Color photographs printed in magazines, newspapers, or books consist of a series of dots in cyan, magenta, yellow, and black (CMYK) that fool the eye into seeing the millions of colors that make up the original image.
Black & White Halftones
Black and white continuous tone photographs contain millions of shades of gray. When printed, these shades of gray convert to a pattern of black dots that simulates the continuous tones of the original image. Lighter shades of gray consist of fewer or smaller black dots spaced far apart. Darker shades of gray contain more or larger black dots with closer spacing.
In traditional prepress, when you supply your printer with the actual photographs, rather than digital scans they create the halftoning necessary for printing by photographing the photograph through a special screen. For color images four separate screens are superimposed on each other in each of the four process colors.
When using scanned images or images from a digital camera, you can produce digital halftones direct from the software to the printer. Digital halftoning depends on the lpi (lines per inch, or screen frequency) and the resolution of your output device (printer). The screen used may be specified in your printers PPD (PostScript Printer Driver) or set specifically in your software program.
Color images are recreated through a combination of color separations and specific screening patterns and screen angles. When creating color separations, use the settings that match your output device. Talk to your service bureau about the correct settings to use with their imagesetter.
In addition to screening patterns, different printing presses and the paper you print on require specific screen frequencies the number of dots used to create the image. This is that often misunderstood concept of LPI (lines per inch). In general, a higher screen frequency produces a smoother, more detailed image. However, for certain types of paper a higher screen frequency is not better. Typically newsprint uses an LPI of 85. 133 is the norm for glossy paper such as in magazines. Check with your printer to determine the best LPI settings to use for their equipment and your paper. See this LPI Chart for some common LPI settings and further explanation of this and related terms.
In practice, you may not need to do anything special with your images other than insuring you use the correct printer driver. The printer driver provides all the instruction needed.
However, there may be instances when you want to override default settings or create your own halftones. In the print dialog options for your document you can choose a different PostScript Printer Driver (PPD) or overide the LPI and screen angle settings for your document.
In a graphics program such as Adobe Photoshop or Corel Photo-Paint you can convert your image to a black and white image using a specific halftone screen or dithering. This is necessary when supplying camera ready artwork using digital images. Both Photoshop and Photo-Paint allow you to specify LPI, screen angle, and a dot shape. Remember, size and rotate the image in your graphics program before converting to a halftone. Don't resize or rotate the halftone after you place it in your page layout program.
This brief overview provides basic information only. For more in-depth technical specifications and tutorials as well as advice on proper file preparation, see the sidebar resources.
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