Page #133 (103. About Color Management)

104. About Adobe Gamma

Before You Begin

104 About Color Management

See Also

106 Ensure That What You See Is What You Get

Keeping your monitor calibrated is the best way for you to reduce the likelihood that what you see when viewing an image in Photoshop Elements becomes something other than what you print. Using Adobe Gamma, you should calibrate your monitor (as explained in 105 Ensure That What You See Is What You Get) at least twice per year, plus every time you replace your video card or update your video drivers. Monitor calibration affects not just Photoshop Elements but everything you see and use in Windowsespecially your imaging applications.

Selecting the Right Chromaticity

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Proper calibration of color on your screen has a direct effect on how sharp your text looks in your word processor. How do colors affect contrast? Windows uses an anti-aliasing technique to make black letters look smooth on a white background. It's an optical illusion that involves "lacing" the stair-stepped notches in curves and diagonal lines with blue and orange dots (optical opposites) so that your mind actually blanks out the notches. If your monitor isn't calibrated, these colored dots become more evident, enhancing an illusion of low contrast rather than one of high contrast.

Photoshop Elements includes a tool for calibrating your monitor called Adobe Gamma. You'll learn how to use Adobe Gamma in 105 Ensure That What You See Is What You Get. Before you jump to that task, however, there are some technical terms you must understand to complete the steps, and the first one is called monitor chromaticity. Basically, every monitor has its own idea of how to display pure red, green, and blue, and that information is stored in the monitor's ICC color profile. The particular definition of pure red, green, and blue is called the monitor's chromaticity.

A monitor's chromaticity chart based on its ICC color profile, as produced by a program called ICC Inspector.

KEY TERM

Monitor chromaticity A particular monitor's definition of pure red, green, and blue. A monitor's chromaticity is stored in its ICC color profile.

A CRT monitor's profile contains this chromaticity datathe precise hues of red, green, and blue produced when its phosphors are struck by only one electron gun, at any one point, at full intensity. If you were to spread out your monitor's chromaticity color chart over your dining room table, it would look like a full-color version of the potato-chip shape pictured here. You could make three marks on that chart, and say that "pure" red, green, and blue for your monitor are "here," "here," and "here."

When all three of your CRT monitor's electron guns are directed toward the same point, the monitor produces white. So, the chromaticity of the light at that point on the chart is the white point. In the chromaticity chart shown here, the white point is always somewhere in the middle of the triangle that defines your monitor's gamut, but not necessarily in the geometric center. The five white dots represent the relative location of white points within the color gamut, for the five most commonly used light temperatures.

KEY TERM

White point A representation on a chromaticity chart of the hue produced by a monitor when it is instructed to show pure white. A CRT produces white by aiming its red, green, and blue electron guns at the same point, at full intensity. On an LCD monitor, white is produced by transparency. For accuracy, the white point is often expressed in terms of the temperature of the lighta measure of the actual heat it produces. Adobe Gamma enables you to set the basic white point in degrees Kelvin, although you can also express it in coordinates if you have them.

An LCD monitor produces its color in an entirely different manner, although its three optical primaries are also red, blue, and green. A crystalline compound whose physical state is described as "more liquid than solid," circulates between a pair of glass substrates. Light from a fluorescent source is shone through these substrates, which act as polarizing filters. In its natural state, the crystal also polarizes the light along the same axis as the glass. Certain points along the substrate have been tinted red, green, and blue. As some of these points are electrified, the crystal twists, blocking the passage of light and producing color. The precise hues produced can be plotted on the same chromaticity chart as for the CRT monitor.

Because white is produced on a CRT by red, green, and blue put together, the chromaticity of those three points directly affects the white point. But on an LCD, white is produced by red, green, and blue taken away. So, the white point there depends only on the wavelength of light produced by the fluorescent source behind the LCD screen.

The chromaticity of your monitor is not something you adjust, as you can with brightness or contrast. In the Adobe Gamma dialog box, you can set the exact chromaticity (selected from the Phosphors list) and white point (set in the White Point frame) for your monitor, and you can help it calibrate your entire color system so that what you see onscreen is what you get when you print an image. This might sound easyand it is, as long as you've located and installed the appropriate ICC profile for your monitor. If you can't locate the ICC profile (either on the manufacturer's disc or its Web site), you must enter something, and here are some clues to help you:

• When your color is more than just a tiny bit off, but not completely wrong, and if you know for certain that your CRT is a Trinitronespecially if it was manufactured by Sony, but also if your manufacturer licenses Sony's technologyselect Trinitron for your chromaticity setting.

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  Although Adobe Gamma allows you to do so, you should never attempt to guess at the chromaticity settings by manually entering a string of numbers.

  
  

• If your CRT is not a Trinitron, try the sRGB setting, which Adobe Gamma calls EBU/ITU. In Wizard mode, you can test your Before and After settings against one another to see whether the results look right to you. Of all the possibilities, EBU/ITU is the most likely to yield acceptable results if you have a non-Trinitron CRT.

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  In some casesespecially if your monitor is more than a few years oldthe EBU/ITU (sRGB) chromaticity setting might look somewhat better than even your monitor's own designated profile settings. You do not risk damage to your monitor if you use chromaticity settings other than those specified by the manufacturer. But for the sake of image quality, do not override the ICC color profile unless the results look preferable to your own eyes.

  
  

• If the EBU/ITU setting doesn't result in true colors, and neither your monitor manufacturer nor the Internet can help you locate your chromaticity settings, your best course of action is to cancel Adobe Gamma and make gamma and white point adjustments to your video driver's chromaticity settings directly. In the Windows Control Panel, double-click the Display icon. Click the Settings tab and click the Advanced button. Click the tab for your video driver (such as GeForce or nVIDIA), and then adjust the gamma settings for each channel or for all channels simultaneously. The nVIDIA ForceWare driver panel shown here enables you to load a monitor profile (including chromaticity data and white points) into the video driver software directly (from the Custom Color Settings list), overriding Windows' own color management. You might find this to be a blessing or a curse, but in my experience, never anything in-between.

The monitor calibration panel supplied with a recent version of nVIDIA''s ForceWare video card driver. Go here to manually calibrate a monitor if choosing EBU/ITU as your chromaticity (Phosphors) setting produces less than optimal results.

If, no matter what happens, the color on your monitor always looks wrong, consider upgrading your video card driver software. Whatever your card's manufacturer is, check its manual to see who produces its internal video chipset (most likely, nVIDIA or ATI). You can also look for the chip name on the video card, or try looking in the Properties dialog box for your monitor, which will contain a tab for the video card: Double-click the Display icon in the Control Panel, click the Advanced button on the Settings tab, click the tab for your video card, and look for a logo for nVIDIA or ATI. After determining who makes the chipset for your video card, go to that manufacturer's Web site and download its latest benchmark drivers. These drivers use the latest technology and are generally updated far more frequently than the brand-specific drivers for your video card. They are probably better drivers than what you're using now and will probably reset your chromaticity settings to sRGB specifications, or something at least remotely pleasing. Even so, you can tweak the results of using this new driver by changing the color correction settings of the video driver as explained earlier.

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If you decide to adjust the color correction settings of your video driver, make sure that you do so after making whatever changes you want to make to the ICC color profile using Adobe Gamma. Because the video driver's color corrections are applied over top of whatever the ICC color profile is telling the monitor to display, you'll want to make video driver adjustments last.

Adjusting Monitor Gamma

Another term you must be familiar with before calibrating your monitor is gamma. Because a monitor's gamma affects the brightness of images displayed on that monitor, you'll find this term used a lot in graphics editors such as Photoshop and Photoshop Elements. Because a monitor does not respond in a linear fashion to changes in brightness in an image (its response looks more like a sharp curve), by properly adjusting the gamma value on your monitor (the point where its luminance curve begins to bend), you can create a near 1:1 relationship between the tonal values in an image and their brightness onscreen. In other words, when the monitor gamma is set correctly, the brightness and contrast of the midtones within an image will also appear correct.

The increase in power needed to display colors ranging from pure black up to pure white is a gentle curve known as the monitor's gamma.

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Software manufacturers don't make this very clear, but for any one monitor setup, you might actually have to deal with three different sets of gamma curvesone each for red, green, and blue. Windows has a basic curve of its own, whose gamma point is set at 2.20. Adobe Gamma enables you to adjust this starting value and also set gamma variations that pertain to the three color channels individually. Your video card driver might include a separate gamma setting that pertains to color correction for your monitor; it does not affect the choices you make in Adobe Gamma or the values saved in the resulting profile. The video card gamma is simply an additional adjustment to the profile result; typically this gamma is left at a neutral setting of 1.0 so that it does not affect color management.

To properly calibrate a CRT or LCD monitor, you must adjust the gamma for each color channel individually. In Adobe Gamma, you adjust these gamma points by sight. Luckily, because of the way this adjustment is accomplished, you can actually be color blind and still make the proper choices. Adobe Gamma presents you with red, green, and blue squares. Each square is framed with a region of thin horizontal stripes, alternating between full color intensity and black. In the center is a block of color plotted at half intensity. As you squint at each square (so that you're looking at it fuzzy rather than in focus), you adjust a slider to minimize the distinction you see between the striped region and the solid center. If you consider "half intensity" to be a blend of "full intensity" and black, squinting provides you with a reasonable specimen of how half intensity should appear. (If you're color blind and can't see red, green, or blue, you can still compare the striped shade you do see with the solid shade.)

In Adobe Gamma, you adjust the gamma of each color channel using these three squares.