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Scanning Line Art
Step 1: Set the scanning mode to Line Art
In the scanner dialogue box (varies dependant on your scanner and its software) you should be able to select a mode called Line Art. When you use this mode you are actually acquiring an image that is only all black and white pixels as opposed to tones.
Step 2: Setting the Resolution
Scanning line art is a different bird altogether than any other type of scan. Line art isn't screened during output, and is not converted to half tone dots. With line art what you see is what you get, as it uses tiny solid black squares in the print. This is one of the reasons why line art is scanned at such a high resolution. The resolution needs to be high enough that the small black squares are not visible or detectable to the human eye.
You might be tempted to start with 300dpi as that's probably what everyone will tell you that you should scan at. We want to start with the highest quality scan first.
900 dpi is the minimum you would want to use, however, 1,200 dpi is the industry standard for this purpose. Anything larger doesn't noticeably affect the detail and serves only to increase file size.
Step 3: Preview the image
Your scanner should allow you to preview and 'crop' the area you wish to scan. Make sure you have all the line art and leave some white space around just incase, you can crop later if necessary.
Step 4: Acquire the Image
Pretty simple step here.Click scan, and then open it in Photoshop (if this isn't already the case). You will want to make sure the line art acquired is okay so preview the pixels at 100% to see if you have any jaggies. The final result depends a lot on how clean the original line art is, and what type of method/ink and paper is used so a judicious eye may be needed to decipher the difference between jaggy and intended information.
Step 5: Clean up
Now you will need to examine the image for stray dots and clean them up. The standard method in this mode is to use the pencil tool with foreground color selected being white. This is one of the reasons I scan in line art in line art mode, as it is far easier and more effective to clean up the 'dirt' assuming you have a fairly clean piece of paper to begin with. You can also paint in black any areas of the line art that have holes or patches missing as is sometimes the case.
Test the line art by printing at 100% to a good quality laser printer or other device to see what the output may be like, and to see if any further adjustments need to be made, or if you need to rescan.
For the purposes of Coloring Line art in Photoshop
All the above steps may seem a bit extreme for this purpose and you may in some cases be correct, for the purpose of coloring line art in Photoshop your final output resolution may only be 300-350 dpi (150-175lpi) so you will need to resample the line art to the appropriate resolution. Do this only after you have cleaned up all other unnecessary information from the original scan and have it cropped the way you want