O.K. What you are seeing is called a moire pattern. A laser printer prints in a series of dots, usually called a “screen”. A deskjet prints solids, so if there is moire, it would be a lot less noticeable or even impossible to detect. They are however noticable on the monitor none-the-less. These patterns are caused by (Before I start, go and get yourself a magnifying glass... seriously.):
Take the magnifying glass and look at your document you’ve just scanned. Do you see all those little colored dots? Those are how presses print media, a series of patterns, typically called “rosette patterns”. These rosettes are needed to effectively “screen” the same color of ink to become different shades of different colors when mixed with other inks. But all you need to know is, that pattern is there, and your scanner picks it up.
When you scan this document that contains the “screens”, the scan is made into digital format by the photosensitive elements in your scanner. Now the higher your resolution in the scanner (i.e.. 600dpi) the more photo elements that are used. For instance if you had a 4 inch document, scanning at 300dpi, 1200 photo elements would be used. With the following information you could surmise that the scanner too, has it’s own grid. So, when you scan this document, it’s “patterns” are superimposed into the scanners “grids”, giving you “moire”, or what looks like uniform “candy stripes”. To reduce this there are 2 reliable tools.
1) The easiest, try rotating your document in the scanner. Experiment with the angle to get the lowest amount of “moire”. Even if your scanner is low resolution 600dpi, scanning at 1200 or higher will help get rid of the moire.
2) Once that is done, the “Gaussian Blur” filter in Photoshop (5.0 and higher) is your friend here. Whatever the rotation of the document didn’t cover, this will help compensate. Gaussian Blur simply blurs this image resulting in loss of sharpness. When adjusting the radius, remember that you can use decimals instead of whole numbers. Also if carefully done, the “Dust and Scratches” filter can help here too.
It is always good practice to try to scan originals when possible. Try to scan photos or slides as often as you can. Try not to scan out of magazines, or other printed media.
Also you may look into a scanner with a “screen filter I use a Umax for my photos, slides & negatives, and it has a “screen filter” built into the program called a “descreen” filter. This is also a viable option in helping you get rid of those dreaded moire patterns!
Hope this helps!