All About Color Management

8/6/2019 All About Color Management

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All About Color Management

Note: This is a simplified, easy to understand explanation of what colormanagement on the computer is all about. We have tried to minimize the

jargon and technical explanations as much as possible. Highly technical

writings on color management can be found on the Internet if one wishes to

pursue this subject further. Our intent is to give an explanation that can be

understood by the average person. A basic understanding of color

management will help you, either directly or indirectly, make better digital

prints. This is our "first edition" of All About Color Management. We will

periodically add to, edit and update, and improve this writing. We welcome

your feedback ([email protected]). If you find some errors, areas that are

confusing, or need further explanation, please let us know.

Why We Need Color Management . . .

When importing, creating, editing, and viewing a digital photo it is critical we

have consistent color all the way from the source (scanned image, digital

camera image, etc.) to the final print. A color management system can

reconcile the color differences among these devices (digital cameras,

scanners, monitors, printers) so we can be more certain the final print is whatwe want.

Color Space . . .

Each device operates within a specific range of colors. This is the devices

color space or gamut. No device whether it be a digital camera, monitor, or

printer, is capable of producing the full range of colors seen by the human

eye.

Color spaces are defined within two different color modes: RGB (red, green,

blue) and CMYK (cyan, magenta, yellow, black). Scanners, monitors, and

most digital image files function within the RGB mode. Printers can function

in either RGB or CMYK mode. This is one of the most confusing aspects of


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color management and requires further explanation.

Most of the printers we use, including inkjet printers use CMYK inks (or some

variation). When using printer profiles the printer's color space is defined in

terms of RGB and Lab color values. When the printer profile is created the

Lab color values are measured from the CMYK ink patches made by the

printer's test chart. These color values are not saved in CMYK mode, but in

Lab color values. This allows the color space to be translated from Lab to

RGB or RGB to Lab. So, our printers are actually functioning as RGB devices.

This will be discussed further near the end of this writing. More background

information is needed here now to make this area of color management more

clear.

Most RGB devices are capable of producing a greater range of colors than

CMYK devices (our RGB printers!). Computer monitors, for example, can

always display more colors than your printer is capable of printing. Because

of these differences in color spaces, colors can shift when transferring an

image from one device to another. The function of a color management

system is to control and adjust for these differences and shifts in colors. The

goal is for the colors in your final print to look the same as the colors on your

monitor, and for these colors to look the same as the colors in your original

photo. Of course computer imaging gives you the ability to improve upon a

less than perfect original photo (i.e. retouching the photo) and maybe create

a near perfect final digital print.

A Little History . . .

Color management has been around for a long time. They were known as

prepress systems used to separate colors which were then sent to a four-

color press machine for printing. The CEPS installations were very expensive

and proprietary in nature. That is, you had to go to one physical location for

the entire process. There was no cross-platform compatibility. There wereno common standards between different CEPS installations.

Eventually the International Color Consortium (ICC) was formed around 1993

with the goal of creating, promoting, and encouraging the standardization of

an open, cross-platform color management system. In 1998 Photoshop 5.0


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was the first application to implement the ICC standards. This first

incarnation didn't work very well for consumers, so Adobe came out with an

improved version in Photoshop 6.0.

ICC Color Management Lets You Do Three Things . . .

1. Convert color images from one color space to another (e.g. RGB to Lab).

2. See colors on a monitor as they'll appear in print.

3. Create proof images on printers.

That's it. It doesn't fix bad photos. An average photo cannot be turned into agreat photo using this system. It can only help give you some consistency

when going from scanner to monitor to printer.

The two most common color management systems in use now are Windows

ICM and Apple's Colorsync. These are complex "color engines" that process

mathematical data from the various devices and digital images we are using.

The CMS will also function as a feedback system. For example, information

used to create a mathematical definition for the color space of a printed

image can be fed back to the monitor to simulate what the printed image willlook like on the monitor (called a soft proof). Photoshop is capable of

displaying a "soft proof" to simulate what the print will look like before it is

printed. One must be using a calibrated monitor in order for this to work

well. Photoshop can also display "out-of'gamut" colors, or colors the monitor

can show but colors the printer cannot print. These will be the areas of your

photo to look at carefully because they will be the areas where a color shift

will occur. The color shift may not be noticeable or it could be significant, but

it will occur. There is usually very little one can do to avoid the color shifts.

We just live within the limitations of this technology.

Now We Get To The Confusing Stuff . . .

When you need to convert colors from one color space to another color

space, a means of handling "out-of'gamut" colors has to occur. The Color


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Management Module (CMM) is the engine of a CMS that drives color

conversions. The CMM uses look up tables (LUT) to perform these

conversions. The CMM does little more than cross-reference a set of numbers

from one color space to another color space.

Before giving an example of a conversion process we need to talk about

device-dependent color spaces and device-independent color spaces. Both

are used by the CMM. RGB abd CMYK are device-dependent color spaces

because they are usually associated with a device (scanner, monitor, printer,

etc.). Lab Color and XYZ Color are theoretical color spaces that represent the

visible color spectrum. Lab and XYZ are device-independent color spaces

that can represent any color in the spectrum. We need to use Lab or XYZ

color spaces within color management because we do not want to impose

any unnecessary limitations on the conversion process by using a color space

(like RGB or CMYK) with a limited range of colors.

Our Example . . .

We want to convert the RGB image we see on the monitor to CMYK for

printing purposes. First, Photoshop sends the RGB values of a pixel in the

image to the CMM. The CMM finds these values in a LUT in the image's RGB

source profile. The CMM then finds the same color values in Lab or XYZ. This

is done for every pixel in our image. We now have an image defined in Lab

or XYZ color space. Next, CMM looks up the LUT in the printer profile for our

CMYK print image. This is where our color shifts occur because some of the

colors in our original RGB image, and now Lab or XYZ color image, may not

exist in our CMYK profile. So, CMM finds the next closest color values. The

CMM sends these values back to Photoshop and we now have our image

defined within the constrains of CMYK. Remember our image is still residing

in a Lab or XYZ color space. It has NOT been converted to CMYK color values.

The printer profile we are using contains CMYK color values (which define our

range of printable colors), but these values reside within the printer profile in

a Lab or XYZ color space. When our image is sent to print, the Lab valuesconvert to the RGB color space and finally our printer driver converts the RGB

color values to CMYK color values for printing.

About Rendering Intents . . .


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There are four different rendering intents available when converting your

image from RGB to CMYK: Perceptual, relative colorimetric, absolute

colorimetric, and saturation. Each rendering intent tells the CMM how to

convert colors for the desired effect. Most color-managed programs like

Photoshop let you pick a rendering intent, but you can't in some programs,

like QuarkXPress 4.0. You'll need to look at your own application program tosee if it offers the rendering intent options.

Perceptual Intent - - Perceptual intent compresses all colors in the image;

overall saturation is decreased slightly, but the relationship between all

colors is preserved, meaning the image looks very similar to the original. The

main negative of using this intent is that it compresses the color gamut, and

it does this whether or not it is needed for a given image.

Relative Colorimetric - - In relative colorimetric intent, out-of-gamut colors in

the source image are converted to the closest available matches in the

destination image. This can cause some colors to be clipped. In-gamut

colors are unchanged. The main negative here is that out-of-gamut colors

will shift slightly, sometimes causing a loss of detail or posterization in

clipped areas. Images with a lot of detail in highlight areas may be affected

by this intent.

Perceptual and relative colorimetric intent are the two most commonly used

intents. Both remap the media white point of the original to that white point

of the destination. There is no hard and fast rule in choosing one over the

other. If color quality is critical for you then its best to try both ways and see

which you like best.

Absolute Colorimetric - - Absolute colorimetric is the same as relative

colorimetric except it does not remap the white point. This is helpful when,

for example, you want to ultimately print to newsprint but you are proofing

on bright white paper. This intent will simulate on your bright white paperwhat your image will look like on newsprint.

Saturation Intent - - Saturation intent will sacrifice color accuracy for color

saturation. You might use this intent for charts or graphs or anywhere you

need vivid colorful images. It does not work well for photographs.


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What Are ICC Profiles . . .

ICC profiles are a set of numerical tables that define colors within a colorspace. ICC profiles can define color for a scanner, digital camera, monitor,

printer, or a digital image. All digital images are defined within a color space.

The standard ICC profile for a RGB image will contain number tables for all of

its RGB color values. The profile will also contain a set of tables for Lab or

XYZ color values. Using a LUT these color values can be translated back and

forth, RGB to Lab, Lab to RGB.

Printer profiles are used by the CMM engine to produce accurate colors when

printing and when soft proofing.

Custom Printer Profiles are individually made profiles that calibrate a specific

printer to one type of paper using a specific inkset, and a specific set of

printer driver settings. To create the custom profile a set of color charts must

first be printed on this printer with the appropriate printer driver settings.

The color patches from these charts are then measured by a colorimeter or

spectrophotometer. Spectrophotometers are more advanced and accurate

than colorimeters so are the preferred instrument for making quality profiles.

The color values are usually measured and recorded in Lab values, not RGBor even CMYK values. Despite the fact that most printers we use print in

CMYK (or some variation), these printers function as RGB printers. Printer

profiles contain several sets of numbers defining RGB and Lab color values

for all of the color patches that were measured by the spectrophotometer.

They also contain information about rendering intents. The printer profile can

then translate color space back and forth, RGB to Lab and Lab to RGB using a

specific rendering intent. Color values are ONLY converted to CMYK by the

printer driver when sending the image to the printer for printing. The

conversion to CMYK values is basically a hidden process we have no control

over.

How Does a Custom Printer Profile Help? Why Not Use Generic or Free Printer

Profiles?


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** A custom printer profile maintains the closest possible match between

your input image and the final print.

** Print output will be more predictable, reducing the need for more

corrections to your photo.

** Greater predictability means fewer reprints, saving you money.

** Custom profiles may use less ink per print depending upon the printer

used, saving ink costs.

** Custom printer profiles can usually print a wider range of colors than

manufacturer's profiles.

** Grayscale values will be more accurate, making it possible to print quality

B&W prints.

** You can create a custom printer profile for any type of paper so you are

not tied to expensive manufacturer papers.

** Print onto a wide variety of third-party papers, canvas and fabric media

not normally available for your printer.

** A high quality custom printer profile often produces better shadow and

highlight detail compared to generic profiles.

Here is a list of software and hardware color management tools . . .


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Application Software - - Adobe Photoshop, Illustrator, InDesign, Pagemaker,

Macromedia Freehand, CorelDraw, and QuarkXPress.

To Profile Monitors - - Colorblind Prove-It, Gretag-Macbeth iCSync and

EyeOne Monitor, Color Vision Monitor Spyder colorimeter and Optical

software, MonacoEZcolor and Sequel Chroma colorimeter, and Heidelberg

ViewOpen. LCD monitors are becoming more popular but are more difficult

to profile and are usually not as color accurate or as bright as a CRT monitor.

To Profile Digital Cameras - - Pictographics InCamera Professional, and

GretagMacbeth ColorChecker DC and ProfileMaker Pro.

Note: This is not a complete listing. We will try to add to it when possible.


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Color Profiles

Printer Profiles

Printer Profiles

ICC Profiles

Giclee Prints

Printer Profiles

Printer Profiles

Color Profiles

Documents

All About Color Management