Scanner: A device to turn hardcopy (i.e paper) into bits'n'bytes (i.e a computer file)
There are many kinds of scanners (radio scanners, bar code scanners etc),
but this article focuses on flatbed scanners for computers
You have probably seen them standing around in offices, in homes, and you
might have one yourself. Scanners can scan text to editable on-screen text through
OCR (Optical Character Recognition), convert analog pictures (i.e a photograph
or things from a magazine) into digital images (i.e computer files).
Most scanners are quite easy to use; HOW they work, however, might
be a bit more tricky... So that's what I'm having a look at here!
Types of scanners
A Flatbed scanner is basically a big CCD array and a light source under
a glass plate. You put whatever you want to scan on top of the scanner, press
a button, and the image is scanned. It will scan photographs, pictures, and
text - basically anything that's on a piece of paper or on an overhead transparancy.
They are usually in the size of A4 (Lettersize) or A3 (Whatever double
letter-size that is called in the US. if you know, msg me!). You can
scan basically anything you can put on the scanner, provided it isn't so heavy
that the glass plate breaks.
A sheet-fed scanner is really the same as a flatbed scanner, except that instead
of the scanner head moving, the paper moves through the scanner. This means
that those scanners can be very small, and (more importantly) portable. You
can only scan single sheets, as books etc. won't fit in the scanner.
A hand-held scanner is an odd mixture between a sheet-fed scanner and a flatbed.
It works the same as a flatbed, but the user har to move a scanner head over
a piece of paper (think about a paint roller, and that's about how it works).
Because the handheld scanners are generally smaller than the paper you are scanning,
you have to scan in several passes, which can be difficult to align. To
A film scanner is usually a small scanner that scans photo negatives and
slides. It has much higher resolution than a flatbed scanner. it will only
scan translucent things, and usually the largest scan size is 24*36mm (a regular 135 film negative). Medium and large format negative scanners do exist, but are INSANELY expensive.
A dias option turns a flatbed scanner into a film scanner. The dias option
is basically a light source that shines through the negative, to enable the
scanner to read the image. This is "the best of both worlds", although
the quality of the film scannings are usually not good enough for anything but
screen-use. You'd probably be better off scanning the paper copies.
A drum scanner is a very, extremely, horribly expensive type of scanner.
You put whatever you want to scan inside a drum (hence drum scanner).
This drum revolves, and the image is scanned one line at a time. Takes a while,
but the quality is incredible. Used in magazines etc. where the quality is paramount.
In drum scanners, you can scan single sheets, transparancies and film.
- Serial - Not many scanners use this interface anymore, as most
computers have a parallel port that is much faster.
- Parallel - Parallel was for a long time the preferred interface
for scanners, causing a horrible lot of trouble for computer users, because
scanning would lock up the whole computer while the scanning process was going
on. Besides, the printer used the same port.. Imagine the fun.
- USB - Most newer scanners can use USB (Universal Serial
- Firewire (or IEEE 1394) - Firewire is basically a very
fast USB, and is therefore used in a few film scanners. I'm not aware of any
flatbed scanners that use this, but they probably exist.
- SCSI (Small Computer System Interface)- This used to be (and still
is, although USB 2.0 and Firewire are probably taking over)
the top-notch standard for high end scanners and (expecially) film scanners.
Drum scanners also use the SCSI interface. Advantages of SCSI is that it uses
hardly any system resources, and that it is fast.
The information between a computer and a scanner goes something like this:
- Scanner head (ccd)
- Scanner internal processing
- Scanner interface (i.e one of the five from the list above)
- TWAIN driver
- Program (i.e MS Word, Photoshop or whatever)
The TWAIN step above is important, because this is what enables the computer
to communicate with a scanner. In general, TWAIN helps scanner vendors to write
device drivers for their products.
How a scanner works
The most important part of the scanner is the CCD unit. (for an absolutely
fabulous description about how CCD's work, have a peek at my writeup on
the subject. Even though that was written for digital cameras, the same applies
for scanners. Just substitute "digital camera" with "scanner",
and ignore all occurences of "megapixel"). The CCD, in short, is
the "eye" of the scanner.
What happens when the scanner scans:
- Something is put on the glass plate, lid is closed, and scanning process
is started (either via hardware1 or software2)
- A lamp3 is used to illuminate the thing in the scanner
- The scan head (i.e the lamp and the CCD) moves across the document to
make a pass4
- The light from the lamp hits the document and reflects. It then goes via
a set of mirrors (usually 2-3 mirrors) to the CCD array.
- The captured image is processed5 and sent to the computer using
one of the mentioned interfaces
1) via hardware: By pressing a button on the scanner
2) via software: By pressing a button on-screen saying something
3) The lamps used in scanners are usually TL lamps, CCFL's or
Xenon lamps, depending on price and target group
4) Most scanners use single-pass scanning, i.e the whole image is
scanned during one movement across the page. If this is the case, a lens splits
the colours into three, so the CCD array can pick up the various colors on different
parts of the array, or the CCD array have color filters on them (like in a digital
camera. Some scanners, however (ironically, either very old and cheap or very
new and expensive) use three-pass scanning, one for each color (RGB, Red Green
5) Some scanners don't process the data at all, and send everything
to the computer for processing.
Often you see a scanner has the resolution mentioned like something like 2400*1200
pixels (= dots per inch). The first number is determined by how many sensors
there are across the CCD sensor. The other number is determined by the precision
of the stepper motor.. Correct me if I'm wrong, but I believe that a 2400*1200
single pass scanner has 2400 sensors across (one for each colour, i.e. 7200
sensors5), and that the stepper motor moves an inch in 1,200 steps.
5) 7200 sensors sounds like a lot, but compare that to the 4,000,000
million sensors that can be on a 1" Digital Camera CCD
Optical vs. interpolated resolution
The optical resolution is a number describing how much information a scanner
can get out of a lens. Interpolation is a number showing how much information
the scanner can get from the information it got from the picture. In general,
we don't like interpolation (interpolation is generally done inside the
scanner, and all the extra information has to be transmitted to your computer.
A complete waste of time - you might as well get full optical resolution,
and enlarge your picture with image editing software. It seems logical that
a computer can do the interpolation faster than a scanner (it's kinda in the
words.. The scanner scans, and the computer computes)
Bit Depth / Colour depth
This is a number referring to how many colours can reproduce. Most image editing
programs (including photoshop 6.0) has full support for 24-bit colour (see
table below). Most scanners send 24 bit images to the computer, but the
difference lays in how the colours are used internally in the scanner. Some
scanners can output higher colour bit depths
BPC | BD | COLORS
5 | 15 | 33 K
6 | 18 | 262 K
7 | 21 | 2 M
8 | 24 | 16 M
9 | 27 | 134 M
10 | 30 | 32 G
11 | 33 | 8 T
BPC= BITS PER CHANNEL
BD = BIT DEPTH
as you can see, the bit depths increase with increments of three (because an
increase of one bit per channel of each of the RGB color amounts to a three
bit color depth increase), and the number of colours increases exponentially.
My scanner happens to be a 14 bit per channel scanner, which means it has a
bit depth of 42, and a total of.. eh.. never mind. More colors than I'll ever
What determines what is a "good" scanner?
- Scanner software
- Believe it or not, but scanner software and firmware are the most
important factors in scanner quality. A 1200*1200 dpi scanner from Canon
and a 1200*1200 scanner from Hewlett Packard are essentially the same
technically - the difference is mainly in the scanner software.
- Obviously, the more information the scanner can get out of a picture,
- There is a lens splitting the colors - The "cleaner" your colours
are split, the clearer the colours become.
- Rule of the thumb: The brighter your light source is, the better your
picture quality becomes - it will make the pictures sparper.
- The more colours, the more you have to work with. (you'll never be able
to put all those colours to use anyway :)
How to choose a scanner?
Find out what you need. If you are going to use the scanner to scan holiday
snaps and send it off to friends via the internet or put them on your web page,
get the cheapest scanner you can find. If you want some more quality, because
you want to do image editing work etc, go through the checklist above. Start
by getting a decent brand (Canon, Epson, Agfa etc) and look at points
2 and 5 especially. Point 1, 3 and 4 get better the more money you spend :)