Scanning Stamps by Brian Robinson
you want to look at a stamp closely or compare it to other stamps, you
can take it out of its mount, get out the magnifying glass, do your
investigation and then put it back in the mount. You risk
creasing it, or worse, dropping it in your coffee. The best approach is
to scan it once and then put it away where it belongs.
people who want to scan stamps will need a flat-bed scanner.
These have become relatively cheap and the resolution has increased
although the speed of scanning has not. People also use web cams,
digital cameras and even their phone.
scanners usually read RGB red-green-blue data from the photo-sensitive
array. This data is then processed with some proprietary algorithm to
correct for different exposure conditions, and sent to the computer via
the device's interface (usually usb). On the PC it is processed
further and finally saved to a file.
The size and quality of a scanned image depends on a number of factors.
varies depending on the scanning array characteristics, but is usually
at least 24 bits. High quality models have 36-48 bits of color depth.
Another parameter for a scanner is its resolution which is measured in
pixels per inch. Instead of using the scanner's true optical
resolution, many manufacturers refer to the interpolated resolution,
which is much higher using software interpolation.
resolution is the true resolution of a scanner, and is determined by
using a resolution test chart. The effective resolution of most
consumer flatbed scanners is considerably lower than the manufactures'
given optical resolution.
For example, the Canon CanoScan LiDE 120
the optical resolution is quoted as 2400 x 4800 dpi. However,
they say that the resolution is selectable from 25 to 19200 dpi
size of the file created increases with the square of the resolution;
doubling the resolution quadruples the file size. A resolution must be
chosen that is within the capabilities of the equipment, preserves
sufficient detail, and does not produce a file of excessive size. The
file size can be reduced for a given resolution by using "lossy"
compression methods such as JPeg, with some cost in quality. The
time taken by the scanner to produce the image also increases as we go
to higher resolutions.
Most stamps are about 1 inch x 1 inch but
the first thing we do is reach for a magnifying glass as they are too
small to see the real detail that a philatelist wants. When we
scan in a stamp, we can enlarge it to any size we wish. However,
this comes at a cost. If we look at a stamp that was 1 inch x 1
inch, on a PC screen that is 17 inches and enlarge the image to
BIG, the computer has to estimate or interpolate the pixels to
make it fit the new bigger size. In doing so, the image will
often become ‘blurred’ as a color changes from one color to
another. If part of a stamp goes from red to white and only red
ink was used to print it, then this section of the stamp should be
either red or white (assuming the ink didn’t run at all). On a
color scan which is enlarged, you are likely to see the color change
from red, through various shades of pink to white. These pink
colors have been invented by the scanner software or interpolated as
the manufacturers call it. This is easy to observe if you
look at a stamp scan and keep enlarging it. To prevent the
software having to guess the missing pixels