Part Seven - Color Management
by Chuck Gardner
sRGB colorspace
 While those
in graphic arts used Macintosh computers almost exclusively
for professional publishing the use Windows 3.1 and then
Windows 95 computers proliferated in both offices and homes
worldwide. Both Windows 3.1 and Windows 95 lacked lacked
ICC based management, and in any case most corporate users
had video cards which only allowed the use of 256 colors
which was fine for most business graphics but unsuitable for
photos and graphic arts work. In addition to using only 256
colors the PC monitors had default calibrations which were
far brighte, bluer (i.e., higher Kelvin temperature white
point) and more constrasty (e.g., gamma = 2.2) than a
Macintosh. Thus a file created on a Macintosh would look
completely different when viewed on a PC. This
incompatibility became a significant issue in the mid-1990s
with the proliferation of the Internet and the increasing
availablily of low cost scanners.
Rather than adopt one of the existing RGB standards such as the de facto ColorMatch standard used in the graphic arts industry (i.e., 5000 K / 1.8 gamma) Microsoft and Hewlett-Packard instead created a new "Standard RGB" color space when ICC color management was incorporated into Windows 98. As the CIE Yzy diagram illustrates, the sRGB gamut (shown by the red outline) is slightly smaller than ColorMatch RGB. The big difference versus ColorMatch RGB is the default white point (6600 K vs 5000K) and gamma (2.2 vs 1.8). The sRGB gamut matches the color space of most uncalibrated scanners and monitors. The criteria for creating it was finding an RGB colorspace that looked good on like those found on corporate computers where getting the primary colors in the corporate logo or bar graph correct is about as critical as color management gets. As a result the sRGB gamut is the smallest, and the worst possible choice for serious graphic arts work and digital photography as this comparison of sRGB, Adobe RGB (1998) and CYMK colorspace illustrates. Adobe RGB (1998)  Adobe, when
adding ICC profiles and ColorSync to PhotoShop and its other
applications, introduced several new RGB color spaces and is
seeking to establish Adobe RGB (1998) as a new graphic arts
standard to replace the widely used de factor Radius
ColorMatch standard. As the diagram at the left illustrates,
the gamut of one of these new RGB gamuts, Adobe RGB (1998)
shown with the yellow border is significantly larger than
Apple RGB (green border) or ColorMatch RGB (blue). It has
default white point of 6500K, and gamma of 2.2 which are
both significantly different from the de facto ColorMatch
standard of 5000K / 1.8 gamma.
 What is
particularly significant about the Adobe RGB gamut is it is
larger than a monitor can display. The reason why you might
want to use an RGB color space which has colors you can't
see on the screen becomes evident when you compare Adobe RGB and sRGB with the color space of a
CYMK printer such as a large format HP DesignJet 3500
CP. It is easy to see from the three-dimensonal mapping
of the color spaces that the larger Adobe RGB (1998) is
nearly a perfect match to the CYMK colorspace of the
printer. You should also note that sRGB also fits within
Adobe RGB color space.
How ICC Tags work in PhotoShop 6.0 Adobe changed the color management interface in PhotoShop 6.0. In version 6.0 a user is required to first define the desired RGB working space and CYMK output space. Two sets of defaults, web and US Prepress, are are offered, wih "web" being the default. If "Web" is selected, sRGB color coordinates will be used to display the image data. If "US Prepess" is selected the RGB values in the file will be mapped to Adobe RGB color space on the monitor. When a file is opened PhotoShop 6.0 checks for an embedded ICC profile tag. This tag tells PhotoShop, or any other ICC aware application how to display it on the monitor. Regardless of which working RGB colorspace is selected by the user, a pure white in a digital file has a value Lab = (100,0,0) and RGB = (255, 255, 255) regardless what colorspace is used. What the working space profile tells the computer's video card, is how to drive the monitor's RGB guns to produce that pure "white" to match the white point for the selected color space. If the working space is ColorMatch the monitor will output the combination of red, green, and blue to produce a pure white with a color temperature of 5000 K, and create a tonal separation equal to a 1.8 gamma. If the working space is sRGB, a pure white will have a color temperature of 6600K and a gamma of 2.2. If the working space is Abobe RGB the setting will be 6500 K and 2.2 gamma. When a camera file is resaved in Photoshop 6.0 with an embedding ICC profile tag which identifies the working space it was edited in, the actual pixel values of the file are not changed. If a monitor calibrated to the sRGB white point and gamma (6600K / 2.2) is put to the right side of one calibrated to the ColorMatch white point and gamma (5000K / 1.8) and the same file is opened without color management, it will look different. But if you put on a pair of glasses with one yellow tinted lens over the right eye and a clear on on the right and viewed the two monitors with separate eyes the yellow lens on the sRGB eye would compensate for the lower white point of the ColorMatch monitor. Thus, in essence what the ICC profile tag on the file does is pass along an instruction for what color sunglasses to wear when viewing the file. When Photoshop running on a color managed computer with a monitor calibrated to the sRGB white point and gamma (6600K / 2.2) encounters a file tagged as created under warmer ColorMatch conditions it instructs the video card to shift the balance of red, green, and blue which constitute white, thereby shifting the color temperature of "pure" white on the screen. If the profile tag later is removed from the file it does not change the file's color data, it simply removes the the "wear yellow / blue / rose colored glasses when viewing this file" instruction and the file reverts, unchanged, to its non-color-managed state. Working Space In addition to white point and gamma the working space profile selected in PhotoShop defines the maximum range of color which can be displayed on a monitor. There are instances, particularly with the sRGB and ColorMatch working spaces, in which both the camera file and the monitor can display colors which fall outside the working colorspace gamut. When this occurs any color data in the source (camera) file falling outside the limit proscribed by the working gamut standards are altered, or "rendered" according to one of four sets of parameters denoted as "rendering intents which are rules for mapping out-of-gamut colors. Prior to ICC profile based color management the out of gamut data which was clipped would be permanently lost. Now, because of the ability of the color management software to dynamically simulate the appearance of any color space on almost any monitor, it is not necessary to change the pixel values in the file. This is VERY signficant change. It is no longer necessary to convert a file to CYMK mode to preview it. When that is done it DOES permanently remap and clip the out-of-CYMK-gamut RGB colors. Beginning with Photoshop 5 users had the abiity to preview files in "virtual" CYMK mode and Photoshop 6 has refined it to the point were "soft proof" simulation of any CYMK printing process is possible. As mentioned above, the Adobe RGB (1998) colorspace is larger than either the sRGB (PC and Web default), ColorMatch RGB (professional graphic arts standard) and the range of color all but the most expensive scanners can record. Which of the working colorspaces is best for you, or whether you even need to bother with color management at all depends on the color gamut your camera records and how you output the files. Keep in mind that ICC profiles and colors management allow one to very easily shift from sRGB for working on web images, to ColorMatch for files printed on offset presses. If you share your photos via the web or e-mail, you'll be best served by using sRGB standards (6600K / 2.2 gamma) to calibrate and characterize your monitor and as your PhotoShop working space. This will ensure that a picture balanced to look good on your monitor, with or without color management will good on uncalibrated or sRGB calibrated computers. Most low-end printers which print directly from RGB files have default configurations which utilize sRGB. If your photo files are used professionally and printed consult with those who design with and print your photos. Learn if they use color management (many still don't) and how they calibrate their monitors for white point and gamma. Most printers still use ColorMatch (5000K/1.8 gamma) because 5000K lighting is the industry standard for viewing color. Advertising agencies and designers who prepare work for print and Internet may be using Adobe RGB (1998) instead. Based on the three-dimensional renderings of color spaces it would appear that Adobe RGB (1998) is probably the best all round choice as a working colorspace. Because both sRGB and ColorMatch fit inside of it, and there are few CYMK colors in offset or ink jet inks which fall outside of it a file with an Adobe RGB (1998) gamut could be coverted to any of the others without a loss or gross distortion of the color data. However, its theoretical superiority can only be exploited if the input device (camera or scanner) and monitor can utilize the color space. If the colorspace of the source file is smaller than Adobe RGB (1998) I have found that using it as the working space in PhotoShop may not be a good idea for several reasons. I've compared the 3D map of my calibrated monitor with Abobe RGB (1998) and find that the Adobe RGB gamut greatly exceeds what my monitor can display, particularly in the blue / green range. This makes it difficult to predict how these colors will print using on-screen soft proofing. I've also noted that when I standardize on Adbobe RGB the smaller color gamut of my camera files is stretched and distorted with the fleshtones becoming over saturated and neutral tones acquiring a cyan color bias. A side-by-side comparison Here is an example of my experience with my Kodak DC290 camera and PhotoShop 6.0 using a self-calibrating Apple 17" Studio monitor (the one with the clear case) set for a white point of 6500 and gamma of 2.2, identical to Abobe RGB (1998). The photograph was illuminated with two Vititar flash units bounced into silver umbrellas. Camera white point adjustment was set to "auto". The photo was opened with no color management, sRGB, and Adobe RGB. Screen shots were made, opened without color management, combined in one file, then saved without an embedded profile. 
Without Management (Center): When a file is opened without color management PhotoShop defaults to the colorspace of the monitor. In this example my monitor was set with a white point of 6500K and gamma of 1.8, the same parameters for Adobe RGB 1998. The flesh tones of the JPEG camera file appear slightly oversaturated. This is typical of DC290 images. The rich saturation is one of the reasons I selected it over others. The front of the G4, which is dark grey with a slight bluish tint appears in the photo much lighter with a noticable cyan bias. The blue of my jeans is far lighter and more cyan than the actual pants. My conclusion is that while the color rendition of my camera is good overall, it is simply not accurately recording the color of some objects. Adobe RGB (1998) (right): a camera file opened in Adobe RGB working space is nearly identical to opening it without color management due to the fact the monitor was calibrated to the Abobe RGB white point and gamma. The only difference is that when opened with the Adobe RGB profile the color data in the camera file is mapped to the larger color space. Colors which were already very saturated in the camera file became even more saturated when displayed in Adobe RGB space. sRGB (Left): The same file opened in sRGB working space appears less saturated and cooler than when opened without color management. Apart from the cyan in some objects mentioned above it is, overall, a good match to the actual appearance of the scene. Another comparison Next I recalibrated my monitor using the ColorMatch RGB standard used by the professional graphic arts community. It uses a much warmer 5000K white point and a less contrasty 1.8 gamma. The comparison of the same file with no color management, ColorMatch profile, sRGB profile, and Adobe RGB profile. 
Since the monitor was now calibrated to ColorMatch defaults there is very little difference between the unmanaged (i.e., monitor profile) and the ColorMatch version. The sRGB version appears darker overall, and again the Abobe RGB version is oversaturated with an extreme cyan bias.  There could be several reasons for the cyan
bias, but I suspected the source of the problem was that the
DC290 was somehow recording a UV component outside of the
range of human vision produced by Vivitar 285HV electronic
flash having. I suspect the G4 case and pants may contain
dyes that are refecting more UV light than other objects and
that UV light is being recorded as image data by the camera,
predominantly by the blue channel which is closer to UV
wavelengths. I was able to use PhotoShop to remove the cyan
bias in these objects to match what I see visually by using
Image > Adjust > Hue / Saturation, selecting Cyan, and
reducing the lightness by -64. I then reduced the saturation
overall by -12, resulting in the image shown to the left
which is a good match to the actual scene. I saved this
adjustment profile, and when it is applied to other photos
taken with the Vivitars it also corrects for the cyan bias.
Creating a Camera ICC profile  I created
an ICC profile for the DC290 by shooting a standard IT8
color target with the camera, using the same stobe lighting
used for the sample photo. The resulting photo file was
analyzed using ScanOpenICC, a profiling tool we use at work
to profile our scanners. A 3D
view of the gamut of resulting ICC profile revealed some
very stange looking spikes in the cyan colors which I
suspect are the source of the strange cyan color shifts.
Using the "Embed chosen profile" AppleScript which is
included with AppleSync, I embedded the newly created camera
profile into a copy of the sample photo and opened the
orginal without color management and the copy with the
embedded camera profile. The results are shown below:
The photo on the right is the uncorrected copy. The one on the left was opened with newly created camera profile embedded. The profile isn't perfect, but is is certainly closer to the actual colors than the cyan saturation adjustment. Conclusions I've concluded that Abobe RGB is a poor match of working color space with my camera. The profiling process, while of no real value for adjusting photos, nevertheless revealed that there is some abnormal sensitivity to cyan, as evidenced by the spike which appears on the 3d rendering. sRGB appears to clip the camera gamut rendering the picture duller and darker than when no color management is used. Overall, I find that ColorMatch RGB is probably the best working space for my current equipment. I was interested to read in a review of the new Nikon D1x camera that it allows the user to select either sRGB (the default) or Adobe RGB as the camera profile. If I had one of these cameras then Adobe RGB would be he logical choice for my PhotoShop working space. Hopefully more camera makers will follow Nikon's lead and make Adobe RGB the default colorspace for all cameras. Until then, or I can justify spending $5,000 for a D1x, I'll stick with my DC290 and ColorMatch. Goto Next Page > Goto < Previous Page Goto Class Outline Goto super.nova.org my home page. |