images are created using a ruled glass screen which has lines crisscrossing in
opposite directions forming a grid like pattern to break up the continuous tone
image into tiny dots of different sizes. The glass screen consisted of two
sheets of optical glass with fine parallel lines etched on one side of the
glass, those lines filled with a black opaque pigment. Both the glasses when
sealed together with the etched surface facing each other, the parallel lines
crisscrossing each other at right angles left transparent openings in between
them that produced dots. The formation of the dots produced by the glass screen
depended on the distance from the film to the screen, size and shape of the
lens aperture, intensity of light, the length of exposure, and the speed and
contrast characteristic of the film in use.
halftone screening process also employed Contact screens made of color dyed
film based emulsions as against glass plate based screen. Three varieties of
contact screen were made available, namely Magenta, Grey and Orange contact
screens, each by different manufacturers for specific purposes. The contact
screen is made up of vignetted translucent dots on a film base instead of
crisscross lines. Their use is almost extinct now.
contact screening process, as the name implies, they are used in direct contact
with the sensitized material. The use of these screens freed the operator from
setting screen distance, lens openings etc as they have no effect on dot
principal difference between the glass screen and the contact screen is that
each of the dots formed in between the opaque lines in the glass screen are
uniformly transparent and each of the the dots in the contact screen have
Conventional screening is referred to as AM (amplitude modulation) screens, or
amplitude modulation with screen rulings such as 150 lines per inch- referred
to as lpi, 175 lpi, 200 lpi. Amplitude (AM) screening modulates the screen area
by changing the size of halftone dots that are in an ordered pattern in a given
area. This has been the typical method for creating halftones and process
screens for the last 100 years or more. The traditional technique of creating
screens i.e. amplitude modulation (AM) produces a regular grid of dots that
vary in size that represented the tonal gradation. Traditional amplitude
modulation halftone screening is based on a geometric and fixed spacing of
dots, which produced dots of varying sizes depending on the tone color
represented. In AM screening process, the printable dot gets transformed as
irrational imaging i.e. dots transformed into variable width of dots in
numerous permutation combinations like round dots, oval dots etc.
half toning, the density of dot clusters, which we define as the number of
clusters per unit area, is fixed. The most commonly used AM half toning
algorithm is clustered dot screening. One drawback of cluster dot screening is
its limited ability to render fine detail and are vulnerable to moiré patterns.
Thus, it is not fully suitable for half toning images scanned from printed
conventional screening referred to AM screening process, Stochastic screening
(Staccato) process refers to FM screening. Frequency screening (FM) modulates
the screen area by changing the number of randomly distributed dots of the same
size that appear in a given area. This is the method used in the various
stochastically driven methods for creating screens and halftones using special
software. The frequency modulation (FM) processing is a halftone process based
on pseudo random distribution of halftone dots generated through software
application. No special screen like glass screen or contact screens are
separately put to use in this process and only suitable software inbuilt equipment
converts the continuous tone images to printable tonal image.
modulation (FM) changes the density of dots according to the gray level desired
but not the size of dots. The stochastic screening or FM screening uses fixed
size of micro dots and shades are obtained by varying the density of the dots as reflected by the color tone in the original. The micro dots are rendered as
though they are weaved together to create true and smooth tonal variations. The
technique of stochastic screening, which existed since the seventies, has had a revival in recent times thanks
to increased use of computer to plate (CTP) techniques.
The basic micro dots which are fixed in size and produce the shades in the images by sitting one on top of the other, sitting side by side, sitting in half area or one third of other dot or sitting at a distance etc. The shapes of basic dots may differ depending on the selection of screen pattern in the software used.It can be square, round, elliptical or even oval.
In respect of
multi colour separation negatives or positives, the dots sit in
rosette pattern (see illustration) to reproduce the colour effect.
Sometimes recopying the coloured printed original will cause moiré pattern
that will disturb the true reproduction. The FM process eliminates the moiré
pattern as the FM screening process does not create rosette pattern producing
helps printers produce high fidelity, artefact free images that exhibit fine
detail without halftone rosettes, without causing moiré effects, gray level
limitations, abrupt jumps in tonal values etc. It places dots of the same size,
in different concentrations, to reproduce continuous tone. With the FM method,
it is possible to produce very fine detailed print, highlight, and shadow
areas, and eliminate the moiré and broken lines associated with AM screening.
FM screening software also improves colour and halftone stability on press.
In one process of FM half toning, the dot density, spacing between dots and the dot size gets automatically measured to determine the screening process as the printed tonal value depends on the combination of the three factors. This process also allows the operator to incorporate tone correction wherever
necessary i.e. the dot density and size of dots to match the original. Depending on the print process
chosen, the sizes of the dots will be modulated in the software. Remember that the dot
density and dot size are critical parameters of any FM half toning algorithm and
therefore the original i.e print out for reproduction by FM process is
converted into grey level image first.
Some of the FM dots and
Then the the
halftone patterns are generated by printing gray patches with varying dot sizes
and dot densities and they are compared with the tonal values of the original to finalize the absorbance values of the pixels in each shade
to determine final processing.