Color Basics Part 4 – How is Color Reproduced?
Throughout history, reproducing the colors we see in nature has taken many forms. The media and methods used to reproduce color include color paintings, printing presses, color film, color monitors, color printers, etc. There are only two basic ways, however, of reproducing color; the additive system and the subtractive system.
Additive Color System (RGB)
As stated in previous parts of this series, the additive color system involves light emitted directly from a source, before it is reflected off an object. Light of a specific color, or wavelength (for example, a theatrical spotlight) can be created by passing white light through a colored filter. The additive reproduction process mixes various amounts of red, green and blue light to produce other colors. Combining one of these primary colors with another primary color produces one of the secondary colors of light. These include cyan, magenta, and yellow.
To illustrate the Additive Color System, imagine three spotlights, one red, one green, and one blue focused from the back of an ice arena on skaters in an ice show. Where the blue and green spotlights overlap, the color cyan is produced. Where the blue and red spotlights overlap, the color magenta is produced. Where the red and green spotlights overlap, the color yellow is produced. Where all three colors of light overlap, the color white is produced.
As mentioned before, television screens and computer monitors are examples of systems that use additive color to display selected colors to the human eye. All image capture devices also use the additive color system to gather information needed to reproduce a color image. These devices include digital cameras, scanners and video cameras.
Subtractive Color System (CMYK)
The subtractive color system involves colorants and reflected light. Subtractive color starts with an object (Often a substrate such as paper or canvas) that reflects white light and uses colorants (such as inks, dyes or paint) to subtract portions of the white light reflecting from an object to produce other colors. If an object reflects all the white light back to the viewer, it appears white. If an object absorbs (subtracts) all the light reflecting from the surface, it appears black. It is the subtractive process that allows everyday object around us to show color. Remember the example of the red apple? The apple really has no color. It has no light energy of its own. Colorants in the apple’s skin absorb green and blue wavelengths of white light and reflect the red wavelengths back to the viewer, which registers as red in the brain.
Color paintings, color photography and all the color printing processes use the subtractive process to reproduce color. In these cases, the reflective substrate is canvas (paintings) and paper (photography and printing), which is usually white. The subtractive color system produces its own unique range of colors, or color gamut.
Printing presses use color inks that act as filters and subtract portions of the white light striking the image on the paper to produce other colors. Printing (four-color process) inks are transparent, which allows light to pass through to and reflect off of the paper base. It is the paper that reflects any unabsorbed light back to the viewer. The offset printing process uses cyan, magenta and yellow (CMY) process color inks, and a fourth ink, black. The use of black ink will be explained below. The black printing ink is represented by the letter K to avoid confusion with B for blue. Overprinting one transparent ink with another, produces the subtractive secondary colors of red, green and blue.
To be reproducible on press an original color image, such as a photograph, must first be converted into a pattern of small dots for each of the four colors (CMYK). When printed with ink on paper, the small dots fool the eye and give the visual appearance of the original image.
What is Tone?
Tone is actually the lightness or darkness value of an image. The tonal range of an image is the transition from the light areas to the dark areas. In the conventional halftone process, full strength ink colors are always used. However, to create the appearance of tone, each area of an image is converted to a certain dot size of that full strength color, next to the white of the paper, to give the same tonal appearance as the original image. When printed, areas with larger dots (dots that are even touching each other) appear darker than areas with smaller dots. The size of each halftone dot is measured in terms of how much surface area will reflect white light; 1% - 100%.
In theory, a perfect process ink would absorb 100% of one primary color of light and allow reflection of 100% of the other two. If this were true, an overprint of all three process color inks would absorb all portions of white light, reflect no light to the viewer and produce an appearance of black. In reality, however, printing inks contain impurities and no process ink is perfect.
For example, even the best cyan ink does not absorb all the red light, and does not reflect all the blue and green light. Therefore, when the three process inks are combined equally, a brown color is produced, not black. Black is used as a fourth printing ink to add depth to the shadow areas and enhance detail in the image overall.