Graphics Programs Reference
In-Depth Information
are converted to dots in the printer. So if your scanner's software
specifies dpi, it really means spi, but you can see how the two are
closely related.
Tip
Scanner Advice
The most important issue with
scanners is optical resolution versus
interpolated resolution. A scanner
captures
optical resolution
through
hardware.
Interpolated resolution
is what happens after the captured
data is enlarged via software. You
should only care about the opti-
cal resolution when choosing a
scanner.
Lines per Inch (lpi)
In professional printing environments, you'll often hear the term
lines per inch
(lpi).
This is from the traditional process where
images with gradiated tones (such as photographs) are screened
for printing to create a
halftone
. This was originally performed by
laying film with dots printed on it over the film before exposure.
In the digital age, this process and these terms are used less often,
but it is still good for you to have a basic understanding.
These days, the work of converting an image to lines is performed
by an imagesetter. The dots are arranged in lines, and the lpi mea-
surement refers to the number of lines per inch. An increase in lpi
results in smoother images. Table 1.1 shows the most common lpi
settings for different output formats.
Table 1.1 Common lpi Measurements
Output Method
Typical lpi
Screen printing
35-65
Laser printer (matte paper)
50-90
This image has been converted to a
halftone, as is evident by the visible
dot pattern.
Laser printer (coated paper)
75-110
Newsprint
60-85
Offset printing (uncoated paper)
85-133
Offset printing (coated paper)
120-150+
High-quality offset printing
150-300
Within Photoshop, you need to choose from one of eight image
modes when working with a document. The mode you pick will
depend on what you need to do with the image and how you
intend to output it. For example, the mode used for web graphics
will differ from those used for professional printing. The three
most common modes used are RGB, Grayscale, and CMYK, but
it's worth taking a quick look at all eight.
video 2:
Converting Image Modes
