Depending on the type of image you have, you need to determine the right type of printer. This will be a balance of budget and availability. The simplest images, such as line art, use only one color. An illustration may use several colors, and those can be printed using CMYK inks to create the different colors or spot inks that exactly match.
The most complex images are photographs because they use varying colors and tones to simulate the image. These types of images are generally referred to as continuous-tone images.
If your multicolored image is intended for commercial output (that is to say printing on a large press), it will need color separation. This process allows for a master plate to be created for each color. Generally, the plates created are for cyan, magenta, yellow and blackalso called key(CMYK) inks.
These plates can be created in several ways. Usually, the process is handled by a printing professional. However, let's take a quick look at how these separations can be created in Photoshop.
To simulate continuous tones in images, commercial printers break down images into dots. For those images printed on a press, this process is referred to as halftoning. By varying the size of the dots, the halftone screen creates the optical illusion of variations in tone.
While an inkjet printer also uses dots, it's not the same. An inkjet printer's dots are very small and uniform in size.
Quality of Detail
How clear an image prints depends on its resolution and screen frequency. Professional printing devices are often capable of high resolution. As such, they require a finer screen ruling (lines per inch). For more on resolution, you can revisit Chapter 3, "Acquiring Digital Images." It's a good idea to discuss resolution requirements with your printer before starting a job.