Stochastic screening
Encyclopedia
Stochastic screening or FM screening is a halftone
process based on pseudo-random
distribution of halftone dots, using frequency modulation
(FM) to change the density of dots according to the gray level desired. Traditional amplitude modulation
halftone screening is based on a geometric and fixed spacing of dots, which vary in size depending on the tone color represented (for example, from 10 to 200 micrometre
s). The stochastic screening or FM screening instead uses a fixed size of dots (for example, about 25 micrometres) and a distribution density that varies depending on the color’s tone.
The technique of stochastic screening, which has existed since the seventies, has had a revival in recent times thanks to increased use of computer-to-plate
(CTP) techniques. In previous techniques, computer to film
, during the exposure there could be a drastic variation in the quality of the plate. It was a very delicate and difficult procedure that was not much used. Today, with CTP during the creation of the plate you just need to check a few parameters on the density and tonal correction curve. When you make a plate with stochastic screening you must use a tone correction curve, this curve allows one to align the tone reproduction of an FM screen to that of an industry standard.
Given the same final presswork tone value, an FM screen utilizes more halftone dots than an AM/XM screen. The result is that more light is filtered by the ink and less light simply reflects off the surface of the substrate. The result is that FM screens exhibit a greater color gamut than conventional AM/XM halftone screen frequencies.
The creation of a plate with stochastic screening is done the same way as is done with an AM/XM screen. A tone reproduction compensation curve is typically applied to align the stochastic screening to conventional AM/FM tone reproductions targets (e.g. ISO 12647-2).
Halftone
Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing...
process based on pseudo-random
Pseudorandomness
A pseudorandom process is a process that appears to be random but is not. Pseudorandom sequences typically exhibit statistical randomness while being generated by an entirely deterministic causal process...
distribution of halftone dots, using frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
(FM) to change the density of dots according to the gray level desired. Traditional amplitude modulation
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
halftone screening is based on a geometric and fixed spacing of dots, which vary in size depending on the tone color represented (for example, from 10 to 200 micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s). The stochastic screening or FM screening instead uses a fixed size of dots (for example, about 25 micrometres) and a distribution density that varies depending on the color’s tone.
The technique of stochastic screening, which has existed since the seventies, has had a revival in recent times thanks to increased use of computer-to-plate
Computer to plate
Computer to plate is an imaging technology used in modern printing processes. In this technology, an image created in a Desktop Publishing application is output directly to a printing plate....
(CTP) techniques. In previous techniques, computer to film
Computer to film
Computer to Film is a print workflow involving the printing from a computer, straight to film. This film is then burned onto a lithographic plate, using a plate burner. The plate is then put on an offset printing press to make a product...
, during the exposure there could be a drastic variation in the quality of the plate. It was a very delicate and difficult procedure that was not much used. Today, with CTP during the creation of the plate you just need to check a few parameters on the density and tonal correction curve. When you make a plate with stochastic screening you must use a tone correction curve, this curve allows one to align the tone reproduction of an FM screen to that of an industry standard.
Given the same final presswork tone value, an FM screen utilizes more halftone dots than an AM/XM screen. The result is that more light is filtered by the ink and less light simply reflects off the surface of the substrate. The result is that FM screens exhibit a greater color gamut than conventional AM/XM halftone screen frequencies.
The creation of a plate with stochastic screening is done the same way as is done with an AM/XM screen. A tone reproduction compensation curve is typically applied to align the stochastic screening to conventional AM/FM tone reproductions targets (e.g. ISO 12647-2).
Advantages
- The screening of four colors are no longer made with four different angles as with the traditional screen therefore it eliminates screening moiré.
- FM screening does not create rosette patterns.
- Halftone dot sizes can be as fine as 10 micrometres, which gives at the product a quality comparable to that of photographic prints.
- The effects of misregistration is not completely eliminated, but the effect is certainly less apparent than in the traditional screening, this feature is very favorable for printing on rotary machines where the misregistration is very common due to effects such as web growth.
- The use of FM screening allowed ArchantArchantArchant is a publishing company, based in Norwich, England, United Kingdom. The group publishes four daily newspapers, around 75 weekly newspapers, and 75 consumer and contract magazines....
, a UK regional publisher, to switch to fonts with "tiny holes"; such an "eco-font" permitted a reduction in ink without turning fine text grainy.