Film Density and Linescreen:

If you've ever wanted to know about film density or halftone-screening then this is your lucky day. This is a process that allows us to see shades of gray while using only black ink on white paper. It's all about the ratio of black print versus visible white background. If we want to 'see' a light gray we print a bunch of small black dots, evenly spaced. The ratio of black to white, and the distance between the black dots creates a bit of an optical illusion for our brain... we 'see' it as a shade of gray. If we want to see a medium gray, we print larger (still evenly spaced) black dots which means less white background is visible. More black combined with less white makes us 'see' a darker color, so the darker we want to go, the more black gets printed until the point where the white background disappears - and we have solid black.
 

The optical illusion of 'seeing gray' is more convincing when viewed from a distance. The closer you look, the more noticeable the dots are.

Want to get technical about linescreens? The frequency of the dots within a given area is referred to as it's density - this is an important number. In the United States and Canada we measure halftone screens in "lines per inch" or lpi. If the "linescreen" [aka: "screen frequency"] is 100 lpi that means each square inch of a given color will contain 100 lines per linear inch. So 100 in length x100 in width = 1,000 halftone dots.

Different printing techniques and conditions require different screens, but these numbers are typical, not hard and fast.

• Newspapers: 85-100 lpi
• CDman's Silkscreen Printing: 100 lpi
• Magazines: 133-175 lpi
• CD packaging (paper products): 150 lpi
• Art books: 175 lpi
• CDman's Offset Disc Printer: 200lpi
   

 
Device Resolution refers to the number of image elements per inch that a printer or imagesetter can produce. The higher the device resolution the better the quality of the print. A typical image setter is capable of producing over 2400 dpi.

The dots-per- inch combined with the line-screen controls the number of levels of gray that can be achieved with an output device. One dot on a hi-resolution image setter is equal to 1:2400 of an inch