Rip Technology, Colorimetry and Color Management in Digital Textile Printing

TEXTIL PLUS FACHARTIKEL

RIP TECHNOLOGY, COLORIMETRY AND COLOR MANAGEMENT IN DIGITAL TEXTILE PRINTING

Software and workflow topics are often underrepresented in the machine-dominated textile industry, but they have a critical influence on quality and profitability. Professional color management, for example, enables technology-agnostic printing of textiles or networked printing across remote locations.

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Inkjet printing on textiles has many advantages: there is no need to make printing forms or screens, which makes the process ideal for smaller runs and enables the economic production of personalized and individualized prints. Ink- jet-based processes are also preferred for printing photo- OLIVER LUEDTKE graphic images. However, there are also some challenges Dipl.-Ing. to be mastered: on the one hand, ink is absorbed by fabrics, Chief Marketing Officer so the textile or fabric must first be pre-treated. The differ- ColorGATE ent types of fibers, dyeing techniques and finishings of the DE-30171 Hannover base material provide a wealth of changing parameters as [email protected] well. Still, customers have high expectations of the print quality: despite the relatively uneven surface, they expect a de- The RIP: the data preparation hub tailed and well-resolved print with a large color gamut. In All these and more tasks are fulfilled by the RIP and color direct-to-garment printing, the substrate is often a dark or management solution. In the closer sense of the word, a RIP black garment. In such cases, the use of white ink is a basic («Raster Image Processor») is a software or hardware compo- requirement for the printing process, because the CMYK nent that converts print data into the output format of the inks used are not opaque but translucent. Without a white press. This is not trivial because the popular PDF format, underbase, the print would be hardly visible. Sometimes which is often used in prepress, stores information in a white logos, halftone designs or even white photos are resolution-independent way whenever possible. The letter T, printed on dark textiles, which requires control of the linear for example, is not defined as a collection of pixels but as a reproduction of tonal values. In textile roll-to-roll printing, «vector graphic» consisting of basic geometric shapes such on the other hand, printing is mostly done on white or at as lines, circles and cur ves. T he core task of a R IP is to convert least light-colored fabrics so textile roll-to-roll printing sys- such image descriptions into the actual print resolution and tems usually do not feature white ink. raster format of the attached printing system. In practice, In both direct-to-garment and roll-to-roll printing, addi however, machine operators call practically any software a tional colors such as red and green are often used for extend- «RIP» that is used to control the printing system – even if it ing the color gamut. This means that, together with white does nothing more than passing on existing pixel data, e.g. and the subtractive primary colors cyan, magenta, yellow from Photoshop, to the printing system more or less un- and black (CMYK), sometimes up to seven primary colors changed. Professional RIPs, such as the Textile Production- have to be managed. server from ColorGATE, are characterized by the following

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the next steps, such as the creation of color profiles, become difficult. Hence, when linearizing the printing system with a pro - fessional RIP, the first step is to print a tonal value wedge and to measure the resulting density values. The RIP uses this to calculate a transfer curve that ensures, for example, that a color field with 50 % black is actually printed with half of the attainable area coverage.

Color needs to be managed Once this step is completed, the next challenge is tackled: color management, which is traditionally a core function of a RIP. Color management in textile printing follows the same approach that has been used for decades in the (paper) printing industry: it is based on a specification by the Inter- national Color Consortium (ICC). Important for understanding this concept is the distinc- tion between device-dependent and device-independent color. Common color specifications such as RGB and CMYK A RIP converts vector data into pixel data. are usually device-dependent. This means that one and the same RGB value produces different colors on different hard- features, among others: ware. Everybody who has ever compared the advertising ■ Extensive linearizat ion f unct ions for ensuring a dy namic, screens hanging over a row of supermarket check-out desks well-graded ink application on the textile knows this phenomenon: a red that should actually be the ■ Creation and flexible application of color profiles, in order same is sometimes paler, sometimes more intense, some- to factor in the color characteristics of the input and out- times a bit yellowish, sometimes blueish, depending on the put devices, as well as color deviations between produc- device. The reason is the serial deviation of the devices, com- tion lots and temperature / humidity changes bined with different settings for brightness and contrast. ■ High processing power to make sure that all those conver- One can easily imagine that the differences are even bigger sion processes don’t become the bottleneck in the produc- on devices from different manufacturers, with their differ- tion workflow. The objective is a fast, good and high-qual- ent masks, phosphors and controls. The same applies to ity conversion of the print data, optimized for the CMYK-based printing systems, which use inks with drasti- characteristics of the printing system ■ A trusting working relationship between the developer of Black generation settings during profile creation. the RIP solution and of the printing system – to under- stand customer requirements and to fine-tune soft and hardware to each other

The linearization is an elementary prerequisite for suc - cessful color management. Imagine you want to print a gra- dient, for example in black, on a light-colored T-shirt - from zero to 100 percent ink coverage. You would be surprised how «uneven» the result will look when using the default settings of many printer drivers – after all, they are optimized to produce the most «colorful», saturated print image possible. This means that full area coverage is often produced even at tonal values of around 70 percent, and above that there are hardly any visible steps to be seen. However, if at certain values the colors hardly differ from each other,

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cally varying pigments, dyes, viscosities and chemical com- Color management in the sense of ICC now means to positions. Color definitions such as RGB or CMYK are there - «translate» color from the color space of one device to the fore not suitable for professional color management. color space of another device. For each device involved, a In order to be able to «translate» colors between different so-called color profile is created, which represents the «color devices, a device-independent color definition is required. fingerprint» of the device. Simply put, a selection of de- This is usually done using the L*a*b* color space as defined vice-specific color combinations (for example, CMYK values) in DIN EN ISO 11664-4. When printing on surfaces, we are is printed on the device. Then, the resulting absolute L*a*b* dealing with subtractive color mixing: (more or less) white color values are measured with a spectrophotometer. The ambient light falls on a printed surface and certain wave- result is a translation table for each specific device that de- lengths are absorbed or reflected by the surface. The wave- scribes which device-specific color needs to be sent to print length distribution then addresses certain receptors in the in order to obtain a certain absolute color. eye of the observer, thus creating the color stimulus. This In textile printing, there are some additional factors that means that the spectral remission properties of an object influence the color beyond the specific device. Color repro- are one important factor for an absolute color definition, but duction can be changed by different fabrics and textiles, not the only one. Color perception also depends on the am - different pre-treatment methods, different temperatures of bient light and the spectral sensitivity of the observer. In the dryer or the hot press. Theoretically, one would have to order to reduce the number of variables, the latter two are create a separate profile for each changing production con- often standardized: for the light, the standard illuminant dition, even if users practically work with a reasonable com- D65 is usually used for printing purposes in Europe, and a promise. so-called «standard observer» is defined as the viewer. All This is another area where the wheat is separated from three parameters are initially available as spectral curves the chaff: while many «inexpensive» RIPs do not even in- which are multiplied with each other and then mathemati- clude a profiling tool, the ColorGATE’s Textile Production- cally integrated. In the end, each color is represented by server does not only offer a powerful color profiler but also three numerical values, the L*a*b* values. The dimension L* extensive features for «updating» a color profile once it has stands for the brightness of a color (0 for black and 100 for been created. During profile creation, there are a number of ideal white), a* denotes the green or red component and b* assumptions and settings that have to be made, including represents the blue or yellow component. A certain combi- those relating to black generation. These settings do not only nation of these three values (for example, L* = 60, a* = – 52, b* influence the quality of the subsequent print output, but = 41 for a saturated green) denotes a certain color in an ab - also the ink consumption. However, while the creation of solute way, regardless of the device on which it is going to be the basic profile requires qualified personnel and know- output. how, the regular quality control and some adjustments can be carried out, for example, by the operating personnel of the machine. Linearization curves for a six-color textile printing system. Control over color means business flexibility Once you have acquired and calibrated this infrastructure comprising professional RIP, spectrophotometer and profiles, you can actively control color in your production process: if you use several digital printing systems in your production, maybe even from different vendors, color management can ensure that they produce color-identical results. Digital processes can even be color-matched to an analog system such as a screen-printing machine. Once a color standard has been defined and quality as- sured by absolute color measurement, the same color can be guaranteed across several production sites. That way, the user can plan her / his machine allocation according to economic criteria and does not have to print all copies on the same machine or at the same location. This means that pro-

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black, accepting the fact that such a conversion may cause inaccuracies or that the saturation of a particular spot color may be reduced. A good RIP has spot color features on board with which Pantone or HKS colors can be defined device- independently. The user can then have the print file with the additional separation calculated by the RIP without further ado and obtain correct and brilliant results. It is often a good idea to take the color of the fabric into account when printing textiles. Let's imagine we wanted to print a portrait on a black T-shirt where the depicted per- son wears dark sunglasses. As described before, one would usually print a layer of white ink as underbase first, because otherwise the translucent CMYK inks would be hardly visible. The face would be composed of the usual primary colors, in this case probably with high proportions of magenta and yellow, and the sunglasses would be printed with black ink. A professional RIP solution, on the other hand, determines before printing whether the same or an even better impression can be achieved by using the original color of the textile, by leaving out the white underlay in the appropriate places and not using the other colors at all. This Profiling assistant of the Textile Productionserver. is called a «knock-out». The result will not only have much more contrast, it will also have a better textile «feel» due to duction can happen regardless of technology and location. the lower ink application which also reduces the printing Another option is «proofing»: in cooperation with brand costs. The same applies to so-called «semi-transparencies», owners, the textile printer can carry out color matching based on paper printouts produced on a «normal» inkjet Print sample with black knock-out and semi-transparencies. printer. This printer can be located anywhere, even at the customer’s premises. Every print sample and sales material produced according to the standard gives a realistic picture of what is possible in production. In other words, active color management is an absolute prerequisite for efficient and economical work, as it saves faulty productions, com- plaints and disappointed expectations.

Special textile features In addition to pure color management, a professional RIP solution fulfills other important tasks in the production process. Print service providers often receive files from agencies and designers with spot colors, for example with the re- cently announced «Pantone Color of the Year», Classic Blue. In professional prepress, spot colors are not defined as a mix- ture of primary colors but are embedded in the file as an additional «separation». In the days of analogue printing, a separate film or printing plate was created containing the spot color. Some print service providers try to convert such spot colors into the primary colors cyan, magenta, yellow and

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i.e. areas in which the color of the textile is included in the screen-printing carousel) and later produce smaller re-runs design of the printed image and is overlaid by slight amounts on a digital system for economic reasons. It goes without of printing ink. Here, ColorGATE’s Textile Productionserver saying that the products of both production runs should offers numerous setting options for the white coverage as look as similar as possible. This is not only a matter of the well as for the transition between ink and textile. This color, but also of the overall screen-printing impression, enables astonishing results, especially if the edges of the which is mainly created by the printing screen used there. design contain gradients or cloudy structures. The Textile Production Server from ColorGATE’s also offers Last not least, a RIP can also help to resemble an analog numerous flexible configuration options for raster simula- print beyond color. Textile printers and finishers often tion. ■ produce larger orders on an analog process (e. g. on a

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