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Created and Maintained by: The Photoimaging Information Council |
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Andrew Rodney |
| July 24, 2008 | |||
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When you consider that digital images are composed of pixels, all having specific numeric values, it seems logical that those pixels would produce the same color appearance day in and day out. Since the device we view these pixels is a display, how do we know this device is properly reproducing the color correctly and repeatedly? Have you ever gone into an electronics store and seen dozen of TV’s all getting the same signal yet showing vastly different color appearance? It is difficult to know which is producing the correct color. You expect the digital images you view today will appear the same next week and a year from now. Until you alter the numeric values using something like Adobe Photoshop, the image should appear the same every time you view it and on any computer system you may be using. As displays age, the color they produce gradually changes over time. The process of setting a device into a specific (idealized) condition is known as calibration. Without calibration, it is not possible to preview identical numbers the same way on multiple devices like a display. This is true for displays, printers and any device that may change condition over time. Then, as this device changes its behavior over time, we calibrate to this ideal condition so that the device behaves not only as we expect but at an optimal condition.  Figure 1. Each Display is receiving the same signal and thus the same RGB numbers but which is providing the correct color? Without proper calibration, it’s anyone’s guess. The next step is to profile the device. The crux of modern color management is based upon what are known as device profiles or the more familiar name: ICC profiles. These small files are simply used to define how a device behaves; how it reproduces color. This ICC profile defines the color space of the device and provides the necessary numeric values understood by computer systems that result in the correct color reproduction. It is important to understand that ICC profiles themselves are simply descriptors for how a device produces color and should the device change behavior; this profile is no longer useful. We would need to recalibrate the device back to ideal conditions and then produce an ICC profile to define this behavior. Such a profile is then used by software products such as Adobe Photoshop or Elements and used to properly display the numeric pixel values. If you work with others and they calibrate and profile their displays, you can share your images knowing that each user sees the same numbers appearing the same way on multiple displays. The best way to ensure that a device like a display is properly calibrated and that the resulting ICC profile reflects this condition is to use a measuring device. While there are software utilities that attempt to accomplish both tasks without instrumentation by asking you to visually calibrate your display, this is not recommended. The human visual system is excellent for some tasks and poor at others. Viewing an entire image within context, for example viewing a full color image on your display and deciding it is too light or too blue is a task an instrument simply can’t handle and a task best served by our human visual system. On the other hand, placing a device into a consistent and repeatable state by viewing a number of solid colors is very difficult task for the human visual system and easily accomplished using an instrument. An example of this that I like to use for illustration can be seen here:   http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html
©1995, Edward H. Adelson. These checkershadow images may be reproduced and distributed freely. In this example, our visual system is fooled yet an instrument would have no difficulty measuring the two gray patches and letting us know that indeed, they are numerically identical. The instrument doesn’t suffer this (and other) optical illusions! We use instruments all the time to measure items that are not well measured visually. Speedometers, rulers, and measuring cups are examples of tools we use to precisely measure items. The instrument used to measure color of an emissive** device like a display is called a colorimeter. Such devices and the software that drive them to both calibrate and profile a display has come down in costs over the years. You can purchase a package for as little as $80 although those packages in the $150-$200 range are recommended for professional users who need additional control over this process.
http://web.mit.edu/persci/people/adelson/checkershadow_proof.html When calibrating a device, there are some target calibration aim points that need to be specified. The three calibration target values that must be defined are the display’s Gamma, White Point and Luminosity. Hopefully each can be discussed in detail in an future article. At this point, it is important to understand that calibration places our display into a predetermined and optimal condition by specifying and adjusting the display for these three target values. Once this is reached, the colorimeter then measures a series of known colors sent to the display by the host software. The end result is an ICC device profile of that display. This profile is used by applications that are designed to preview colors correctly using ICC color management. Not all applications can do this (few web browsers for example). The more mature image editing applications can use the display profile to aid in producing correct previews of digital images. Calibrating and profiling your display is the first step users should take in reaching predictable color from their digital darkrooms. This is a process that should be conducted at least once a month. The entire process only takes a few minutes and most of today’s color management products provide easy, step-by-step wizard based solution so that virtually anyone can accomplish this necessary task. Some of the packages on the market you may wish to examine are the Pantone huey, the Color Vision Spyder, the X-Rite OPTIX and Eye-One Display-2. **Emissive: The ability to emit energy; usually used to refer to a device like a display (which emits light).
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