Simulated Process, Index, or What?


One of the most often asked questions I get is to explain the difference between the various printing and separation methods and the type of software used for each. Questions like, “is Corel Draw better than Adobe Photoshop” and “when should I use Simulated Process Color over Index Color separations”; are the norm. With so many articles about the different methods and so may automated software programs expounding the virtues of each method I thought it was time to try to clear the air about what they are.

Software Programs
Before we can get into a discussion of separation types we need to have a quick overview of the types of software programs available for color separations.

Vector Based Programs
Vector programs are normally used for spot color separations (see below). They know the math coordinates or “vectors”; between point A and point B. Vector based programs love to work with typefaces, hard edge graphics, and clip art. Without getting into a Mac vs PC debate, the most popular vector program on the Windows/PC platform is CorelDraw. On the Mac platform it is Adobe Illustrator and Macromedia Freehand.

Pixel/Raster Programs
Raster or pixel based programs treat images as small pixels of color. These programs work with Photorealistic images and can be used to lighten, sharpen and enhance photos. The most popular pixel based program on both platforms is Adobe Photoshop. It is not only an extremely powerful program, but also one that has been embraced by the industry as the defacto standard. Most ink companies will provide free of charge their process color ink values as a Photoshop file and there are now a number of automated color separation programs designed specifically for Photoshop.

So which is the best program. There is no correct answer. When you read on you will see that different types of separations are done with different programs. Most computer graphic departments have both vector based programs for their text, cartoons, logos and hard edge graphics and pixel based programs for their photorealistic images. Don’t forget, you can create part of the image in one program and take it into another to add additional elements.

Types of Separations
Spot Color
This is the bread and butter of the industry. Spot color images generally have specific solid colors that can also be made of small dots called halftones (figure 1). Spot color separations/prints are generally used for logos, school designs, clipart, hard edges graphics, cartoons or other images that have a black or dark outline.

A spot color image can be as simple as one-color and as complex as ten colors, and can include lots of shading, gradations and detail. They still generally have a flat, cartoon like look and are not photorealistic.

Spot color separations are done in vector based programs like CorelDraw, Adobe Illustrator and Macromedia Freehand (figure 2).

Process Color
Process Color images are made up of the colors of cyan, magenta, yellow and black (figure 3). Process color prints are generally referred to as CMYK (the “K”; is for the “key”; color of black). All of the photos in magazines like Impressions are made of these four colors. If you took a magnifier to these images you would see that they are made up of small halftone dots that when printed make up most of the colors of the rainbow (figure 4).

If process color will print all these colors then why not use it for all your designs? If a T-shirt image were not photorealistic why would you want to print four colors when for a simple “spot color”; image only two would do? Also, for most of your spot colors images you need a more vibrant, solid image rather than a soft halftone dot print.

Process color prints on T-shirts generally only work well on light colored shirts. The inks used are very transparent and do not work on black, and when printed on an underbase of white ink, will become very pastel.

The problem with printing process is that if you are not a good printer or don’t know how to do the separations, the images will be muddy when printed. Although process color separations are generally done by pixel based programs like Adobe Photoshop (figure 5), these programs were designed for paper/offset printing and the program settings don’t allow for the fact that halftone dots grow in size when printed on a soft object like a T-shirt (dot gain). Process color separations are generally NOT done with vector based programs like Corel Draw.

For this reason, process color is not for everyone. It generally needs better control like properly tensioned, high mesh-count screens and the ability to hold fine halftone dots and print them in register with minimal dot gain. The secret to good process prints is in proper separations and good printing. Yes, you can do it, but plan to experiment a little.

When you see a hot process color print, it almost always has additional spot colors. What you think is just a cmyk print may in fact have cmyk plus two or three spot colors to make the design really jump off the shirt.

If you don’t know how to do the separations you should either use an industry specific color separator, follow directions from articles in this magazine or download how-to-do-it articles from http://www.screenprinters.net/articles, or use industry specific separation software.

Simulated Process Color
This is also known as “fake”; process color. Simulated Process color images have a photorealistic look but are not printed with the process colors of CMYK (figure 6). They look like process, smell like process, feel like process but aren’t process. The color separations for simulated process color are made up of halftone images of spot colors like red, yellow, blue, etc. They are often called “tonal”; or “channel”; separations (figure 7). Simulated process separations can be printed on light and dark shirts and are generally done in Adobe Photoshop.

Because the inks for simulated process color are generally all-purpose, semi-transparent plastisol they give you a bright print even when printed on an underbase of white ink. When done correctly, simulated process prints can be very photorealistic with smooth gradations and bright colors.

Index Color
This is probably the most confusing of the separation methods. For simulated process, real CMYK process and spot colors with gradations, any shading is done with different size halftone dots that have a definite pattern and angle to them (figure 8). Index separations are done in Adobe Photoshop and use a random square dots that are all the same size. These random dot patterns are also often called diffusion dither or stochastic (figure 9).

A Word About Moir�
With traditional halftone dots there is the possibility of getting undesirable patterns called moir�s when the halftones are exposed on screen mesh and printed on shirts (figure 10). These moir� patterns happen because halftones generally need to be printed with a different angle for each color. If the angles of halftones are not correct they create an “interference of two patterns” and give you little checkerboard effects.

In theory, index separations should not give you a moir� pattern because the dots are all the same size and they are random. This is one of the biggest industry lies ever told. Yes, you will not get a moir� pattern within the separations but if you use the wrong screen mesh (200 dpi index separation on a 200 mesh) you will get the worse moir� you have ever seen.

Index color separations are done in Adobe Photoshop by creating a color table of the most prominent colors in your image (and the most colors you are capable of printing) and then letting Photoshop convert the image to just those colors using random square dots. Photoshop will make (or try to make) the image look as close as the original as possible with just the limited number of colors you selected (figure 11).

Index prints can be very bright on light and dark shirts and the separations are easy to do. The downside to index prints is that for the image to be photorealistic you need at least six colors and in some cases eight to ten colors. Index prints can sometimes have a grainy and textured look to them. When printed with a lot of colors index prints can also be very striking (figure 12)! Index separations work great for spot color images too. They are also easy to print because you are placing a dot next to a dot rather than printing halftone dots on top of halftone dots. Just don’t use indexing because someone told you that halftones are hard. Most of the award winning prints you see are still real process color and simulated process color.

When To Use What?
OK, with all that said, when do you use what method?

CMYK Process Color
Great for photorealistic images on white or light shirts. Do not use on dark shirts. Requires good separations, screen making and printing technique. The best process prints have additional spot colors. Prints may be a little duller than a simulated or index print.

Simulated Process Color
Great for dark shirts that need a photorealistic image. Works on light shirts too. Requires good separations, screen making and printing technique. Can print very smooth gradations and hold excellent detail. The most popular method used by award winning printers. Prints are bright because all purpose inks are used.

Index Color
Works on light and dark shirts. Typically requires more colors than simulated or process color (especially if going on black shirts). Very easy to print because all the dots are the same size and you are printing square dots next to square dots rather than halftone dots on top of halftone dots. Separations are easy to do in Adobe Photoshop and screen making and printing can be forgiving. Very production friendly and easy to print. Images can have a slightly grainy (stippled) look. Works well for spot color also.

Summary
Remember, don’t get too caught up in using these terms with your customers. They don’t know and really don’t care if you index it or make it simulated process color. All they want is their image to look good on a shirt.

Good luck, Scott

  Author’s URL: Scott Fresenerhttps://mrroggers.wordpress.com/2008/03/21/who-is-scott-fresener/

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