Page 49 - CS6_ID_Look_Inside

Understanding Resolution Screen Ruling

Raster image quality depends directly on the file’s
INDESIGN FOUNDATIONS to understand the resolution requirements from the very number of available dots in a horizontal or vertical inch.
resolution; when you work with raster image files, you need
The screens used with old graphic-arts cameras had a finite
beginning of the process:
That number was the screen ruling, or lines per inch of the
halftone. A screen ruling of 133 lpi means that in a square
• Pixels per inch (ppi) is the number of pixels in one
inch there are 133 1 133 (17,689) possible locations for
horizontal or vertical inch of a digital raster file.
a halftone dot. If the screen ruling is decreased, there are
• Lines per inch (lpi) is the number of halftone dots
fewer total halftone dots, producing a grainier image; if the
produced in a horizontal or vertical linear inch by a
high-resolution imagesetter in order to simulate the
producing a clearer image.
appearance of continuous-tone color.
Line screen is a finite number based on a combination of the
• Dots per inch (dpi) or spots per inch (spi) is the screen ruling is increased, there are more halftone dots,
number of dots produced by an output device in a single intended output device and paper. You can’t randomly select
line of output. Dpi is sometimes used interchangeably a line screen. Ask your printer what line screen will be used
(although incorrectly) with pixels per inch. before you begin creating your images. If you can’t find out
ahead of time or are unsure, follow these general guidelines:
When reproducing a photograph on a printing press, the
image must be converted into a set of different-size dots • Newspaper or newsprint: 85–100 lpi
that fool the eye into believing it sees continuous tones. The • Magazine or general commercial printing: 133–150 lpi
result of this conversion process is a halftone image; the • Premium-quality-paper jobs (such as art books):
dots used to simulate continuous tone are called halftone 150–175 lpi (some specialty jobs use 200 lpi or more)
dots. Light tones in a photograph are represented as small
halftone dots; dark tones are represented as large halftone If you find this information a bit confusing, don’t worry.
dots. Prior to image-editing software, photos were converted As a general rule for preparing commercial print layouts,
to halftones with a large graphic-arts camera and screens. most raster images should have twice the pixel resolution
The picture was photographed through the screen to create (at 100% size) as the line screen that will be used.
halftone dots, and different screens produced
different numbers of dots in an inch (hence the
term “dots per inch”).

Image Resolution
When a printer creates halftone dots, it calculates
the average value of a group of pixels and
generates a spot of appropriate size. An image’s
resolution controls the quantity of pixel data that
the printer can read. Regardless of the source —
camera, scanner, or files created from scratch in
a program such as Photoshop — images need to
have sufficient resolution for the output device
to generate enough halftone dots to create the Each white square represents a pixel. The highlighted area shows the
appearance of continuous tone. pixel information used to generate a halftone dot. If an image only
has 72 pixels per inch, the output device has to generate four halftone
Ideally, the printer has four pixels dots per pixel, resulting in poor printed quality.
for each halftone dot created.
The relationship between pixels
and halftone dots defines the
rule of resolution for all raster-
based images — the resolution
of an image should be two times
the screen ruling (lpi) that will
be used for printing.

The same raster image is reproduced here at 300 ppi (left) and 72 ppi (right).
Notice the obvious degradation in quality when the resolution is set to 72 ppi.

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