White light

White light is a mixture of all the wavelengths that the human eye is capable of detecting. We generally think of daylight as white. However, daylight is not always white. Because it is filtered by the Earth's atmosphere, it changes color depending on the angle at which the light is entering the atmosphere in relation to our position on the Earth. Particles in the atmosphere also have a bearing. For instance, when there is an abundance of moisture particles in the air, we may see a blue tint to our view.

Color

Color is the sensation produced when electromagnetic radiation of wavelengths within the range 380 nm to 780 nm, reflects from objects into our eyes where electro-chemical reactions cause signals to be passed to our brains. These signals are interpreted by our minds as colors. That is, colored light only appears to us when there is a predominance of wavelengths from just a part of the spectrum.

It is when the objects we look at absorb part of the spectrum to which we are sensitive, that we perceive the color of an object. For instance, if an object absorbs those wavelengths corresponding to the colors red and green, we then perceive blue. If an object absorbs those wavelengths corresponding to the colors green and blue, we then perceive red.

Perception of color

Seeing color is a sensation. Every person does not feel sensations the same way; there is no absolute color that is inherently seen the same way by every person.

Everyone will agree that ripe tomatoes are red. However, a group of people probably won't agree on which tomato is the reddest, or how a group of tomatoes should be ranked in terms of their redness.

Sometimes we have ingrained emotions related to certain colors. Red is an aggressive color in western society. People with red cars tend to drive faster. Police also tend to stop red cars more often, since they drive faster, but also as a response to seeing the color red.

Factors affecting the perception of color

There are a number of other circumstantial and environmental factors that impact how the eye perceives light and color.

They include:

• Color temperature (light intensity)
• Metamerism
• Surrounding color

For some good examples of how perceptions can interfere with reality, visit the sites listed below

•   http://www.sandlotscience.com/
•   http://www-bcs.mit.edu/people/adelson/checkershadow_illusion.html

Color temperature

The intensity of the surrounding light changes the perceived color of any object. Color temperature is a way of measuring the intensity of light radiating from a light source. It is measured by comparison to a specific metal contained in a black box and heated to a specific temperature. The unit used to measure color temperature is called Kelvin.

The standard color temperature for viewing color reproductions is 5.000 Kelvin. Viewing booths suitable for evaluating color must be set to this standard to ensure correct lighting conditions.

Metamerism

Metamerism occurs when two colors match under one light source, but appear different under another light source. Those two colors are called a metameric match.

A metameric match might cause problems when trying to match proofs to press-sheets under different lighting conditions.

Surrounding Color

The color surrounding an object can affect how the color of the object is perceived. If a green circle is surrounded by yellow, it will appear lighter than if it is surrounded by dark blue.

Given the difficulties in perceiving true and accurate colors, it is therefore extremely important to reduce the possibility for error by evaluating color reproductions only in a standardized viewing conditions.

Additive color

White light is the sum of many different colors. When equal parts of each of the three major bands of color are added together, white light is created.

White light is the sum of Red, Green, and Blue light. This is known as additive color. As stated earlier, Red, Green, and Blue are the primary components of white light. The presence of all three colors will result in white, and the absence of all three will produce black.

Additive color involves the manipulation of a light source or multiple light sources to control color. A television monitor uses additive colors. The television begins with a blank, dark screen. Three separate electron beams corresponding to red, green and blue signals are projected onto a fluorescent screen. The screen is made up of tiny triads, or groups of three individual cells. A single triad is also known as a screen pixel. Each cell within a triad is sensitive to one of the three beams and will fluoresce, or give off a colored light, when excited by the beam. The result is a combination of red, green and blue light for each screen pixel, which we see as a single color.

Subtractive color

Printed color begins with white paper illuminated by white light. Take away the light, or print on black paper (with transparent inks), and you will see nothing.

To build a color image, we can selectively subtract some of the light. When we print with red ink, we see it as red because it absorbs blue and green light and reflects only red. In other words, red ink absorbs, or subtracts, two-thirds of the visible spectrum, and reflects one-third. The same can be said for blue and green. They each absorb two thirds of the visible spectrum and reflect one third.

In the printing process, we use colored inks that each reflect not one-third, but two-thirds of the visible spectrum. These special inks correspond to the secondary colors of light; namely Cyan, Magenta, and Yellow.

Since each color absorbs one-third of the visible spectrum, it takes all three inks to make black. Overprinting any two of these colors will produce red, green, or blue. By overprinting selected amounts of cyan, magenta, and yellow, the widest range of colors, within the limitations of paper and ink, can be achieved.

Color Reproduction

In color reproduction, pigmented transparent inks (cyan, magenta, and yellow) are used for printing. These inks are called process colors. A transparent printing ink is made to absorb one component of white light and transmit the other two.

The paper, or substrate, then reflects the transmitted colors back to the eye. Cyan ink absorbs only red light, so appears blue-green. Magenta ink absorbs only green light and appears bluish-red. Yellow ink absorbs blue light.

Color Absorption and Reflection

In theory, when each of the process colors - cyan, magenta, and yellow - are overprinted, absorbing all the visible light reflected from the paper, the result should be black.

In reality, solid layers of all three printing inks do not absorb all the available light, and a brownish grey colour is produced. This is because of impurities in process inks. Cyan ink not only absorbs red as it should, but also absorbs some green and blue light.

The magenta ink should absorb only green light. It also absorbs some blue and red. Yellow ink is nearly ideal. To overcome this problem, black ink is also used. When black ink is added to the reproduction, it will add detail and enhance contrast, making the dark areas appear darker and the light areas appear lighter.

Cyan, magenta and yellow can therefore define the printed color space. This representation of the color-space is referred to as the CMY color space -a three-dimensional model, resembling the RGB model, yet distorted due to the above-mentioned impurities.

Color separation

In color reproduction, the process of translating a color photograph or transparency into its cyan, magenta, yellow and black components is called color separation.

The principle of color separation is similar in both photographic and electronic scanning processes. Each process utilizes the concepts of additive and subtractive color for filtering the individual components of white light.

The photographic process consists of illuminating the original copy with white light and then separating it into three images by placing red, green, blue filters in between the copy and the film.

• A red filter is used to generate the cyan separation
• A green filter is used to generate the magenta separation
• A blue filter is used to generate the yellow separation

The black separation is made from a combination of the other three.

CMYK

The printing industry generally refers to the four process color separations as CMYK where: C is for Cyan, M is for Magenta, Y is for Yellow and K is used for Black, to distinguish it from Blue.

Different color languages

The devices comprising an open computer imaging system may all process colors differently. Each type of device can be thought of as using its own 'language' when interpreting and rendering color. This means that there can be no automatic way of guaranteeing color fidelity throughout a system. Also, device characteristics, such as age and wear, and operating conditions, such as temperature, can affect color rendition. This can lead to disappointment when, for instance, what was seen on the monitor looked fine, or when a screen representation of a scanned artwork looks markedly different from the original.

Color spaces

Unfortunately, we are often not going to be able to faithfully reproduce an object as seen by the human eye. Devices and systems available today are simply not capable of doing so.

Monitors can represent more colors, operating in the RGB color space (red, green and blue phosphors), than desktop printers and the offset lithographic processes.

A scanner records, or samples, an image as a set of RGB values. The accuracy of the color measurements taken from the image depends on the scanners manufacture, maintenance and settings. A monitor will display that image to us in a way that depends on viewing conditions, the screen phosphors used in its manufacture, the state of its settings and on its condition. Further, when an image is printed, the colors reproduced and perceived will depend on the inks, as well as the paper used to print and the conditions under which the print is viewed.

A system is required to compensate for the color handling differences between the devices. This system is required to enable predictable reproduction of an original image, in which the output is as close as possible to the original. However, it must be acknowledged that the output may be comprised of a smaller set of colors than that in the original scanned artwork. In other words, some colors may be changed or lost.

Resources

The websites of most of the well-known imaging product manufacturers and distributors now have pages dedicated to this issue. A small selection of these and others providing information are listed below:

•   http://www.agfa.com/
•   http://www.heidelberg.com/
•   http://www.microsoft.com/
•   http://www.apple.com/colorsync/
•   http://www.color.com/
•   http://www.color.org