PHOTOSHOP COLOUR MANAGEMENT
By Ian Lyons
/A Computer Darkroom Essay
I published my first essay onPhotoshop Color Managementback in the days of version5. Back then ICC based color management was in its infancy and many Photoshop users needed all the help they could get to understand the myriad of options, dialogs, alerts, etc. With versionPhotoshop CS6now in the hands of users it's time to see what has changed.
The first and most obvious change since CS5 is the UI. However, color management remains very similar in both look and feel to versions dating back as far as CS2, which is a good thing because existing users will be up and running fairly quickly. It's for this reason that the essay is, for the most part, simply an update of earlier versions rather than a complete rewrite. The one exception being the changes introduced to the newPrint dialog.
Section 1 - Color Management Primer
Components of a Color Management System
A typical imaging system will consist of input and output devices, for example: scanners, digital cameras, monitors, and printers. Unfortunately, with such a diverse range of device types, technologies, and gamut limitations, it's inevitable that they will each reproduce the same color differently (i.e. color isdevice dependent). Obviously, this will present significant problems when working with documents originating from different sources, and will be further complicated when the same document is destined for different types of output device. Therefore, some means of ensuring that color data is reproduced in a predictable way throughout the entire imaging system is essential. This is the purpose of aColor Management System(CMS).
A color managed system comprises three basic components, namely: -
· A device-independent color space - this is usually referred to as theWorkingorReferencecolor space.
· ICC profiles for each device (i.e. printer, scanner, monitor, digital camera, etc.) that will accurately describe the unique color characteristics of each device.
· AColor Matching Module(CMM) that will interpret the information contained within the device profiles and carry out the instructions on how the color characteristics of each device should be treated.
Color Numbers, their Meaning and ICC Profiles
A digital image will usually comprise many millions pixels, each of which is represented by a numeric values. The values assigned to each pixel will describe many attributes but in this essay it's the color value or mix (e.g. RGB value) that we're most interested in. As I have already mentioned, when a color is device-dependent, the appearance of pixels with identical values will very often differ because each device has its own unique way of translating the color "value" or "number" into visual color. The role of ICC profiles is to ensure that discrepancies that result from the widely differing color characteristics of each device are known to the color management system. If we were discussing spoken language rather than color, then the ICC profile would be synonymous with a translator.
Device profiles come in two basic forms, i.e.InputandOutputprofiles. Input profiles typically describe the color 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 they represent the source device. This means we can never convert a document into the color space of our scanner or digital camera. Output profiles, on the other hand, are two-way meaning we can convert From or To them. For example, we can convert a document with an embedded monitor profile into one that has a color profile describing a printer, or vice versa.
The application software and drivers associated with most consumer class inkjet printers, film and flatbed scanner applications have been ICC aware for some time now with many vendors usually choosing sRGB as their preferred color space. This is a bit of a misnomer because sRGB is not a device color space. Clearly, these vendors are doing some work behind the scenes so as to keep things simple for the user, which isn't necessarily as wise as it might first appear. The reason being that sRGB isn’t generally regarded as appropriate for high quality image editing, especially when print or film output is required. So, to overcome this some vendors also provide the user with the option of choosing from a small selection of alternatives. For example, printer vendors such as Epson provide generic ICC profiles with their photo class printers, although its generally accepted these profiles are rarely as accurate as users would like. Nevertheless, they are getting better with each new generation of printer. As a rule, truly accurate color matching usually requires customised ICC profiles for each device and/or media type. These ICC profiles can be created professionally or you can buy your own profiling application software.
Why bother with Profiles and Color Management?
Even though color correction and color management are not the same thing they're often confused with each other, especially by the novice Photoshop user. Explaining the difference can very often lead to even more confusion, but hopefully the following explanation will provide some help.
We have already established that the color characteristics of most imaging devices tend to be unique to that device. Likewise, it's very rare for them to be truly linear (i.e. R=G=B=Neutral). Sometimes this second characteristic is referred to as the device being "badly behaved"; with scanners and printers being good examples of badly behaved devices. Obviously, it would be extremely difficult to edit a document where a group of pixels with values of R=G=B=128 (i.e. grey) actually appeared on the monitor to be significantly non-neutral. In such circumstances color correction would an absolute nightmare. To overcome these discrepancies we usually carry out all our editing in color spaces that are "well behaved" or independent of any specific device. In Photoshop "well behaved" color spaces are more usually referred to as theWorking Spaces, and are always characterised by RGB values that appear neutral when all three are equal. TheseDevice Independentcolor spaces do not behave like, nor are they influenced by, any real world device. So, in this respect it could be argued that Working Spaces are based onsynthetic color spaces.
So, we haveDevice ProfilesandWorking Space Profiles- how do they interact?
The first thing we need to understand is that for any color profile to be useful to the color management system it must conform with the ICC standard. In fact, you'll more often see such profiles referred to as just ICC profiles. Anyway, without the aid of device profiles the accurate translation of the document color data (the RGB numbers) from the scanner or digital camera into the Working Space will prove very difficult, if not impossible. Likewise, without the aid of accurate media specific printer profiles, the translation from the Working Space into the color space of a digital printer will prove equally difficult. We also need an accurate monitor profile so as to ensure that what we see on the monitor is a true representation of the document color.
The following flow diagram demonstrates a typical imaging workflow, with the document being passed between devices: - from scanner/digital camera - to - computer - to - monitor - and printer.
Typical Imaging System
So, the main benefit offered by color management is that the process of color correction can be undertaken in the knowledge that the document displayed on the monitor is an accurate visual representation of the original subject, and that the final print will accurately reflect the colors of the document being displayed.
Document Specific Color
Just like its more recent predecessors Photoshop CS6 continues to use document specific color settings, Actually, the Working Space that's chosen in theColor Settingsdialog has a direct bearing on only three types of document, viz.: -
1. The default color space of new documents created via theNewcommand found in theFilemenu
2. Existing documents without an embedded ICC profile
3. Imported documents with no embedded ICC profile (i.e. untagged documents), which might include scanned documents or those emanating from digital cameras.
Document specific color means that it's the ICC profile embedded within the document that determines how it will be displayed (it's appearance) and not the default Photoshop Working Space. With Photoshop CS6 you can have multiple documents, each in its own unique Working Space, open at the same time, and each will be displayed accurately. Of course all of this assumes you're using an accurately calibrated and characterised monitor.
Section 2 - Monitor Calibration and Characterisation
Monitor calibration and characterisation (profiling) is probably the most important aspect of a color managed workflow; yet many users seem oblivious to the issues poor monitor calibration can can have on their documents. So, what is calibration, why is it so important, and why is it different from characterisation?
Calibration is a process whereby a device is brought to a standard state (e.g. a color temperature of 6500K and gamma of 2.2), whereas characterising the monitor is the process of determining how the monitor represents or reproduces color. We characterise the monitor by measuring how it displays known color values, then creating an ICC profile. The ICC profile is simply a data file that includes a description of the monitors’ color handling characteristics (i.e. its gamut). The calibration data will also be written into the ICC profile. As I've already mentioned, Photoshop then uses the monitor profile to automatically optimise the display of documents. It does so by carrying out an on-the-fly conversion between your document profile (e.g. ProPhoto RGB, Adobe RGB, sRGB, ColorMatch) and your monitor profile. This conversion does not alter the actual document in any way; just its appearance on the monitor.
Adobe stopped shipping Adobe Gamma with the Mac version of Photoshop a few versions back, but for a while kept it for Windows. This was because there was no software only alternative. Since Apple Display Calibrator Assistant was still installed within System Preferences Mac users never really found the absence of Adobe Gamma to be a problem. However, given that Windows Vista and Windows 7 don't play well with certain utilities, it was inevitable that Adobe Gamma would eventually be dropped from the Windows version of Photoshop. So, it came as no surprise that as of Photoshop CS3 Adobe stopped shipping Adobe Gamma. For what it's worth, I think we can safely assume that Adobe will never again ship a monitor calibration utility with Photoshop.
Obviously, software only monitor calibration applications use the human eye to determine tone and color differences between a series of white/grey/black/color patches. However, it should go without saying that the eye isn't the most accurate method of measuring these differences. Therefore, my recommendation would be to use a hardware based system such as theDatacolor Spyder,X-rite Photo ColorMunkiorX-rite Photo i1 Display Pro.
Tip for Mac OS X users:a tutorial describing the process of calibrating a display with the Apple Display Calibrator Assistant can be foundhere.
Useful Information on location of ICC/ColorSync Profiles
Photoshop CS6 is only compatible withWindows XP with Service Pack 3orWindows 7 with Service Pack 1on the PC platform andOS X 10.6.8or higher on the Mac platform. The upside of this is that theICCandColorSyncprofiles are more easily found.
Profile locations:-
· Windows XP and Windows 7 - sub-folder named Windows\system32\spool\drivers\color
· Mac OS X - ColorSync profiles are generally located in either the Library/ColorSync/Profiles or Users/~/Library/ColorSync/Profiles folder
Section 3 - Photoshop CS6 Color Settings
The Color Settings dialog is the control room for the Photoshop color management system, and like all control rooms it can appear complicated. If you already use Photoshop CS5 or earlier you'll likely have Color Settings configured to meet your needs, and it's probably best that you stick with those settings. On the other hand new users should pour themselves a cup of strong coffee and pull up a chair because what follows is not always easy to understand, and may take a few attempts to sink in.
Whether you use Mac or Windows the appearance and list of options within the Color Setting dialog, and elsewhere within Photoshop CS6, is to all intents and purposes identical. So, begin by opening the Color Settings, which is found towards the bottom of the Edit menu.
Figure 1 - Photoshop CS6 Default Color Settings
The first thing that I will draw your attention to is the box at the bottom of the dialog labelledDescription. As the mouse is moved across the various pop-up menus, etc. you should see a short but informative explanation of what each menu does. Also note the button labelledMore Options; it's probably best that you select it now, as it will let you see the complete Color Settings dialog rather than the simplified version.
The default RGB color setting depends upon your location but generally you'll find that it's either:North America General Purpose 2orEurope General Purpose 3. If you're working with documents that are primarily destined for the web then either is perfectly acceptable. However, if you're documents are destined for print then these settings are generally regarded as being less than ideal. So, if theGeneral Purposesettings are not ideal, what is?
I could answer the above question by simply writing "USorEurope Prepress Defaults", but doing so doesn't really help explain why nor would it be strictly true. Therefore, I will work my way through each section of theColor Settingsdialog in turn.
The first section is labelledSettings, and is a simple pop-up menu with a list of preset Photoshop settings plus any that you may have saved previously. You needn't worry too much about this section just yet.
Figure 2
Working Spaces
The next section (figure 3) is labelled Working Spaces, and as I discussed earlier the selections made here will determine the Working Space profiles used for color handling of your documents.