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A Basic Guide to Color Management eBook

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Basic Guide to Color Management - Chapter 01

1.0 Imaging System

A digital Imaging system will consist of Imaging Input Devices, Image Display Devices, and Image Output Devices. It may be represented diagrammatically in a straight forward form as shown below and consist of:-
  
            DISPLAY DEVICE
 
            |
            |
                                          
            COMPUTER 
            WITH 
            DIGITAL PHOTO IMAGING SOFTWARE 
                              
            /               \
            /                 \

            INPUT DEVICE              OUTPUT DEVICE 

          
Or in a more elaborate fashion as shown below:-

INPUT DEVICES may be :-

  • Scanners
  • Digital Cameras

DISPLAY DEVICES may be:-

  • Monitors
  • Projection TV Displays

OUTPUT DEVICES may be:-

  • Inkjet Printers
  • Film Recorders

The technical performance of these devices will be elaborated on in subsequent sections of this ebook. In this context we will give you an understanding of how to relate the Digital Hardware and associated Software to the photographic process that you are used to.

For the moment we are concerned to establish firmly in your mind the need of working in a well defined, calibrate Digital Processing environment and removing a common attitude that "If I Stuff it up in the camera - I can always fix it later on in the computer". This is not the case!

In this context we will also distinguish between recovering a worthwhile image and manipulating an Image for Effect.

1.2 Color Gamut

Every color device, whether it be an Input device such as a Digital Camera or Scanner, a Display Device such as a Monitor or TV Projection Display, an Output device such as a Photo Quality Inkjet Printer is capable of responding to or reproducing a restricted proportion of the colors in the visible spectrum which the human eye is capable of detecting.

The range of colors detected or produced is called the devices "Color Gamut". When discussing "Color Gamut" it is customary to abbreviate it to "Gamut" for short.

The total electromagnetic magnetic spectrum along with the visible spectrum to which the human eye responds to is shown below:

1.3 Device Dependent Gamut

We now see that every device has its own Gamut.

Thus:-

  • A Digital Camera has its own Gamut.
  • A Scanner has its own Gamut.
  • A Display Monitor has its own Gamut.
  • An Ink jet Printer has it own Gamut.
  • etc, ...,

To make matters worse the same Brands and Models of Input, Display, and Output devices have different Gamuts usually due to manufacturing variations and the environment in which they are working. These variations in the performance of different Color Devices explain in part why it can be difficult or frustrating when trying to get prints which match the original scene. To compound this the characteristics of the paper will also come into the color calibration of the system.

1.4 Color Space

Color may be defined digitally using different numeric units which are called "Color Spaces". One way of representing all the colors in nature is shown below:

The different color Input, Display, and Output devices often use a different "Color Space".

Some Important Color Spaces are:-

  • RGB--------------Red, Green, Blue---------------------------Device Dependant.
  • sRGB-------------Red,Green, Blue----------------------------Device Dependant.
  • CMY--------------Cyan, Magenta, Yellow--------------------Device Dependant.
  • CMYK------------Cyan, Magenta, Yellow,Black-------------Device Dependant.
  • HSV---------------Hue, Saturation, Value
  • CIE Lab-----------Luminance, with a, b chrominance--------Device Independent.
  • CIE XYZ---------Trichromatic Values------------------------Device Independent.

Scanners and Monitors usually work in RGB Color Space.

Digital Cameras often work in sRGB Color Space.

Printers usually work in CMY or CMYK Color Space.

Thus Images scanned and displayed on RGB devices must be converted from RGB Color Space to CMY or CMYK Color Space at some stage in order to obtain photographic hard copy on a printer output device. This conversion is sometimes done in the computer and sometime done in the output device.

Likewise it follows that an image held in a digital CMY or CMYK file on a computer must be converted to RGB Color Space before being displayed on a Monitor. This conversion is usually done in the computer.

So we now see that we have diferent Color Spaces associated with different Input, Display, and Output Devices with each of them having a different Gamut!

Problems often occur when the Input Device(Digital Camera, Scanner etc.) has a larger Gamut than the Output Device (Say Ink Jet Printer).

Most original Transparencies and Color Prints have a larger Gamut than a typical Output Device such as an Ink Jet Printer and the associated paper type being used.

1.5 Mapping

1.5.1 Color Space Mapping.

Mathematical operations based upon matrices are used to convert from say, RGB Color Space to CMYK Color Space. This is called Color Space Mapping and will be part of an Image Processing computer program. Your computer program may also enable you to work in different Color Spaces. Advantages to photographers in selecting a Color Space when manipulating Images will be considered when we distinguish bother sections of this course.

1.5.2 Gamut Mapping.

Mathematical operations again based upon matrices are used to redistribute the color values associated with one Imaging Device eg. Scanner to fit the Gamut of another Imaging Device eg. Monitor.

This is called Gamut Mapping.

The most common type of Gamut Mapping is "Gamut Compression".

Fortunately we are not concerned with the detailed mathematics required and will only need to appreciate the Digital Image Manipulation Process through the use of computer programs and Third Party Imaging Consultant Services.

Gamut mapping is achieved through what is called "Profiling".

1.6 Profiling

Profiling enables us to perform Gamut Mapping in the Image Processing Software we may be using.

Typical Image Processing Software such as ADOBE PHOTOSHOP recognise and use international standards which assist us in the calibration process.

The creation of International Color Consortium (ICC) profiles for color Input , Display, and Output Devices is the foundation of Digital Color Management and embodies Digital Image Processing System Calibration. Color Management in the Mac and Windows environment has some diferences which will be addressed in later Section of this course.

Profiling has the following objectives common to both Operating Systems:

  • Match the appearance of a Scanned original image to that Printed on a hard copy Output Device.
  • Match the physical appearance of a Scanned original to that of the digital image displayed on the Monitor or VDU.
  • Match the appearance of an image displayed on a Monitor to that to be obtained on a Printer.

Other objectives may include:-

  • Simulating the appearance of image to be obtained on an Output Device on a Monitor or VDU. This is called "Soft Proofing".
  • Simulating the appearance of an image on one Output Device to that which would be obtained on a different Output Device. This is known as "Digital Proofing". This could be extended to visualising an image on the same Output Device but taking into account different paper characteristics.

Example:-

As most transparencies and Color Prints have a larger Gamut than a typical Output Device such as an Ink Jet Printer we aim through the Profiling Process to reproduce the Original Image realistically by "Compressing" or Mapping the Gamut of the Input Image to fit the colors available from the Output Device.

1.7 The Profiling Process

Our Digital Imaging Computer System can be represented diagrammatically as shown below.

The Device Independent Color Space was established in 1931 by the Commission International De L'Eclairge (CIE).

Two Common device independent Color Spaces are:-

  • CIE Lab Color Space
  • CIE XYZ Color Space

In order to map Input Color Space & Gamut to the Display Color Space & Gamut and Display Color Space & Gamut to the Output Color Space & Gamut we need a " Device Independent Color Space".

As already mentioned RGB, CMYK and CMY are all device dependent.

In 1931 an international group of scientists called the Commission Internationale De L'Eclairage (CIE) after researching the characteristics of human vision established an objective way of expressing color which specifies the entire visible color gamut.

Two common numeric Device Independent Color Spaces are:- CIE L.a.b. and CIE XYZ

As can be seen from the above diagram Device Independent Color Space is an intermediate Color Space through which Digital Cameras, Scanners, Monitors and Printers are calibrated and through which images are Mapped from one device to another.

The process of converting the characteristics of Device Dependent Color Space with its associated Gamut into CIE-L.a.b or XYZ Device Independent Color Space is called Profiling.

A Device Profile essentially consists of a List of Device specific RGB, CMY, or CMYK numbers and their equivalent CIE Device Independent Values.

This List of numbers identifies all of the colors capable of being Imputed (Scanner, Digital Camera, etc...), Displayed or Printed by the Device.

This List is called a LOOK-UP TABLE [LUT].

Thus:-

  • A Digital Camera will have its unique Input Profile [LUT]
  • A Digital Scanner will have its unique Input Profile [LUT].
  • A Monitor or VDU will have its unique Display Profile [LUT].
  • A Printer will have its unique Output Profile associated with a particular paper type [LUT].
  • ... and so on.

Profiles are useless with out some way of applying them to an image or color document.

1.8 Applying Profiles.

Profiles are applied using a "Color Management Engine". An organisation, called the International Color Consortium [ICC] has created a well defined Profile Format [ICC3.0] which is gaining rapid acceptance as a Standard for Device Profiles.

An ICC Color Management Engine applies Profiles by joining two profiles, one for the Input device and one for the Output Device. The joining of these Profiles is done via an ICC-AWARE application program such as ADOBE PHOTOSHOP under these circumstances. The Digital Image is then passed under control of the computer program picture element (pixel) by picture element (pixel) through the Profiles.

When Profiles are used, your Digital Imaging software such as ADOBE PHOTOSHOP with its associated ICC Color Engine is Mapping the CIE-L.a.b numerical values of the Input Profile to the CIE-L.a.b numerical values of the Output Profile and then substitutes the Input and Output Device values through their Device Dependant Look-up tables for the CIE values.

This is all handled in your Digital Imaging Processing software such as ADOBE PHOTOSHOP.

The end result for example is a single Look-Up Table which converts Input device Gamut to Output device Gamut which may or may not involve a change of Color Space.

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The Digital System is Calibrated and the creative photographic digital Image process occurs in a well defined and repeatable environment .

Methods of Calibrating your Digital Imaging System and obtaining the ICC profiles to incorporate into ADOBE PHOTOSHOP will be covered in subsequent sections of this ebook.

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