What is an ICC profile?
A. ICC profiles help you to get the correct colour reproduction
when you input images from a scanner or camera and display them on a monitor or
print them. They define the relationship between the digital counts your device
receives or transmits and a standard colour space defined by ICC and based on a
measurement system defined internationally by CIE. Thus, if you have a profile
for each of your scanner, camera, display and printer, the fact that they refer
to a standard colour space lets you combine them so that you obtain the correct
colour as you get images from the scanner or camera and print or display them.
An ICC profile is one that conforms to the ICC specification. By conforming to this specification profiles
may be exchanged and correctly interpreted by other users. The two main types
of profiles are source (input) and destination (output) profiles and
essentially consist of tables of data that relate the device co-ordinates to
those of the standard colour space defined by ICC. There are various relationships
defined in each profile (known as rendering intents ? see a later question).
Special types of profiles (devicelink, and abstract) are defined for special
workflow applications.
What is spectrophotometry?
A. Spectrophotometry is the measurement of the reflectance or
tranmittance of a sample at discrete wavelengths. Spectrophotometers usually
provide illumination of the sample by white light and then contain a
diffraction grating to refract the reflected light and enable measurement of
the amount of light reflected at discrete wavelengths.
What are the limitationss of spectrophotometers?
A. Different spectrophotometers are more or less accurate than
others. Most of the cheaper instruments available today are reasonably accurate
for reflecting media without steep changes in spectral reflectance, which is
the case for most pigments.
Although measurement typically takes a second or so on a hand-held
device, if you have a large number of colour patches to measure this can add up
to long time. Some manufacturers offer automated devices which move the
measuring head across the chart automatically, or allow you to 'swipe' the
instrument across a strip of patches.
How do I implement ICC colour management on my system?
A. To apply colour management, you need a profile for each of your
scanner and/or digital camera and another for your monitor and/or printing
device. Each of these relates the device colour data to the standard colour
space which allows them to be combined to produce an overall transformation.
To combine profiles you need a Colour Management Module (CMM). At
its most basic this is nothing more than an interpolation engine for combining
LUTs. ICC do not specifically recommend a single CMM as some CMMs attempt to
'add value' for specific applications by picking up private tag information in
the profile.
Many colour management-aware applications such as high-end RIPs
and Adobe Photoshop contain an internal CMM. CMMs are also built in to the OS
on the Mac (ColorSync) and Windows (ICM and WCS).
Where can I
find profiles for my devices?
A. Most often these are available from the manufacturer of the
device.
How do I make ICC profiles?
A. The main requirement is a software application that will
generate profiles from measurement data. For output profiles, you also need a
measurement instrument to measure your prints or display. For more details, see making profiles. For a list of software and
instruments available from ICC members, see profiling tools.
What is an output profile?
A. Output profiles are of the LUT type, and are used in
conjunction with hard copy output device, such as printers and film recorders.
Output profiles translate between the PCS and the output colour encoding. In
the case of a printer profile, the output colour encoding might be monochrome,
CMYK, RGB or n-colour, where n can be up to 16 (although in practice is rarely
greater than 6 or 7).
In some workflows there is a further conversion from the output
colour encoding of the profile to the actual colorants used by the printer,
which is usually performed in the printer driver.