The 4-Color Printing Process: A Better Understanding
Once a job is designed on the computer it is digitally separated in a process that prepares the file for printing on a traditional printing press. This process takes all of the colors you see on your screen and separates them into the four main colors— C (cyan), M (magenta), Y (yellow) and K (black).
Plates are then created for the printing press to correspond with the 4 different colors – traditionally this was done using a process that involved film and transfer to metal plates, but now can be accomplished by printing directly to CTP plates. The four plates are then placed on the press with the appropriate color ink associated with each plate.
“4-Color” or “Process Printing”, is the procedure in which four ink colors are printed on the press. The combination of these four colors— C (cyan), M (magenta), Y (yellow) and K (black), together, simulate the 16.7 million colors that the human eye can differentiate.
The the many different colors you see in the final printed piece is accomplished by printing these four colors as tiny dots. The darker the color, the closer and larger the dots are printed, the lighter the color, the smaller and further apart. At even a small distance, the human eye combines the tiny dots into a composite image.
How We Do It
There are a wide variety of art production and publishing tools available on the market today. Many of our customers come to us very frustrated after having attempted to produce their documents using any one of the off-the-shelf software packages that are typically provided to a consumer.
Although, many in-expensive, readily available and easy to use document preparation kits frequently get acceptable results at home on the family inkjet, they are just no match to the top quality and very powerful publishing programs used by graphics professionals. Yes — the high end software is more expensive and the programs take some time to learn how to fully utilize, but there is a reason for that. You would not expect an off-the-lot economy car to compete on the same race track as a very expensive and fully equipped sports car. To achieve powerful — quality results with your printed documents, you should make your software choices with the same logic.
The graphics programs we use most frequently based on a balance of cost, reputation, accuracy and efficiency are Adobe Photoshop CS2 for image production and Adobe Illustrator CS2 for line illustrations. For page assembly and text input and to prepare items to go to a high-end, professional imagesetter for film output we use QuarkXPress and Adobe InDesign, on the Macintosh computer platform. These combinations of software are generally considered the work horses of the graphics industry and have been developed specifically to give consistent quality in a printed product.
Our staff has decades worth of combined experience in graphic arts at your full disposal. We’ve learned a lot in that time and we are constantly in the process of learning more as the industry evolves.
RGB Color and CMYK Color Explained
Scanners, and now Digital Cameras, are becoming commonplace. It’s becoming easy to get the images you want onto your computer and incorporate them into your documents with any number of computer programs. Unfortunately, computer images are typically for, well … computers. This leads to a problem as new and easy to use consumer technologies begin to collide with the long standing professional sciences of offset lithography. That new digital camera you bought is designed to produce beautiful and bright pictures to display on the monitor of your home PC or print out on your new Photo Quality Inkjet. High quality 4-Color printing, however, has some very different and specialized needs.
We see a couple of common errors occur almost on a daily basis. One is customers supplying the images they want to use in an Image Mode, commonly referred to as RGB, which translates to literally Red, Green, and Blue. Red, Green, and Blue are the colors of the phosphers that illuminate your monitor and form the images that you see. The other error is that images are frequently much too dark to print on paper. This is a problem that occurs when your computer’s monitor is not properly calibrated to display the image data as it will look when it is actually printed. This is because computer monitors are designed to display computer programs, and video games for the most part. The actual display is much brighter than printing inks can physically produce. RGB (Red, Green, Blue) is not CMYK (Cyan, Magenta, Yellow, Black).
Computer monitors illuminate colored light while printed inks reflect light to show color. It would be similar to turning on a light bulb in a dark room and comparing that to a printed picture of a light bulb. The printed picture of the light bulb has no ability to light a darkened room and your computer’s display has to be adjusted to compensate for these physical differences.
Here is how RGB color works. RGB color is formed when the Red, Green and Blue phosphers of your monitor are lit up in different values of brightness. The brighter the phosphor is lit, the brighter the color. The less energy that is applied, the darker the phosphor is and the darker the color. Different combinations of the 3 colors can produce a wide variety of colors when your eye naturally blends these tiny combinations into one large image. This is about the only thing RGB and CMYK have in common.
You can see in the above image that as different colors and amount of light are added to each other the colors change and become brighter. This is why RGB color is sometimes referred to as “Additive” color.
The opposite of that would be “Subtractive” color which is the science that CMYK inks produce when your eye sees an image printed on paper. Printed images and almost everything else we see in the natural world are a result of light being reflected off of surfaces and into our eyes. The sun or other light source shines one common collection of lightwaves on an object and that object absorbs certain lightwaves depending on its surface properties and reflects the others back. This blend of leftover lightwaves reflected back into our eyes is what creates the color we see. The CMYK inks we use to take advantage of this principle are not the three Red, Green, and Blue colors of your monitor, they are 4 separate inks called Cyan, Magenta, Yellow, and Black. Cyan is a light blue color, Magenta is pink and Yellow and Black are, of course, yellow and black respectively.
You can see as these inks combine, their pigments mix and the colors actually become darker. As these combinations of ink on paper begin to blend together they reflect and absorb different wavelengths of light and form the colors you see. Since printing inks need a variety of 4 colors to produce images and cannot possibly be brighter than the paper they are being printed on and RGB images are created with only 3 very specific colors and can be as bright as the amount of energy being used to make them, CMYK and RGB are two very different things.
The above images are both from the same source. The one on the left is in RGB. You can see its very bright, almost neon in its vibrant display of color. The one on the right is a CMYK conversion of the one on the left. You can see that much of the neon quality dies out as the computer knows that CMYK inks cannot possibly produce those brilliant intensities and it changes the image data to show you what it will really look like when printed.
You can learn about printing inks in our Printing Education section.
This is much trickier and a little more difficult of a problem. You should work with your Service Bureau in order to begin to fully compensate for the effects of monitor calibration. You should still be aware of this problem and make some effort at adjusting for it.
As we said earlier, monitors are much brighter than printed inks so many of the images you see on an uncalibrated monitor are much brighter than the actual image data will really produce when printed.
How often have you worked with something like the image on the left? You’ve spent hours adjusting it to get exactly what you want only to have the image on the right returned to you by your printer. It’s not the image that’s causing the problem, it’s your computer’s monitor (Your desktop inkjet printer is also a proofing culprit since it’s designed to print out images representative of a video monitor). Until you tell your monitor otherwise, it does what it was designed to do by displaying video images, not images that are destined for print. The image data changed exactly as you instructed it to, but the monitor was too bright to really show you what that data would produce in CMYK printing inks.
Don’t fear, you can conquer this problem and get much more reliable and less surprising results. Consult your service bureau to learn more.
How to Deal With Image Files
The first thing you need to be aware of when dealing with digital images is the resolution of the image. Simply put, an images resolution is determined by how many pixels of information exist per inch within the image at 100 percent of the size it will be printed. Your digital images are very similar to a mosaic made with colored tiles. Pixels are the tiles with which the image is built. The more tiles you can use per inch, the more refined the detail can be within the image.
Here’s an illustration of the differences you might expect to see in image quality between a 72 pixels per inch image and a 300 pixels per inch image. The image on the left shows a detailed look at what 72 pixels per inch looks like up close.
2 pixels per inch inch is considered low resolution although it is perfectly fine for a website or other on-screen display, it is not really good enough for a high-resolution image setter which will reproduce your artwork to be printed on a printing press. The 300 pixels per inch image has over three times as much detail.
When working with images destined for a printing press you should be using a general rule of thumb of 2 to 1. The digital image should posses twice as many pixels per inch as the line screen in which your artwork will be printed at. So Cal Graphics likes to print images at 175 line screen or 150 line line screen. So, using the 2 to 1 rule, your images should be scanned at 350 or 300 pixels per inch for maximum quality. Any less than that and you risk having the golfer on the left in your foursome.
One of the most common form of images that people come across today is the JPEG image format. A JPEG image is an image that has a particular form of compression applied to it in order to make the file size as small as is practical and still have something leftover that looks like what you started with. Unfortunately, JPEG compression is a form of “lossy” compression which means that image data considered unnecessary is thrown out, or “lost”. JPEG image files break an image down into regular common sectors, throw out detail and then relay on the human eyes tendency to blend common or expected things together as it thinks they should look. This can have some very bad results when an image is destined for a high-resolution image setter as is used in the printing industry.
The image above is of a nice sunny day in Southern California. You can plainly make out the palms, blue sky, whispy clouds and distant hillsides. However, if you take a closer look at a detail of the image which is shown below, you can see that the image is made up of globs of colors arranged in a certain sequence that our eyes put together and view as the scene above.
These defects will show if you use this image for your printed piece and it will have disastrous results. You can make use of JPEGs in the printing industry if the images are saved with the MAXIMUM amount of detail preserved while the image is compressed. Remember, once information is thrown out from a file, a computer does not know how to recreate it again and you may have to rescan the image to regain its truest detail.
GIF image compression is also meant for internet use. A GIF image will compress its file size by throwing out color data instead of image data and shift all similar colored pixels to its choice of 1 distinct color.
Image file formats that we use most frequently in the printing industry are EPS and TIF. Either one is suitable and both have distinct technical advantages for advanced computer users. If you are manufacturing a “cutout” or “clipping path” in an image program like Adobe Photoshop® it’s typical to save the image as an EPS file since an EPS file format is used to deal with path data commonly called “vector” art. Adobe Illustrator® and Macromedia Freehand® files must be saved as EPS format files for this same reason. You may find that a TIF format is a little easier to work with since it has a form of compression available to it called “LZW”. “LZW” is a form of “lossless” compression meaning it looks at the actual data in the file and finds exact patterns of info which it can account for in easier terms. For instance, if a large area of your image made up of thousands of pixels is all white, instead of remembering the color of each individual pixel a thousand times over, it remembers that this particular area of pixels are all the same. Much more efficient.
If you are saving files as EPS’s in Adobe Photoshop, please use BINARY as your encoding method NOT JPEG.
To Sum Image Formats
When in doubt, you should be safe saving your images at 100% of the size they will print, CMYK (or grayscale if your printing 1 color), 300 Pixels per inch for photos and 600 pixels per inch for hard-edged line art, and in TIF format. If you are unsure, just call and describe your situation and we will help you.
Text in Photoshop
The basic rule of thumb here is don’t. Photoshop is for image manipulation. You can do a variety of things with text in Photoshop, but unless you’re an experienced user, I would recommend avoiding it. You could produce some very unexpected and costly problems for yourself without even realizing it.
Photoshop sends artwork to an image setter as a bitmapped graphic. You should use an application like QuarkXPress to assemble your text and images together on the page for maximum results. Page layout applications will send text to an image setter as vector information and it will have the capability to produce razor sharp results.
In the Text Detail graphic above, you can see the kind of edge you may end up with if you incorporate your type elements into a Photoshop image. There are reasons to handle text in Photoshop and there are ways to get excellent results, but, unless you’re an experienced user of image creation applications, it’s best to avoid this type of approach.