Color separations using two colors or more can be created from any design. Most designs are created using computer graphics programs such as Illustrator and Photoshop. It is also possible to create simple color designs in word processing programs. A multi-colored design that has been created on paper can be scanned into a computer. Once the design exists as a computer file, a color separation may be created.
Film positives are printed in black no matter what color they represent in the design. The film positive is used to put the image on a silk screen stencil that has been coated with light sensitive photo emulsion. The film positive must be printed in opaque black to stop light from hitting the screen in the image area during the exposure process.
Simple designs with bold areas of color are separated using a spot color technique. In Photoshop, a color is sampled using the eyedropper tool. The select pull-down menu is used to navigate to the color range dialog box to isolate the color, and then the screen printer clicks, "OK." Next the channels menu is opened to generate a new spot channel using the upper right hand pull down menu button. This process is repeated for each color in the design. Film positives for each color are printed from the channels menu.
Full color images can be color separated to four process colors that can create secondary colors when printed. A film positive is created for each of the process colors, cyan, magenta, yellow and black. Screen printers print these film positives from Photoshop using the channels menu. After the screen printer clicks "print," the print dialog box opens. From there the screen button opens the halftone screen dialog box. The frequency is set at 55-65 lines per inch, the printer clicks "OK," and the film positives are ready to print out.
Screen printers are careful to position each film positive in a centered position as they image screen printing stencils. This enables them to set the imaged screens in the press in perfect registration so that each separated color prints in the proper place relative to the other colors.