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The SGITM ColorLockTM System:
Professional Color Management for Color-Critical Applications

 

The SGI ColorLockTM System:
Professional Color Management for Color-Critical Applications

When we launched the Silicon Graphics 1600SW flat panel display last fall, we told you about the engineering mission at the heart of the product: to build a wide-format LCD that gives you an expansive view into your work with absolutely no compromise of image quality. We explained why we made certain critical design decisions--such as using thin film transistor (TFT) technology for exceptional contrast ratios, compensation films rather than in-plane switching to counteract positive birefringence without blurring moving images, and digital interface technology to eliminate flicker. And we described the science behind the exceptional resolution, crispness, and richness of the display: 8-bit subpixel addressing to yield more than 16.7 million true colors, and saturation of those colors at over 62% of the National Television System Committee (NTSC) standard--the highest in the industry.

But all this engineering innovation adds up to more than just an uncommonly pretty picture. To apply our flat panel's display capabilities to real-world tasks, we've now enhanced its ability to manage, match, and create color palettes, so you can precisely control, reproduce, and synchronize color parameters in today's electronic imaging applications and workflows. We call this color management technology the ColorLockTM system, and it is available for Silicon Graphics 1600SW flat panels running on our new Silicon Graphics 540 and Silicon Graphics 320 platforms for Windows NT® 4.0 and for our O2TM UNIX® machines.

ColorLock technology gives you precise control over your flat panel through software and hardware that let you calibrate the display, adjusting it to known settings or to your own customized settings, then "lock in" these parameters. Once locked, these parameters can be quickly, easily, and exactly reproduced by you or other ColorLock system users on SGI platforms. They can also be matched by anyone else creating color with ICC-compliant applications and devices. Using Silicon Graphics 1600SW in a workflow, you can consistently reproduce colors from system to system and across the Web. If you're using or developing applications that depend on color for their core utility, such as medical imaging, film, animation, e-commerce, or print production, the ability to quickly and accurately match color can be an invaluable tool for both improving the quality of your product and getting it to market faster.
 

ColorLock System Components

The ColorLock system consists of a sensor, applications software, the Silicon Graphics 1600SW flat panel display and its adapter card plus a Silicon Graphics 320, Silicon Graphics 540, or O2 platform. The ColorLock sensor and the adapter card ship with the Silicon Graphics 1600SW flat panel display for these systems.
 

Color Science, the Standard Observer, and the ColorLock System

To appreciate the technology behind the ColorLock system, it helps to understand several color science basics: colorimetry, color space, and the CIE standard. If you're already well-versed in the concepts and terminology of color science, you may want to jump to the Showing--and Calibrating--Our True Colors section below.

The first term, colorimetry, relates the measurement of light to the trichromatic responses, or visual perception of light by the standard human observer. How reliable is the "standard observer"? Surprisingly, very. Although many factors influence color perception, studies show that the human factor in the colorimetry equation is indeed quite stable. In fact, in color-matching experiments, test subjects with normal color vision who are asked to mix red, green, and blue light (RGB) until it matches a reference color select similar amounts of separate RGB values to create the color (within the limits of experimental error).

Based on these psychophysical color-matching experiments, in 1931 the Commission Internationale d'Eclairage (CIE) devised a coordinate system for comparing colors that defines colors numerically according to the amounts of RGB that must be combined to match the reference color. This internationally accepted standard color model, called the CIE trichromatic coordinate space, is our basis for developing the colorimetry tools that the SGI ColorLock system uses to measure, match, and lock color in the flat panel display. Using the CIE model, the ColorLock system converts RGB values to XYZ coordinates. X and Z are color-defining chromaticity values. Y represents luminance, an absolute measure of the intensity of a light source, weighted by the color matching function of the visual system that most closely correlates with the monochromatic sensation of brightness. You can see the CIE chromaticity diagram, which illustrates the CIE trichromatic coordinate space, in the ColorLock Settings screen shots below.
 

Display Characterization for Precise Control

Even with a standard color-matching system in hand, there's another crucial variable that must be controlled if colors are to be exactly reproduced: the physical characteristics of the display medium itself. In today's digital environment, we have to characterize, or measure and stabilize the computer platform and the computer display so there can be an unambiguous basis for representing color information.

As part of the ColorLock system, Silicon Graphics 1600SW monitors are individually characterized at the factory by a special process developed by SGI. Each panel's colorimetric profile and specific viewing parameters are programmed into its onboard memory. Based upon this data, a much more exact model can be used during the calibration process--one that describes the exact nature of the changes in color for any given adjustment of the panel colorimetry by the user. Our careful, individual characterization process provides the reference data that increases the accuracy of color matching during calibration.
 

Device Gamut

The range of colors that a device (i.e., monitor, scanner, camera, or printer) can reproduce accurately is called a device gamut. The illustration below shows device gamuts for different types of hardware within the CIE standard color space.

The CIE color space and device gamuts
The CIE color space and device gamuts

The gamut of the Silicon Graphics 1600SW flat panel display references the full CIE color model, and, in terms of output, its gamut is as large if not larger than those of most commercially available display devices. You can use the ColorLock system to restrict the Silicon Graphics 1600SW monitor's color parameters so they do not exceed the gamut of another device in your workflow. This lets you preview or soft proof your final product and see output as it will appear on or after processing by the output device.
 

ICC Profiles and the ColorLock System

Just as the CIE model describes the RGB coordinates of color to provide a common means for defining color space, there is a cross-platform, worldwide standard set of values that defines how specific devices reproduce the hue, brightness, and saturation in their color spaces: the International Color Consortium (ICC) profile. Color management systems reference the ICC profile to reconcile color spaces and gamuts between different devices in a workflow. ICC profiles are an integral part of the ColorLock system; when you calibrate your monitor's color space, the ColorLock system can save the corresponding ICC color matching descriptors. These ICC profile tags enable matches of display output with other ColorLock users and maintain color matching at each stage of the workflow. (Later on we'll see a workflow example of how Adobe® PhotoshopTM5 references ICC profiles to preview images calibrated on Silicon Graphics 1600SW.)
 

Showing--and Calibrating--Our True Colors

Perhaps the most significant value of the ColorLock system is that it wraps the intricacies of decades of color science research--colorimetry, color space, the CIE model, and digital ICC profiles--in a combination of hardware and software that is quick and easy to use, relatively inexpensive, nearly maintenance free, and extremely accurate.

For example, the ColorLock system's specialized color management software and hardware sensors are photopically corrected, which means they take into account the fact that the spectral sensitivity of the human eye is peaked at green with greatest perception of wavelengths in the 550 nm range. Photopic correction results in color calibration that can accurately determine the precise amount of red, green, and blue components required to produce a specific color.

The CIE photopic curve
The CIE photopic curve

The ColorLock system light sensor, which plugs into the back of the flat panel and clips over the top bezel, has better linearity than measurement instruments costing thousands of dollars. For example, many instruments of this type have to be recalibrated regularly. But not so with our sensor. Completely self-contained and epoxy-encapsulated, it uses the same kind of filters found in the motion picture industry, making it extremely durable. In fact, in terms of exposure to the environment the sensor shows no noticeable degradation after two years of use.

The sensor's calibration accuracy is also impressive. Our tests show that with the ColorLock system, we can set any pair of Silicon Graphics 1600SW monitors to display a color difference that is less than the smallest increment of color change perceptible to the human eye.

In addition, the ColorLock system sensor uses the colorimetric profile data previously described to self-correct the panel to which it is attached, and track aging and changes in the panel. The result: automatic maintenance for your flat panel's calibration utility, so that calibration accuracy will never degrade.
 

Ease of Calibration

install_sensor connect_cable

Aligning and connecting the sensor

Calibrating the flat panel display is fast and easy. To calibrate, first select a preset color workspace and then align the sensor on the screen and plug it into the back of the monitor. The five preset color workspaces have standard, fixed values for color temperature, gamma, and brightness and consist of the following:

sRGB HDTV broadcast specification; approximates the average PC monitor
Broadcast The SMPTE-C standard for broadcast television in the U.S.
GraphicArts_D50 Graphic arts specification that displays images with a warm yellowish tint; the standard for pre-press work
Mac® Legacy Image Macintosh® display specification
WebViewing Specification for optimum Web viewing

During the calibration process, ColorLock software displays a series of known colors and gray levels for each primary color as well as for white and black. You see flashes of red, green, and blue light and a range of colors as the ColorLock system checks your monitor's RGB information (the colorimetric profile and viewing parameters that were created during characterization).

This RGB information is then sent to your computer, which calculates the proper gamma values to load into lookup tables for your Silicon Graphics 1600SW monitor as well as for the appropriate adjustments to its lamps. At the end of the process the ColorLock system has calibrated or matched your display to preset parameter values you've selected, and, if you've instructed it to do so, saved an ICC monitor profile in your system. Since the ColorLock system references the CIE and ICC models, you can precisely adjust your flat panel display to known color space and device settings.
 

Creating Custom Settings

If you need to work with a color-matching parameter that isn't specified by any of the five standard presets, you can create your own custom setting by changing the display's color temperature, gamma, brightness, and luminance values. And after you change any of these values, you can use the ColorLock system to recalibrate your monitor and save your customized settings in an ICC color matching profile. Or, if you just want to view a new setting, you can simply adjust values and then cancel the process without saving them.

Here are the adjustable values and their impact on color:

    Color temperature (also known as white point). This control specifies how your monitor displays the color white. This reference point is the most crucial factor in color judgment. The ColorLock system presets use the CIE standard daylight value (locus) for the white point, since this matches most modern color specification systems. You may, however, adjust color temperature in a custom preset that you create.

    Gamma. The gamma control specifies the amount of contrast affecting mid-level grays, or mid-tones, of the displayed image. As you adjust the gamma, your system loads the corresponding mid-tone correction function onto your system's graphics controller.

    Brightness. The brightness control determines the intensity of the back light, affecting the level of light transmitted through the liquid crystal display LCD filter. The luminance window, to the right of the brightness control, shows the measured value that approximates our perception of brightness. For example, when all other factors in a viewing environment are equal, a color with a higher measured luminance appears brighter than the identical color with a lower measured luminance.

 

settings1

settings2

Moving the color temperature slider changes white point, shifting relative intensities of red, green, and blue. Here the CIE chromaticity diagram shows this shift, as the displayed color now corresponds to a different point on the CIE Daylight locus.
 

Color-Critical Application and Workflow Examples

If you use or develop code for color-critical applications that require precise color management, such as for the textile, print, e-commerce, government command/control/communication, or medical imaging industries, a typical workflow may involve a myriad of display systems and output devices. To further complicate the task, it's not uncommon for the workflow to include independent artists in different locations who are contributing graphics or designs that need to be integrated onto the same page--literally or figuratively. With the trend toward geographically dispersed workflows increasing daily due to the explosion of Web intranets, internets, and extranets, it is even more essential that all participants view color in the same manner and, in effect, use the same palette.

For example, in the textile industry, a design can be created in Milan and manufactured in Japan for a company based in New York, with raw materials coming from Singapore. The separation of design, sourcing, production, and marketing facilities dictates that color must be communicated accurately between each location. Using calibrated electronic displays conserves time and money along the entire process--giving the designer a way to generate accurate electronic representations of an artistic vision, giving the factory manager a design template for transforming the raw materials into textile, and giving the sales organization a critical marketing tool for previewing and promoting the new product prior to its completion, without any physical generation of samples.

A calibrated display, with a wide adjustment range of display parameters, is a basic tool for synchronizing judgment of color reproduction and for streamlining the entire workflow. What's more, since all modern color production and color reproduction processes recognize the need for adjustment and control of display color temperature to minimize errors in color judgment, a user-adjustable white point is a critical requirement.
 

SGI Value Add: User-Adjustable White Point

Of all the calibration systems currently on the market, the ColorLock system is the only one that offers a separate system for a user-adjustable white point--making Silicon Graphics 1600SW especially useful for graphic arts, multimedia, and film production applications, which rely on color temperatures referenced to the CIE Daylight locus. In fact, the adjustment of white point is the most critical and basic requirement for these applications' color judgments, since the viewer notices hue differences with greater ease than any other color change.

The way white point is adjusted also impacts the accuracy of the display and its success as a tool for color judgments. The Silicon Graphics 1600SW flat panel component of the ColorLock system uses a separate optical system to adjust white balance (from 5000 to 7000 K). By contrast, CRTs and other flat panel monitors make the adjustment by changing only the brightness of RGB, reducing the dynamic range of gray scales, which in the case of CRTs can make the display unstable. (And for other flat panels, this method relies on setting output display lookup tables to correct for color temperature, simultaneously lowering the luminance and color resolution of the displayed data.) The ColorLock system instead manipulates color temperature via the flat panel backlight, which in turn changes the actual spectral content of the screen's color filters and minimizes impact on display luminance and color lookup tables. The precision afforded by this process gives the user a whole new dimension of color control, with color temperature adjustment accuracy within a range of 25 K.
 

SGI Value Add: Status Monitoring

Many industrial and military control applications require the reprogrammable aspects of a flat panel display attached to a computer. For instance, when devices are used in punishing yet control-critical environments such as transportation and battlefield coordination or power plant monitoring, the operational status of the display itself must be constantly monitored to ensure accurate color display. The ColorLock system includes this built-in measurement capability through integral monitoring of its lamps by its on-board colorimetry profile.
 

The ColorLock System in a Publications Workflow

Today, nearly all design and publishing activity is physically separated from the printing of final copy, so synchronization of workflow processes across a wide area network is crucial--as is a reliable method of predicting how the document will print prior to actually inking the presses. When these predictions are accurate, they can greatly improve the quality of decision making, eliminate mistakes, and save an enormous amount of cost in terms of time and production dollars.

To meet these requirements, we've built several capabilities into the ColorLock system that make it a particularly valuable tool for color management in a publications workflow. Panels 1 through 6 of the illustration give you a quick tour of one such workflow and show how the ColorLock system handles ICC profile tagging and integration with Adobe Photoshop 5.

1. pub_workflow1 2. pub_workflow2
3. pub_workflow3 4. pub_workflow4
5. pub_workflow5 6. pub_workflow6

  1. ColorLock your Silicon Graphics 1600SW flat panel display to a known set of calibration standards. For the presets that ship as part of the ColorLock system, and for any new presets you create, ColorLock software generates an ICC monitor profile that you can use with Adobe Photoshop 5.

  2. Capture an image (i.e., using a scanner).

  3. Open the image in Photoshop 5. Each time you open Photoshop 5, your system references the current profile and gives you a preview of your color workspace, which you can embed directly in your Photoshop 5 image files. This is your soft proof. In addition, when you import your embedded Photoshop files into another ICC-compliant application, you retain your color workspace.

  4. Edit the image in Photoshop, save the edited image, and tag it with an embedded Photoshop ICC profile. ICC profiles establish the fundamental color descriptors that are automatically translated between ICC-compliant applications and devices using Color Management Systems (CMS).

  5. Import the tagged Photoshop file into an ICC compliant application (i.e., QuarkXpress, PageMaker) to proof the image in a page layout application that color-matches the image using the Photoshop ICC profile. Save the image in the page layout application. You can also generate an ICC color profile that can be used as a standard for other flat panels.

  6. Send the page layout file, which includes the tagged Photoshop image, to a printer or other output device. Use a CMS to translate between the Photoshop ICC profile and the ICC profiles for all other devices in your workflow. This maintains color matching with the original Silicon Graphics 1600SW flat panel display soft proof all the way through to the final output. The CMS performs the necessary color adjustments as your image moves from device to device within a workflow.

    file_transfer

  7. Synchronization across the Web. The ColorLock system lets you synchronize color output with all the devices you use to capture and reproduce color, calibrating screens in multiple locations and over the Web.

 

Silicon Graphics 1600SW Flat Panel Update

In addition to the ColorLock system, we've made several other enhancements to the flat panel's utility.

Lamps

Our vendor warrants that our lamps will last 20,000 hours before reaching half brightness; now test data shows that even after 40,000 hours of use the lamps are still at over half their original brightness. Conservatively estimated with moderate power management, that means you have potentially about 10 years of use, compared with only two or three years for a color-managed CRT. And after those first 10 years, you can simply replace the lamps and get another 10 years of use from your flat panel.

Contrast Ratio

Contrast ratio has been enhanced to 350:1, which is several times that of a typical CRT in ambient light.

Industry Awards and Reviews

We've garnered several industry awards since the Silicon Graphics 1600SW product launch:

  • PC Computing MVP Award
  • PC Computing Breakthrough Award for our digital flat panel solution pack
  • Maximum PC Gear of the Year Award
  • Maximum PC Kickass Award
  • VAR Business Top 100 Products: Best Flat Panel designation
  • Computer Reseller News: Inclusion in the Ultimate PC
  • "The SGI screens are the best I've ever seen." --Russell Brown, Adobe Systems
  • "Next time you're in the market for a new monitor, don't limit yourself to the same old thing--check out the Silicon Graphics 1600SW flat-panel display." --Art Liddle, CADalyst
  • " . . . I hadn't seen one to make me say, "I've got to have it" until this bundle came along." --VAR Business

Silicon Graphics 1600SW was also featured on the cover of 3D Design, January 1999, and Animation Magazine, November/December 1998
 

Web Resources for Color Science Information

If you'd like to read more about colorimetry and color management, we recommend the following Web sites:


Copyright © 1999, Silicon Graphics, Inc.