Inks used in color printing presses are semi-transparent and can be printed on top of each other to produce different hues. For example, green results from printing yellow and cyan inks on top of each other. However, a printing press cannot vary the amount of ink applied to particular picture areas except through "screening," a process that represents lighter shades as tiny dots, rather than solid areas, of ink. This is analogous to mixing white paint into a color to lighten it, except the white is the paper itself. In process color printing, the screened image, or halftone for each ink color is printed in succession. The screen grids are set at different angles, and the dots therefore create tiny rosettes, which, through a kind of optical illusion, appear to form a continuous-tone image. You can view the halftoning, which enables printed images, by examining a printed picture under magnification.
Traditionally, halftone screens were generated by inked lines on two sheets of glass that were cemented together at right angles. Each of the color separation films were then exposed through these screens. The resulting high-contrast image, once processed, had dots of varying diameter depending on the amount of exposure that area received, which was modulated by the grayscale separation film image.
The glass screens were made obsolete by high-contrast films where the halftone dots were exposed with the separation film. This in turn was replaced by a process where the halftones are electronically generated directly on the film with a laser. Most recently, computer to plate (CTP) technology has allowed printers to bypass the film portion of the process entirely. CTP images the dots directly on the printing plate with a laser, saving money, and eliminating the film step. The amount of generation loss in printing a lithographic negative onto a lithographic plate, unless the processing procedures are completely ignored, is almost completely negligible, as there are no losses of dynamic range, no density gradations, nor are there any colored dyes, or large silver grains to contend with in an ultra-slow rapid access negative.
Screens with a "frequency" of 60 to 120 lines per inch (lpi) reproduce color photographs in newspapers. The coarser the screen (lower frequency), the lower the quality of the printed image. Highly absorbent newsprint requires a lower screen frequency than less-absorbent coated paper stock used in magazines and books, where screen frequencies of 133 to 200 lpi and higher are used.
The measure of how much an ink dot spreads and becomes larger on paper is called dot gain. This phenomenon must be accounted for in photographic or digital preparation of screened images. Dot gain is higher on more absorbent, uncoated paper stock such as newsprint.
Other articles related to "screening":
... These procedures include screening activities which include but are not limited to review of group-based data (cumulative records, enrollment records, health records, and report cards) hearing ... Child-Study, Early Intervening or Instructional Support Team, the above screening activities may lead to consideration by the teams to move the next level of screening activities ... When screening results suggest that the student may be eligible, the District seeks parental consent to conduct a multidisciplinary evaluation ...
Screening can also mean preventing access of something by some sort of barrier. Particular cases:
- Electric field screening
- Electromagnetic shielding in physics, the exclusion of electric, magnetic, or electromagnetic fields by a metallic screen or shield
- In atomic physics and chemistry, the screening effect or atomic shielding is the reduction of effective nuclear charge by intervening electron shells
- Screening (printing), a process that represents lighter shades as tiny dots, rather than solid areas, of ink by passing ink through a perforated screen
- Screening is a process stage when cleaning paper pulp
... This can either be achieved through a screening process of potential recipients, or else by making the benefits of the transfers so low only the most ... are also many problems associated with this method as the transaction costs of screening are very high, due to the need to pay for assessment, the travelling cost of candidates to and from the ... the programme and avoids the transaction costs of screening ...
... confer a higher risk of subsequently finding colorectal cancer and warrant more frequently screening ... The screening guidelines are the same, as for other colonic adenomas ... The screening interval is dependent on (1) the number of adenomas, (2) their size and, (3) whether high-grade microscopic features are present ...