Callibration & Profiling

• Calibration is the act of changing a device's behavior to achieve some desired state. We calibrate for many reasons, but for color management, the most important reason is to make the device behave consistently so that the profile that describes it remains accurate.

• Profiling, which is often called characterization, is the process by which we record a device's behavior into a profile. It doesn't change the device's behavior; it just records how this device makes color and what colors it can (and cannot) reproduce.

Calibration is like performing a tune-up on your car. You're actually changing the behavior of the device (in this case, your car's engine) by adjusting something. Profiling. on the other hand, is like getting a printout oi the mechanic's report-the car's current levels of hydrocarbons and carbon monoxide emissions, RPM measurements of idle speed, and so forth-that describe the car's current behavior. lust as you can get an emissions report with or without a tune-up first, with color management you can profile (characterize) a device with or without calibrating it first. Calibration actually changes the way a device behaves, whereas profiling simply describes how the device was behaving at the time the profile was created.

If a device isn't stable--if its behavior changes over time--then simply profiling it, as Bruce likes to point out, is like trying to measure a moving target with a rubber ruler.Thepoint is not that it's impossible; it's just much harderthan measuringastationary target with arigid ruler. Ideally, process control is accomplished through calibration alone. It's easier to calibrate an unstable device to make sure that it's behaving the way the profile says it is. In some cases, though, we simply have to accept a certain amount of instability. In those cases, process control will require either more frequent profiling or basing the profile on a device's average behavior.

Calibration and profiling are often confused because the steps involved seem very similar on the surface. Both processes involve sending a target of known color signals-the stirnulus-to a device, measuring the color that results--the response and then feeding the results into some software. With some kinds of devices, such as monitors, calibration and profiling are performed at the same time by the same pieces of software or hardware.

But calibration and profiling are separate processes with different goals. When you profile, you simply record the response and encode it in a profile. When you calibrate, you use the stimulus and response to actually change the device's response. For example. when you set a monitor's white point, the calibration software sends a stimulus-RGB 255, 255,255-to the monitor, examines the response that's directed by the measuring instrument, and keeps adjusting either the monitor guns or the lookup table in the video card until the response matches the white point that you requested.

As said earlier, Calibration means adjusting device's behaviour so that it produces a specific, known response to stimuli--in plain English, it always produces the same color in response to a given set of numbers. But we don't just calibrate our device to any ld state. In fact, there are three possible goals for calibration, which we list here in order of importance for color management:

• Stability
• Optimization (linearization)
• Simulation

Summary:
You spend a modest amount of time before profiling to make sure that the device is behaving properly, adn after profiling to make sure that it keeps behaving that way.

source: Real World - Color Management by Bruce Fraser, Chris Murphy, and Fred Bunting

What is Device Link Profile?

In a normal workflow situation, the color space of the input device is transformed to the color space of the output device via the device-independent L*a*b* color space (known as the profile connection space). This process requires two different profiles — a source profile and a destination profile. 

Color conversion using source and destination profiles

A device link profile is a special kind of ICC profile that converts the color space of the input device directly into the color space of the output device, whereby the output device can be either a physical printer or a file format. Unlike ordinary source or destination profiles, they do not describe a specific color space, but define the conversion from a source color space to a destination color space. The basis for creating a device link profile is, therefore, always an ordinary ICC profile. 

Device link profiles are most commonly applied to direct CMYK-to-CMYK transformations because converting via a device-independent color space can lead to undesirable effects, such as unsmooth color gradients. In a device link profile the color separations are maintained, thus preserving the black channel of the source profile. 

Color conversion using a device link profile

Indeed, preserving the black channel of the input color space is one of the main advantages of using a device link profile. For this reason, device link profiles are often used during data preparation, e.g. to convert from ISOcoated to ISOuncoated. 

Furthermore, device link profiles enable you to by-pass the L*a*b* color space, thus preventing contamination by other colors, e.g. 50% black stays 50% black. 

However, you should be aware that device link profiles are not as flexible in use as other ICC profiles. Each device link profile can only be used for the specific combination of source and destination profiles for which it was created. 

source: http://www.efi.com/