer screen, colours are created using an additive process. That is to say, the screen starts out black and then colours are added to it through illumination of red, green and blue (RGB) pixels. Various combinations of these three colours create the entire gamut of the display.

A wide-format inkjet printer, on the other hand, creates colours using a subtractive process. The substrate starts out white and then inks are laid down on it to block light from reflecting back to the observer’s eye. Through printing combinations of cyan, magenta, yellow and key/black (CMYK) inks, less of the white material becomes visible. It may seem counterintuitive, but in theoretical terms, all of the colours of the job are in that white material to begin with.

Fortunately, technology can overcome the shortcomings and frailties of human eyes and the differences between additive and subtractive colour creation processes. Instead of building an ICC profile based on ‘measurements’ taken by eyesight, for example, a spectrophotometer is used to measure predefined colour swatches output by a printer or displayed on a monitor, taking subjectivity out of the process of measuring and reporting colour values.

A light booth should also be used when viewing colours for final approvals. This allows viewing conditions for the sample to be carefully controlled, including the viewing angle and illumination. A light booth can simulate daylight, store lighting or even incandescent residential lighting.

It is also important to use a high-quality computer display that can be calibrated. When a monitor is properly calibrated for colour, it will become possible—under controlled viewing conditions—to simulate finished print colours. This is no small feat, as it combines elements of both additive and subtractive colour processes, but with the proper colour measurement software and equipment, such calibration can be completed by just about anyone within a matter of minutes.

Printer calibration
The creation of a custom ICC profile should begin with the calibration of the printer. One way to achieve this is to use the linearization functions built into the RIP software, but in most cases, these functions do not linearize to a known standard printing condition. This can mean a problem after the custom colour profile has been created, as when the colour drifts, it is impossible to get back to the exact baseline printer calibration that was used to build the profile in the first place. The solution to this problem would be to relinearize and reprofile, a process that can take upwards of an hour to complete.

An alternative approach is to replace the RIP’s own linearization routine with a grey-balance calibration routine, such as the International Digital Enterprise Alliance’s (IDEAlliance’s) G7 methodology (see Sign Media Canada, April 2015, page 70). There are several significant benefits to this approach. First, the baseline printer output—even without a custom profile—will be grey-balanced and visually pleasing in terms of perceived colours. Secondly, since the G7 method achieves grey balance by calibrating the printer to a set of published standard aim points, it is possible to hit the same targets again and again.

There is also a benefit when using a calibrator tool that automatically generates ink restrictions to achieve G7 calibration. This way, when creating profiles in different locations, it will be easier to achieve very similar results, because the setting of ink restrictions is handled by software instead of human interpretation.

With both a custom profile and automatic ink limiting in the printer’s calibration software, it becomes possible not only to ensure precisely the right amount of ink is laid down on the substrate, but also to maximize the printer’s gamut so as to hit custom spot colours. And when colour drift occurs with a custom profile and a G7-calibrated printer, the profile can be recalibrated and rebuilt without having to print and measure an extensive custom profile target, turning a one-hour-plus process into a 15-minute fix.

Colour verification
One way to detect colour drift is visual observation, but this is usually not sufficient. By the time colour has drifted far enough to be noticed by the human eye, chances are the sign shop has already shipped poorly colour-matched prints to its clients.

Rather than wait until colour drift is visually noticeable, a spectrophotometer should be used periodically to measure a verification target. This will allow the shop to ensure it is still producing colour at qualities defined by an industry or in-
house standard.

A verification target can contain enough colour patches to enable evaluation of conformance to the G7-related ‘known good print conditions’ and updated printer calibration if a colour has drifted too far. The updated calibration can be imported into the RIP software to help generate an updated profile. Then, another verification target can be printed and measured to confirm the adjustments were sufficient to bring the colour back into specifications.

Process control
The key to the success of the G7 methodology is process control. Just as coffee shops control such variables as the power of hydrogen (pH) level of their water to ensure their products taste the same across many locations, so too can sign shops maintain in-house processes at a consistent and repeatable manner.

Most of the ‘hard work’ of process control is already done once a sign shop has generated its baseline printer calibration and begun measuring its verification targets on a regular basis. The last step is to collect all of the measurements in a database for ongoing evaluation. Fortunately, many of today’s RIP software packages are designed for this purpose and will proactively provide the evaluation results. By taking note of trends in printed output, it even becomes possible to predict colour failures and recalibrate a printer before a verification target fails an inspection by spectrophotometer. The goal, after all, is to prevent failures before they happen.

Improving colour consistency
In these ways, small investments in software, measuring instruments and training can achieve better colour consistency for sign shops creating wide-format printed graphics. And these costs can quickly be recovered.

For one thing, the time and effort that would otherwise be spent on the phone with suppliers trying to track down a canned profile to get the desired colour out of a given ink-and-substrate combination can now be saved by developing and using custom ICC profiles. For another, a shop that follows through with regular colour verification measurements, ensuring all graphics going out the door showcase consistent, repeatable colour, is much less likely to have customers complain that one print run does not match another. Indeed, many such shops find clients bringing them more work to print.

Successful colour consistency begins with the creation of custom ICC profiles, continues throughout a workflow based on printer calibration, colour verification and process control and benefits from ongoing analysis over time.

Jim Raffel is co-founder and CEO of ColorMetrix Technologies, which develops cloud-based, cross-platform printer calibration, colour verification and process control software for wide-format inkjet printing. This article is based on a session he presented at the Specialty Graphic Imaging Association (SGIA) Expo in November 2015. For more information, contact him via e-mail at raffelj@colormetrix.com[1] and visit www.sgia.org[2].

Source URL: https://www.signmedia.ca/wide-format-printing-understanding-colour-management/

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