Image scanning recommendations
Use a calibrated scanner to capture 2000 to 3000 pixels on a size. Save the image as a TIFF file in IBM-PC byte order with LZW (lossless) compression. Use Zip or StuffIt to reduce the file size (without compressing the image itself.) The details of the final LOC American Memory “3UA” standard for uncompressed archival images follows:
Uncompressed Archival Images: Save as a TIFF file in IBM-PC byte order with LZW (lossless) compression. The minimum size is 3000 pixels on the image’s long side, with the other side falling between 1900 and 2200 pixels. The resulting file size is 5-10 mega. (Scan black-and-white images in color to capture the quality of aging: sepia tone, etc. Capture the border of the image in order to have a reference as to pure “white.”)
There are two ways of determining the “resolution” of images: either by “dots or pixels-per-inch” or “the total number of pixels” (mega-pixels or pixels-per-side).
With pixels-per-inch, the value changes depending on the size of the original. For instance, when scanning a 35mm slide or negative, the ppi is usually much higher than scanning a 20×30 inch map. Part of that is practical — the map at 300 ppi would be 6000×9000 pixels, resulting in a huge file! — and part of that is based on preservation — to capture the details in a slide, you need to capture far more pixels of data.
Now, with the pixel measurement — referenced in the Library of Congress American Memory standards — what is important is the resulting number of pixels captured. For instance, in the two scenarios above, specifying approximately 3000×4000 pixels would determine the resolution for the scan itself. Video and digital cameras use this method; as shown in the Library standards, digital cameras are measured in mega-pixels, which is simply a multiplication of the pixels-per-side. 1600×1200 is 1,920,000 pixels, or about 2 mega-pixels.
PPI does become very relevant when you are considering the output device when printing an image. For example, the 3000 pixel side of an image, when printed to 10 inches, would be 300 ppi, which is high enough for a magazine print, but not quite high enough for continuing tone. Pixels-per-inch and dots-per-inch become very important for output.
So, in order to keep apples and apples together, let’s measure all image resolutions as pixels-per-side, or total pixels (in mega-pixels).
4/19/02, 8/29/05 (JC)