Baby Steps To Print Purchasing Know How
The Printing Crash Course - Printing School
What Are Halftones and Line Screens?
When type is printed in black ink, it is either 100% black (image area) or 100% white (non image area). When pictures are printed, the same process is utilized, but the images are screened (turned into dots).
An image that is screened is called a half-tone due to the tonal effects it gets from the screening process. The number of dots per inch is the screen value or line screen value of the image. The line screen value is identified by a number of Lines Per Inch (LPI ). Line screens are normally 80 lines, 100 lines, 133 lines, 150 lines and 175 lines for most jobs, although extremist perfectionists have gone as high as 400 lines, but past around 200 lines most of the difference is only noticed in very fine images, or under a high powered magnification.
Frequently confusing acronyms such as DPI (dots per inch) are used to talk about the line screen value used on the image. Dots per Inch is a term to denote the scanning value to be utilized in digital imaging and digital processing of scanned images to be printed and should not be confused here with line screens. DIP values required in print for scanned images is 304 dots per inch.
The process of screening turns the solid areas (shadows) of the photograph into 95% black dots, and 5% white dots, but no solid patches. The middle-tones (mid-tones) are 50% white areas in-between 50% black dots - a chequered board effect. The white areas (highlights) of the photograph are rendered to 95% white areas with 5% black dots. One can further define the 25% black and 75% blacks as quarter tones and three-quarter tones and so on. A smooth curve gives contrast to the image between the quarter and the three-quarter tones, and still maintaining the fidelity of the highlights and shadow areas.
To further explain the detail of the half tone - one has to look closely at a black and white picture, ideally in a newspaper, as they use a lower value line screen due to the absorbency and dot gain properties of the newsprint that is utilized in the newspaper industry. It is easy to see scattered small black dots in the highlight areas, where as equal areas of black and white in the midtones, and scattered small white dots in the shadow area. This process is repeated four times in CMYK printing at different screen angles to achieve a rosette-patter (not a moray patter which is offensive to the eye and not pleasing) that blends the colors of the inks into a very soft and smooth image.
As all printing processes described in Printing 101 are either solids or blanks, each and every process in print utilizes the use of half toning and line screens to achieve the tonal effects and the fineness of the screening process to control the quality.
The higher the line screen value, the finer the dots in the screen, and therefore the more detail it can carry in the image. Although caution must be exercised as too high a line screen value on a soft or absorbent stock will result in a higher than normal dot gain on the stock, thereby making the shadow areas solid and therefore unpleasant.
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