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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.

RIP TECHNOLOGY, COLORIMETRY AND COLOR MANAGEMENT IN DIGITAL TEXTILE PRINTING

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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-

graphic images. However, there are also some challenges

to be mastered: on the one hand, ink is absorbed by fabrics,

so the textile or fabric must first be pre-treated. The differ-

ent types of fibers, dyeing techniques and finishings of the

base material provide a wealth of changing parameters as

well.

Still, customers have high expectations of the print qual-

ity: despite the relatively uneven surface, they expect a de-

tailed and well-resolved print with a large color gamut. In

direct-to-garment printing, the substrate is often a dark or

black garment. In such cases, the use of white ink is a basic

requirement for the printing process, because the CMYK

inks used are not opaque but translucent. Without a white

underbase, the print would be hardly visible. Sometimes

white logos, halftone designs or even white photos are

printed on dark textiles, which requires control of the linear

reproduction of tonal values. In textile roll-to-roll printing,

on the other hand, printing is mostly done on white or at

least light-colored fabrics so textile roll-to-roll printing sys-

tems usually do not feature white ink.

In both direct-to-garment and roll-to-roll printing, addi-

tional colors such as red and green are often used for extend-

ing the color gamut. This means that, together with white

and the subtractive primary colors cyan, magenta, yellow

and black (CMYK), sometimes up to seven primary colors

have to be managed.

The RIP: the data preparation hubAll these and more tasks are fulfilled by the RIP and color

management solution. In the closer sense of the word, a RIP

(«Raster Image Processor») is a software or hardware compo-

nent that converts print data into the output format of the

press. This is not trivial because the popular PDF format,

which is often used in prepress, stores information in a

resolution-independent way whenever possible. The letter T,

for example, is not defined as a collection of pixels but as a

«vector graphic» consisting of basic geometric shapes such

as lines, circles and curves. The core task of a RIP is to convert

such image descriptions into the actual print resolution and

raster format of the attached printing system. In practice,

however, machine operators call practically any software a

«RIP» that is used to control the printing system – even if it

does nothing more than passing on existing pixel data, e.g.

from Photoshop, to the printing system more or less un-

changed. Professional RIPs, such as the Textile Production-

server from ColorGATE, are characterized by the following

OLIVER LUEDTKE

Dipl.-Ing.

Chief Marketing Officer ColorGATE DE-30171 Hannover

oliver.luedtke@colorgate.com

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features, among others:

■ Extensive linearization functions for ensuring a dynamic,

well-graded ink application on the textile

■ Creation and flexible application of color profiles, in order

to factor in the color characteristics of the input and out-

put devices, as well as color deviations between produc-

tion lots and temperature / humidity changes

■ High processing power to make sure that all those conver-

sion processes don’t become the bottleneck in the produc-

tion workflow. The objective is a fast, good and high-qual-

ity conversion of the print data, optimized for the

characteristics of the printing system

■ A trusting working relationship between the developer of

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 opti-

mized to produce the most «colorful», saturated print image

possible. This means that full area coverage is often pro-

duced 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,

the next steps, such as the creation of color profiles, be -

come 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 managedOnce 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

are usually device-dependent. This means that one and the

same RGB value produces different colors on different hard-

ware. Everybody who has ever compared the advertising

screens hanging over a row of supermarket check-out desks

knows this phenomenon: a red that should actually be the

same is sometimes paler, sometimes more intense, some-

times a bit yellowish, sometimes blueish, depending on the

device. The reason is the serial deviation of the devices, com-

bined with different settings for brightness and contrast.

One can easily imagine that the differences are even bigger

on devices from different manufacturers, with their differ-

ent masks, phosphors and controls. The same applies to

CMYK-based printing systems, which use inks with drasti-

A RIP converts vector data into pixel data.

Black generation settings during profile creation.

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cally varying pigments, dyes, viscosities and chemical com-

positions. Color definitions such as RGB or CMYK are there-

fore not suitable for professional color management.

In order to be able to «translate» colors between different

devices, a device-independent color definition is required.

This is usually done using the L*a*b* color space as defined

in DIN EN ISO 11664-4. When printing on surfaces, we are

dealing with subtractive color mixing: (more or less) white

ambient light falls on a printed surface and certain wave-

lengths are absorbed or reflected by the surface. The wave-

length distribution then addresses certain receptors in the

eye of the observer, thus creating the color stimulus. This

means that the spectral remission properties of an object

are one important factor for an absolute color definition, but

not the only one. Color perception also depends on the am-

bient light and the spectral sensitivity of the observer. In

order to reduce the number of variables, the latter two are

often standardized: for the light, the standard illuminant

D65 is usually used for printing purposes in Europe, and a

so-called «standard observer» is defined as the viewer. All

three parameters are initially available as spectral curves

which are multiplied with each other and then mathemati-

cally integrated. In the end, each color is represented by

three numerical values, the L*a*b* values. The dimension L*

stands for the brightness of a color (0 for black and 100 for

ideal white), a* denotes the green or red component and b*

represents the blue or yellow component. A certain combi-

nation of these three values (for example, L* = 60, a* = – 52, b*

= 41 for a saturated green) denotes a certain color in an ab-

solute way, regardless of the device on which it is going to be

output.

Color management in the sense of ICC now means to

«translate» color from the color space of one device to the

color space of another device. For each device involved, a

so-called color profile is created, which represents the «color

fingerprint» of the device. Simply put, a selection of de-

vice-specific color combinations (for example, CMYK values)

is printed on the device. Then, the resulting absolute L*a*b*

color values are measured with a spectrophotometer. The

result is a translation table for each specific device that de-

scribes which device-specific color needs to be sent to print

in order to obtain a certain absolute color.

In textile printing, there are some additional factors that

influence the color beyond the specific device. Color repro-

duction can be changed by different fabrics and textiles,

different pre-treatment methods, different temperatures of

the dryer or the hot press. Theoretically, one would have to

create a separate profile for each changing production con-

dition, even if users practically work with a reasonable com-

promise.

This is another area where the wheat is separated from

the chaff: while many «inexpensive» RIPs do not even in-

clude a profiling tool, the ColorGATE’s Textile Production-

server does not only offer a powerful color profiler but also

extensive features for «updating» a color profile once it has

been created. During profile creation, there are a number of

assumptions and settings that have to be made, including

those relating to black generation. These settings do not only

influence the quality of the subsequent print output, but

also the ink consumption. However, while the creation of

the basic profile requires qualified personnel and know-

how, the regular quality control and some adjustments can

be carried out, for example, by the operating personnel of

the machine.

Control over color means business flexibilityOnce you have acquired and calibrated this infrastructure

comprising professional RIP, spectrophotometer and pro-

files, 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 man-

agement can ensure that they produce color-identical re-

sults. 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 eco-

nomic criteria and does not have to print all copies on the

same machine or at the same location. This means that pro-

Linearization curves for a six-color textile printing system.

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duction can happen regardless of technology and location.

Another option is «proofing»: in cooperation with brand

owners, the textile printer can carry out color matching

based on paper printouts produced on a «normal» inkjet

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 featuresIn 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

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 ma-

genta 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

is called a «knock-out». The result will not only have much

more contrast, it will also have a better textile «feel» due to

the lower ink application which also reduces the printing

costs. The same applies to so-called «semi-transparencies»,

Print sample with black knock-out and semi-transparencies.

Profiling assistant of the Textile Productionserver.

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i.e. areas in which the color of the textile is included in the

de sign of the printed image and is overlaid by slight amounts

of printing ink. Here, ColorGATE’s Textile Productionserver

offers numerous setting options for the white coverage as

well as for the transition between ink and textile. This

enables astonishing results, especially if the edges of the

design contain gradients or cloudy structures.

Last not least, a RIP can also help to resemble an analog

print beyond color. Textile printers and finishers often

produce larger orders on an analog process (e. g. on a

screen-printing carousel) and later produce smaller re-runs

on a digital system for economic reasons. It goes without

saying that the products of both production runs should

look as similar as possible. This is not only a matter of the

color, but also of the overall screen-printing impression,

which is mainly created by the printing screen used there.

The Textile Production Server from ColorGATE’s also offers

numerous flexible configuration options for raster simula-

tion. ■