SCANNING TIPS

Compiled and liberally edited by Gerard Bowles, 1997, with testing of principles and techniques. Much of this material is based on information from Epson Corporation, Agfa-Gevaert N.V., and Jeff Bone of Infomedia, Inc. The width and layout of this page is designed to be economically printed, as it is used by two universities.

Introduction:
This papers offers tips for scanning. Grasping it all at one time might be difficult. Therefore, I have written each section in layers of difficulty--grasp the first layer and apply it, and so forth. If after considerable study and scanning parts are still unclear to you I suggest studying the scanning software manual with your scanner while referencing this paper.


SCANNING COLOR OR GRAY SCALE

According to Epson don't scan and print at the same resolution. Printer and scanner resolutions are different. For all images except test and line art, for 1-1 (100%) of image size, scanning at 120-dpi resolution gives good results with 360-dpi printer resolution, and scanning at 240-dpi is good for 720-dpi resolution. If you are enlarging use a higher scanning resolution. Images scanned at very high resolution increase storage space without producing a noticeable improvement in printed output.

For color or gray scale images, ignore the dpi (dots per inch) resolution of your printing device. For example, for a service bureau's 2400 imagesetter printer, a scan resolution of only 150-250 dpi will produce the highest quality gray-scale or color images the imagesetter can print. Similarly, even tho your laser printer may have a 300 dpi resolution, it required a scan resolution of only 75 to 100 dpi to obtain the best quality gray scale images it can print. (1)

Calculating more precisely the required resolution for gray scale or color on an electronic color printer or a printing press:
1. Find the lines per inch (lpi) of the printing device.
2. Lines per inch x 2 = the best scanning resolution. For example, for 133 lines per inch (common for magazines and books), scan at 266 dpi (2x133=266)

For even smaller file sizes, try scanning at about 1.5 x the lines per inch. This may cause little or no perceptible loss of output quality.

Agfa offers this formula: Scan res. = Output device res. X Sizing factor

Note: The size of the image on the monitor will probable be different from the size of the image when you print it.

Critical color scanning or color separations for professional use should be done at a service bureau. (2)

Agfa offers this formula for gray scale or color resolution:
Scan res. = Screen ruling X Quality factor* (qf) X Sizing factor**
*Quality factor is 2 for 133 lpi, or 1.5 for less than 133 lpi (Quality factor is also known as halftoning factor)
**Sizing factor = Desired size divided by Original size
An image to be output using a 175 lpi printer at 75% of its original size should be scanned at 197 ppi (Scan res. = Screen ruling X Quality factor X Sizing factor = 175 lpi X 1.5 X 0.75 = 197 ppi).

"Unsharp originals are given the impression of being sharper by the application of unsharp masking (USM). This process does not add detail but heightens the contrast at edges of objects to make them more visible." USM is similar to traditional photographic methods in that it combines the unsharp image with an even more blurred copy (mask) to produce the sharpening effect. (3)


GRAY SCALE

  • Do not exceed 256 shades or gray.
  • If you reduce the image by 50% this doubles the resolution.
  • 1.3 to 1.5 X the lpi. Example: A 1200 dpi Linotronic produces 133 lpi, so don't exceed 200 dpi when scanning.
  • DPI = LPI X increase or decrease in original size X 1.5
  • Sharpening the image in an image modification program (Photoshop) gives better results than a high resolution scan. Oversharpen a little--softening occurs in reproduction.
  • Scanners reduce contrast so always sharpen.
  • Add 1 or 2 units of random noise. This will especially help low-resolution scans or reproduction from and low resolution image source. (2)

    SCANNING LINE ART

    Resolution as high as output, not to exceed 600 dpi. Example: At 100% (same size), 600 dpi printer = 600 dpi scan resolution. (Read above about printer resolution)

    Agfa offers this Resolution rule:
    Scan res. = Output device res. X sizing factor

    Use a tracing software, especially if you intend to blow up the art. "Using Corel Trace, Adobe Streamline, or the AutoTrace function in FreeHand, you can convert the line art scan into Encapsulated postscript and thus allow you to increase the image to any size with out the fear of the Jaggies (of course, this is not a flawless step, you may well have to spend a considerable amount of time correcting for obscure autotracing errors)." (2)

    Agfa lists 4 factors important in scanning line art:
    1. Sizing
    2. Output resolution
    3. Sharpening (It is best to do this in the scanning if the software is capable)
    4. Black-white threshold value ("How much detail is retained in light areas, and whether or not small highlights in shadow area fill in.")


    SCANNING HALFTONED ART

    Halftoned images should be scanned at less lpi than the printing to avoid a moire.


    Resources (Reproduced with permission to Gerard Bowles):
    (1) Epson Corporation
    (2) Jeff Bone, President, Infomedia, Inc.; © 1993-1995, All Rights Reserved
    (3) "An Introduction to Digital Scanning," Agfa-Gevaert N.V., 1994

    Additional resources:
    Consult the manual for the scanner, scanning software, and printer you are using.
    "The Scanning FAQ," Jeff Bone, President, Infomedia, Inc; © 1993-1995, All Rights Reserved published/WWW, 1996
    "An Introduction to Digital Scanning," Agfa-Gevaert N.V., 1994