Computer input scanning devices
| Pages | 233-238 |
| DOI | https://doi.org/10.1108/eb045169 |
| Date | 01 April 1992 |
| Published date | 01 April 1992 |
| Author | Howard Falk |
| Subject Matter | Information & knowledge management,Library & information science |
Hardware Corner
Computer input scanning
devices
Howard Falk
135
.Watchung
Avenue,
Montclair,
NJ
07043,
USA
Scanning devices make it possible to transfer images from
paper onto disc, where they can be processed, displayed and
printed.
Drawings
(line art)
or photographic
images
(halftones) can be
transferred, making it possible to use them in computer-printed
publications such as newsletters, leaflets, advertisements, bro-
chures and
books,
or
in
computer-displayed presentations.
Printed or typewritten text can also be transferred. Key-
boarding the whole text of a book or pamphlet is very expens-
ive and time-consuming, but scanning devices can capture a
page of text in a few seconds with relatively little manual
effort. Once captured in this way, the text can be analyzed by
optical character recognition (OCR) software and trans-
formed into strings of computer characters, which can then be
processed as if they had been typed into the computer key-
board.
Light is converted into digits
The scanning process
is
straightforward. The scanning device
illuminates the image and measures the amount of light re-
flected from each small element of the image. The output of
the light-sensitive unit (usually a charge-coupled device) in
the scanner is then converted into digital form for computer
storage and processing. Types of scanning devices which are
of interest include flatbed scanners, sheet fed scanners, print-
head scanners and hand-held scanners.
Flatbed scanners
Flatbed scanners operate in a similar way to office copiers.
The image page to be scanned is laid, face down, on a glass
platen.
A
cover is closed over the image page and the scanner
goes through its operating cycle, producing a computer file
which can be stored, displayed or printed.
Flatbed scanners hold the image still while the scanner
mechanism moves the light-sensing device across the image
and so are conducive to accurate scanning. Pages of
a
bound
book can also be scanned in this
way.
The disadvantage is that
each page needs to be inserted by the operator unless a separ-
ate document feeder is acquired.
Sheet fed scanners
Sheet fed scanners can handle several pages at a time, requir-
ing less operator attention. Once one page has been scanned,
the machine automatically selects the next. However, the
sheets being scanned can twist slightly as they are fed into the
machine, distorting the image on disc. Moreover, the sheets
have to be separated.
Printhead scanners
A printhead scanner mounts directly on the operating mech-
anism of
a
dot-matrix or daisy-wheel printer. The material to
be scanned is loaded like blank paper in normal printer oper-
ation and the printer scans it line by line, without doing any
printing.
The motions of the printer are controlled by the scan-
ning software.
Mounting the printhead scanner can be
a
finicky
process.
It
must be aimed at just the right angle and maintain just the
right distance from the paper, while the printer mechanism
bounces about. If the printhead moves unevenly across the
page,
the quality of scanner output will be affected.
Hand-held scanners
Users of hand-held scanners have to be careful to move the
equipment evenly and smoothly across the images being
scanned. Uneven motion can result in elongation of the result-
ing image in those areas where the scanner was moved more
slowly. Similarly, side-to-side wrist motion, while scanning,
can cause skewing of
the
resulting image.
A line-art image, which consists of black lines on a white
background, is simple compared to
a
photograph of a face, yet
may be more difficult to scan using a hand-held scanner. Just
a small twitch by the user can create jagged lines in the output.
For scanning line-art images, while avoiding the appearance
of jagged lines, the resolution of the scanner should be as high
as
possible.
Typically,
a
resolution of 300 dots per inch will be
satisfactory for laser-printed line-art images.
Small dots combine to make up images
Computer output displayed on a screen, or printed by a dot-
matrix or laser printer, is made up of tiny dots, all the same
size.
A small cluster of these dots serves to represent each
The Electronic Library, Vol. 10, No. 4, August 1992 233
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