Technical Advisory Service for Images
Putting you in the picture – digital images in education © TASI · Last reviewed - July 2002
? All scanners have the following components in common: an optical system, a
light sensor, an interface and driver software. Exactly what form these components
take and how they are arranged is what differentiates the scanner types. The quality
of the scan obtainable is primarily dependent on the quality of the optics and the
light detector, but the productivity is mainly reliant on the interface and driver
Scanners are the most common digital capture device, their purpose being to
convert light (which we see when we look at something) into ‘0’s and ‘1’s that we can
store and use in a computer. Or put more simply, scanners convert analogue
images into digital image files.
The large fall in cost of scanners over the last decade means that they are no longer
the specialist item they once were. However the range of prices and specifications
of the scanners on the market can easily confuse the potential buyer. The different
types of scanner (flatbed, film, drum and others) only adds to the confusion.
Before examining the different types of scanner, lets take a look at the underlying
technology and consider some of the issues that need to be considered when
choosing and using them.
Resolution is probably the specification that causes most confusion for newcomers
to scanning. It is simply a measure of the ability to capture detail within the original
Scanner manufacturers normally specify two figures for this parameter: ‘optical’ and
‘interpolated’ – however only the ‘optical’ figure gives a true indication of a scanner’s
For scanners that use Charge Coupled Devices (CCD) as the sensor, the optical
resolution is limited by the number of elements (pixels) in the detector array and how
the array is moved relative to the image. Resolution is specified by the number of
separate and distinct samples that the sensor can make in every inch and therefore
measured in samples per inch (spi), although often the more generic ‘dpi’ (dots per
inch) is given by manufacturers.
The figure given for ‘interpolated’ resolution is a result of the scanner software
'guessing' the values between pixels and presenting these intermediate values as
'real' values. This process gives no extra image information but merely smoothes
the visual information and increases the size of the file. There are very few times
when this process is in any way useful and when it is; it can be better undertaken
within image processing software. So, remember that when assessing a scanner’s
resolution, it is only the ‘optical’ figure that should be taken into account.