Geva Barash, VP of Sales & Marketing, NUR America NUR Macroprinters is the market leader in wide format digital inkjet printing system for production environments. Thecompany designs, manufacturers and markets wide and superwide format digital printing systems and consumables.

As wide-format digital production printing evolves, new technologies and paradigms are constantly being added to the continuous flow of new products. These advances have not been limited to changes in the core inkjet technology. They also encompass new concepts in colour systems and colour management, some of which were developed solely for the wide-format domain.

While just a few years ago, four-colour printing was the only way to go wide format, today’s buyers are exposed to many colour concepts and many different terminologies that are often used to describe the same thing. Likewise, a variety of related terms are used in conjunction with these colour concepts - sometimes correctly and sometimes totally out of context.

This article attempts to sort out some of this confusion. It reviews the different colour concepts available today and explains each concept’s merits and disadvantages. It also
explains the relationship between the different systems available and other related terms such as colour gamut, colour space and apparent resolution.

6/8 Colours: The Dark/Light and Hexachrome Systems
Let’s review the different printing systems available today, in order to better understand the pros and cons of each. Digital Process Four- Colour - Four-colour CMYK printing system (cyan, magenta, yellow, black). Digital Dark/Light Six-Colour - Six-colour printing system that uses two diluted or “light” colours in addition to the four dark colours: CMYK, light cyan, light magenta

Figure 2: 8C (left) vs. 4C - Sharp contours on baseball glove stitches and sharp text on ticket.
Figure 3: 8C (left) vs. 4C - Sharp details on red pen.

Digital Dark/Light Eight-Colour - Eight-colour printing system that uses four diluted or
“light” colours in addition to the four dark colours: CMYK, LC, LM, light yellow, light black. Digital Dark/Light systems (both sixcolour and eight-colour) enhance image quality in several aspects (explained later in this article).

Digital Hexachrome® Six-Colour - Sixcolour printing system developed by Pantone that uses orange and green in addition to the four dark CMYK colours. The Hexachrome colour system attempts to match spot colours, also known as market- standard PMS (Pantone® Matching System), commonly used in offset and screen printing systems.

Note: Hi-Fi colour is any printing technique that uses more than four process inks to expand the tonal range of the printed piece. The term: “Hi-Fi” is commonly used when referring to Digital Dark/Light six-colour, Digital Dark/Light eight-colour or Digital Hexachrome six-colour.
The following section explains common terms used in the industry:

6/8 Colours: The Advantages and Some “Sacred Cows”
Colour Gamut
In the following illustration (Figure 1), the three polygons, each with six points, correspond to the six primary colours: cyan, magenta, yellow, red, green, blue (C,M,Y,R,G,B). The area inside each polygon represents all the colours that can be produced with that particular set of inks. This is the colour gamut, also known as the colour space, of two different colour reproduction systems, and provides a handy way for colour scientist to display the relationship between different gamuts.

Colour gamut differs from ink to ink because of the different ink pigmentation each company’s colour lab produces. In the above illustration, the gray polygon corresponds to the market standard Chromalin®’, which is commonly used as a reference base. The blue polygon corresponds to one type of four-colour printer and the red polygon corresponds to adifferent four-colour printer. In this example you can see that one set of inks can deliver a wider range of greens, while the other can deliver a wider range of reds.

The type of media used is another factor that contributes to colour gamut. For in- stance,
the better absorption characteristic of Flex media means it produces a wider “apparent” colour gamut than paper. The Dark/Light system is composed of either two or four additional diluted colours, namely: LC, LM (for the six-colour system) or LC, LM, LY and LK (for the eight-colour system), combined in printing with the traditional CMYK Dark Colours. The alternative Hexachrome Orange and Green six-colour system has an evident wider colour gamut than that of the Dark/Light systems. However, the Hexachrome system has suffered - and currently still suffers - from slow market acceptance, primarily due to difficulties in customising colour management.

6/8 Colours Reduce Graininess
Two parameters are mainly responsible for the graininess phenomenon in digital printing: ink density and positioning of printed dots on media. The graininess phenomenon is most noticeable in the highlight and mid-tone areas, mainly corresponding to skin tones, pastel colours and gradient images.

One of the goals of the eight-colour system is to reduce the graininess phenomenon. Using light inks means more printed dots are produced in the same printed area, thereby achieving an ink density level comparable to that of four-colour printing.

All in all, this formation contributes to a much smoother, homogeneous look, resulting in photo realistic image quality without graininess.

Obviously, an eight-colour system will produce smoother images than a six-colour system, especially in areas rich in K and Y.

Addressable & Apparent Resolution
Print resolution is conventionally expressed as“dpi,” the number of dots printed per inch. The term “addressable resolution” refers strictly to the number of dots that the printer can place - or address - on each linear inch of printed surface. An eightcolour system produces twice as many gray levels per printed dot. It is therefore said that an image printed with a eight- colour system at an addressable resolution of 370 dpi looks just as good to the human eye as the same image printed at 740 dpi from a four-colour system. The human eye cannot perceive these slight nuances because of natural limitations. Hence, both prints would appear the same. One can claim that an eight-colour printer with an addressable resolution of 370 dpi apparently produces a printout with quality of 740 dpi, and thus has an apparent resolution of 740 dpi. When comparingthe overall benefits of the eightcolour system to those of the four-colour system, the following conclusions can be drawn:

Figure 4: 8C (top) vs. 4C - Smooth gradient shades on golf ball and metal look of tennis racket.

The eight-colour system:
• Increases apparent resolution
• Produces finer details
• Ensures smoother gradient transitions
• Produces vivid and crisp colours
• Reduces graininess

The six-colour system is similar in all aspects, although to a lesser extent. However, eight-colour printing does have some disadvantages:
• Extra ink consumption
• Lower speed

Eight-Colour vs. Four-Colour: Take a Closer Look at the Difference
Printed Samples
Figures 2, 3 and 4 are samples of two similar files, one printed with an eight-colour printer, the other printed with a four-colour printer. Take a closer look at the following nuances found in the eightcolour sample (left side of each image): Sharp contours on baseball glove stitches and sharp text on ticket (Figure 2); Sharp details on red pin (Figure 3); Smooth gradient shades on golf ball and metal look of tennis racket (Figure 4).

Reprinted with Permission. Copyright 2003 Screenprinting and Graphic Imaging Association International. All Rights Reserved. This article was published in 3rd Quarter, 2003 issue of the SGIA Journal. Visit www.sgia.org for more information about the SGIA Journal and many other resources enjoyed by members of SGIA International.”

Summary : The Variety of Printing Systems
The following table describes the common four-color and eight-color printing systems on the market today:
Print System	No. of Colors	General Features	Advantages	Disadvatages
Offset/Screen- Process (four-color+spot)	4: CMYK + optional spot colors	Spot colors nable printers to chieve special colors that are not in the color gamut	Spot colors	Time consumin process (prepress and hardware) of modifications for spot colors
Digital - Process (four-color)	4: CMYK	Prints within the limitations of four-color	Lower ink consumption, faster Speed	Graininess, Lower "apparent" color gamut
Digital - exachrome (six-color)	6: CMYK + optional O,G	Produces a smoother im- age than four-color	Wider color gamut	The OG systm suffered- andcurrently still suffersfrom limited market acceptance because of difficulties in customizing color management
Digital - Dark/Light (six-color)	6: CMYK + LC, LM	Produces images with apparently low granularity, smoother than four-color	Enhances the overall image look, producing a more photo realistic image.	Higher ink consumption, Lower speed
Digital-Dark/Light (eight-color)	8: CMYK + LC, LM, LY, LK	Produces images with apparently low granularity, much smoother than four-color and six-color	Enhances the overall imagelook, producing a photo-realistic image more than four-color or six-color	Higher ink consumption, Lower speed

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