Scanning is one of those subjects that causes a deal of confusion, especially
to newcomers. The notes below are intended to help users understand some of the
fundamentals of scanning. If, however, all you want to know is what resolution
to set your scanner at then go to the summary section at the bottom of this
page. Don't worry if it seems confusing, it is!
It is the subject of resolution that causes the most confusion, there are four
terms you will come across when discussing this, they are:
SPI: Samples per inch. This is a measurement of scanner and digital image
resolution. To scan an image the scanner takes a sampling of portions of the
image. The more samples it takes per inch, the closer the scan is to the
original image. The higher the resolution, the higher the SPI.
PPI: Pixels per inch. This is the number of pixels displayed in an image.
A digital image is composed of samples that your screen displays in pixels. PPI
is the display resolution not the image resolution, though SPI and PPI are often
misleadingly interchanged.
DPI: Dots per inch. This is probably the most familiar and most misused
measure of resolution. For desktop publishing, it is probably the least
important measure as well. It is not the resolution of images on screen. It is
the measure of how many dots of ink/toner/laser a printer can place within a
square inch. It is the resolution of an output device such as a laser printer or
imagesetter. A standard office laserprinter is 300 DPI and a standard
imagesetter is usually 2400 DPI. In general, you should not be too concerned
about this, as we set the appropriate resolution. If you think of PPI (pixels
per inch) as screen resolution, then DPI is the printer resolution.
LPI: Lines per inch. The line screen of a printing press. This subject
often gets little, if any, attention from graphic designers. This is
unfortunate, because it is the most crucial element in determining the finished
quality of print, especially where there are photos involved. LPI refers to the
way that printers reproduce pictures and tints, simulating greys or colours by
printing lines of halftone dots. The number of these lines per inch is the LPI.
Understanding LPI is crucial in setting up the correct resolution for your
scanner.
It is unfortunate, then, that the software and manuals that came with your
scanner may use SPI/PPI/DPI interchangeably. You will have to look at the terms
in context to help you decipher whether they really mean samples, pixels or
dots.
In Litho printing, photographs and graphic elements such as tints are broken
down into small dots. If you look at a printed photograph through a magnifying
glass you should be able to see these "rows" of dots. Generally the higher the
number of these "rows" or dots per inch, the higher the quality. The number of
these dots per inch is determined by two main factors.
1. The actual printing press
The presses the Russell Press use can produce line screens of up to 200 lines
per inch. A press can produce a lower line screen well, but cannot go above its
optimum line screen without a loss of quality. The image will tend to get
"muddy" because there is more detail then the press is designed to handle.
2. The type of paper ... this is the most significant factor
Printing at, say, 200 lines per inch on very soft, porous paper will produce a
disaster. This is because the dots of ink would spread out (because the ink
would be absorbed into the paper), the dots would join together producing an
indistinct mess.
For example, newspapers, which use very soft porous paper, usually print
photographs at 80 to 100 lines per inch because the images look better than at
150 lines per inch, even if, technically, there is less detail.
On denser, glossy paper, higher line screens can be used to achieve extremely
detailed images. This is because the ink is less likely to spread out as it
dries.
The Russell Press, generally uses line screens of 120 LPI and 133 LPI for
greyscale work and 150 LPI to 175 LPI for 4-colour work.
This is where the line screen of the final printed document is important. The
optimum resolution required for scans is TWICE the line screen of the final
document. So if your document is going to be printed at 133 lines per inch, you
should set your scanner to 266 SPI*. (300 is OK). You can, of course, set your
scanner to a higher resolution, but this will result in a much larger file, with
no perceptible difference in printed quality. For instance, in the example
above, if you set your scanner resolution to 600 the resultant digital image is
4 times the size, requiring more computer memory and disk space, with no real
advantage in terms of quality.
If you are not sure what the line screen will be, assume it is 150lpi and set
your scanner resolution to 300.
There is an exception to this rule, though, and this is for line artwork, where
we would advise you to set your scanner resolution to 1200, otherwise you may
get jagged edges on the digital image.
Before scanning your image, it is advisable to know what size it will be used
at. That way, you won't have to scale it in your page layout program. The
biggest danger in scaling scans in your page layout program is that, if you
scale up, you lose resolution. For example an image scanned at 300 SPI and
scaled to 200% is effectively 150 SPI which is too low for general printing.
Scaling a scan down is not a problem. If you are in doubt about the final size,
scan your image larger than you will need. Scans can always be reduced, but they
cannot be enlarged without loss of quality.
*This is the resolution of your scanner, in your software package, it may be
called PPI, SPI, or DPI, just to cause more confusion!
1. Find out the final line screen the job will be printed at (if you are not
sure, ask us!). Set your scanner to twice that resolution for greyscale or
colour images. If you are not sure what the line screen will be, then set your
scanner resolution to 300.
2. Scale your picture to the correct size when you actually scan it. Not in your
desktop publishing program. Scaling down the image on the page is fine, scaling
up is only OK if essential, you will get away with small percentage increases,
say of up to 10%.
3. If you are scanning line shots, set your scanner resolution to 1200.
4. Save your files in TIF or EPS format. Never use JPEGs unless absolutely
necessary, as this format may cause loss of detail.
5. If your job is in four-colours, then make sure your scans are CMYK and not
RGB (red, green, blue). Many scanners scan images to RGB, you may need to
convert these images to CMYK, see your software manual for details on how to do
this.