User`s guide

PowerRIP 2000 Manual
iProof Systems, Inc.
29
Calibration
What is color and why do I have to calibrate
The science of color can be very complicated if
described using mathematics to describe human vision.
It is much easier to understand color if we use broad
concepts instead of those difficult, complicated mathe-
matical formulas.
The human eye has only three color receptors. These
three color receptors perceive and react to red, green,
and blue (additive primary colors). The human eye is
very much like your scanner, in that it generates signals
for the red, green, and blue parts of the picture. By the
way, for those math wizards these three receptors are
mathematically described as XYZ.
But we don’t usually define color using XYZ. We use
hue, saturation, and brightness. Hue is the color name.
So if we ask for the color red we are asking for the hue.
Saturation describes the strength of the color.
Brightness describes how light or dark the color is.
There are many other things that affect our perception of
color such as what color background the color will be
appearing on. As an example, if you put red on a blue
background it would not look as intense as it would on a
white background.
Another influence on how we perceive color is the way
our eye registers the color values depending on the rela-
tive position of other colors in an image. As an example
dark colors are less noticeable to the eye then light col-
ors.
The type of paper used to output the image can also
influence the look of the color. Different degrees of
whiteness and glossiness will affect how we perceive
(see) color.
And finally, the type of light used to perceive the color
very much affects how we see the color. The contrast
and saturation of the image is very much affected by
the light we use. So if you look at an image in daylight
and then look at it under florescent lights the colors will
look different.
Note: Chapter 7 includes information about ColorSync and Calibration.
The features discussed in Chapter7are dependent on your printer and version of PowerRIP 2000. Not all
features are available on all printers or on all revisions of PowerRIP 2000.
We can go on and on about the science of color, but the
short and simple of the matter is that the observer, the
light source, the paper, and how the colors are arranged
on the page all affect the final output. Add to that mix the
fact that no two human eyes perceive color the same way
and you have a host of variables that take many books to
explain. So, let’s look at how we can produce good color
that is pleasing to the eye and true to the colors in the
image, as we perceive them. Again, keep in mind that no
two people agree on what is good color. So the following
information is a guide for you to follow to calibrate your
PowerRIP 2000 for your particular needs.
But before we begin, let us explore some of the proper-
ties of color that you will be exposed to when consider-
ing calibration. When printing in color different color
inks are placed on the paper to create an image. So the
ink and paper are very important factors in printing good
color images. But you also need to know which output
color process is best suited for your color images. There
is one basic method used to lay ink on paper. That is
CMYK. But RGB and Spot colors will also need to
addressed in this discussion. So before we begin to cali-
brate, let us understand the difference between RGB,
CMYK and Spot colors.
You are going to be looking at the image two ways. On
the screen and on the printed page. The goal is to be able
to predict from the monitor display what the output will
look like. But this is not easy to do. The monitor uses
RGB to display the image. But generally images are out-
put in CMYK. So we want to take an RGB screen image
and output as a CMYK and maintain its color integrity.
There is no universal printing standards for PC RGB,
CMYK, monitor white, paper white etc. This is particu-
larly apparent when you import an image in one applica-
tion, print it for proofing purposes and then save it as an
EPS. If you import this EPS into a second application,
very often you will see a color shift because the second
application’s color model is slightly different and affect-
ed the images color during output. So it is important that
you take all these factors into account when calibrating
PowerRIP 2000. You want to create a calibration test file
using the application and paper you will be outputting
Chapter 7 - Color Matching