Guides (Scanning)

Scanning - Resolution

• Resolution is a really important issue in a scanner, and is measured in Pixels per inch (PPI). Dots per inch (DPI) are used to measure resolution on a printer. The collection of pixels is called it a bit map (shown left).
• Setting scanning resolution depends on;
  • Type of Media you are scanning
    • Reflective material ( not transparent) includes pictures and photos, may be contone - continuous tone (example photograph), half tone (example newspaper photos) or line-art (example cartoon, signature).
    • Transparency material (Allows light to pass through - e.g. 35mm slides, OHT's).
  • Final output device
    • Printer, Computer monitor, Fax

• Most manufacturers give resolution as some number by some other number, typically 300-by-600, or 600-by-1,200. The true optical resolution, applies in one direction only (the x axis, or horizontal plane), and is determined by the number of CCD elements and the quality of the lens. The second number given is sometimes called the mechanical resolution, since it indicates the minimum movement of the scanner's mechanics--the number of steps per inch that the scanner takes in the y direction. This figure is typically double the optical resolution. (Note that some marketing departments like to put the larger number first, claiming, say, 1,200 by 600 dpi, because it looks more impressive. Its safe to assume that the actual optical resolution is the lower number.)
• The reason you need the extra steps of mechanical resolution is because of problems you'll otherwise run into with interpolation. Interpolation guesses the values for pixels at a finer level than the scanner samples them, based on the values of nearby pixels.
• It's easy to interpolate between two measurements on the same scan line because the scanner measures the entire line and has all the information available. It's harder to interpolate in the other direction—to fill in an interpolated line—because the scanner hasn't scanned the lines after the interpolated line yet. By taking extra steps in the y direction, you eliminate the need to interpolate in that direction.
• Scanners that offer higher interpolated resolutions than the scanner's optical and mechanical resolutions (and that's most of them) do their interpolation for the higher resolutions at the computer. That enables them to receive later lines in the image before they interpolate between lines
• Unfortunately, both the scanner-based and software-based interpolations can be less sophisticated than the interpolation routines in a sophisticated program such as Adobe Photoshop, the premiere image-editing tool for high-end users. You'll often get better results by scanning at the maximum optical resolution for the scanner, and then resampling at a higher interpolated resolution in your image editor. The only way to find out, though, is to try it both ways. You may also find that the best choice depends on the type of image, so that you may be better off using the scanner's interpolation for line art and the image editor's interpolation for photos.
• For most purposes, a high-quality 300-dpi optical resolution is enough. A 600-dpi optical resolution is needed only if you plan to scan small targets—such as slides—and enlarge them. If you scan a 1-inch target at 600 dpi and then enlarge it to 4 inches, the resolution of the enlarged image will be 150 dpi. That's  enough for a good quality printout on any desktop printer. But if you started with 300 dpi, the enlarged image would be 75 dpi. That's appropriate for viewing on-screen but not for printing