For those unfamiliar with the printing industry, the screen being referenced here is the dot pattern created by the printing device to create the tonality of your image, and is also called a halftone. The finer or higher the screen frequency, the smoother and cleaner the image.
When scanning, remember that the higher your dpi, the more detail will be picked up by the scanner. With the exception of line art, the best way to scan is to use the “millions of colors” option even when scanning grayscale images. The reason for this is that grayscale will only pick up thousands of shades of gray, whereas you will get millions by scanning in color.
Color modes
Light emanating from various sources, from the sun to the fluorescent tubes above my head, projects waves and particles of light that bounce off objects, enter my eyes, hit cones and rods in the back, and presto, I see shape and color. The computer does a pretty good job of recreating this effect, and uses several methods to recreate natural appearances for us.
RGB, which stands for “red, green and blue”, is the basic color model for your screen. Little laser guns in the back of a CRT monitor fire their rays through red green and blue phosphors that reproduce color on your monitor.  Flat panel screens are based on similar principles, but we don’t need to get into that just now.
These three colors can produce millions of colors, from pure white (r255 g255 b255) to pure black (r0 g0 b0). The colors in this range are called the RGB gamut. Just as combining all colors of the spectrum creates white, and an absence of light creates black, combining RGB colors forms Cyan (r0, g255, b255), Magenta (r255, g0, b255) and Yellow (r255 g255, b0).  RGB is what is known as a subtractive color scheme: you take away to add more tone. However, inks don’t work that way, which brings us to CMYK.
CMYK (Cyan, Magenta, Yellow and Black) is the 4-process-color model used in full color printing. Ideally, 100% Cyan, Magenta, and Yellow would create Black but due to impurities that exist in all inks, the actual result is a muddy brown. Thus, black ink (K) is added for that purpose (K is used for Black so it is not confused with Blue).
Sadly, the CMYK color gamut is smaller than the RGB gamut, and covers a slightly different area of the color spectrum, so there are a few colors that can be printed but not rendered in RGB, and many colors that cannot be printed at all. (That is why when you paint an image with a wide range of red hues, you end up with a much flatter range.)
Photoshop has a great feature to use in RGB mode, which is called Gamut Warning.  When it is enabled, it will put a gray color over any colors which are out of printing range. This is a good way to proof your colors before conversion.
LAB color
This is an interesting color mode that doesn’t usually get used a lot. It stands for Lightness A and B. What it gives you is 3 channels that have separated out your gray tones (Lightness) and your colors, which are letter-coded A (from Green to Red) and B (from Blue to Yellow). It was created to be device-independent, for consistent color regardless of whether you are printing or viewing on your monitor. Lab color is supported by postscript level 2 and 3 printers so high end printers can create separations from these files. However, most print shops will prefer that you bring your file in CMYK.
Grayscale
Uses only the luminosity or lightness of an image measured from 0 (black) to 255 (white) or ink coverage 0%(white) to 100% (black)
Duotone mode
Creates multiple-tone grayscale images using two to four custom inks which you specify. These can be very tricky files, and I would avoid them like the plague if you can.
Indexed color
This is used for web graphics (primarily GIF format) and only allows 256 colors. Not useful for printing at all.
Multichannel mode
This is used for very specialized printing.
Creating the best grayscale image for printing:
Scan at full color. Convert your file from RGB to Lab. Select the lightness channel (the other two channels will drop out of view.) Go to Image>Adjustments>Curves.
You will get a straight line going from 0 to 100 on a graphed box. For photographs (or any images of continuous tone), you will first need to determine your darkest area. There is an eye dropper tool available to pick color and find your darkest tone. You don’t want anything over 90; solid black will flatten the printed image, and anything over90 will “plug up” the halftone on the printing plate, the result being a big black blob where your darkest details once were. Now, find your lightest point. It should not be lower than 5; lower than that, and it may get no tone at all and you end up with a bald spot on your image (Think local newspaper photo, and you’ll understand.)
You now have a halftone image ready for printing.
Full color
Rule #1: ask for a color key. These neat little guys are made from the film that will be used for printing, and are very accurate renditions of what you will get (they do go a little dark). They will cost around $13 to $15 or so, depending on where you are. Some shops will just include it in your printing price. Color keys do not lie, so if you see a serious change in color, remember:Â it’s your machine with the problem, not their film.
In all printing, always use either TIFF files or PSD files. You will be much happier with the outcome. EPS files will work; however, they can be tricky to work with, and don’t always like to print right. JPEGs are a very bad idea altogether. They are for web graphics only and will, in fact, drop out important color information every time you save them.
When saving your files to disk, it is best to put your finished image on a clean CD so there is no confusion. Doing these things saves you time and money, since you now won’t have to pay anyone to do it for you.
Many of these little tidbits can be found in your Photoshop manual, if you have one.
Article by Jason Wilson.
