Scanning Stamps by Brian Robinson

If 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.
 
Most 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.

Scan quality
Color 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.

Color depth varies depending on the scanning array characteristics, but is usually at least 24 bits. High quality models have 36-48 bits of color depth.

Resolution 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.
The effective 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

The 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