Photoshop and the Monitor

In the days of film, when you could find out what the color should look like by looking at the film on a light table (assuming that the film was available), you could argue that monitor profiling and calibration was in the "nice but not essential" category. But with the advent of digital capture, the monitor is the first place where the image comes into existence in any meaningful way, so monitor calibration becomes an absolute mission-critical necessity!

When you work in any space except monitor RGB, Photoshop uses the monitor's profile to transform the data on the fly as it gets sent to the video card so that the monitor displays the color correctly. The great benefit of this approach is that it makes it possible for people using very different monitors on different platforms to view the same image virtually identically.

Remember: Photoshop displays everything through your monitor profile. If the profile doesn't describe the real behavior of your monitor accurately, everything you see, and hence everything you do to your images, will be off by a little or a lot, depending on how inaccurate the profile is.

Tip: If You Just Want to Go by the Numbers

It's possible to do good work with Photoshop using an uncalibrated, uncharacterized monitoryou just can't trust what you see on the screen. If you want to simply go by the numbersreading the RGB levels and the CMYK dot percentagesyou can use the Info palette to check your color and simply ignore what you see on the monitor. Even with a calibrated monitor, it's usually a good idea to check those numbers anyway.

If you aren't concerned with the monitor appearance, open Color Settings, pull down the RGB menu in the Working Spaces section, and choose Monitor RGB. We don't advocate thiswe much prefer being able to work visuallybut it is possible, particularly if you're working in a closed-loop environment where you always go to the same output conditions. Of course, if you do this, you may as well ignore the rest of this chapter....

But to make this magic happen for you, you need an accurate profile for each monitor, and you need to let Photoshop know which profile it should use for the monitor. To display color accurately, Photoshop needs to know how your monitor behaveswhat color white it produces, what sort of tonal response it has, and what actual colors it produces when it's fed pure R, G, or B. Photoshop gets all its information about the monitor from the display profile. If you want the color on your monitor to be accurate, you must have a customized ICC profile that accurately describes the behavior of your monitor.

Evaluating Your Monitor

Monitors lose brightness over time, and eventually they simply wear out. Long before the menu bar is burned into the screen, the monitor has lost so much of its brightness range that it probably can't be accurately calibrated to ideal settings.

Calibration utilities work by selectively reducing the brightness of the red, green, and blue channels (making them dimmer). So when you calibrate your monitor, the first thing you'll notice is that it's not as bright as it was in its original uncalibrated state. If it wasn't very bright to begin with, it's a problem.

Here's our simple rule of thumb: turn the contrast control all the way up. If the monitor is brighter at that setting than you like, it's a worthwhile candidate for hardware calibration. If it isn't as bright as you'd like, it's a candidate for replacementit's only going to get dimmer over time, and you'll find it very difficult to bring it to a specific white point. You can still get some life out of the monitor by running it in its raw state and simply profiling that state, but it won't last forever.

Creating a Consistent Environment

Three factors combine to produce the sensation we describe as color: the object, the light source that illuminates that object, and the observer. You are the observer, and your color vision is subject to subtle changes brought on by things as disparate as age, diet, mood, and how much sleep you've had. There isn't a lot you can do about those, and their effects are relatively minor, but it's good to bear them in mind because they make the phenomenon of color very subjective. The other factors that affect your color vision are, fortunately, easier to control.

Lighting. Consistent lighting is vital if you want to create a calibrated system. In the United States, color transparencies and print proofs are almost always evaluated using light with a controlled color temperature of 5000 Kelvins (K). In Europe and Asia, 6500 K is the standardit's a little more blue. (Strictly speaking, the relevant standardsD50 and D65are daylight curves that aren't absolutely identical to the black-body radiation described by the Kelvin scale, but for most practical purposes they're interchangeable.)

You need to provide a consistent lighting environment for viewing your printed output; otherwise the thing you're trying to matchthe original image or the final outputwill be constantly changing. You can go whole hog and install D50 lighting everywhere, bricking up any offending windows in the process, but for most of us that's impractical. But you can situate your monitor so that it's shielded from direct window light, turn off room lights for color-critical evaluations, and use a relatively inexpensive 4700 K Solux desk lamp for evaluating photographs and printed material. Be careful, thoughmany D50 lamps require a special fixture to avoid overheating, because the unwanted wavelengths are reflected through the back of the lamp into the fixture.

Theoretically, the ideal working situation is a low ambient light (almost dark) environment. This maximizes the apparent dynamic range of the monitor and ensures that no stray light is distorting your color perception. However, some shops that have tried this have noted a significant drop in the productivity of the employees forced to work in dark windowless rooms, so go as far toward approaching that ideal as you feel is reasonable.

Consistency is much more important than the absolute color temperature of the light sourcethe variations we've measured in the color temperature of viewing booths at various commercial printers are strong evidence of that. If you work in a studio with a skylight and floor-to-ceiling windows, the color of the light will change over the course of the day, and hence so will your perception of color. In a situation like that, you really need to create an area where you can view prints and transparencies under a light source that's shielded from the ambient light.

A hood to shield the monitor from stray reflections is also very worthwhilea cardboard box spray-painted matte black may not be elegant, but it's every bit as effective as more expensive solutions, and doesn't distort the color the way most antiglare shields do.

Context. Your color perception is dramatically affected by surrounding colors. Again, you can go to extremes and paint all your walls neutral gray. (Bruce wound up doing this because his office was painted pale pink when he first moved in, and he found that it was introducing a color cast into almost everythingincluding his dreams.)

It's easier and more important, however, to make your desktop pattern a neutral, 50-percent gray. Pink-marble, green-plaid, or family-snapshot desktop patterns may seem fun and harmless, but they'll seriously interfere with your color judgment. We also recommend not wearing Hawaiian shirts when you're making critical color judgments. Designer black, you'll be happy to know, is just great.

You also have to maintain your monitor profile. Monitors drift over time, though LCDs tend to drift much more slowly than CRTs, so a profile that was accurate when it was created may not be accurate a week, a month, or a year later. In theory, there are two distinct ways to compensate for monitor drift.

  • You can create a new profile regularly, a process technically known as characterization.

  • You can adjust the behavior of the monitor regularly to bring its behavior into agreement with the behavior described by the profile, a process called calibration.

In practice, most monitor profiling tools do both, and make no clear distinction between the two. The practical distinction boils down to the aim points you choose, and the reasons for preferring one approach over the other stem entirely from the features offered by the monitor.

How White Are Your Whites?

For several years, we advocated calibrating monitors to a white point of D50 and a gamma of 1.8 to match the proofing illuminant and dot gain of the commercial printing industry. Hard lessons taught by bitter experience made us back away from that recommendation.

It's very difficult for a CRT monitor to achieve satisfactory brightness when calibrated to a D50 white point, because the blue phosphors are the most efficient of the three, and calibrating to D50 invariably involves turning down the blue channel. Often, the result is a monitor that looks dingy and yellow. Our eyes respond to brightness in a quite non-linear way, and when the brightness of the monitor is too low (below about 75 candelas/m2) we see yellow instead of white.

But even with LCD displays that can produce a blindingly bright D50, a second problem seems to arise when you attempt to compare an image on the monitor side-by-side with hard copy in a D50 light box. We've been able to achieve close matches, but we've also noted that the highlights on the monitor tend to appear redder than those of the hard copy, even when both monitor and light box are calibrated to D50 and balanced to the same level of illumination.

We haven't yet heard a technical explanation of this phenomenon that completely satisfies us (and we're not sure we'd understand one anyway), but we've experienced it ourselves, and we've heard enough reports from others that we believe it's a real issue. Part of the explanation may be that, while a theoretically ideal D50 illuminant produces a continuous spectrum, both the lamps in light boxes and the phosphors in monitors produce spiky, discontinuous output that's concentrated in fairly narrow bands. There are many different combinations of wavelengths that produce the tristimulus values that add up to a D50 white point.

Bruce believes that there's also a perceptual effect in play. One of the well-documented tricks our eyes play on us is something called "discounting the illuminant"if we look at a red apple under red light, we still see it as red rather than white, because we know it's red, and we discount the red light. But when we look at a monitor, we can't discount the illuminant because the image is the illuminant!

One solution is to separate the monitor and the light boxBruce has taken to working with the monitor in front of him and the light box behind him, switching from one to the otherwhich seems to resolve the problem in large part. Interestingly enough, though, others have reported that calibrating the monitor to D65 rather than D50 creates a much better match with a D50 light box.

Another factor that nudges us towards D65 and away from D50 is that, whenever we measure the color temperature of daylight within a thousand meters of sea level, we invariably find that it's much closer to D65 than to D50. Our eyes seem to adapt easily to a D65 monitor white.

Obviously this subject needs a great deal more research, but we've come to the conclusion that it makes more sense to calibrate the monitor to D65 than to D50. Most LCD displays have a native D65 white point anyway, so for LCDs, just use the native white point. If you're happy with a D50 monitor white, don't fix what isn't broken; but if you're running into any of the aforementioned issues, we strongly recommend that you try D65 instead.

Calibration Parameters

Monitor profiling packages typically ask for the following parameters:

  • White luminance: the brightness of pure white on the monitor, specified in candelas per square meter (cd/m2), or foot-lamberts.

  • White point: the color of monitor white, specified either in Kelvins or as a daylight temperature such as D50 or D65 (see the sidebar, "How White Are Your Whites?"). For practical monitor calibration purposes, you can treat 5000 K and D50, or 6500K and D65, as interchangeable.

  • The tone response curve, usually specified as a gamma value.

Some packages also let you set a separate black luminance value, but only for CRT displaysLCD displays have a fixed contrast ratio, so the black luminance depends entirely on the white luminance.

Display Adjustments

The ability to calibrate a display depends on the controls that can affect its behavior. You can calibrate any display by changing the lookup tables in the video card that drives the display, but the glaring weakness in this approach is that all current video card lookup tables (LUTs) are 8 bits per channel. As you'll learn in the next chapter, whenever you edit an 8-bit-per-channel image you end up with fewer levels than you had when you started, and the same holds true for tweaking the video card LUT.

Some displays allow you to make adjustments that change their behavior in the display itself, avoiding the losses inherent in tweaking the 8-bit video card lookup tables. With those displays, it makes sense to calibrate to a specific white point and/or gamma value.

Other displays, including most but not all LCD displays, have no physical adjustments other than the brightness of the backlight. With this type of display, it makes the most sense to profile their native unadjusted behavior, and let the color management systemwhich typically uses 20 bits per channel instead of the video card LUT's 8 bitsdo the work of correcting the displayed colors.

CRT Monitors

CRT monitors are pretty much an orphaned technology now. Sadly, the high-end CRTs such as the Sony Artisan and the Barco Reference Calibrator have been discontinued. Manufacture of high-end CRT displays has largely ceased.

But there are still plenty of CRTs in good working order out in the field (Bruce hopes to get at least another year out of his Sony Artisan). Most CRT displays allow separate control over the RGB guns. (Some only let you control two of the three, in which case the master gain control, usually labeled "Contrast," controls the third.)

With CRT displays, we recommend adjusting the RGB gains to achieve the desired white point and target luminance. The gamma value, however, can only be achieved by adjusting the video card LUT. If the profiling package offers native gamma as an option, use it. Otherwise we recommend choosing gamma 2.2, because it's closer to the native gamma of CRT displays than any of the other likely choices, and hence involves the smallest tweaks to the video card LUT.

Basic LCD monitors

Most current LCD monitors, including the Apple Cinema Displays, allow only one adjustment, which is the brightness of the backlight. On these types of displays, it makes sense to set the brightness to a comfortable level (bearing in mind that, as with CRT monitors, the higher you set the white luminance, the faster you'll wear out the display), then just profile the monitor at its native white point and gamma. If the software forces you to choose an explicit gamma value, use gamma 2.2.

High-end LCD monitors

Some high-end LCD monitorsnotably the EIZO FlexScan and ColorEdge seriescontain their own lookup tables, independent of the video card, with 10, 12, or even 14 bits of precision. The extra bits don't let the monitor display more colorsthe OS pipeline through which applications communicate with the display is only 8 bits per channel widebut they do let you calibrate the display to a specific white point and gamma without incurring the losses inherent in doing so in the 8-bit video card LUT. For these displays, we recommend a white point of D65, native gamma if it's an option, and gamma 2.2 if it isn't.

LED-backlit monitors

Just to confuse matters, there's a new kid on the block. LED-backlit monitors use arrays of red, green, and blue LEDs for the backlight instead of a fluorescent tube. Since the colors are produced independently, you can adjust the white point by varying the strength of the red, green, and blue LEDs.

This type of display typically also has its own internal 12-bit (or higher) LUT. We prefer profiling the native tone response of the display when the profiling software lets us do so, but we typically use gamma 2.2 when it doesn't.

This type of display is just starting to appear on the market. (Bruce is typing these words on a hand-built pre-production model of the NEC/Mitsubishi 2180 WG. The WG stands for Wide Gamut, and they ain't kidding!) These displays will likely be expensive for the next couple of years, but we expect them eventually to replace fluorescent-backlit LCDs for serious imaging workthey're brighter and have much larger gamuts than fluorescent-backlit displays can produce.

Profiling Tools

If you're serious about working visually with Photoshop (rather than just going by the numbers), a profiling package that includes a hardware measurement device is highly recommended. (Bruce wanted to say "essential," but David persuaded him to take the softer line.) Various eyeball-based profiling utilities (such as Adobe Gamma) are available, but they have two major drawbacks:

  • Most are designed for CRT displays, and don't do a good job of estimating the tonal response of LCD displays.

  • They use the user's eyeballs as the measurement device. Our eyes are highly adaptable, which is great for a mammal living on planet Earth, but distinctly suboptimal when the goal is to return the monitor to a known state. Because human eyes involuntarily and uncontrollably adapt to the current ambient lighting conditions, they aren't accurate enough for consistent color.

Colorimeters and spectrophotometers have none of the eye's wonderful adaptability, so they always produce the same answer when fed the same stimulus. For monitor calibration and profiling, that's a big advantage!

However, if you must use eyeball-based tools, these guidelines may help improve the results:

  • Don't try to use Adobe Gamma on LCD displays. It was designed for CRT monitors, and the mechanism it uses for estimating gamma doesn't work at all well on LCD monitors.

  • Minimize your eyes' adaptability by profiling under the same lighting conditions each time you make a profile. Ideally, the monitor should be the brightest thing in your field of view. (This is always true, but it's particularly critical during profilingsee the sidebar "Creating a Consistent Environment," earlier in this chapter.)

  • Give the display at least a half-hour warm-up time before profiling.

  • Many eyeball-based profiling tools take an existing profile as their starting point. Often, if you take an existing display profile built with the eyeball-based tool as your starting point, the end result is very bad indeed. Start with a known good profilesee "Bad Monitor Profiles," later in this chapter.

A good many users are still reluctant to spend money on display profiling hardware and software. If you don't care how the image looks on the monitor and you're happy to just go by the numbers, you don't really need a custom monitor profile or the gear to build one. In all other cases, and especially if you're shooting digital cameras, trying to save money by doing eyeball calibration and profiling is a classic example of being penny wise and pound (or euro?) foolish. As with most things, you tend to get what you pay for with monitor profiling tools, but even the least expensive instrumented package will return more accurate and more consistent results than any of the visual tools.

Setting Aim Points

Use the capabilities of your monitor as a guide in setting aim points for calibration. The goal is to change the video card LUT as little as possible so that you get the full 256 shades per channel that the OS allows you to send to the monitor.

Aim points and the working space

We should make it abundantly clear that the white point and gamma of your display are entirely independent of the white point and gamma of your RGB working space. The color management system translates working space white point and gamma seamlessly to that of your display. The goal in setting white point and gamma for the display is simply to make the display behave as well as it can.

White luminance

The trade-off in setting the white luminance is that you want it high enough to be comfortable, but low enough to avoid wearing out the display prematurely. If a display can't reach 75 cd/m2 after profiling, it's a candidate for replacement. LCD displays are typically much brighter than CRTs, but overdriving them will wear them out just like CRTs. A reasonable rule of thumb is to set the luminance at about 80 percent of full power (less if it appears too bright), until that setting becomes too dim. Then you can crank it up while starting to shop for a replacement.

Reasonable starting points are around 80-95 cd/m2 for CRTs, and around 120 cd/m2 for LCDs (though if the display can produce a much higher luminance, you may want to set it higher).

Target white point

On displays with genuinely adjustable white pointswhich means CRTs and a very few exotic LCDs at this pointwe recommend adjusting the display to a D65/6500K white point. See the sidebar, "How White Are Your Whites?" for our detailed rationales for doing so. Some combinations of profiling package and display let the profiling package control the display's internal controls via a DDC (Display Data Channel) connection, either through a separate USB connection or through the monitor cable itself. Besides being easier than adjusting the display through the front panel, DDC connections often allow the profiling package to make finer adjustments than the front-panel interface allows.

If the white point isn't adjustable in the display itself, as is the case with most LCD displays, we recommend profiling with the native white pointit's usually very close to D65 anyway.

Target gamma

With those profiling packages that allow it, we generally prefer to use native gamma as the aim point. Sometimes this is a hidden featurefor example, the Sony Artisan software in Expert mode lets you enter "---" (three dashes) to use the display's native gamma.

If native gamma isn't an option, we use gamma 2.2 for CRT displays. LCD displays are a bit more complicatedthe tonal response curve of an LCD display doesn't really match a gamma curve. With most LCD displays and most profiling packages, if forced to choose a gamma value, we'll use gamma 2.2.

Perhaps in recognition of the fact that LCDs don't really follow a gamma curve, some profiling packages now offer more exotic tone response curves. With "standard" LCD displays that don't have their own internal LUTs, we still prefer using native gamma if possible, but if that isn't an option, or if you're using a display with internal LUTs that the profiling software can address, we encourage you to investigate these options. We've obtained good results using the L* curve in Integrated Color Solutions' BasICColor Display and the DICOM curve in Mitsubishi/NEC's Spectraview II.

Bad Monitor Profiles

There's one further gotcha. A fairly large number of users wind up creating profiles with Adobe Gamma or Display Calibrator that turn all colors in Photoshop into a psychedelic mess. If accepting the situation and calling it art works for you, fine. Otherwise, start over. The problem is almost invariably caused by starting out with a bad profile. Pick a known good profile like sRGB or Apple RGB as the starting point, and repeat the whole exercise. This time it should work.

Also, if your monitor profile is invisible to Photoshop, it's probably badPhotoshop rejects monitor profiles with white points that cannot reasonably be interpreted as white. We haven't seen this happen with any of the current versions of the popular profiling packages, but that doesn't mean it can't happen!

Real World Adobe Photoshop CS2(c) Industrial-Strength Production Techniques
Real World Adobe Photoshop CS2: Industrial-strength Production Techniques
ISBN: B000N7B9T6
Year: 2006
Pages: 220
Authors: Bruce Fraser © 2008-2017.
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