Scanners have become an important part of the home office
over the last few years. Scanner technology is everywhere and
used in many ways:
Flatbed scanners, also called desktop scanners,
are the most versatile and commonly used scanners. In fact,
this article will focus on the technology as it relates to
flatbed scanners.
Sheet-fed scanners are similar to flatbed
scanners except the document is moved and the scan head is
immobile. A sheet-fed scanner looks a lot like a small
portable printer.
Handheld scanners use the same basic technology
as a flatbed scanner, but rely on the user to move them
instead of a motorized belt. This type of scanner typically
does not provide good image quality. However, it can be
useful for quickly capturing text.
Drum scanners are used by the publishing industry
to capture incredibly detailed images. They use a technology
called a photomultiplier tube (PMT). In PMT, the
document to be scanned is mounted on a glass cylinder. At
the center of the cylinder is a sensor that splits light
bounced from the document into three beams. Each beam is
sent through a color filter into a photomultiplier tube
where the light is changed into an electrical signal.
The basic principle of a scanner is to analyze an image and
process it in some way. Image and text capture (optical
character recognition or OCR) allow you to save information to
a file on your computer. You can then alter or enhance the
image, print it out or use it on your Web page.
In this edition of HowStuffWorks,
we'll be focusing on flatbed scanners, but the basic
principles apply to most other scanner technologies. You will
learn about the different types of scanners, how the scanning
mechanism works and what TWAIN means. You will also learn
about resolution, interpolation and bit depth.
On the next page, you will learn about the various parts of
a flatbed scanner.
Anatomy of a Scanner Parts of a typical
flatbed scanner include:
Charge-coupled device (CCD) array
Mirrors
Scan head
Glass plate
Lamp
Lens
Cover
Filters
Stepper motor
Stabilizer bar
Belt
Power supply
Interface port(s)
Control circuitry
Close-up of the CCD
array
The core component of the scanner is the CCD array.
CCD
is the most common technology for image capture in scanners.
CCD is a collection of tiny light-sensitive diodes,
which convert photons (light) into electrons (electrical
charge). These diodes are called photosites. In a
nutshell, each photosite is sensitive to light -- the brighter
the light that hits a single photosite, the greater the
electrical charge that will accumulate at that site.
Photons hitting a photosite and creating
electrons
The image of the document that you scan reaches the CCD
array through a series of mirrors, filters and lenses. The
exact configuration of these components will depend on the
model of scanner, but the basics are pretty much the same.
On the next page, you will see just how all the pieces of
the scanner work together.
The Scanning Process Here are the steps that
a scanner goes through when it scans a document:
The document is placed on the glass plate and the
cover is closed. The inside of the cover in most
scanners is flat white, although a few are black. The cover
provides a uniform background that the scanner software can
use as a reference point for determining the size of the
document being scanned. Most flatbed scanners allow the
cover to be removed for scanning a bulky object, such as a
page in a thick book.
In the image above, you can see the
fluorescent lamp on top of the scan
head.
The entire mechanism (mirrors, lens, filter and CCD
array) make up the scan head. The scan head is moved
slowly across the document by a belt that is attached
to a stepper
motor. The scan head is attached to a stabilizer
bar to ensure that there is no wobble or deviation in
the pass. Pass means that the scan head has completed
a single complete scan of the document.
The stabilizer bar is very durable and
tightly secured to the body of the
scanner.
The image of the document is reflected by an angled
mirror to another mirror. In some scanners, there are
only two mirrors while others use a three mirror approach.
Each mirror is slightly curved to focus the image it
reflects onto a smaller surface.
The last mirror reflects the image onto a lens.
The lens focuses the image through a filter on the
CCD array.
Look carefully at the image above and you can
see all three of the mirrors plus the lens assembly in
this scan
head.
The filter and lens arrangement vary based on the scanner.
Some scanners use a three pass scanning method. Each
pass uses a different color filter (red, green or blue)
between the lens and CCD array. After the three passes are
completed, the scanner software assembles the three filtered
images into a single full-color image.
Click on the green Scan button to see the scanning
process.
Most scanners today use the single pass method. The
lens splits the image into three smaller versions of the
original. Each smaller version passes through a color filter
(either red, green or blue) onto a discrete section of the CCD
array. The scanner combines the data from the three parts of
the CCD array into a single full-color image.
Another imaging array technology that has become popular in
inexpensive flatbed scanners is contact image sensor
(CIS). CIS replaces the CCD array, mirrors, filters, lamp and
lens with rows of red, green and blue light emitting
diodes (LEDs). The
image sensor mechanism, consisting of 300 to 600 sensors
spanning the width of the scan area, is placed very close to
the glass plate that the document rests upon. When the image
is scanned, the LEDs combine to provide white light. The
illuminated image is then captured by the row of sensors. CIS
scanners are cheaper, lighter and thinner, but do not provide
the same level of quality and resolution found in most CCD
scanners.
We will take a look at what happens between the computer
and scanner, but first let's talk about resolution.
Resolution and Interpolation Scanners vary
in resolution and sharpness. Most flatbed
scanners have a true hardware resolution of at least 300x300
dots per inch (dpi). The scanner's dpi is determined by
the number of sensors in a single row (x-direction sampling
rate) of the CCD or CIS array by the precision of the
stepper motor (y-direction sampling rate).
The precision of the stepper motor determines
the y-direction sampling
rate.
For example, if the resolution is 300x300 dpi and the
scanner is capable of scanning a letter-sized document, then
the CCD has 2,550 sensors arranged in each horizontal row. A
single-pass scanner would have three of these rows for a total
of 7,650 sensors. The stepper motor in our example is able to
move in increments equal to 1/300ths of an inch. Likewise, a
scanner with a resolution of 600x300 has a CCD array with
5,100 sensors in each horizontal row.
Most scanners have a scan area that is either
letter size (8.5x11 inches, 21.6x27.9 centimeters) or
legal size (11x14 inches, 27.9x35.6
centimeters).
Sharpness depends mainly on the quality of the optics used
to make the lens and the brightness of the light source. A
bright xenon lamp and high-quality lens will create a much
clearer, and therefore sharper, image than a standard
fluorescent lamp and basic lens.
Of course, many scanners proclaim resolutions of
4,800x4,800 or even 9,600x9,600. To achieve a hardware
resolution with a x-direction sampling rate of 9,600 would
require a CCD array of 81,600 sensors. If you look at the
specifications, these high resolutions are usually labeled
software-enhanced, interpolated resolution or something
similar. What does that mean?
Interpolation is a process that the scanning
software uses to increase the perceived resolution of an
image. It does this by creating extra pixels in between the
ones actually scanned by the CCD array. These extra pixels are
an average of the adjacent pixels. For example, if the
hardware resolution is 300x300 and the interpolated resolution
is 600x300, then the software is adding a pixel between every
one scanned by a CCD sensor in each row.
Another term used when talking about scanners is bit
depth, also called color
depth. This simply refers to the number of colors that the
scanner is capable of reproducing. Each pixel requires 24 bits to
create standard true color and virtually all scanners
on the market support this. Many of them offer bit depths of
30 or 36 bits. They still only output in 24-bit color, but
perform internal processing to select the best possible choice
out of the colors available in the increased palette. There
are many opinions about whether there is a noticeable
difference in quality between 24-, 30- and 36-bit scanners.
Image Transfer Scanning the document is only
one part of the process. For the scanned image to be useful,
it must be transferred to your computer. There are three
common connections used by scanners:
Parallel - Connecting through the parallel
port is the slowest transfer method available.
Small Computer System Interface (SCSI) - SCSI
requires a special SCSI connection. Most SCSI scanners
include a dedicated SCSI card to insert into your computer
and connect the scanner to, but you can use a standard SCSI
controller instead.
Universal Serial Bus (USB) - USB scanners
combine good speed, ease of use and affordability in a
single package.
FireWire - Usually found on higher-end scanners,
FireWire
connections are faster than USB and SCSI. FireWire is ideal
for scanning high-resolution images.
A scanner may have more than one way of
connecting to your
computer.
Did You
Know?
TWAIN is
not an acronym. It actually comes from the phrase "Never
the twain shall meet" because the driver is the
go-between for the software and the scanner. Because
computer people feel a need to make an acronym out of
every term, TWAIN is known as Technology Without An
Interesting Name!
On
your computer, you need software, called a driver, that
knows how to communicate with the scanner. Most scanners speak
a common language, TWAIN. The TWAIN driver acts as an
interpreter between any application that supports the TWAIN
standard and the scanner. This means that the application does
not need to know the specific details of the scanner in order
to access it directly. For example, you can choose to acquire
an image from the scanner from within Adobe Photoshop
because Photoshop supports the TWAIN standard.
In addition to the driver, most scanners come with other
software. Typically, a scanning utility and some type of image
editing application are included. A lot of scanners include
OCR software. OCR allows you to scan in words from a document
and convert them into computer-based text. It uses an
averaging process to determine what the shape of a character
is and match it to the correct letter or number.
The great thing about scanner technology today is that you
can get exactly what you need. You can find a decent scanner
with good software for less than $200, or get a fantastic
scanner with incredible software for less than $1,000. It all
depends on your needs and budget.
For more information on scanners and related topics, check
out the links on the next page.
