Colour Management – Research Questions
1. What is the purpose of
colour management?
The purpose of colour management is to
obtain colour reproduction. This can
be a challenge because each separate device used, such as printers, monitors,
cameras and scanners all respond or produce colours differently. This is why it
is important that we are able to manage colour between these devices to enable
reproduction of colours.
2. What problem makes colour
management necessary?
Like I stated in the previous
question; there is a difference between colours in different devices, but then
there is also the human’s perception of colour and how each person’s brain may
process colours differently. Without the use of colour management, reproduction
of colour is not going to be accurate. In this case it is important to keep
calibration standards to ensure colours are true across various devices with
different colour gamut.
A profile describes the behaviour of a device like a
scanner, monitor, or printer. For instance, a profile can tell the CMS,
"This is the reddest red that this device can output." A profile can
also define a virtual color space that's unrelated to any particular device (the Adobe
RGB space is an example of this; we'll see how it's useful later on). Profiles
are the key to color management. Without a profile, 100 percent red has no specific
meaning; with a profile, the color management system can say, "Oh, this
color is supposed to be red in the specific way that red appears on that
printer." Profiles conform to the standard ICC (International Color
Consortium) specification that lets them work with all CMSs on all platforms.
ColorSync profiles on the Mac and .icm or .icc profiles in Windows both follow
the ICC spec and work on both platforms.
What's the Difference Between Document, Device,
and Working Spaces? These
are all color spaces that are simply used in different ways. One way to think
about this is that they go from general to specific:
The working space that you
set in the Edit > Color Settings dialog is
the default color space you set in Photoshop. If
you start a new document or open a document that doesn't have a profile, the working space is the profile that will
be associated with the image. It
also means that if you're always opening images that already contain the right
profile, the working space never comes into play. You'll notice that there are
four working spaces in the Color Settings dialog—that's because each color mode
gets its own default. RGB has its own working space, CMYK has its own, and so
on. When picking an RGB working space, it's usually best to choose one that's
built into Photoshop; for more information see "About the Built-In RGB
Working Spaces" later in this chapter.
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The working space is most important when you open untagged images,
because the working space is
essentially a default color space for
documents that aren't already tagged with a profile. If you usually work with
tagged images, the working space may not come into play often.
The document
color space is just another way of saying "the
profile that's embedded inside an image." If there is no profile embedded
in an image, you can either let your Photoshop default working space take over,
manually assign a profile to it, or tell Photoshop to leave it untagged (that
is, don't color-manage the document). Photoshop handles document profiles very
intelligently: If you have five documents open and each has its own correct but
very different profile, there won't be any need to apply the working space to
any of them, and in addition, Photoshop will maintain each document's profile
separately. Photoshop won't let one document's profile affect another document.
A device color space represents
the range of color produced by a
device you use to create or output images. On
the creation side, it could be a digital camera or scanner. On the output side,
it could be a printer. As we've discussed, device
color spaces are valuable for precisely describing the colors of the device that
an image came from or is going to, but they are not good for editing, so you'll
typically run into device color spaces (device
profiles) when you first create or finally output an image.
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The one device
profile Photoshop uses all the time is the display profile describing your
monitor. That's why it's so
important to calibrate your monitor and generate a profile for it.
In a
typical image-processing workflow among color spaces, an image begins its life containing a device-specific source profile
and gets converted to a standard, perceptually uniform workspace (such as Adobe
RGB or sRGB) for editing and archival storage. Media-specific copies of the image are then converted to the color
spaces for the media where they'll be used (the Web, print, video, and so on).
5. What is a reference
colour space and how are they used? Give an example of one.
A reference colour space is also known
as device-independent space. These express colour in absolute terms. They often
serve as universal reference colours so they are useful as a backdrop for comparing
other devices. Otherwise these are usually an unseen colour space.
"... a colour
managed system will require three basic components, namely: -
·
A device-independent colour
space - this is usually referred to as the Working Space or Reference
Colour Space.
·
ICC/ColorSync device
profiles for each device (printer, scanner, monitor, digital camera,
etc.) that describe the colour characteristics of the specific
device.
·
A Colour Matching Module (CMM)
that will interpret the information contained within a device
profile and carry out the instructions on the way the colour gamut of each
device should be treated."
(Lyons
I 2002, Photoshop 7 Colour Settings, viewed 22 March 2012
<http://www.computer-darkroom.com/ps7_colour/ps7_1.htm>)
In Photography and digital printing, calibrating
means to bring a device such as a flat screen monitor, to a repeatable operating
state. Usually with the expectation that it will maintain that state over several
days or months without having to be re-calibrated.
It is also noted that the environment in which your
monitor is being operated also plays a major role in the way your images will
look. Light from windows, overhead lighting, and even the colour of the walls
and ceiling in the space where you work will all influence the overall look of
your display. Some hardware devices allow you to take ambient light
measurements, which can be helpful.
Profiles are generally categorized in the following ways: input profiles (for scanners and cameras); display profiles (for monitors and projectors); editing space (or working space) profiles, which are often
embedded in digital files, such as JPEG or TIFF files; and output profiles (also known as printer profiles).
A printer profiles for example are small files that describe the gamut (number of
printable colours) of a specific printer's paper and ink combination (they have a .icc or .icm extension). Profiles are
often downloaded when you install a printer driver, or you can download them
online from the websites of many paper manufacturers, companies, or individual
photographers.
Using calibration and profiling together ensures
that what you see on the computer monitor is what you will see on paper when
printed.
If a
device has a larger colour gamut than the final device, some of the original
colours will be outside the final devices colour space. These ‘out-of-gamut’
colours occur with nearly every conversion and are called a gamut mismatch.
Each time a gamut mismatch occurs, the CMM uses the ‘rendering intent’ to
decide what qualities of the image it should prioritize. Common rendering
intents include: Absolute and relative colorimetric, Perceptual, and saturation.
Each of these types maintains one property of the colour at the expense of others.
Perceptual – Tries to preserve the overall colour appearance by converting all the
colours in the source space so that they all fit in the target space, while
still preserving the colour relationships. Ie all the colours shift but
none of their relationships do. Good choice for images with many out of
gamut colours. Shifts the extreme colour into gamut, along with the like
colours - everything shift to a smooth gradation.
Relative Colorimetric – Default, similar to Perceptual, except no adjustment is made for in
gamut colours, so far fewer colours shift, however colour relationships
change. Maps source white point to target white point. Reproduces all in
gamut colours exactly, and clips out of gamut colours to the closest available
hue in the target. May be a better choice than Perceptual when original
colours needs to be preserved and there are fewer out of gamut colours. - Clips
whats out of Gamut.
