Scanning is the process of recreating an original copy of something to a digital file format. This file once saved in the proper format can be edited and manipulated for your specific needs whether it be for a brochure, newspaper, flyer or even a letterhead.

To begin this overview you must first understand the different results achieved by scanning an original as colour, grayscale or line art. (Seen below).

COLOUR Colour scans are typically scanned as RGB on your scanner. RGB (colour) work should be scanned between 170 dpi - 300 dpi at 100%. Once the scan is complete it should be converted into CMYK mode in an image editing program.

GRAYSCALE Grayscale is made up from three areas, black (shadows), gray (midtones) and white (highlights). Grayscale work should be scanned between 170 dpi - 300 dpi at 100%.

LINEART Line Art is made up of only 100% black and 0% white, having no midtone values. The original art is typically scanned as line art or line work on your scanner. Line art should be scanned at a higher resolution than grayscale or colour work (600 dpi - 1200 dpi at 100%) is recommended.

Now you know if your original is colour, grayscale or lineart. This will be the mode you select in your scanner software (depending on the original, line art as line art, grayscale as grayscale etc..)

You must know three more things before you finally scan your original.

1. What is the original size? (our example will be 3" x 5" ).
2. What is the finished size going to be in your project? (Our example is going to be 6" x 10").
3. Finally what media is your project being printed on? (This is where the lines per inch of the printers specs come into play. Oxford Webs specs are listed here.)

Now it is time to put all this information together and make a good scan from an original.

Below is the equation for determining the resolution needed to scan an image larger than the original size.
In our sample we have the final image size of 6" X 10" divided by the size of the original image 3" X 5". From there you multiply the result (which would be 2) by the desired linescreen (85 lpi in this case, resulting in 170) and then multiply by 2 to get the final resolution for output which would be 340dpi.

If you are going to keep your original image size at 100%, then as a general rule you double the lines per inch that the job is to be printed at. This will give you the dots per inch for scanning the image.
For example we want a grayscale image scanned at 100% for our webpress. Our webpress uses 85lpi, therefore we scan the image at twice the line screen resulting in a 170 dpi scan.
85 lpi X 2 = 170 dpi.

DPI
DPI refers to dots per linear inch. An example would be if you scanned a picture at 300dpi, this means that there will be 300 printer dots per inch of the picture which gives you the total image of the picture.

Knowing the correct dpi is crucial when you want your scan to appear crisp and clean. Dpi is based on the type of image to be scanned whether it be a full colour scan, greyscale or line art scan, and what type of stock is being used on the press or printer.

For example a 72dpi picture (say, from the internet) would not work well on a full colour glossy magazine stock, which would require a 300dpi scanned image. The more dpi, the finer the dot when printing, which results in better quality images.

When using an image editing software such as Adobe Photoshop, you may notice the term ppi, this refers to pixels per linear inch. Pixels are what your monitor uses and how your scanner reads the image that it's scanning. The greater the number of pixels per inch, the crisper the image will appear on your screen. The number of ppi is directly related to the number of dpi, so 300ppi (on your screen) equals 300dpi (from your printer or press).

Note: The higher the dpi or ppi of an image, the larger the file size will be of that image.
If scanning at 100% you would never need to scan more than 350dpi/ppi for a colour/grayscale image.

A Low DPI image has less dots to define the picture

A High DPI image is clearer at the expense of a larger file size.

LPI
LPI refers to lines per inch, this is how many rows or lines of dots there are used to create an image on film or paper. The relationship between the output resolution (dpi) and the screen ruling (made up of lines per inch) determines how fine or coarse an image appears on the printed output.

An example would be the printing of a full colour magazine on coated paper. The scanned pictures are 300dpi, this would make the lpi 150, as the lpi equals half of the dpi (as mentioned above). Higher screen ruling results in higher quality images on the printed output.

Typical lpi screen rulings are 85lpi for printing on a web press and 133-175lpi for printing on a sheetfed press.

Note: If you have a 300dpi image (at 100%) and it is going out on a web press (85lpi) your image will not look better than if it was scanned 170 dpi (at 100%).

MOIRÉ
Moiré patterns are formed when you have two similar screen angles overlapping each other. They are a common problem in scanned images due to the interference between the printed halftone pattern and the scanned pre-existing halftone pattern. This can happen when printing a scan of a preprinted original.

When the dots of an image are positioned in a symetrical pattern, the result is a clean, sharp image. This is called a rosette pattern. This is what you want when reproducing a picture on a press.

A moiré pattern

A rosette pattern