Constant Plate Varieties 101: Scanning Engraved Postage Stamps

Occasionally, I receive scans of some lovely stamp varieties where the quality of the scans has made them unusable on the site. The most frequent problems I see are caused by insufficient scanner resolution or too much sharpening caused by scanning at different resolutions than the scanner's native optical resolution. Below are some of my thoughts based on years of experience scanning stamps. Note that these instructions and comments relate to high resolution scans for showing details and varieties on engraved stamps where colour accuracy is not critical. If you just want a low resolution scan to post as a small image on a forum or to send via email, you probably do not need to worry about many of the details discussed here.

High Resolution Scanning

Most of today's scanners intended for consumers, as opposed to production and graphic professionals, are designed to perform medium resolution scanning for use in digital documents that will be viewed on a computer screen or printed via a standard inkjet printer. This is especially true of the so-called "all-in-one" printers where the scanner is mainly designed for use in making copies or faxing. Very few of these types of scanners are suited to scanning stamp varieties where the objective is to scan at high resolution so that the stamps can be zoomed or viewed at a much larger size onscreen or in print.

The resolution of a scanner is usually measured in pixels per inch (ppi) or dots per inch (dpi). Although one of these units is really an onscreen measurement and the other is used for print, they amount to the same thing when referring to scanner resolution. They each refer to the maximum number of data points that the scanner can resolve per inch. In this article, we will use pixels per inch. Scanners usually range from 300 ppi at the very low end to 4800 ppi or even higher for dedicated scanners aimed at photographers and other professionals. It is very important to compare the native or optical resolution of the scanner when comparing different models or making a purchase decision. Most scanners include software that can "enhance" the resolution through a process known as interpolation. This means that the images are enlarged by the software which uses a mathematical algorithm to guess what the missing information would be between the actual pixels/dots that the scanner can scan. While interpolated images can be useful for some applications and uses, especially when printing via an inkjet, the interpolated resolution should never be used to evaluate the maximum resolution of a scanner. It should also be noted that third-party software such as Photoshop can usually do a better job of enlarging an image than the software that comes with most inexpensive scanners.

If you want to scan engraved stamps to examine them in detail for yourself or to illustrate some aspect of them for others, then you need an absolute minimum resolution of 600dpi. To see fine details, 1200dpi is a better resolution and is probably the best for viewing stamps onscreen. Most of the scans on this site are approximately 1200dpi. Scanning at a higher resolution than 1200dpi will make the size of your image files significantly larger and provides little benefit unless you intend to enlarge the image considerably (for a sign or poster) or if you are preparing images for professional printing and publication. Remember that doubling the scanning resolution will make your file sizes four times larger and that much harder to store, upload, or email to others. Examples of different scan resolutions are shown below.

So my primary recommendation is to select a scanner that has an optical resolution of at least 1200dpi and to use this setting for your stamp scans, and never use the scanner's software to interpolate higher resolution. Although many scanners perform best at their maximum optical resolution, it may also be acceptable to scan at a lower resolution if your scanner has a very high maximum. Unfortunately this is not always the case. Several scanners, including most of the previous generation of all-in-one units made by HP, perform very poorly when scanning at anything less than their maximum resolution. Most of HP's newer units scan at a maximum of 1200dpi so this is suitable for stamps. However, several of HP's older models, which many of us may have in our homes, actually scan at higher resolutions up to 4800dpi. The problem is that scanning at anything less than the maximum resolution can produce exceptionally poor scans. See below for images scanned on my own HP Officejet 6500A.

The poor quality of the scans at reduced resolution forces me to scan at the maximum resolution and then reduce the size of the images in Photoshop. Although this works well, it greatly increases the time to scan my stamps and requires the extra Photoshop processing. Below is an example of an HP scan at 1200dpi compared to one at 4800dpi and then reduced to 1200dpi in Photoshop. Notice how the artificial sharpening of the lower resolution scan by the scanner software makes the stamp image and perforations look extremely coarse. There are also all sorts of digital artifacts and pixelization in the native 1200dpi scan. Unfortunately, this is typical of many scans that I receive for the website. On older used stamps that are already on very coarse paper, the scans are sometimes so coarse and over-sharpened that I cannot use them on the site.

While I have singled out HP in this article, it should be noted that the results you receive will depend on a combination of your scanner resolution, the software used with it (which may perform differently with different operating systems), and the settings you are using within the software. For scanning stamps, I recommend that you use the manual/advanced/professional mode that may come with your scanning software. Images should be saved in a lossless format such as TIFF or BMP. JPEG/JPG may also be acceptable as long as it is not compressed heavily. Your software should have settings for this. You should also turn off such features as Image Adjustment, Sharpen, Unsharp Mask, Colour Correction, etc., and leave options such as Saturation, Colour Balance, Brightness, and Contrast at their default settings. Even using manual/advanced/professional mode, some scanning software still makes decisions for you based on the type of original document you are scanning. This is usually selected via a choice of "Document" or "Photo" for the source of the scan. I usually find that Document is the best for engraved stamps, but you may want to try Photo if this does not give you satisfactory results.

Those Annoying Scan Lines

Another common problem I see in many of the scans I receive and in my own HP all-in-one scans is the scan lines that appear periodically. These are usually vertical (in the same direction that the scanner carriage moves) but they can also be horizontal or even both. If these lines show as a colour problem or even as solid black or white lines, they can sometimes be eliminated by cleaning the scanner glass surface and/or by resetting the scanner by unplugging it and the plugging it back in after a few minutes. My experience is that this does not usually work.

The lines I am accustomed to seeing are caused by a small skip in the scan due to problems with the carriage that moves the scan head or even problems with the actual CCD/CIS sensors in the scanner that cause it to skip certain lines while scanning a document. Power fluctuations, quality and length of the scanner cable, and several other factors can influence this, especially if the problems are intermittent. However, my experience is that these lines are usually quite regular and there is no way to eliminate them other than to move the stamps to other positions on the scanner surface and hope for the best. Of course, scanning at a higher resolution and then reducing the resultant scan images using software such as Photoshop will help to conceal the scan lines, but they will usually still show to some extent. The only sure way to eliminate scan lines is to get a better scanner that does not produce them.

My Scanner Recommendation

My need to produce high resolution scans of stamps has increased significantly since I launched this website. So I thought I would try a few different scanners to see if I could make the job easier. I have settled on the Canon CanoScan LiDE 220. The benefits of this scanner are:

The only real downside I have experienced with this scanner is that the software is not particularly simple to setup and use for advanced scanning functions. The in-context help information is only accessible online and is not particularly informative. If you are not comfortable working with scanning software, this may not be the best solution for you. To get the best results for stamp scans, you need to do the following:

Other Scanning Tips

Here are a few other tips for getting better scans:

Good luck. If you have other scanning tips or advice to share, please Contact Us.