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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 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
Moiré
With traditional halftone dots there is the possibility of getting
undesirable patterns called moirés when the halftones are exposed
on screen mesh and printed on shirts (figure 10). These moiré
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 moiré 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 moiré 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 moiré 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.
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