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Understanding Resolution Image Resolution
Raster image quality depends directly on the file’s
INDESIGN FOUNDATIONS to understand the resolution requirements from the very average value of a group of pixels and generates a spot
resolution; when you work with raster image files, you need
When a printer creates halftone dots, it calculates the
beginning of the process:
of appropriate size. An image’s resolution controls the
quantity of pixel data that the printer can read. Regardless
• Pixels per inch (ppi) is the number of pixels in one
of the source — camera, scanner, or files created from
horizontal or vertical inch of a digital raster file.
scratch in a program such as Photoshop — images need
• Lines per inch (lpi) is the number of halftone dots
to have sufficient resolution for the output device to
produced in a horizontal or vertical linear inch by a
high-resolution imagesetter in order to simulate the
continuous tone.
appearance of continuous-tone color.
The relationship between pixels and halftone dots defines
• Dots per inch (dpi) or spots per inch (spi) is the generate enough halftone dots to create the appearance of
number of dots produced by an output device in a single the rule of resolution for raster-based images — the
line of output. Dpi is sometimes used interchangeably resolution of a raster image (ppi) should be two times the
(although incorrectly) with pixels per inch. screen ruling (lpi) that will be used for printing.
When reproducing a photograph on a printing press, the In the image below, each white square represents a pixel.
image must be converted into a set of different-size dots The highlighted area shows the pixel information used to
that fool the eye into believing it sees continuous tones. The generate a halftone dot. Ideally, the printer has four pixels
result of this conversion process is a halftone image; the for each halftone dot created.
dots used to simulate continuous tone are called halftone
dots. Light tones in a photograph are represented as small
halftone dots; dark tones are represented as large halftone
dots. Prior to image-editing software, photos were converted
to halftones with a large graphic-arts camera and screens.
The picture was photographed through the screen to create
halftone dots, and different screens produced different
numbers of dots in an inch (hence the term “dots per inch”).
Screen Ruling
The screens used with old graphic-arts cameras had a finite 72 ppi 300 ppi
number of available dots in a horizontal or vertical inch. If an image only has 72 pixels per inch, the output device
That number was the screen ruling, or lines per inch of the has to generate four halftone dots per pixel, resulting in
halftone. A screen ruling of 133 lpi means that in a square poor printed quality.
inch there are 133 1 133 (17,689) possible locations for
a halftone dot. If the screen ruling is decreased, there are In the images below, the same raster image is reproduced
fewer total halftone dots, producing a grainier image; if the at 300 ppi (left) and 72 ppi (right); notice the obvious
screen ruling is increased, there are more halftone dots, degradation in quality in the 72-ppi version.
producing a clearer image.

Line screen is a finite number based on a combination of the
intended output device and paper. You can’t randomly select
a line screen. Ask your printer what line screen will be used
before you begin creating your images. If you can’t find out
ahead of time or are unsure, follow these general guidelines:
• Newspaper or newsprint: 85–100 lpi
• Magazine or general commercial printing: 133–150 lpi
• Premium-quality-paper jobs (such as art books): For line art, the general rule is to scan the image at the
150–175 lpi (some specialty jobs use 200 lpi or more) same resolution as the output device. Many laser printers
and digital presses image at 600–1200 dots per inch (dpi);
If you find this information a bit confusing, don’t worry.
As a general rule for preparing commercial print layouts, imagesetters used to make printing plates for a commercial
most raster images should have twice the pixel resolution press typically output at much higher resolution, possibly
(at 100% size) as the line screen that will be used. 2400 dpi or more.

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