The colors we see in nature represent an extremely wide range of colors. When it comes to reproducing color, however, we run into limitations. No color reproduction system (color film, color monitors, printing presses, etc.) can reproduce the entire range of colors we see in nature.
Color gamu is another term for “range of colors.” Each color reproduction system has its own color gamut. For example, the gamut of colors that can be reproduced on photographic film is greater than the gamut of colors that can be produced with process color inks on paper using the offset printing process. Computer screens display more – and different – colors than can be produced on color film or most color printing devices.
How many colors are there?
If you are a human eye – billions
If you are a computer screen – 16 million
If you are a photographic film – 10 to 15 thousand
If you are a printing press – 5 to 6 thousand
Additive Color vs. Subtractive Color
Video monitors use the additive color system. Offset printing uses the subtractive color system. Computer screens display a larger gamut of colors than can be produced on press and by most color printing devices. This is important to know when using the computer as a design tool. The color you see on your computer monitor is probably not what you will get when the job is printed.
The limitations of the offset printing (subtractive) process are due in part to the image screening process and in part to the type of paper used to print the image. The screening process converts an original continuous-tone image, such as a color photograph, into a pattern of small dots for each process color so the image can be printed with a pigment (wax, toner, ink) or dye on paper. A continuous-tone image shows a continuous density range between lighter and darker areas. An ink-printable image (screened image) is made up of small dots which creates the illusion of lighter and darker tones. A screened image can be produced using a fixed grid pattern of different-sied dots, or by varying the number of randomly placed, same-sized dots – or a combination of the two.
Screening Process and Tone Compression
In offset printing, to be ink-printable, a continuous-tone image such as a photograph is converted into small dots of varying sizes using a camera and a halftone screen or, more commonly, a digital scanner. The original color image is separated into four separate halftone images – one for each of the three process colors and one for black. Historically, reproduction of continuous-ton images has relied on halftone screening methods that produced dots of different sizes in a fixed grid pattern. To be reproducible on press, each area of the original image is converted to a certain dot size to give the same visual appearance as the original image. When printed, areas with larger dots appear darker than areas with smaller dots. The size of each halftone dot is measured in terms of dot area percentage, from 1% to 100%.. In a conventional halftone image, the dot size changes proportionately to the tonal value of the original image.
The coarseness of the grid, or screen ruling, determines the distance form the center of one dot to another. Newer digital screening methods produce very small, similar-sized dots randomly placed, not on a fixed grid. In these screened images, the number of small dots in a given area changes proportionately to the tonal value of the original image. Regardless of the screening method, a continuous-tone image must be converted into small dots to be reproducible on press.
The goal of four-color process printing is to create the illusion of continuous-ton color. Reproducing good tone is considered the first and foremost objective in achieving good color reproduction. The primary factor that limits color reproduction with subtractive color systems is tone compression.
What is tone? Tone is actually the lightness/darkness value of an image. The tonal range of an image is the transition from the highlight (or minimum density) tot he shadow (or maximum density) areas. On a printed sheet, the highlight areas have minimum ink coverage and the shadow areas have maximum ink coverage.
The density range between the highlight and shadow areas can vary from one image to another. One image may have a narrow tonal range while another image can show a wide tonal range. Regardless of the tonal range, the number of density levels in a screened image. In other words., the number of density levels of an original is usual far greater than what is achievable on press. This means the tonal range of an original image must be compressed during the image reproduction process. The result is tone compression which requires that certain parts of the tonal range must be emphasized at the expense of others. Because of this inevitable compromise, a decision must be made as to what parts of an original image are the most important to reproduce accurately. The entire tonal range of an original image is usually difficult to reproduce on press. Detail in the highlight ares may have to be sacrificed to hold the detail in the shadow ares or vice versa.
Tone compression is more manageable if the original image is produced using special photographic techniques. An experienced photographer can adjust the lighting of a subject to change the contrast, or reduce the tonal range, of the original image to match the capabilities of the reproduction process. A low-contrast image requires less tone compression than a high contrast image and is easier to reproduce on press.,
Another factor that affects the amount of colors reproducible by the subtractive process s the type of substrate – usually paper – used to print the image. As discussed earlier, offset printing uses transparent color inks that act as filters an subtract portions of the white light striking the image on paper to produce other colors. It is the paper that reflects any unabsorbed light back to the viewer. Paper stocks vary in color, gloss, brightness, texture and absorbency. A press that prints on coated paper produces a wider range of colors than a press that prints on uncoated paper. This is because the rougher surface of the uncoated paper scatters the light and reduces the amount of light reflected back to the viewer. Smooth, glossy white paper returns more light back to the viewer. The range of colors on a substrate such as newsprint, which is usually rough, uncoated and yellowish, is more limited. A paper with a bluish cast will absorb some red and green wavelengths and cause colors to appear grayer than if printed on white paper. The effect of the paper base is so important to the appearance of a printed sheet that it can be considered a fifth color.
The Role of a Color Proof
Once a print job is on the press, changes are costly. The primary role of a color proof is to predict what a job will look like when printed. Using a color proof – and knowing what to look for in a proof – saves time and money by allowing changes to be made before a job goes on press. A proof serves as a communication and quality control tool at many steps in the production process. It is used within a production environment to monitor how a job is progressing. It is used with the customer to determine if color correction is necessary. It is used with the printer to check image quality and sere as a pressroom guide. A color proof often serves as a “contract” between the printer and the customer. This means the customer expects that the printed sheet will look like the proof. In the pressroom, the press operator makes adjustments to produce printed sheets that match the proof.
To be useful, a color proof must match the folor, tonal range and visual appearance of the printing process. First, the colorants of the proofing system must simulate both the primary and secondary hues produced by the printing inks. Next, the proof must simulate the tone compression inherent in the printing process used. Finally, a roof must match the overall appearance of the printed sheet. Factors that affect overall appearance include the substrate, or paper stock, and gloss level.
A common pitfall is to judge a proof on how pleasing it is to the eye without considering how well it represents the printing process If a proof cannot be matched on press, the proof creates frustrations for the press operator and unrealistic expectations for the customer.