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Pro Lab - Basics of Color Management
Color Management Primer: The Basics of Color Management
Color management is a technology that helps you solve many of the problems associated with reproducing the same color on different devices, within different applications, and across different platforms.
Implementing color management into your lab's workflow helps to resolve the differences in colors that result when you reproduce the same image using different devices, platforms, applications, materials, and processes. Adding color management to your workflow and adopting the International Color Consortium (ICC) industry standard workflow will help you:
Color Communication
Communicating color information between different applications and devices requires a 'common language' that each component in the workflow understands and is able to interpret accurately.
Most digital files express colors either as an RGB value (red, green, and blue) or CMYK value (cyan, magenta, yellow, and black) for each pixel. On a well-calibrated and stable printer, the same RGB (or CMYK) numbers will always produce the same results. But as soon as you try to print the same file on another machine you will get different colors. This is because the colors that each device produces depends on the exact mix of inks, paper, and technology that they use. RGB and CMYK colors are "Device Specific" in order to get prints that look the same (or at least similar) on a variety of printers it's necessary to modify the RGB (or CMYK) numbers used to make the prints. This can be done manually by changing the color balance of images in an image-editing program or in printer-driver software. Or, color management can be used to change the device-specific numbers.
The device-specific nature of RGB or CMYK values makes them unsuitable for communicating color information between different devices. Fortunately, the Comission Internationale de l'Eclairage (an international organization responsible for developing standards for illumination systems) has spent the last 70 years developing unambiguous methods of describing colors. Among these standards are the CIEXYZ and the CIELab systems; it is these device-independent color spaces that color management systems use when communicating information about how particular colors should appear.
Profiles
The ICC has defined a file format for linking device-specific color data from scanners, printers, and monitors to color data in "Profile Connection Space," or PCS. This PCS is always in an unambiguous device-independent CIE color space (either CIEXYZ or CIELab). By using a series (usually two) of ICC color profiles, it is possible to convert from one device-specific color space (e.g., a scanner) to another (e.g., a printer) through the unambiguous device-independent PCS.
Different Sized Gamuts
One of the challenges that labs experience when trying to reproduce color on various devices is the issue of color gamut. If a device can record or reproduce very saturated colors, it is said to have a large color gamut. Photographic transparencies, for example, have a large color gamut. Conversely, if a device cannot reproduce saturated colors well, it is said to have a small color gamut. Newspaper printing systems, for example, have a relatively small color gamut. If you want to print an image that originated on color transparency film in a newspaper, it won't be possible to reproduce the full color gamut of the original. It is one of the functions of a color management system to compress the color gamut in source files to fit within the color gamut of an output device.
The ICC specifies three ways in which this can be done:
The Three Cs of Color Management
There are three key ingredients in a color-managed workflow. If any single one is missing, your lab won't get the expected results. Think of them as "The Three Cs of Color Management."
Calibration
Color management can only be truly effective when as many components as possible in your imaging workflow are calibrated. That is, these devices are working within the manufacturers' tolerances and are in a known and stable condition.
Calibration is typically a two-step process:
The frequency with which a device must be calibrated depends on your quality requirements and on how quickly it moves out of calibration.
Characterization (or Profiling)
Once the individual components of your workflow have been calibrated, you are ready to "Characterize," or make ICC color profiles for them. To do this, you will need access to measuring devices (probably a spectrophotometer and a colorimeter) and compatible ICC profiling software.
Conversion
Once all your lab equipment is calibrated, you need to look at the individual elements in your workflow and make sure that each is set up to use the correct profiles and to make the appropriate color conversions.
Color management is a technology that helps you solve many of the problems associated with reproducing the same color on different devices, within different applications, and across different platforms. Implementing color management into your lab's workflow helps to resolve the differences in colors that result when you reproduce the same image using different devices, platforms, applications, materials, and processes. Adding color management to your workflow and adopting the International Color Consortium (ICC) industry standard workflow will help you:
- Establish consistent and predictable color throughout all parts of the color reproduction chain. This results in fewer remakes.
- Reduce materials and labor costs. In large-scale imaging operations, media waste and lost production time can result in thousands of wasted dollars or Euros per annum.
- Establish standard operating procedures for color production that will reduce color deviation caused by variation in techniques.
Color Communication
Profiles
Different Sized Gamuts
The Three Cs of Color Management
Color Communication
Communicating color information between different applications and devices requires a 'common language' that each component in the workflow understands and is able to interpret accurately.
Most digital files express colors either as an RGB value (red, green, and blue) or CMYK value (cyan, magenta, yellow, and black) for each pixel. On a well-calibrated and stable printer, the same RGB (or CMYK) numbers will always produce the same results. But as soon as you try to print the same file on another machine you will get different colors. This is because the colors that each device produces depends on the exact mix of inks, paper, and technology that they use. RGB and CMYK colors are "Device Specific" in order to get prints that look the same (or at least similar) on a variety of printers it's necessary to modify the RGB (or CMYK) numbers used to make the prints. This can be done manually by changing the color balance of images in an image-editing program or in printer-driver software. Or, color management can be used to change the device-specific numbers.
The device-specific nature of RGB or CMYK values makes them unsuitable for communicating color information between different devices. Fortunately, the Comission Internationale de l'Eclairage (an international organization responsible for developing standards for illumination systems) has spent the last 70 years developing unambiguous methods of describing colors. Among these standards are the CIEXYZ and the CIELab systems; it is these device-independent color spaces that color management systems use when communicating information about how particular colors should appear.
Profiles
The ICC has defined a file format for linking device-specific color data from scanners, printers, and monitors to color data in "Profile Connection Space," or PCS. This PCS is always in an unambiguous device-independent CIE color space (either CIEXYZ or CIELab). By using a series (usually two) of ICC color profiles, it is possible to convert from one device-specific color space (e.g., a scanner) to another (e.g., a printer) through the unambiguous device-independent PCS.
Different Sized Gamuts
One of the challenges that labs experience when trying to reproduce color on various devices is the issue of color gamut. If a device can record or reproduce very saturated colors, it is said to have a large color gamut. Photographic transparencies, for example, have a large color gamut. Conversely, if a device cannot reproduce saturated colors well, it is said to have a small color gamut. Newspaper printing systems, for example, have a relatively small color gamut. If you want to print an image that originated on color transparency film in a newspaper, it won't be possible to reproduce the full color gamut of the original. It is one of the functions of a color management system to compress the color gamut in source files to fit within the color gamut of an output device.
The ICC specifies three ways in which this can be done:
- Perceptual rendering. This rendering intent moves the most saturated colors in source files to the gamut boundary that the output device allows. It then maintains a saturation difference between these most saturated colors in the source files and other less saturated colors. The result is that photographic images look better, but absolute color accuracy may be sacrificed.
- Colorimetric (absolute and relative) rendering. Like perceptual rendering, colorimetric rendering moves the most saturated colors in source files to the gamut boundary of the output device. Other less saturated colors are printed as accurately as possible with the result being that all out-of-gamut colors of a particular hue are reproduced identically. This type of rendering is suitable for conversions between similarly sized color gamuts where color accuracy is the top priority. The difference between absolute and relative colorimetric rendering is that the former attempts to simulate the white point of the source color space. A typical use of this is to show the color of the paper when making contract proofs in the CMYK world.
- Saturation rendering. This intent is very seldom used. Theoretically, it would be used to maximize color saturation when preparing business graphics (graphs, charts, etc.).
The Three Cs of Color Management
There are three key ingredients in a color-managed workflow. If any single one is missing, your lab won't get the expected results. Think of them as "The Three Cs of Color Management."
Calibration
Color management can only be truly effective when as many components as possible in your imaging workflow are calibrated. That is, these devices are working within the manufacturers' tolerances and are in a known and stable condition.
Calibration is typically a two-step process:
- The evaluation or measuring of the device's characteristics that you are interested in.
- Then (if necessary), adjusting the device so that its characteristics change and it produces the required results.
The frequency with which a device must be calibrated depends on your quality requirements and on how quickly it moves out of calibration.
Characterization (or Profiling)
Once the individual components of your workflow have been calibrated, you are ready to "Characterize," or make ICC color profiles for them. To do this, you will need access to measuring devices (probably a spectrophotometer and a colorimeter) and compatible ICC profiling software.
Conversion
Once all your lab equipment is calibrated, you need to look at the individual elements in your workflow and make sure that each is set up to use the correct profiles and to make the appropriate color conversions.