Ever since the beginning of colour reproduction, colour management has
existed in one form or another. The basic concept underlying Colour
Management is to ensure that colour data is processed in a consistent
and predictable way throughout the entire imaging workflow. A typical
Imaging System will consist a wide range of Input and
Output Devices, and each device will reproduce colour differently.
This means that a colour represented by one device will rarely if ever
match the same colour represented on another device. In other words,
colour is Device-dependent. So expanding upon our earlier
definition we can say that the purpose of a Colour Management System
(CMS) is to maintain the consistent and accurate "appearance"
of a colour on different devices (e.g. scanners, monitors, printers,
etc.) throughout our imaging workflow.
Components of a Colour Management System
In order that we can achieve the above aims 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.
The following diagram demonstrates a typical "colour-managed
workflow" and shows the image being passed along the chain -
from scanner/digital camera - to - computer - to -
monitor - and printer with the ICC profiles
ensuring that the colour data from/to each device is correctly described.
Colour Numbers, their Meaning, and
Profiles
A digital image will comprise pixels each of which is represented by a
number. This number will describe the location of the pixel within
the image and its particular colour value (typically an RGB
value). We have already noted that since colour is device-dependent the "appearance"
of each coloured pixel will vary for each device. We also noted that this
is because each device has its own unique way of translating the raw
colour value into visual colour. To minimise the discrepancies that
result from the widely differing colour characteristics of each device we
use the device profiles to inform the CMM how the colour values
produced by that device should actually appear. This may be on our
monitor, in print or on film output. In simple terms it is the device
profile that conveys the "meaning" of the raw colour numbers
associated with each pixel.
Whilst "Consumer "class film and flatbed scanner software
applications are now ICC/ColorSync aware they tend to be based
upon sRGB, likewise consumer digital cameras. This colour space is
not normally regarded as appropriate for high quality image editing,
especially when print or film output is required, but is quite often all
we can expect to get. We should not confuse matters by assuming that the
scanner or digital camera is an sRGB/Adobe RGB device - they most
certainly aren't. The fact that the image delivered into Photoshop by the
scanner or digital camera application software is already in a
device-independent colour space means that it has already undergone a
considerable amount of data processing.
Some digital hardware vendors allow scanning/capture of images in any
user selected "working space" colour space. Unfortunately, the
image is rarely delivered into Photoshop with an embedded ICC/ColorSync
profile, which can lead to confusion and error.
Device profiles come in two basic forms, i.e. "Input"
and "Output". Input profiles typically describe the colour
characteristics of scanners and digital cameras, whereas Output
profiles describe devices such as monitors, printers and film recorders.
Input profiles are often referred to as one-way since it is
only possible to select them as the "Source" meaning we can
never convert an image into the colour space of our scanner or digital
camera. Output profiles are two-way meaning we can convert "From"
or "To" them.
Many "Prosumer" class scanners and digital printers are now
supplied with some form of generic or "canned" profiles. Whilst
these profiles are useable they are rarely accurate. For truly
accurate colour matching you should seriously consider getting "Customised"
profiles for each device and/or media type. These profiles can be created
professionally or by buying your own profiling software. Sadly very few,
if any digital camera vendors have adopted the "canned" profile
approach instead opting process the images into a device-independent
colour space as discussed above.
Why bother with Profiles?
Even though colour correction and colour management are not the same
thing they are often confused with each other, especially by the novice
Photoshop user. The colour characteristics of most imaging devices are
such that it is very rare for them to be truly linear (i.e.
R=G=B=Neutral). Sometimes this characteristic is referred to as the
device "not being well-behaved". Scanners and printers are good
examples of "badly behaved" devices. Obviously it would be extremely
difficult for a Photoshop user to edit an image where a group of pixels
with values of R=G=B=128 (grey) actually "appeared" to be
non-neutral. In such circumstances colour correction would be an absolute
nightmare. To overcome these discrepancies we usually carry out all our
editing in a colour space that is well-behaved. Well behaved colour
spaces are more usually referred to as the "working space", and
are always characterised by having R=G=B appearing neutral.
Without the aid of accurate device profiles the accurate translation
of the raw colour data (the numbers) from our scanner/digital camera into
the working space will prove very difficult, if not impossible. The
translation from the working space into the media specific colour space
of a digital printer will prove equally difficult without the aid of
media specific printer profiles.
So the benefit offered by colour management is that the process
of colour correction can be undertaken by the user in the
knowledge that the image displayed on their monitor is an accurate
visual representation of the original subject, and that the final
print will accurately reflect the colours of the displayed image.
"EXIF" is a familiar term to many digital camera users, but
until Photoshop 7 it had little use. EXIF is a standardised format
agreed by various digital camera vendors for incorporating data such as
the exposure time, aperture, lens, picture resolution, colour space, etc
into the image file. However, Photoshop 7 now reads the EXIF
information embedded within the JPEG format images captured by
digital cameras and can also write new information to the EXIF
file. There are a number of quirks in the way Photoshop 7 manages the
colour space of these image files, and so those already familiar with
Photoshop 6 should be prepared for a few surprises.
The screenshot below is presented for those readers who've never seen
EXIF data before. I captured the information file from the new
File Browser window but you can also get it by selecting "File
Info > EXIF" from the Photoshop File menu.
So far as colour management and Photoshop 7 are concerned EXIF
holds some useful information (circled red in the screenshot), and this
information. The first piece of information is the potential
availability of an embedded "Colour profile" that describes the
actual colour space of the image. The second is the EXIF "Colour
space" to be assumed in the event of no colour profile being
embedded. The embedded colour profile can be anything the camera vendor
chooses to allow or nothing at all. At time of writing this essay it
appears that only Nikon have chosen to allow embedding of colour
profiles, and so many users will never see an embedded profile. On the
other hand the EXIF colour space appears to be widely used.
It is worth noting that the EXIF colour space recorded within
the EXIF information can only be one of two values, i.e. sRGB
or Uncalibrated. Generally digital camera manufacturers have set
their software to automatically write sRGB as the EXIF
colour space. This means that in the absence of an embedded colour
profile Photoshop 7 will attempt to open JPEG format digital camera
images into the sRGB colour space. If the EXIF colour space
has been set to Uncalibrated Photoshop will rightly assume that
the image has no predefined colour space information and the image will
not be colour managed (the image will be defined as Untagged RGB).
The screenshot shows the EXIF file for a JPEG image captured on
a Nikon D1X digital SLR camera. The owner had configured the camera
software to output the image in Adobe RGB and as we can see the
Nikon software has been very obliging and embedded the Adobe RGB
colour profile. Prior to Photoshop 7 this colour profile could not be
read and so the image would have opened with the "Missing Profile"
dialog flagging a potential problem - not anymore! The image will now
open directly into Photoshop or present the user with a "Profile
Mismatch" warning! Note: Each of these
terms will be discussed in Part 2 of this essay.
Also notice that the screenshot shows the EXIF colour space is
set for sRGB - so we have an image with two conflicting sets of
colour space information. Thankfully Adobe has coded Photoshop 7 to give
priority to the colour profile and when it's not present Photoshop will
use the EXIF colour space information.
2 September 2002
It appears that some users were getting confused with the appearance
of the Profile Mismatch dialog so in late August Adobe released a
Plug-in designed to make Photoshop 7 behave in the same way as
earlier versions, and so ignore the presence of the EXIF colour
space information. The new plug-in is called Ignore EXIF Colour Space
and can be downloaded from:
PC platform -
http://www.adobe.com/support/downloads/detail.jsp?ftpID=1881
Mac platform -
http://www.adobe.com/support/downloads/detail.jsp?ftpID=1882
Photoshop 7 continues to use document/image specific colour settings,
which means that the colour space of each document is independent of
others that may be open on the Photoshop desktop. As with Photoshop 6,
the working space defined in "Colour Settings" only has a bearing
on three types of image, viz.: -
-
new images/documents;
-
existing images/documents without an embedded profile;
and
-
images/documents with no embedded ICC/ColorSync profile (i.e. "untagged
images/documents").
"Image specific" colour means that it's the profile embedded
within an image that determines how the image will be displayed (it's
appearance) and not the Photoshop working space. With Photoshop 7 we can
have multiple images, each in its own unique working space open at the
same time and each will be displayed accurately (of course this assumes a
well calibrated monitor).
Note that the "Assign Profile", "Convert to Profile" and
"Proof Setup" commands have undergone minor alteration. I will
discuss each of them in detail later.
Also note that except for the obvious difference in user interfaces;
Photoshop 7 for Mac OS9.x, Mac OSX and Windows share
a virtually identical feature set. Therefore the Windows based
screen shots that follow should be equally useful to Mac users.
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