Image Scanning Information
Outline
Basic Definitions and Concepts
A number of variables have to be taken into consideration when scanning images for printing or for the
Web.
- DPI
- Dots Per Inch. A measure of the output resolution produced by laser printers or
imagesetters. See also LPI.
- LPI
- Lines Per Inch. A measure of the frequency of a halftone screen used in printing. The
archaic, and now misleading term line harkens back to the diffraction line etchings used to create
analog halftones
- SPI
- Samples Per Inch. Used here to indicate the number of data points per inch that a scanner
captures (i.e. input). As opposed to DPI (dots per inch), PPI (pixels per inch), or LPI (lines per inch)
- all of which refer to output.
Resolution
Most scanners have two different resolutions: the optical resolution (the actual number of pixels
a scanner can acquire) and the interpolated or enhanced resolution (a means of mathematically
enhancing the resolution).
Things to keep in mind:
- Never scan grayscale or colour images above the optical resolution of the scanner.
- Always scan line art at the highest possible resolution of the scanner (i.e., the interpolated
resolution), and then downsample the image to match the output device.
Optimum Resolution Settings
The optimum resolution for a scan primarily depends on the type of artwork being scanned (line art,
grayscale, colour), and the final output of the artwork.
Line art
Line art (black and white illustrations, text, logos, etc.) should be scanned at the highest possible
resolution and then resampled to match the final output. For instance, printing line art at 1200 dpi to
a 600 dpi printer will not produce a better image.
Grayscale
Grayscale images should never be scanned beyond the optical resolution of the scanner. The actual
resolution depends on the halftone frequency of the final output device. A formula to keep in mind is
resolution = halftone frequency x 2
You may even want to try scanning at less [halftone frequency x 1.25, 1.5, or 1.75) to minimize the size
of the file. For instance, a 300 dpi laser printer has a maximum halftone frequency of 60 lines per
inch, which means that the scanning resolution needs to be no more than 120 dpi (60 x 2 = 120). Sending
a higher resolution image to the same printer will not increase the quality of the output.
The halftone frequency of the HP LaserJet 5mp Printer at the CHASS Electronic Publishing Centre is ca.
75 lpi. Thus, if you are scanning grayscale or colour images for final output on this printer, you
should scan your images at no more than 150 dpi.
Colour Images
Scanning colour images follows the same guidelines as scanning grayscale images (i.e., resolution =
halftone frequency x 2).
Note: Scanning for final output with imagesetters or film separations requires high-end scanners.
If you are scanning for final output to a colour laser printer you will need to find out whether the
printer is a halftone or continuous tone printer (a continuous tone printer produces an image without
dots and with almost photographic quality). If the latter, then you can often scan your images at less
than the final output. For example, for output to a 400 dpi continuous tone colour laser printer you
could scan your images at 150-200 dpi and get great quality (though you will get better quality if you
scan at the same dpi as the final output).
Scanning for the Web
The same basic principles noted above apply when scanning images for the Web. The optimum resolution is
dependent on the resolution of the final output, which in the case of the Web is monitor resolution: 72
dpi (or more technically 72 ppi - pixels per inch).
The only time you may want to scan at a higher resolution is when you are scanning line art. In this
case you may want to over scan the image, i.e., scan at a higher resolution and then resample the image
to a lower resolution in an imaging program like Photoshop. This may yield a higher quality image.
Another consideration when scanning for the Web is the bit depth of the scan. If the final output
for the scan will be a GIF (Graphic Interchange Format), which has a maximum bit depth of 8, you may
want to scan with that bit depth (= max 256 colours). Note that you can also reduce the bit-depth in an
imaging program like Photoshop (you will probably want to further reduce the bit depth to lower the file
sime of the GIF image).
If you are scanning photo-realistic JPEG images ten you will want to make sure that you are scanning at
24 bit (millions of colours) or higher.
Scanning with DeskScan II Software
ScanJet is the scanning software available with the HP ScanJet IIc in the Scanning Room at the
Electronic Publishing Centre (14th floor Robarts).
Use the following table as a guideline to scanning at CHASS:
| Type of Artwork | Final Output | DeskScan
Settings |
| Type | Path |
| Line Art GIF | Web page | B&W Drawing | Screen (image at 75 dpi) |
| Grayscale JPEG | Web Page | B&W Photo | Screen (image at 75 dpi) |
| Colour graphic - GIF | Web page | Color Drawing/ Color Photo (8 bit) | Screen
(image at 75 dpi) |
| Colour photo - JPEG | Web page | Millions of Colors (24 bit) | Screen (image
at 75 dpi) |
| Line Art | LaserJet 600 dpi laser printer | B&W Drawing | LaserJet 5
(image at 150 dpi) |
| Grayscale image | LaserJet 600 dpi laser printer | B&W Photo (256
grays) | LaserJet 5 (image at 150 dpi) |
| Colour image | ScanJet 600 dpi laser printer | Millions of Colors (24
bit) | LaserJet 5 (image at 150 dpi) |
| Line Art | Colour CT laser printer | B&W Drawing | Color LaserJet (image
at 300 dpi) |
| Grayscale image | Colour CT laser printer | B&W Photo (256 grays) | Color
LaserJet (image at 150 dpi) |
| Colour image | Colour CT laser printer | Millions of Colors (24 bit) | Color
LaserJet (image at 150 dpi) |
More details about scanning at CHASS
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