The Critically Important Color and Gamma Calibration Article
This article will explain how to understand and work with Photoshop's Color Management System (CMS), including how to create a custom profile, how to create an ICC profile for a monitor, how to set Photoshop's CMS defaults, how to use the Color Settings dialog box, and much more. It is excerpted from Inside Photoshop CS, by Gary D. Bouton. (Sams, 2004, ISBN: 0672326442).
The Critically Important Color and Gamma Calibration Article - Translation, Please (Page 4 of 16 )
The Color Management Module (CMM), mentioned earlier, is a color transformation engine that translates data from one device's color to another via an independent color space. The CMM receives the necessary information from the profiles so that it can accurately transform a color from one device to another. The CMM interprets the data and, in essence, says to the monitor, "The file data says to display a blue that has the RGB values of 66, 66, 150, but I've looked at your profile and you always make things too red. You will display RGB value 60, 66, 150 so that it looks like the ideal 66, 66, 150. Photoshop doesn't have to know about our arrangement. When it does something to the color 66, 66, 150, use the color in the file data."
When Photoshop performs an operation that requires it to calculate a new color value, that color must be contained in the current working space. The working space's color mode and the position and size of the color gamut of the working space are determined by the ICC profile. Successfully moving color values from one color space to another is a difficult problem because color spaces do not generally share the same color gamut. How is this conversion done?
To bring all the possible color spaces into a common space where accurate color translation becomes possible, a very large device-independent color space is needed. For example, to translate monitor colors to printer colors, a space must exist that is large enough to encompass all device-dependent spaces (spaces that are unique to a specific device) as well as standard color spaces. At the heart of Adobe's Color engine is a Profile Connection Space (PCS) that is device-independent and has a large enough color gamut to hold both the source and the target color spaces. That PCS is based on the CIELAB color space.
Figure 2.2 shows a representation of LAB color space. It is tongue-shaped because visible light is broken down into uneven amounts, with green being predominant. Our eyes are more sensitive to greens than to blues or reds—hence, the distortion in the shape of a color space. In the color section in this book, you'll find an image (a color image, natch') of the CIELAB color model in two dimensions and three dimensions. It also shows how Adobe RGB (1998)—our recommendation for working spaces—neatly fits inside the gamut of the LAB space.
Figure 2.2 Different color spaces often fit inside one another and often overlap.
In the illustration, the working space is Adobe RGB 1998. This color space, larger than anything your monitor can show you, fits within the translation color space making it possible for every point in the space to be mapped—without color loss—to the translation space (LAB). The colors in printer space, whether it's a commercial press or an inkjet, are CMYK. They represent a smaller space than RGB color and have a few areas that RGB cannot reproduce. Fortunately, the translation layer can reproduce those colors. That is why, in the illustration, the printer space eclipses the monitor space slightly, but overall, fits within the working space.
Note -When to use sRGB
Less expensive digital cameras use the sRGB color space, and this throws a monkey wrench into our argument for Adobe RGB 1998 as a default working space. When you use a digital camera that uses the sRGB color space, you might choose to edit the image in sRGB and print to an inkjet whose gamut is within the sRGB color space. Check your manufacturer's documentation.
Note: At the time of this writing, Adobe Gamma is available in Photoshop CS only for Windows. Macintosh users might want to skip this section, but you should refer to the ColorSync information that came with your OS.
The term gamma is the error measured between a straight, linear mapping of voltage applied to your monitor's circuitry and brightness you see on your monitor. Huh? Okay, example time:
Suppose you were holding a lever that applied voltage to your monitor's blue phosphors. In theory, the screen would be black at zero voltage, medium blue (half blue) at half power, and as bright a blue as possible at full voltage. This is a linear plotting of voltage versus brightness, and it does not exist. There is a sag in the midrange of the voltage versus brightness graph, which is why we perform gamma correction.
Note -Gamma settings debate rages on
You may have heard that Windows gamma is 2.2, and 1.8 for the Macintosh. Do not be confused by the Control Panel's default settings for gamma; don't argue with them. Monitor gamma is different than the gamma of working spaces. Some CMS experts tend to favor 2.2 for both platforms. You very well could have a profile for your monitor that is 2.2 but find that Photoshop's working space displays images with deeper midtones than you would expect.
When you installed Photoshop, the setup program installed a feature called Adobe Gamma in your Control Panel(s) folder. In Windows, the Control Panel is under Start, Settings, Control Panel. You can use this Adobe Gamma Control Panel and your eye to calibrate your monitor or adjust its gamma.
The Adobe Gamma Control Panel actually does more than just adjust your monitor's gamma. It also builds an ICC profile of your monitor that your operating system CMS can use instead of the default one that was installed when you installed your monitor or operating system. In fact, the Adobe Gamma Control Panel provides a much better profile, one that is tailored to the way your monitor actually operates, and not just what the engineering specification for the monitor said should be happening.
Note -Use third-party software if your work is color critical
If your work is color critical, you may want to use a more precise, less subjective method to calibrate and profile your monitor. Third-party folks like Monaco, X-Rite, and Gretag Macbeth all offer special color measurement hardware and software that produces a more precise result, but a result that can cost anywhere from $300 to tens of thousands of dollars. For most people, however, the Adobe Gamma software does good enough profiling for the monitor. And the price certainly can't be argued.
For most computers, Adobe Gamma's control of the monitor's white point and gamma affects your view of everything and every program displayed onscreen, whether it uses color management or not. (The white point, or highlight, is the lightest part of the picture; it is the point along the range of tones in an image after which light tones appear white.)
In some situations, Adobe Gamma doesn't have full control: Windows NT 4 and some Windows 95 machines with video cards do not allow software to manipulate display hardware on a global basis. If you use Windows NT 4 or Windows 95 and have one of the stubborn video cards, you're not totally out of luck; Windows allows programs, such as Photoshop and PageMaker that "understand" color management, to alter the display and use the information Adobe Gamma provides (along with the info in the ICC profile) to adjust the information sent to the display, even if every program and the system itself do not permit that alteration and adjustment. On the Windows NT 4 operating system, the ICM must reside in the Color folder within the System32 folder to be available to graphic applications that support system-level profiles.
The good news is that Adobe Gamma works under Windows 2000 and Windows XP.
Warning -Watch out for double color management
If you use Adobe Gamma to calibrate and set the gamma of your monitor, don't use another program's software to do the same thing. Pick one system and use it.
You now know that, for physical reasons, every device—monitor, printer, camera—is limited in the number of colors it can access. The group of colors available to a device—its color space—differs from that of other kinds of devices (monitors and printers, let's say). When one color space can express 10 shades of green, for example, you will see nice transition or good detail in parts of the image that use green. But what happens when you ask a different device to display the same image, and that device has a smaller color space or one with less variety in the number of greens available to it? What if it can express only two shades of green? All 10 of the greens will have to be expressed as one or the other of the two available greens in the new color space. The result will be a poster effect—areas of flat, saturated color where lots of different colors (green, in this example) used to be. This posterization of color is called color clipping. The way a color that is clipped is mapped into the new gamut is called rendering intent. We talk more about rendering intent later.
Color clipping can happen when you move an image between standard color spaces, such as RGB and CMYK. Color clipping also can occur when you move between subsets of each color space (Adobe RGB to sRGB, for example), or between the color gamuts that different inks or dyes can produce. The color management engine uses the ICC profiles for the different devices and color gamuts to determine the best way to resolve the differences in color capability.
If you're striving for a lighter color or a more saturated one and nothing seems to work, you've reached the greatest possible parameters for such a color when it is output using the media and methods you've chosen. To get the color you want, you would have to step outside the image's color gamut. In the CMYK print world, that usually means adding an extra ink, a spot color. A spot color is a custom ink that is prepared to represent a color that's unavailable in the CMYK color space. It is applied as a separate color plate, in addition to the cyan, magenta, yellow, and black printing plates.
This chapter is from Inside Photoshop CS, by Gary D. Bouton. (Sams, 2004, ISBN: 0672326442). Check it out at your favorite bookstore today. Buy this book now.