Graphics Programs Reference
In-Depth Information
Drum scanners
When top image quality is a must, pros turn
to drum scanners. These units are expensive
(starting at $5,000 and increase significantly).
This is the oldest scanning technology. It calls
for the image to be mounted on a drum. The
drum is then rotated in front of a photomultiplier
tube. The tube is much more sensitive than the
CCDs used in flatbed scanners. Drum scanners'
primary advantage is resolution, and they should
be used when you need to significantly enlarge a
scanned image (such as museum archival pieces
or for magazine output). Because the machines
are expensive and very complex (as well as
potentially destructive), users will often send
images to a service bureau for drum scanning.
A drum scanner is a highly specialized piece of equipment.
These machines are expensive and are usually found only in
high-end, service bureau facilities. ©iStockphoto
COMMON PPI REQUIREMENTS FOR
FINAL FILES
What Size to Scan?
Think in Pixels
People often get confused when determining
which settings to scan with. Too little informa-
tion and the picture goes soft. Too much informa-
tion and the scanner slows to a crawl. The answer
is to know your intended output resolution as
well as your device.
Output Method
Typical ppi
Onscreen (web/slides)
72-96
Laser printing
150-250
Newsprint
120-170
Offset printing
250-300
High-quality offset printing
300-600
For example, if you need to create a 20-inch-wide poster that
will be printed on a high-quality press requiring 300 ppi, use
this calculation:
20 (inches) × 300 (ppi) × 1.25 (pad for flexibility) = 7500 pixels
NoTe
More Advice on Scanning
On the DVD you'll find a bonus PDF
called Scanner_Operation.pdf in the
Chapter 3 folder.
Do not adjust your scanner's dpi (or ppi) settings. Rather, crop the
image after running a preview scan. You can then adjust the scan-
ner's resolution by looking at the output size of the scanned file. As
you adjust the output file size, the scanning software will automati-
cally determine the appropriate settings for samples per inch. All
scanners tell you just how many samples you are about to capture.
Looking at these numbers gives you a truer sense of the end result.
Tot a l pi xel cou nt i is much more i mpor t a nt t ha n dpi , es pec ia l ly
when scanning images of various original sizes.
