Color
Color is a property of objects.
Color is a property of light.
Color happens in the observer.
The color event occurs when we have three
ingredients:
A light source
An observer
An object
Light is energy.
Light is electromagnetic energy. It has been imagined
to be a particle (Newton), or a wave (Huygens).
Einstein and others resolved the situation by defining
light energy as a “wavicle” called a photon.
It is a pulsating packet of energy traveling through
space. The frequency of the pulsing is what we detect
to be colors. Wavelengths describe the distance
between pulses as the photon move along at the
constant speed of light, 186,000 miles per second.
The full range of energy is called the spectrum.
Our eyes detect a very small portion of the energy
spectrum, the visible spectrum.
Light sources
Blackbody radiators-when objects are heated they will
produce light. A tungsten light bulb is an example.
Daylight
Electric discharge lamps-gas in a tube, can include
phosphors. Neon signs and fluorescent tubes are
typical.
Computer monitors-electrons striking phosphors light
up the tiny red green and blue phosphors.
Illuminants
The most common types of light have been labeled by
the CIE standards organization . We often encounter
Illuminant A, Tungsten light; Illuminant D, Sunlight
of different color temperatures (D50, D65, etc.)’ and
Illuminant F, fluorescent lights.
Objects reflect light, transmit light, or they may absorb
it and regenerate it as a lower energy wavelength
(fluorescence).
Trichromacity
The combination of red, green and blue can be
combined to create our visible spectrum. Our
monitors and television sets use trichromacity to create
a full spectrum of color.
Tristimulus
Our eye is stimulated by the red, green and blue
wavelengths and our brain interprets the results into
the visible spectrum. The rod and cone cells in the
retina sense light; three types of light-sensing cells
detect the red, green and blue zones of the spectrum.
International Commission on
Illumination
The International Commission on Illumination abbreviated as CIE from its French title Commission
Internationale de l’Eclairage - is an organization
devoted to international
cooperation and
exchange of
information among its
member countries on
all matters relating to
the science and art of
lighting.
The CIE has attempted
to define the visible
spectrum. The most
common versions of their color models are called CIE
LAB and CIE XYZ.
The monitors and printed paper we commonly use
produce a range of colors, a color gamut, that fits well
within the gamut of the CIE model.
Monitors, printing presses, desktop printers, and other
devices have gamuts falling within the visible spectrum.
Additive and subtractive primaries
The red green and blue wavelengths can be added
together to create what we see as white light. These are
the Additive Primary colors.
The Subtractive Primaries, cyan, magenta, and yellow,
when printed onto paper, remove portions of white
light to reveal their colors. As they are combined, they
subtract all light to create black. (In practice, we have
to add a little black ink, the K in CMYK.)
The basics of Color Management
A complete explanation, with neat pictures, is available
online in a PDF file, Color Management in Mac OS X,
found in the Resources list at http://www.apple.com/
pro/.
The CIE color models of all visible light give us the
basic tool to control color as it appears on different
devices.
To consolidate a number of proprietary color
management schemes throughout the industry, Apple
Computer introduced Colorsync, a color-management
architecture built into the Macintosh operating system.
Apple also started the ColorSync Consortium which
later became the International Color Consortium, or
ICC. The ICC extended the color profile architecture
to the Windows and Unix platforms and allowed
consumers and vendors to use one set of standards to
describe color.
The ICC color management systems use four basic
components:
PCS. The Profile Connection Space allows us to give
a color an unambiguous numerical value in CIE XYZ
or CIE LAB that doesn’t depend on the quirks of the
various devices we use to reproduce color, but instead
defines the color as we actually see it.
Profiles. A profile describes the relationship between a
devices’s RGB or CMYK control signals and the actual
color that those signals produce. Specifically, it defines
the CIE XYZ or CIE LAB values that correspond to a
given set of RGB or CMYK numbers.
CMM. The CMM (Color Management Module),
often called the engine, is the piece of software that
performs all the calculations needed to convert the
RGB or CMYK values. The CMM works with the color
data contained in the profiles.
Rendering intents. The ICC specification includes four
different rendering intents, which are simply different
ways of dealing with “out-of-gamut” colors—colors that
are present in the source space that the output device
is physically incapable or reproducing.
How it works
The PCS is the device-independent color model, the
CIE definition of visible light.
Profiles are the descriptions of how devices (scanners,
printers, monitors) and abstract color spaces (Abode
RGB (1998) or sRGB) handle color information.
A profile of your scanner will evaluate what your
scanner saw when scanning a test target and explain
your scanner’s behavior to the PCS. For example, the
scanner may record a red value of 170 and the profile
would let Colorsync know the color the scanner really
saw was a value of 177.
The CMM is the engine that actually converts the RGB
and CMYK numbers from a device into the corrected
color values of the PCS.
Rendering intents are methods of dealing with out-ofgamut situations. If you ask a printer to print colors
it can’t print, how does it “fake it”? A Perceptual
rendering intent would bring all the out-of-gamut
colors into the printable gamut and move the in-gamut
colors proportionally to preserve the relationships
between colors. Relative Colorimetric intents will
preserve the true colors of the in-gamut colors and clip
the out-of-gamut colors to the closest in-gamut color.
It is a good idea to review the behavior of Perceptual,
Saturation, Relative Colorimetric, and Absolute
Colorimetric intents. Your applications’ color settings
may allow you to choose the rendering intent or the
profile building application may define the intent.
Putting Color Management to use
To manage color you need to turn on the color
management in the operating system, acquire profiles
for as many devices as possible, and begin associating
profiles to files, color spaces and devices.
A graphic file can have a color profile embedded when
it is created or have a profile assigned later. This will
not change any RGB or CMYK numbers in a file; the
profile tells the CMM engine what compensations are
needed to produce true color.
A file may also be converted to a certain output
profile. If your color managed system is sending
a color file to an output system that lacks color
management, it will be necessary to convert the color
to fit that output device’s tendencies. You will need to
obtain a profile for that output device and convert the
file. The real color numbers in the file will be changed
to acheive true color from the output device.
When a conversion takes place, four things happen.
1. The CMS looks at the source profile
and builds a table that correlates source
RGB or CMYK with PCS values, using the
selected rendering intent.
2. The CMS looks at the destination profile
and builds a table that correlates PCS
values with destination CMYK (or RGB)
values, using the selected rendering
intent, or, if none is selected, the
profile’s default rendering intent.
3. Using the interpolation algorithm
defined in the CMM, the CMS connects
the two tables together through the
common PCS values and builds a table
that relates the source to the destination.
4. The CMS passes each pixel in the source
image, or color in the source artwork,
through the table, converting the values
from source to destination.
A word of caution; converting from space to
space does real math conversion to the file data. If you
were to convert mutiple times, your data would begin
to suffer from data loss.
If the entire workflow is color managed, then the data
will only be adjusted once when the final image is
made.