A colorful explanation: a color photo in print is really a combination of four images created through a process known as color separation.

A Colorful Explanation

Color photography is becoming so common in publications that one almost takes it for granted. But making it happen is not as easy as it looks.

By the time a color photo makes it into print, it has been through a process that blends complex computer manipulation with the simple principles of mixing colors that most people learned as children. That color photo in print is really a combination of four images created through a process known as color separation.

To understand color separation, one must understand the theory of "three-color vision," the way the eye perceives color. It is not unlike the theory of mixing paint, where any color can be created with combinations of red, blue and yellow. The difference is that with light, the primary colors are red, blue and green, and all three added together create white light.

The human eye contains three types of light receptors, each sensitive to one of the three primary colors of light. A color scene will stimulate the different receptors, and the brain recreates the color scene by combining the signals it gets from the receptors. In a similar fashion, to make a color separation a technician will process the original color photo through red, blue and green filters.

Exposing the original photo through a red filter creates a negative from which a printing plate is made, in a process not unlike making a regular photo from a negative. The image on that plate depicts everything but the red, which leaves a mixture of blue and green called cyan. That plate, called the cyan printer, is used to apply cyan-colored ink to the page.

The original photo also is processed through a blue filter and a green filter, and the resulting negatives are used to make printing plates that will apply yellow ink and magenta ink. Yellow is a combination of red and green light (everything but blue), while magenta is a blend of red and blue light (everything but green).

In theory, printing the cyan, yellow and magenta images on top of one another should create a perfect reproduction of the original photo. It isn't quite that simple, however. There isn't enough contrast, and limitations of the colors of the inks cause errors in the reproduction.

The errors in the reproduction are remedied by color...