Digilabs Technologies Blog

…Find me a find, catch me a catch.

Posted on: June 25, 2010

The  previous post discussed t the difficulties of color matching.  To sum it up.  Color is a variable of light and light conditions which vary. Its production depends on the material used to produce it and the device we produce it on. We can not define all colors. We can not print all colors, and each machine will print a different set of colors.

So, if we can not define it, print it, or in may ways even agree on it, how can we even start to match colors?

There are many consultants and color experts out there that will be happy to help you. This is because the details of implementing color management is real life is complex. In this entry I do not intend to do that.  I will try to explain the fundamentals of  color management and what need to happen for it to work. When it comes to the details, you will probably still need this consultant, but at least you will know what you are paying for.

First we need to agree on the meaning of color. In 1931, the International Commission on Illumination (CIE) has defined the CIE_XYZ color space.  The CIE_XYZ color space represents color mathematically as the sensation of color that people with normal color vision will experiance when fed with defined light under defined viewing condition.  In the years later, other CIE profiles were developed, but we will skip directly to the CIE_LAB.  The CIE_LAB color space is designed to approximate human vision and represents all the colors we can see.  We don’t need to know more about it except that its model is such that  it is good as a definition and can be supported by todays computing power on desktops and printers. The CIE_LAB (or the CIE_XYZ) are Device Independent Profiles. They do not define colors by the specific numbers user by a specific device to display a colors.

This is in contrast with Device Specific Profiles. A Device Specific Profile represents color by a specific color we get from a specific device that is producing the color, such as RGB or CMYK devices. By definition they represent a specific device, but we will get to how they do it later.

The actual color management boils down to a simplistic process:

Know where you came from. The producing device (camera, screen, scanner) knows about himself enough to attach his own Device Specific Profile which can tell us the device (color) perception of the world.

Know were you are going to. The receiving device (printer, monitor) knows his own profile, so he knows what is his (color) perception of the world.

Know the way. Translate between those two different (color) perception of the world the best way you can.

And yes, it is simple as that.

The Device Specific Profiles describes each input and output device that participates in the process. A profile also has an additional function. Not only it defines the way the device understand the color values, it also provides a way to map between the device colors space and the CIE_LAB color space (and back). The Device Independent Profile (also knows as PCS, for profile connection space) defines color in a deice independent way and will act as a mediator between the two profiles. Since every profile can translate from its own color space to the PCS and back, the PCS is the Rosetta Stone of color. For example, we can take an RGB value in the screen space, convert it to a CIE_LAB value, and convert it from CIE_LAB to a device specific CMYK value. Note that we did not need the two devices to know about each other in advance. We did not need to match the specific screen to the specific printer (which was the way it was done in the past).

The actual mapping between the profiles via the profile connection space happens in the Color Management Module (CMM). The CMM is a software  application that does the mapping in a way the colors remains consistent across the transformation. The key challenge for the CMM is how to handle gamut mapping, or colors that cannot be reproduced on a device and still get as close as  if it were visually the same color. To do that mapping the CMM will need not only the profiles, but some rules about mapping missing colors. Those rules are called rendering intent, for example, do you want the colors to match perfectly and the others to be “clipped” (you will choose this for a logo, where the color accuracy is key) or would you prefer to map all colors and keep the relationship between the color the same (as in a photo).

As you noticed, all of this process depends on each device having its own device specific profile, but I never told you actually how do we get it. And make no mistake, this is the most important part of it all. If the device profile does not describe the device color abilities correctly, we will not get matching colors regardless of how fancy of a system we use! The art of setting the device profile is called calibration. The calibration process measures the device actual outputs against a predefined set of inputs to build the device profile. Monitor calibration is typically done by connecting a sensor to your screen and running a piece of software that produce a series of colors on the screen and reading them from the sensor. Since the application knows what color value it used and what color values did the sensor sense it can build a map between the device actual colors and the expected colors, or a Device Specific Profile. With a printer, the process typically involves printing a page with a known set of colors and measuring each color to build this mapping table. This is were it gets complicated and labor intensive. You need to profile for each paper type, and machines will move out of calibration based on volume, age, load, and humidity, to name a few, so this is a never ending process.

So, to have a good color managed process you will need to have the right systems in place, which is the simple part. You will have to use those systems correctly. You will need to educate your users to use calibrated monitors and to keep them calibrated. You will need the software they are using to support the profile information on screen and when creating the files to print. You will need your printers to be properly calibrated on a regular basis and you’ll need to pay close attention to the system settings (such as rendering intents or what profiles apply to what paper / job).

Not rocket science, just plain old consistent craftsmanship.

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June 2010


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