BARCO Introduction to Color Theory, Monitor Calibration and Color Management.

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INTRODUCTION

Almost every human being is very familiar with color, yet it is difficult to define what it really is. Although color is very real and concrete, we are compelled to reach for abstract terms when we want to grasp its nature. In the following chapters we will try to clarify the complex combination of different factors through which color is produced. Next, we explain how a color display basically handles and creates colors. Finally we will illustrate the problems with color in a complete color production system and how BARCO's innovative approach to Display Calibration and our Calibrator Talk software provides the tools to solve them.

1. WHAT IS COLOR?

We talk about color when we have a particular sensation in the brain, caused when light radiation of a certain wavelength reaches our eyes. This definition is built up from two parts that are quite different in kind. The first one is of a psychological nature. It deals with the way the sensation of color is processed by the mind. The second one is merely the eyes detection of physical radiant energy. So, color is in fact a psycho-physical phenomenon. It interrelates both a psychological and physical process.

THE PSYCHOLOGICAL POINT OF VIEW

Artists and designers have a wealth of knowledge of the complicated interaction of color: how colors are affected by their environment in a composition; how they can create harmony, contrast and rhythm. They talk about colors in subjective terms: hue, saturation and brightness. The terms hue and saturation are collectively referred to as the color's chrominance. The hue of a certain color is its intrinsic nature. It is, when we observe a colored surface, that attribute of the visual sensation, that makes us say a surface is red, or magenta, or whatever color. .

The saturation of a given color is expressed in chromatic color intensity. A vivid color is highly saturated, as opposed to a pastel color of the same hue and with the same brightness. The colors of the rainbow have the highest saturation a color can ever have, whereas the complete absence of saturation results in a shade of neutral gray, regardless of the hue. The shade of gray depends upon the brightness.

Brightness, relates to the sense that a color appears to be reflecting more or less light energy. We talk about bright colors, in contrast to dim colors. Black is the complete absence of brightness for any hue. Every horizontal cross-section of the cone is a color wheel over which the different hues are spread The highly saturated colors are located on the circumference of the wheel. Moving along the radius towards the center of the wheel, the saturation of every hue decreases, with a certain shade of gray as absolute minimum The brightness, at last, is represented by the height of the cone. The apex corresponds to the absence of any brightness, thus black. The base of the cone is the color wheel that corresponds to the highest possible brightness. .

THE PHYSICAL POINT OF VIEW

Scientists and technicians have a completely different approach on color. They want to express the human perception of colors in objective, quantifiable and mathematical terms. But since all human beings vary in the way their minds process the perception of color, scientists can't describe this phenomenon in terms of a particular human being. They have to consider them in terms of an imaginary individual representing the "standard observer."

From a purely physical point of view, the production of color requires three elements: you need a source of light and a kind of light recording element. This can be the human eye, a photosensitive device, etc. In many cases, you additionally have an object that (partly) reflects the light. Each of the three elements can be characterized by a specific spectral curve, i.e., a curve indicating its relative sensitivity towards light of different wavelengths.

The way in which our brain interprets our visual perception as color depends upon the spectral interaction of these three elements.

THE LIGHT SOURCE
In 1666 Isaac Newton discovered that white sunlight is composed of all the colors of the rainbow. Physically spoken, this means that white sunlight is a mixture of several electromagnetic rays with different wavelengths. Every color corresponds to another wavelength. The complete visible light spectrum, as you can see in the figure below, ranges from 380 nm (violet) to 780 nm (red).

 

The curve shown in the above figure plots the relative intensity of light at each discrete wavelength in the visible bandwidth, and is called a spectral response curve.

In fact, we make a mistake by talking about "white light"; many 'white' light sources exist, going from reddish to bluish white. The reason is that each light source is characterized by it own unique spectral response curve. Hence it is easy to understand that the spectral characteristic of the light source influences the way we perceive the light as color. When we observe an object, its color looks quite different under a reddish light source than under bluish light.

Light sources are characterized by their color temperature, expressed in degrees Kelvin(K). A 100-watt tungsten lamp, as standardized by the CIE (Commission Inernationale d' Eclairage) in 1931, has a color temperature of 2856 K. Indoor fluorescent light is represented by 4100 K and daylight from an overcast sky by 6500 K. ANSI D5000 is a reddish light which was derived and standardized by the pre-press industries to allow for a standard light source for all images to be viewed under and is a compromise between the CIE D6500 Daylight Illuminant standard and the D3200-4100 K of photographic and office lighting.

COLOR TEMPERATURE

Color temperature is expressed in degrees Kelvin, a temperature scale with zero point at 0273.16°C, or absolute zero. The link between temperature and color is made by a theoretical light source called a blackbody radiator whose emitted colored light depends on its temperature.

An example of this phenomenon occurs when a tungsten filament is heated until it melts. The light that it emits changes from red over white to blue. The higher the temperature of the metal, the more bluish its emitted light. The color temperature of a particular white light corresponds to the temperature of the metal that emits that particular white. E.g., the white that is displayed on a computer monitor usually is 9300 K, which is rather blue. This is exactly the same color of a blackbody radiator heated to 9300 K (9026.84°C).

Depending on the circumstances, e.g., the time of the day, white sunlight has a different color temperature. The set of color temperatures of normal daylight is depicted on a the "daylight locus" or daylight curve. They are represented by Dc, where c refers to the blackbody color temperature they most closely simulate. E.g., D65 is daylight that slightly differs from 6500 K.

THE INFLUENCE OF THE OBJECT

We perceive a tomato as red and a lemon as yellow. This perception not only depends on the spectral characteristics of the light source and the human eye, but especially on those of the object itself.

The color of an object is due to the characteristic that it absorbs light from certain wavelengths and reflects light from others. The reflected light from different wavelengths results in the object's color. The tomato absorbs greenish and bluish light while it reflects reddish light.

This characteristic can be described in a spectral reflectance curve, showing the relative intensity of the reflected light at each discrete wavelength.
 

HOW OUR EYES PERCEIVE COLORS

Just like any physical light source and any physical object, every light detector is more sensitive to certain wavelengths than to others. This can also be expressed in a spectral response curve. The detector we are especially interested in is the human eye.

The mechanism of human color vision is based on the presence of light sensitive cells in the retina of the eye called rods and cones. The cones are sensitive to color. In fact the eye contains three types of cones. The first type is sensitive to red, the second to green and the last to blue light. Figure A-5 shows the spectral response curve of the three cones.

So when red light hits the eye, the cones sensitive to red are excited, causing a particular sensation in the brain. That is what we call "red." This is what physically happens when we observe a tomato in neutral daylight: the sunlight, composed of electromagnetic waves with wavelengths from 380 nm to 780 nm, shines on the tomato. This one absorbs the light from 380 nm to 580 nm and reflects the light from 580 nm to 780 nm. This light reaches our eyes, where it excites the cones that have their sensitivity centered on 570 nm. The signals from these cones are finally processed by our brain. This particular sensation called "red", we are taught.

Experiments have shown that every color sensation can be created by exciting two or three types of cones simultaneously. E.g., we perceive a color as magenta when both the red and blue cones are excited. This is the principle on which colorimetry is based, and is called the additive tri-stimulus theory. It states that a wide range of colors can be matched by adding different amounts of three primary colors. Color displays are also based on this theory. The additive primary colors are red, green and blue.

2. THE STANDARDIZATION OF COLOR

In 1931 the CIE began to describe color in a quantitative, mathematical way. The basis of this work was formed by two scientists named Wright and Guild who independently performed experiments on human color matching in the 1920s.

COLOR MATCHING - THE STANDARD OBSERVER
Wright and Guild's experiments were focused on discovering how the different colors of the spectrum can be matched by mixing the three additive primaries. An observer was put into a darkened room to observe a circular field that was split in two halves. One half was illuminated by the test color, the second one was illuminated by three independent, adjustable light sources: red, green and blue. The observer's task was to adjust the intensity of the three primaries until he thought the color on the field matched the test color.

Finally the scientists had an idea of the color vision of a 'standard human observer.' They knew how all the colors of the spectrum could be matched by mixing certain quantities of red, green and blue, according to the vision of a standard observer.

As you can see, the RGB model from the color matching experiments had a drawback: it contains some negative values. This means that you should add a negative amount of red to match a color with a wavelength of 500 nm. Of course, this is very undesirable from the point of view of building an electro-optical device that reproduces what a human would see under the same circumstances (e.g., a color display). This phenomenon seemed to occur with all physically realizable sets of primaries. For that reason, in 1931, the CIE standardized to a set of primaries that were not physically realizable, but could match all colors with entirely positive values.

CIE - XYZ Color Space - (x,y) - chromaticity diagram Based on the laws of Grassman, the CIE transformed the RBG values from the experiments of Guild and Wright by mathematical formulas into a set of new coordinates: x, y and z.
The x and z values have no specific perceptual correlates, but y is a measure for the perceived brightness. In other words, y represents luminance.

As all the colors have three coordinates (x,y,z), we can present them in a three dimensional space, called a color space. The advantages of representing colors in a mathematical space are obvious: each color can be unambiguously defined by its coordinates and the visual difference between two colors can be expressed in a physical distance in the color space. This is especially interesting for the color industry, where people have to trade in printed products, specify the exact color they want to order and specify the tolerance in which the delivered color can differ from the ordered one.

A three dimensional figure, however, is difficult to work with, so the CIE transformed the XYZ values into xyz, where the sum of x+y+z=1. This means that, if two values are known, the third one can immediately be calculated. So the z parameter is usually omitted. The result is a two dimensional representation of the color spectrum, the well known (x,y)-chromaticity diagram. This color space is the only true universal color space. It represent all of the colors which occur in nature and are visible to the "Standard Human Observer". All other color spaces(e.g RGB,CYMK,) are subsets of the CIE 1931 Color space and the full range of these color spaces can be represented in the context of the CIE 1931 x,y,Y model.

CIE - L*a*b & L*u*v*color space: The XYZ color space has one major drawback in practice: It is "non linear" the Euclidean distance between two colors that are just noticeably different, is not the same over the whole color space. This is due to the fact that the human eyes response to color stimuli is non-linear. Such a color space is said to be non- uniform.

This means that in the green area of the space, there can be a relatively large distance between two colors before a human observer notices a difference between both. In the blue area, on the other hand, differences are seen very quickly. The practical consequence is that color industry cannot use the same tolerance for every color. This phenomenon is shown as ellipses in the (x,y)-chromaticity diagram. Each ellipse represents the difference in color that is not noticeable by a standard human observer.

This problem is solved by the uniform color spaces. These are mathematical transformations of the XYZ color space where the Euclidean distance between two colors is proportional to the perceived difference between both.

The two uniform color spaces accepted by the CIE are called CIE L*a*b* & CIE L*u*v*, both accepted in 1976. The former is mostly used for flat reflective color, whereas the latter is used for color displays.

3. HOW A DISPLAY CREATES COLOR

A color display, like the BARCO Reference Calibrator, is a kind of light source. In fact, it contains three light sources, red, green and blue phosphors, that cover the glass of the picture tube. Due to additive color mixing, a whole range of colors can be created. This range is called the picture tube's gamut of reproducible colors.

A display needs electron beams to activate its phosphors. The signals which control the electron beams are called video signals and, are created by the graphics board inside the computer connected to the display.

Additive vs. subtractive color mixing

In case of additive color mixing, Red with Green results in Yellow. Green with Blue results in Cyan, and Blue with Red results in Magenta.

In case of subtractive color mixing, mixing Magenta with Cyan results in Blue. Cyan with Yellow results in Green, and Yellow with Magenta results in Red.

As you can see , the primary colors used in additive color mixing are Red, Green and Blue. For subtractive color mixing, they are Cyan, Magenta, and Yellow.
Additive color mixing occurs when mixing light. Subtractive color mixing occurs when mixing paints.

THE PICTURE TUBE(Cathode Ray Tube)

In a color display, the picture tube or CRT (Cathode Ray Tube) is the component to physically create colors. It therefore contains two important parts: an electron gun and a phosphor screen.

The glass of the picture tube is covered with millions of red, green and blue phosphor dots. They emit light when they are hit by moving electrons. The harder the electron beam strikes the phosphor the more light is produced. This is the role of the electron gun in the neck of the tube: to constantly fire electrons towards the phosphor screen.

The electron gun consists of three elements that independently fire different electron beams. The picture tube is designed so that the three independent electron beams hit different colors of phosphor: one beam only hits red phosphors, the second one only hits green phosphors and the third one only hits blue phosphors.

In case the three beams have the same strength, the three colors of phosphors glow in equal intensity. The result is a white color on the screen. It is obvious that, when we change the intensity of one of the beams, the intensity of the corresponding color changes, and so does the composite color on the screen.

In this way we can create all the colors that can be matched by mixing the colors of the phosphors used in the picture tube. Those are not all the existing colors in nature because the colors of the phosphors of a picture tube are less saturated than the primary colors red, green and blue of the rainbow. The complete range of colors that a picture tube can create is called the tube's color gamut. Colors that the tube cannot create are called out-of-gamut colors. E.g., the highly saturated, primary Process colors (Cyan, Magenta, and Yellow) are lying beyond the limits of today's picture tube's gamut. When a user wants to display such a color, his software application usually substitutes this color with the closest possible in-gamut color.

We can depict the colors in a two or three dimensional figure. The triangle inside the figure contains all the colors that a picture tube can display. Colors outside the triangle are out of gamut

The Computer's Graphics Board
The signals that eventually determine the intensity of the electron beams, thus determine the color of each point of the screen, are called the video signals. They originally come from the computer's graphics board and are processed by the display's electronic circuits before being applied to the picture tube.

Suppose you are working with a design program and making a color drawing. How does this drawing get to your display's screen? The image you are making is always stored in the computer's image memory, where it is divided into a large number of picture elements. The color information for each element is called its color index value. Every color index value is separately stored in the image memory for every point in the image.

The computer's graphics board contains a color look up table (LUT). This table transfers all the board's possible color index values to three digital values: for red, green and blue. If the color index value is expressed in 8 bits, then the graphics board is a so-called 8-bit board. In this case the LUT contains 2 (=256) positions, through which it can display 256 colors simultaneously. These are derived from a palette of 16 million colors. The LUT decides which colors will be selected and under which circumstances. This means that different colors are continuously swapped in and out the LUT by different software applications. A16-bit board can display 32,768 colors simultaneously and a 24-bit board 16,777,216. However, this is only theoretical. The number of colors displayed simultaneously can never be higher than the number of pixels of the picture tube (e.g., 1 million). Moreover the human eye cannot discern the difference between many of these different colors.

The three digital numbers at the output of the LUT are finally transformed into analog voltages by the DACs (Digital-to-Analog Converters) on the graphics board. These voltages are the video signals that enter the color display, that transforms them into electron beams that hit the phosphors in the picture tube. This is how the color gets on the screen.

White Point Setting: Absolute or simulated

As we have shown above, the way in which we perceive the color of an object partly depends on the light source that illuminates the object. The color of a dress might look different in plain daylight than in the shop where you bought it because the difference in spectral characteristics between both light courses. Therefore it is important to view photographs and other objects under standardized light sources in color production industry. This happens in a viewing booths. On color displays that are used as 'soft proof' devices in these environments, this very same condition is obtained be setting the so-called white point or color temperature The color on a display's screen is white when the red, green and blue phosphors glow identically. When the intensity of one or two phosphors slightly changes, the color tone of the white also slightly shifts towards red or blue. In this way the color temperature of the white changes. When you have an image on your screen, e.g., a scanned photo, it is important to set the white point of the display equal to the color temperature of the light source that illuminates the original photo.

There are two ways to set the display's white point. The first is to actually change the parameters of the video amplifiers inside the display. This is a real alignment, referred to as absolute white point setting. The second technique is to simulate the desired white point by controlling the digital values supplied to the DACs on the graphics board. This is called simulated white point setting because the display is electronically aligned to another color temperature than the desired one.

The Reference Calibrator displays allow the user to calibrate to an absolute white point setting in an easy and quick way.

4. COLOR PRODUCTION SYSTEM

The complete system and the role of the display
Let us consider a color production system that consists of a scanner as input, an image processing and page assembly workstation, a proofing printer and an image setter as output. The work station is connected to a color display.

The color display serves as a visual reference to the system operator during activities such as color correction, image enhancement and cleanup, retouching and page assembly. At the various points in the operator's workflow where color accuracy is important (e.g., color correction), the display serves as a soft proof device. In terms of production cost, every output of the system, even a proof print, is expensive. The image on the display, on the other hand, is relatively cheap. Therefore the operator is relying on the display to accurately and predictably depict how a final pre-press proof of the color will appear. Now what are the conditions under which the display can fulfill this role?

The first condition is that the color gamut of the display approaches that of the hard copy device as close as possible. If not, the display cannot create most of the possible colors that the system can print out, thus color fidelity is not possible.

Second, the display must have the same surround conditions as will be used to observe the hard copy output. As we have discussed above, this is met by setting the white point of the display. It must be equal to the color temperature of the light source under which the hard copy output is observed. Moreover it is obvious that this white point must remain stable in the long term.

Third in order for the image displayed on the monitor to match the proof a gamma correction must be applied to the monitor which alters the natural gamma response of the monitor, CRT and allows the display to track with the natural S shape of the tonal curve of the printed image.

GAMMA OF EVERY COMPONENT

An important characteristic of each component in the color production system is its Gamma. It is a number that indicates the relationship between the signal values at input and output of a particular device. A Gamma of 1 indicates a linear behavior. This means that the device's output is directly proportional to its input.

This is the case with scanners and digital camera's. They convert light into RGB video voltages. If the scanned image contains a color of 60% of full-scale red (thus medium red), the resulting output will also be 60% of full scale red. A color display, on the other hand, has a non linear color behavior. If the video signal at the input contains a value of 60% of full-scale red, the image on the screen may be only 30% of full-scale red (thus dark-red). This is why a scanned image normally looks darker when it is shown on a display. This behavior can be graphically displayed in a Gamma curve.

Gamma correction can change the non-linear behavior of a CRT. This is done by recalculating the values in the graphics board's Look Up Tables so that the overall Gamma of graphics board - display is altered. In this way we can obtain an overall Gamma of 1, so that the display seems to behave in a linear way. E.g., if the display's Gamma is 2.2, the Gamma correction should be 0.45 to obtain an overall Gamma of 1 (2.2 x 0.45). As a general rule we can say that the higher the Gamma value, the darker the image will be when displayed on the monitor.

Gamma correction is fairly easy to do between monitor and scanners. A linear response can be displayed easily on the monitor by measuring the natural gamma of the displays RGB electron guns and then placing the inverse values to the display gamma in to the color look-up table of the graphics board.

Gamma correction to film and printed media becomes more difficult due the fact that when an image is captured on film or printed by a press or proofing system the highlights and lowlights (3/4 tone to high light/ Low light to black.) become compressed. The resulting tone curve takes the form of a dynamic S-curve (See below). This is further complicated by the fact that similar to a display where the red green blue electron guns don't have the exact same gamma values. The Cyan Yellow Magenta channels don't have the exact same S-curve. Also the K or black channel does not deliver the same "Black" as the monitor black. Printed black has a base "black level" above black because black is caused by reflected light off a black ink. Monitor black, if the display is properly calibrated is a true digital black , by this I mean when the graphic board is set to deliver a LUT DAC value of 0,0,0 RGB the phosphors are not excited and produce no light black by definition being the absence of light. If set for a linear response when the DAC of the graphic board delivers 1,1,1 RGB the video signal produced displays the first step of gray above black. A Color Management System "CMS" measures the differences between the S curve of the printed piece and the exponential gamma of the display and calculates the appropriate CLUT to be applied to the DAC of the graphic board to deliver the gamma corrected image on the screen. Once the CLUT is in place and the monitor white point is set to match exactly the white point of the viewing light illuminating the image the monitor will match the printed piece.

THE PROBLEM WITH COLOR

A color production system consists of several devices that all deal with color at some level. Because all of these components in the system handle color in different ways, color communication between them is not so obvious. They use different color models, have different color gamuts and different Gammas. Moreover their colors are influenced by calibration and environment.

DIFFERENT COLOR MODELS

The first problem is that the different devices describe color in device-dependent terms. Color displays and most scanners work in RGB color space because they are based on the additive color mixing theory.

Printing devices, on the other hand, are based on subtractive color mixing (see above). The inks they use serve as filters. They absorb certain colors and reflect others. The color of the ink as we perceive it is the result of the reflected color. This is in fact subtractive color mixing. The primaries used here are Cyan, Magenta and Yellow. They are combined with blacK, hence we talk about CMYK when talking about printing devices.

DIFFERENT GAMUTS

The second problem is that the different devices in the system all have different color gamuts. Colors that can be created on one device can be beyond the capabilities of the other devices in the workflow.

GAMMA DIFFERENCES

Every component in the system has a different Gamma or tonal (gray scale) response. As we have explained above, this has an important influence on the way an image is processed by our eyes.

Moreover the combination of all the Gamma's in the system results in an overall Gamma. If you transfer an image from one system to another one with another overall Gamma, the image will look quite different on both systems.

DEVICE CALIBRATION

There are not only differences between different devices in a system, there can also be color differences between identical devices of the same brand and model. This problem occurs if they are not calibrated in the same way. Even when devices of the same make and model are calibrated they do not create the color in exactly the same way. Every device in the color work flow puts it own unique color thumbprint on the image file as the image moves between the analog world we live in and the digital world of the computer.

ENVIRONMENT

Surrounding light is very important when it comes to judging color. Therefore accurate color matching must take the surrounding light color temperature into account. But the light a scanner uses to scan an image can be different from the white point setting of the color display.

THE BARCO SOLUTION

Absolute White Point Calibration

A common technique in color displays is the simulated white point setting (see Radius Pressview). However, it is shown that this technique results in a loss of color gamut of the display. Hence it is obvious that absolute white point setting is a basic necessity for the optimum reliability of a soft proof display in color critical applications.

The BARCO Reference Calibrator displays allow quick and easy absolute white point calibration thanks to the built -in Optisense calibration device Moreover it offers the choice among 5 different white points: three factory defined and two user definable color temperatures. D5300K
D6500K
D9300K
and 2 User defined pre-sets which can be calibrated to any white point from
D5000K- D20,000K

External Calibration

The CalibratorTalk software allows for the performance of an external calibration of the Reference Calibrator displays. This means that the video levels from the graphics board are used during calibration to generate the test patterns the display needs to calibrate the light output of the display. In this way the combination graphics board - display is in fact calibrated so that all the errors between board and display are eliminated.

Gamma Corrections

The Calibrator Talk software allows the setting of the overall Gamma of the graphics board-display combination. You can choose between a number of preset Gamma's or set the Gamma manually. In case of Arbitrary Correction, you can even form any correction function you want and automatically load saved gamma values for different output device and scanners
.

Wide Color Gamut

The Reference Calibrator displays are equipped with special CRTs. Their phosphors guarantee a wider color gamut.

Stability of color circuits

The video amplifiers inside the Reference Calibrator(r) are very stable thanks computer controlled feedback loops. The Optisense optical feedback system allows computer controlled re-calibration of light out put values to a CIE Lab Color model that is stable for the life of the display. Accurate, repeatable, and measurable color which is stored as a ICC color profile in side the display's on board computer and is available to all popular Color Management Technologies. AGFA Foto/Color Tune, Color Solutions Color Blind, SGI ICC, EFI,Kodak KCMS as a point and click export function. Short term color stability

The Enhanced Automatic Kinescope Bias (EAKB a BARCO Patented process )system inside the Reference/Personal Calibrator guarantees short-term stability of the black point and gray scale. The display has no warm up time and is completely immune to environmental factors (humidity, ambient room temperature, supply voltage) which cause short term color instabilities in all other monitors.

The Calibrator Characteristic Parameters(CCP) file: The BARCO display's unique passport for Color Management Systems

A Color Management System (CMS) can be implemented to solve the above-mentioned drawbacks of a color production system by translating the color behavior of all the devices in the system into a universal, device-independent color space. The CIE Lab space is both device independent and is based on the human eye's response to color stimuli. Much the way a language translator speaks and understands many different languages a Color Management system interprets the color input from the different devices in the work flow and translates them into one universal color space or language. Therefore the CMS needs a kind of passport or color profile for each individual device in the color work flow. In most cases manufacturers deliver a standard or generic color profile to the CMS, hereby ignoring the existing differences between devices of the same brand and model.

Each individual Reference Calibrator display has its own unique color profile. The CCP file. The Calibrator Talk software can export this file to the CMS. Each BARCO monitor is specially measured during the final phases of manufacture to determine how the display generates color and the affect that the display has on the image file. The CCP file is unique to the serial number of your BARCO Calibrator Display This file which is delivered in the ICC(International Color Consortium) format is point and click exportable to all popular color management systems and describes mathematically to the CMS how each Reference Calibrator display renders color giving the CMS the necessary information to build a profile which describes to the CMS the color space and gamma characteristics of you individual BARCO monitor

The heart of a Color Management System package is called the color engine. The "color engine" is a complex mathematical equation called an algorithm. In this case the answers which are derived from inputting variable data into the algorithm are delivered by the color management system in the form of a Color Look-up-Table(CLUT). The first thing the color management system does is to convert the color data it receives into a useable form. The RGB and Gray scale information from the scanner and monitor (ICC) profiles are translated into a mathematical CIE Lab representation. The CIE Lab color space is used because all other color spaces (e.g. RGB, CYMK, Pantone, HSV) can be translated and fit into the CIE Lab. A digital file is then sent to the output device. The out put of the digital file is measured using a reflective measuring device and a profile is generated for the proofing device.

The CMS is takes the profile data and runs it through the color engine. The answers generated by the color engine are stored in a CLUT which is applied to the LUT of the DAC of the Computers graphic board. The LUT modifies the image data which is sent to the monitor but does not affect the image file directly. The result is with the appropriate CLUT the image displayed on the monitor will match the color proof. As long as the devices in the workflow remain stable and there scales can be calibrated back to the characteristics of the profile created then the result is a soft proof which is accurate to the hard copy.

As with any computer program the resultant output is only as good as the data it process. The old adage garbage in equals garbage out is especially true of Color Management Software. If the device profiles the CMS uses as inputs are custom and accurate to the actual device you have in your work flow then the color match between display and output device can be very accurate. If however the device profiles used are generic in that they represent the entire population of a manufactures product line then there will be errors between the device profile and the actual color performance of the device. This will yield a result which is less then desirable and color matching is not possible.

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