Technologies

Color Management

July 12, 2011

Technology has moved color reproduction into the realm of science, where the right tools are available to achieve consistency throughout the workflow.

Color Management


Technology has moved color reproduction into the realm of science, where the right tools are available to achieve consistency throughout the workflow.



Managing color means that the precise color output for all devices involved in the reproduction of an image are known and used to control that reproduction. These devices, all of which have their own color characteristics, include cameras, scanners, computers, monitors, proofers, and printing presses. The primary goal is to control color, to reproduce it as precisely as possible across the spectrum of devices so that the finished product looks the same as the original.

Cambridge in Colour (www.cambridgeincolour.com), an educational community for photographers, makes a delightful comparison of color management to the consumption of hot peppers, making it quite clear that the mere number of a color does not guarantee that it will look the same everywhere:

“Let’s say that you’re at a restaurant and are about to order a spicy dish. Although you enjoy spiciness, your taste buds are quite sensitive, so you want to be careful that you specify a pleasurable amount. The dilemma is this: Simply saying ‘medium’ might convey one level of spice to a cook in Thailand, and a completely different level to someone from England. Restaurants could standardize this based on the number of peppers included in the dish, but this alone wouldn’t be sufficient. Spice also depends on how sensitive the taster is to each pepper:

“To solve your spiciness dilemma, you could undergo a one-time taste test where you eat a series of dishes, with each containing slightly more peppers. You could then create a personalized table to carry with you at restaurants which specifies that 3 equals ‘mild,’ 5 equals ‘medium,’ and so on (assuming that all peppers are the same). Next time, when you visit a restaurant and say ‘medium,’ the waiter could look at your personal table and translate this into a standardized concentration of peppers. This waiter could then go to the cook and say to make the dish ‘extra mild,’ knowing all too well what this concentration of peppers would actually mean to the cook.

“As a whole, this process involved (1) characterizing each person’s sensitivity to spice, (2) standardizing this spice based on a concentration of peppers, and (3) being able to collectively use this information to translate the ‘medium’ value from one person into an ‘extra mild’ value for another. These same three principles are used to manage color.”

The growing complexity of technology in the printing industry has caused a more scientific approach to replace the craftsmanship of former days. Printing is no longer simply an art, but a measurable means of transferring digital images to ink on paper. Good color reproduction requires sensitivity greater than that which the human eye can achieve.

Consistency in color reproduction is critical. To arrive at the point at which color can be predicted throughout a system requires the removal of variation through process control, established through the careful profiling of all devices in the reproduction chain.

The proper control of color, and of printing in general, starts long before the image is being manipulated into a plate. All products that come into the plant should be certified by their suppliers as consistent, be they plastic films, foils and paper substrates, inks and coatings, photopolymer plate materials, and so forth.

Similarly, process control means adhering to standard procedures at press side. Anilox rolls should be clean, inks should be checked for proper balance and viscosity, web tension adjusted for optimum performance. Such procedures should be standard for all shifts, otherwise color management will become just another good idea that didn’t work.

FIRST 4.0 has a clear description of color management systems and the necessity for the use of color spaces that are independent of any device. FIRST – Flexographic Image Reproduction Specifications & Tolerances – is a publication of the Flexographic Technical Association that provides detailed guidelines for accurate flexo reproduction.

“Color Management Systems (CMS),” according to FIRST, “are a collection of software tools that quantify and reconcile the color differences between various color output devices (such as monitors, scanners, proofers, and printing presses) to help ensure consistent color throughout the reproduction process. Typically, the available color gamut of the printing press is smaller than other devices further upstream in the process, like proofing systems and displays. A CMS will identify the colors outside the gamut of the printing press and limit, or map, the colors on larger-gamut devices (proofer, display). Therefore, colors that can’t be reproduced on the press are not generated. This creates a realistic representation of the printed result at the proofing stage.”

FIRST goes on to explain the benefit of managing color using the CIELAB space. CIELAB refers to a standardized three-dimensional color model with coordinates that describe three characteristics of a color – L representing lightness, A representing where the color is on the red-green axis, and B representing the yellow-blue axis. It is an objective, device-independent way to define a color. (Device-independent means the color can be defined without the color-rendering capabilities of any specific device, such as a monitor, scanner, or printing press.)

“CIELAB-based color management uses the CIELAB color space to quantify color, independent of the device used to produce that color. Colors are numerically defined; this allows the use of mathematical models and algorithms that permit color conversion based on the measured color capabilities of an identified process. A device-independent color space enables conversion from one ingredient-based color space to another. Examples of ingredient-based color systems are RGB and CMYK. RGB color ingredients refer to phosphors in a monitor, dyes in a transparency, photomultiplier tubes in a scanner, or charged-coupled device array processors in a digital camera.

They can all have the same numerical RGB value, but not match because of the different ingredients. CMYK color ingredients include toner powders or dies in digital proofs, wax in thermal proofs, and different pigments in solvent, water and UV inks. The device-independent model produces the closest color match when moving from one ingredient-based system to another. Both input and output color ingredients are specifried colorimetrically, using a spectrophotometer, and are easily compared for conversion between ingredient-based color systems.”



The profile
A critical concept in color management is the profile. A profile can be thought of as a highly detailed description of the relationship between device color numbers and real-world colors for a scanner, camera, monitor, or printer. The profile also describes the gamut of a device – the range of colors the device is capable of capturing, displaying, or printing. The boundaries of a device’s gamut envelop what is referred to as the device’s color space.

An ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards set forth by the International Color Consortium (ICC). Every device that captures or displays color can have its own profile. Some manufacturers provide profiles for their products, and there are several that allow end users to generate their own color profile, typically through the use of a colorimeter or a spectrophotometer. The ICC defines the format precisely but does not define algorithms or processing details. This means there is room for variation between different applications and systems that work with ICC profiles.

Every color device has its own color space and gamut. No two devices – even if they are the same brand and model – display colors exactly the same way. Monitors are notorious for color variations, and tend to get worse over time as their light sources grow old. Printers tend to be more consistent, changing usually because of the inks or toners used. Scanners vary: LEDs last longer than fluorescent lamps.

Profiles accurately document the relationship between device color numbers and standard colors, making it possible to capture, display, and print colors consistently using devices from many different manufacturers. To print a certain shade of blue, the printer profile will tell you the color numbers to send to the printer to get that shade of blue. A monitor profile will tell you what color numbers to send to the monitor to make it display that shade of blue. Color managed applications like Adobe Photoshop utilize monitor and printer profiles to ensure that monitor and print colors match.

A color management system makes use of two profiles: input and output. “The input profile represents the colors desired, and the output profile represents the colors achieved on the device being used,” according to FIRST. “For example, compare an image on a computer monitor and the same image printed on a desktop printer. The input profile is the profile of the monitor (an RGB profile), and the output profile is the profile of the desktop printer (a CMYK profile). Converting between the input profile and the output profile requires a CMM (Color Matching Module). The CMM, sometimes called the ‘color engine’, is the part of the color management system software that maps one color gamut to another. A CMM uses device profiles and render intents to convert the look-up tables between devices. The CMM does this by mapping the out-of-gamut colors into the range of colors that can be produced by the output device.”

The beginning step in calibrating the color management system is to characterize the press. This is accomplished by preparing plates that feature the IT8.7/4 characterization target. “The result of this pressrun creates the press ICC profile (characterization). The accuracy of this pressrun is paramount to the successful implementation of color management,” says FIRST.

The guidelines also state, “In order to realize the benefits of a color management system, the printing process must first be in control. Gray balance, dot gain, density, ink trap, and print contrast, along with other print attributes, must be routinely monitored and controlled. If it is unclear what the specifications are, or if they have been achieved, color management is not appropriate at this time.”



The IT8.7/4 random-layout characterization target contains combinations of CMYK data placed at random so that ink takeout across the target area is more uniform.

Color matching modules
Prepress specialist EskoArtwork produces the Color Engine, which enables color management across the packaging and print supply chain. At its core is a central color database for all color critical data.

Color consistency is critical in brand recognition. In combination with the color database, the Color Engine Pilot provides a tool set for creating and maintaining brand colors, creation of spot color profiles and other color conversion settings. It maintains consistency throughout the production environment by ensuring that every application and operator works with the same color critical data.

The Color Engine utilizes spectral color information, which means that the user can specify the required light source when managing color. EskoArtwork maintains that “it is now possible to manage color for every device used within packaging and print production, including monitors, proofers, press, digital press. The Color Engine also makes it simple to convert between different color spaces: RGB, CMYK, spot or multicolor.”

Kodak’s Prinergy workflow system offers color management tools for the prepress specialist. It allows color definitions in a variety of calibrated and uncalibrated color spaces, which must be transformed to the color space of the output device before output. Prinergy’s Color Management is a tool that does the color conversion from all the defined color spaces to that of the desired output device.

When Prinergy’s Color Conversion box is selected, color management is performed during refining, separating the input data for press so that the refined PDF contains the color needed within the digital master for susequent trapping and final output. The color information is provided in the form of profiles, which can be already appended to input files or applied during refining.

In addition, color management is used during proofing to make the proof simulate the final output to press. This can work only if the press and proofer are consistent and calibrated.

Kodak says that because of Prinergy’s process automation, the calibrated and profiled color management process requires no adjustment for typical jobs. Specialized jobs might require desktop tonal adjustment to obtain results comparable to conventional closed loop color control.
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