For philatelic purposes, a flatbed scanner is undoubtedly the most versatile. You can scan anything from a single stamp to an entire stock book or album page. A hand scanner may be a viable alternative, particularly if your budget is tight. Their width limitation does not matter so much for stamps and covers. I have also seen single sheet scanners and photo scanners used successfully for philatelic purposes, although they require that the item being scanned be sandwiched between clear plastic sheets for feeding into the scanner.

Choosing Scanner Settings

When you scan a stamp or cover, there are two settings you will have to make in the scanning program: resolution and color depth. Here are some tips to help you make the best settings for your purposes

Color Depth

There are three color depth settings that will be useful when scanning philatelic materials. The one you'll probably use most often goes under different names in different manufacturers' scanner programs: True Color, 24 Bit Color, and Millions of Colors are some of the terms I have seen. Images scanned with this color depth can display over 16 million different colors permitting accurate rendition of all the color variations in the item being scanned.

Scanner programs usually offer a 256 Color mode. Images scanned with this color depth setting display a maximum of 256 different colors. 256 Color images can be very realistic, particularly for originals that have a limited range of colors to begin with. The advantage of 256 Color images is that the files are significantly smaller than true color image files. This color depth may be suitable when scanning stamps that are printed in only one or at most a few colors.

Gray Scale or 256 Grays mode discards all color information and creates an image that can display black, white, and 254 shades of gray in between. Gray Scale images can be useful for specialized purposes, such as when you are creating an image for evaluation of pert condition or centering.

When in doubt, I suggest that you always scan in True Color Mode. You can always use your graphics program to convert a True Color image to 256 Color or Gray Scale mode.

Resolution

Selecting the resolution of your scan is perhaps the most important choice you'll make. In selecting a resolution, you need to take into account the intended use of the scanned image. Here's why.

Just like the scan you make has a dots per inch resolution, what the image is displayed on does too. Whether the image is displayed on a screen or is printed on paper, there is a resolution associated with the output device. A computer monitor might have a resolution of 72 or 96 dpi, while a laser printer might be 600 dpi and an ink jet printer can be anywhere from 300 to 720 dpi and even higher. The optimum results are always obtained if the scanned image's resolution is selected based on the resolution of the output device. There's more to consider, however – specifically, the desired output size. Let's consider the situation for images that are to be displayed on-screen.

The default display of images on the screen uses one screen pixel for each pixel in the image. This gives the ideal viewing quality. Of course you can force the image to display at a different size, larger or smaller than its "natural" size. Let's see what happens when an image is displayed on-screen at a size other than its natural size:

• If the image is displayed larger than its natural size, the display software will have to interpolate the extra pixels that are needed and the final result will lose detail and sharpness.
• If the image is displayed smaller than its natural size some of the information in the image will be discarded. You could have gotten the same result with a smaller (lower resolution) image and saved on disk space (and download time too, for the Web).

When scanning images for screen display, you can calculate the ideal resolution as follows. First, determine the relative size at which the image will be displayed. If the image will be displayed at half size, this factor will be 0.5; if it will be displayed at twice its actual size, the factor will be 2.0. Then, multiply this factor by 96 (the most common screen resolution). The result is the scanning resolution you should use. If your scanning software does not offer the precise resolution you calculated, select he nearest value. For example, if the ideal resolution is 192 dpi, use 200 dpi.

There's a simple formula you can use to determine the ideal scanning resolution. First, let's define some terms:

SR = ideal scanning resolution in dots per inch

DR = resolution of final display device in dots per inch (96)

OW = width of the original being scanned in inches

DW = width at which the image will be printed or displayed in inches

Then:

SR = DR ´ DW / OW

With this formula, you can easily determine the ideal resolution at which you should scan. Unfortunately, things usually aren't that simple. There are a number of factors that can, and usually do, prevent you from using that "ideal" resolution when scanning:

• You are not sure of the final use of the image - how it will be reproduced and at what size.
• There are multiple uses intended for the image - for example, you want to make printed copies as well as display it on a Web page.
• The calculated ideal resolution is an intermediate value, such as 117 dpi, that is not supported by your scanner.

In these and other cases, the general rule is to "move up." In other words, you should always move to a higher resolution rather than to a lower one. Thus, if you cannot use the ideal resolution of, say, 117 dpi, you should scan at 150 dpi rather than at 100 dpi. Likewise, if you plan to use the image for printing at 300 dpi as well as for screen display at 72 dpi, use 300 dpi when scanning if possible. Scanning at a higher resolution captures the maximum amount of detail, from the original. You can always throw away some of that information by reducing the image's resolution after scanning (in your graphics program), but you cannot regain information if the image was scanned at too low a resolution. The process of changing an image's resolution is called resampling. If, for example, you scanned an image at 300 dpi for printing, you could resample it at 72 dpi for screen display.

Another advantage of scanning at high resolution is that the resulting images are easier to edit. Certain editing operations, such as rotating an image, give better results with high resolution images. Rotating an image can be useful if the item was not perfectly aligned in the scanner, and is usually easier than rescanning.

File Format choices for Scanning?

For saving local copies of your scanned images you have many choices of file format. The Windows Bitmap (BMP) format is widely used, particularly by people using the Windows operating system. Perhaps the most widely recognized file format is Tagged Image File Format (TIF). Unless you have a specific reason to use another format I doubt you can go wrong using BMP or TIF. The resulting files are relatively large but unless you are scanning a large number of stamps this should not be a problem.

For Web publishing your choices are much more restricted. At present there are only two graphics file formats with wide support on the Internet: Graphical Interchange Format (GIF) and Joint Photographic Expert Group (JPG) format. GIF format can be used only with 256 color and gray scale images. Since many stamps, particularly older ones, were printed in a limited range of colors, the GIF format is perfectly adequate. When the purpose of an image is to show the stamp's centering and perforations, GIF will always serve perfectly well. Its advantage over the JPG format (discussed next) is that the file size is often smaller. GIF files are compressed to save space using a loss-free compression scheme, which means that the image you get out is exactly the same as the image you put in.

How can a GIF file, with only 256 colors, give decent reproduction? Here's how it works. When you save a file in GIF format, the software analyzes the image and selects the 256 colors, from some 16 million that are available, that the image will use (this is called the palette). For example, with a stamp that uses shades of red on a white background, the palette will consist entirely of shades of white and red - enough different shades to give an excellent reproduction.

For True Color images you must use JPG. A True Color image can display over 16 million different colors. For accurate reproduction of stamps with many colors, such as those that use photographs, JPG is required. The JPG format introduces one additional complication. Like GIF, JPG is a compressed format. Unlike GIF, JPG uses a lossy compression algorithm which means that some information is lost during compression - in other words, the image you get out is not as good as the image you put in. When you save an image as a JPG file you must specify the level of compression to use. More compression results in a smaller file and lower image quality.

Image Size and Resolution

I have noticed a lot of confusion about size and resolution of digital images. I'll try to provide some clarification here. Note that this discussion applies only to bitmapped digital images, which are the kind you are dealing with when you use a scanner or digital camera. Vector images are another matter but they are not relevant in this context.

First of all you need to realize that a digital image really does not have a physical size. To say that a digital image is, say, 4x6 inches is meaningless. After all, the image is just a collection of pixels stored in memory or on your hard disk. A digital image does, however, have a pixel size - the number of pixels horizontally and vertically. This is the only "size" that is inherent in any bitmapped digital image.

It can be useful, however, to assign a physical size to a digital image. Suppose that you scan a 4x6 inch postcard. If the resulting image is assigned the size "4x6 inches" then people viewing the image will know the actual size of the postcard. It also means that when you print the image at "normal" size it will print at this size (at least with all the software I have seen). All digital image files include this size information with an image. Scanned images are assigned a physical size that corresponds to the actual size of the area that is scanned - which makes perfect sense.

Then there's the matter of resolution, which is usually expressed in terms of pixels per inch (ppi, sometimes referred to as dots per inch or DPI). Perhaps you have already realized that once a digital image is assigned a size it automatically has a resolution. Here's the formula for horizontal resolution:

Horizontal pixels per inch = (number of horizontal pixels) / (horizontal size in inches)

Note that the vertical resolution can be different from the horizontal resolution, but usually they are the same.

When you are scanning, here's how it works. There are two things under your control. One is the size of the area being scanned - a single stamp, a postcard, whatever. The other is the resolution that will be used for the scan. With some scanner software you set the resolution directly - 96 dpi or 150 dpi, for example. With other software you select the type of document being scanned (color document, color photograph, etc.) and the use you will make of it (printing, web page) and the software selects the best resolution for you. In any case, when you make the scan the resulting file has a pixel size that is determined by these two settings:

number of horizontal pixels = (horizontal pixels per inch) x (horizontal size in inches)

Thus, if you scan a 4x6 inch area at 150 ppi the resulting image will be 600x900 pixels in size (4 x 150 is 600, 6 x 150 is 900).

What about digital photographs, which unlike scanned images do not have an inherent physical size? These images are assigned an arbitrary resolution either by the camera or your software. For example, when I open an image from my digital camera in Photoshop it is assigned a resolution of 72dpi. Since the image is 2560x1920 pixels, this results in the image having a "physical size" of about 36 x 26 inches. This "size" is essentially meaningless, of course.

In your graphics program you can change any of these three image size parameters: physical size, pixel size, or resolution. Because they are all linked to one another, changing any one of them means that one or both of the other parameters must change as well. Let's look at an example. Start with the following image:

Size: 4x6 inches
Pixel size: 400x600
Resolution: 100 ppi (pixels per inch)

Suppose you want to change the size to 8x12 inches. In order to do so you must also do one of the following:

• Change the pixel resolution to 800x1200 (resolution remains at 100 ppi).
• Change the resolution to 50 ppi (pixel size remains 400x600).

Your graphics program will do this automatically, but you must tell it which one to do - change the pixel size or change the resolution. Note that the process of changing an image's pixel size is called resembling. The specific instructions for this differ from one graphics program to another, but the concept is the same.

Resembling of the image is controlled by the Resample Image option. If this option is on:

• Changing the width or height changes the pixel size while leaving the resolution unchanged.
• Changing the resolution changes the pixel size while leaving the width and height unchanged.
• Changing the pixel size changes the width and height while leaving the resolution unchanged.

Scanning and Uploading Images for On-line Auctions

A common need for scanned imaged of stamps is for on-line auction listings. In my experience, selling many hundreds of lots at on-line auctions, a good image is an important factor in whether or not a lot sells. In addition to the general principles of scanning that have already been discussed, here are a few pointers for creating images for auctions.

• Use the JPEG format for your files. In theory you could use GIF for stamps that have only one or a few colors, but the saving in file size is minimal and in my experience not worth the effort of switching back and forth between formats.
• Use a scanning dpi setting that is appropriate for the item being scanned. I usually use 150dpi for single stamps, which means that they will be displayed at about one and a half times life size on the monitor screen. For small lots of 4-8 stamps, I'll use 100 dpi. For entire stock sheets or album pages, 50dpi is good. In cases where a small detail of the stamp design is an important part of its identification, I sometimes go as high as 300 dpi.
• If you are scanning multiple items you can sometimes save time by scanning them all at once into a large image, then using cut-and-paste to create the individual images.
• Trim your images closely, and be sure the stamp is "square" in the frame. Which of these images looks better?