Investigation on halftoning methods in digital printing technology

International Journal of Graphics and Multimedia (IJGM), ISSN 0976 – 6448(Print),
ISSN 0976 – 6456(Online) Volume 4, Issue 2, May - August 2013, © IAEME
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INVESTIGATION ON HALFTONING METHODS IN DIGITAL
PRINTING TECHNOLOGY
Rossitza Sardjeva
Bulgaria
ABSTRACT
Nowadays the growth of digital printing has paved the way for short run print jobs
and personalized prints.
One of the best example of digital printing is electrophotographic (EP) where is
possible to print with dry powder toner colorants, which can be different as dispersion size. It
is master-less technology where there is not intermediate carrier of information. The great
advantage of it is possibilities to change printing elements after cycle by cycle of process, e.g.
to print so called variable data of information (VDP) and to produce personalized products or
printing on demand (PoD).
Digital dry toner electrophotography is the eligible print technology for applying
different screening methods in half-toning - traditional amplitude modulated (AM) screening
and non-periodic frequency modulated (FM) screening.
Key words: digital printing, electrophotography, screening, amplitude, frequency
1. INTRODUCTION
In the printing process in order to print a continuous tone image, it must be transformed
into a binary image, so called bitmaps. The transformation from continuous tone into a binary
bitmaps images is referred to as half-toning process. The result of half-toning is dots are
small enough and the halftone pattern will not be visible at normal viewing distance, the
image is visually integrated as varying of gray or colour shades. This is continuous tone. One
can simulate continuous tones with screening in different ways.
Digital screening is considered an algorithmic process that creates the illusion of con-
tone images from an arrangement of small, binary dot elements. As a result of the current
“computer to print technologies, there are virtually no limits to the practical application of
variable screening.
INTERNATIONAL JOURNAL OF GRAPHICS AND
MULTIMEDIA (IJGM)
ISSN 0976 - 6448 (Print)
ISSN 0976 -6456 (Online)
Volume 4, Issue 2, May - August 2013, pp. 01-10
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Generally in the different approaches for half-toning are two main screening methods:
Amplitude Modulated (AM) and Frequency Modulated (FM). In AM the distance (frequency)
between the halftone dots is constant, e.g. this is periodical, and the different tonal values are
reproduced by varying the size (area) of the dots. In FM the dot size is constant, but the
frequency of them is variable and the number of dots varies depending on original. With
traditional amplitude modulated screening (AM) the individual dots are spaced at equal
distances from one another and only the dot size changes (amplitude). In FM screening dots
have the same size, but are irregularly spaced, referred to as stochastic screening. Usually FM
method is accepted as a better way for reproducing fine details, specific structure (human
skin, face, fabrics), while the AM method is better in reproducing slowly varying tones. Both
half-toning methods are implemented due to complex algorithm of raster image processor
(RIP). The concept of AM and FM half-toning for different tone levels is shown on the Fig.1.
In AM half-toning, the image is built from a number of halftone cells, each assembled
of a number of pixels. The screen ruling or screen frequency denotes the number of halftone
cells per inch (lpi). The print resolution denotes the number of micro dots per inch (dpi). The
ratio of the print resolution and the screen frequency determines the number of micro dots per
halftone cell, and thus the number of reproducible gray levels. FM half-toning methods
generally do not use halftone cells and therefore only the print resolution (dpi) is relevant,
while the screen frequency (lpi) is not used here. In order to reproduce a colour image, it
must first be separated into three or more colour and then make half-toning.
Separations, the subtractive primary colours cyan, magenta and yellow (CMY) are
most often used in three process colour with the additional black (K) for colour printing.
Printing four colour separations on top of each other introduces additional considerations
regarding interference between them. The result is an appearance of unwanted moiré patterns,
accepted as a colour error. To reduce the sensitivity to moiré effects in AM screening
method, different screen angles are involved. Usually, the four angles correspond to 75°, 15°,
0° and 45° for cyan, magenta, yellow and black, respectively. Rotated screens reduce the
effect of moiré, but introduce a new rosette structure, visible for lower screen resolutions.
In FM colour half-toning there is no need for rotated screens and the rosette and moiré
patterns are generally avoided, since the screen structure is no longer periodical and regular.
However, FM colour halftones can sometimes give a somewhat “grainy” appearance.
Normally dependent FM colour half-toning increase the print quality and at the same
time reduce the amount of ink. What is more, in FM screening colour separations make
multicoloured images less susceptible to very seldom problem caused by register
deviations.[1]
Fig.1 AM and FM screening principal for different gray levels: 1.) AM dots; 2, 3, 4) different
way of FM screening

3
In digital printing the image for every page is created print by print, without necessity
of printing plate, e. g. this is a master-less technology. Digital printing has introduced the way
for short run print jobs and personalized prints and all this allows to print so called variable
data of information (VDP) or prints on demand (PoD).
Electrophotography (EP) is a type of digital printing, one of so called computer-to-
print technology, which runs on the base of dry toners. Here there is image carrier,
photoreceptor, coated with photoconductive layer, imaged with digitally controlled laser
impulses or LED. The latent charged image, stored on the photoreceptor, is inked with dry
toner particles and then transfers directly or via intermediate belt to the paper. Inking takes
place by transfer the fine toner powder to the photoconductive drum through electric potential
differences (electric fields) and thus image becomes visible. Requirement here is toner to be
anchored on the paper in order to obtain stable print image, through melting by heat and
pressure (fusing and fixing). In this way toner is fixing on the substrate. The quality is
affected by toner’s particle size, geometric form and chemical and physical structure. In EP it
is of fundamental importance that the toner images are absolutely dried after printing process.
So finishing jobs (folding, collating, stitch) can be done in-line. Now are known already very
narrow particle-sized toners, with stable electronic process, with constant re-imaging and thus
can produce print by print very high quality, free of quality fluctuations, with good image
reproducibility. [2]
Nowadays the print quality generated by EP system is definitely at high level, including
substantial improvements. For the purpose of this research has been used industrial dry toner
digital EP system Kodak NexPress2100 Plus. (Fig.2)
Fig. 2 Kodak NexPress2100 Plus dry toner digital printing press (photo by Kodak corp.)
2. PROBLEM DEFINTION
In the current digital printing (Computer-to-Print) technologies there are virtually no
limits to the practical application of variable screening methods. Generally the change to the
tone value can be done three-dimensionally: through the area of the dots (amplitude), through

4
the space of the equal dots (frequency) and through the thickness of the ink film like in
conventional gravure printing and in non-impact printing technologies such as
electrophotography or inkjet.[1] With EP printing technologies in which the ink can be
transferred to the paper in varying amounts from dot to dot, a tone value is created by
modulation of the ink film thickness and optical density modulation is achieved. Also
halftone gradation of images in EP depends on the type of toners, their quantity, penetration,
smoothness of the printed substrates and thermal fusing process as well.
What is important, the print quality here depends also on the kind of screening
method, size and shape of the individual image element. In the different approach of
screening (like to conventional printing), there are two main methods: Amplitude Modulated
(AM) and Frequency Modulated (FM) (see above).
Frequency modulated screening and its importance for graphic arts increased
definitely in last ten years, when has been totally involved computer-to-plate and computer-
to-print technologies and respectively digital workflow in prepress. Appearance of second
generation of FM screens such as Staccato Satin FM screen, definitely improved print quality
of this type of screening. With FM technology the smallest reproducible pixel can generate
the dots. The tone values are simulated by varying the dots distance in the spacing of the
pixels cell size. Individual dots are combined into clusters according with very complex RIP
algorithm. Small equal dots are distributed on a surface in such a way that the required gray
value appears as an average value, whereas distribution is completely random, non-
periodical.
In amplitude modulated screening (AM) because of the constant distance between the
halftone dots, this method is a periodical screening. Here the different tonal values are
reproduced by varying the size, e.g. amplitude of the halftone dots. While in AM screening is
applied super-cell processes which tries to match the specified screen angles in order to
minimize additional pattern (moiré), in FM screening has no screen angles. Here appearance
of disturbing moiré and rosette pattern coming from traditional periodical screening is almost
impossible, which is the great advantage of FM screening. Only in some cases there is a
minimal risk for additional pattern because of the periodic pixel cell structure. [1]
On the other hand in digital EP systems the print quality depends on addressability
data of the imaging system (dpi, number of pixels per inch), number of gray values, given per
pixel and the toner technology used. The possible reproduction of very fine structures is
determined by the addressability and the reproduction of tonal values and gamut as gray
values per pixel.
3. PROBLEM SOLUTION
This article is a trial to show the comparison of the results obtained of two screening
methods using dry toner EP commercial digital printing system - Kodak NexPress2100
Plus.(Fig.2) This is a 5-coloured single-pass digital system, comprises of successive five
imaging and printing units for each process colour. To print a multicolour image, the printing
sheet passes through five printing units to receive toner and this happens in one pass. The
imaging speed corresponds to the printing speed. It is a highly versatile EP digital press,
providing the best all-runs performance. This press is able to provide standard CMYK finish
and Clear Dry Ink outputs.
The study is processed by help of FM Kodak Staccato DX Screen (Table 1.) and AM
screen with screen ruling of 300 lpi. For screen performance is used Kodak square spot

5
Imaging technology devices, where depending upon its capabilities and screening algorithm,
for FM we have used 20 µm stochastic screening dots. [3, 4]
It is accepted as a modern PDF-based, dynamic multi-stage workflow with numerous
advantages. This type of workflow starts with the transfer of single digital pages and
controlling them as far as outputting. This means that: the ready-made files, written in
PostScript software, are received in prepress via Kodak In-Site Storefront Web-to-Print
connection, avoiding the risk of eventual losses of data of physical carriers like CD, DVD,
flash etc. Also is used a Kodak Darwin special software for personalization of print products,
which is one of the main advantages of digital printing like EP.[5] In the same time such
procedures as input, editing, control of online files, PDF, virtual color proof can be done
automatically. Then follow the various steps like imposing, trapping, RIP-ing and print
outputs, all these doing in various locations at the same time. The structure of electronic
system is compatible to requirements of working area, where operator in prepress can work
individually or in a team.
Table1. Kodak Staccato FM Screening
Exposition – very fast
Staccato raster screening is rendering, converted and
exposed on the full speed, without negative effect on
the software and the productivity
Raster options,
min light dot in resolution of
2400 dpi
10,6 µm staccato raster*, 21,2 µm staccato raster,
21,2 µm staccato raster, 31,8 µm staccato raster,,
42,3 µm staccato raster, 63,2 µm staccato raster,
*for several thermal plates (specification)
Workflow
Kodak FM Staccato can be integrated in Kodak
Prinergy workflow
Controlling (on the subject level)
Screening process can be applied by using Adobe
Acrobat plug in on the level of the separation, image,
page or document.
Redefinition of Calibration
curves
Staccato software has his standard curves for tonal gain
and test plates for his evaluation. Can be used
alternatively redefined calibration curves by using
Kodak Harmony software compatible with Prinergy.
Suitable for different printing
applications
Staccato screening can be used with different types of
papers (coated, uncoated), cardboard, recycled,
newsprint, folia, films
Sheet fed or Web fed offset
printing machines, Digital
printing
Staccato screening assures high qualitative printouts
and improving an effectiveness of commercial printing,
packaging, direct mailings and newspapers as well.
Color proof
Staccato screening can be applied on Kodak Matchprint
Inkjet and Matchprint Virtual system for color proof.
Independent raster for 4 and 7
color separations
Four independent raster for CMYK separations, seven
independent raster for proof of low resolution. * Are
possible seven separations for Kodak Spotless
software. *Seven color separations are accessible only
for specific output devices.

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The consideration that the main parameters, defined the printed quality are Solid Inks
Densities (SID) and Tonal Value Increase (TVI) are absolutely valid here. (After printing, the
halftone dots generally appear larger than their nominal size in the bitmap image, resulting
in increased tonal values than expected, a phenomenon usually referred to as dot gain or
tonal value increase). The quality of a reproduction in terms of tone value rendering will only
be appropriate if it is based on the standard print characteristic curves of the designated
production run. Standards for printing processes without a permanent printing master (NIP
technologies, Computer to Print) are being developed (ISO 12647-7). Since offset printing is
the prevailing method, including for short/medium print runs, it is economically beneficial for
NIP technologies to accept data originally intended for offset printing.
So we have measured these values and received data are accepted as criteria to make
comparison related to half-toning quality obtained by help of AM and FM screening
technology with above described conditions. The measurements of the main parameters have
been done according to ISO 12 647-2:2004, using special control test-form and Deep Eye (X-
rite) spectrophotometer. [6] Measured values of TVI are compared in two cases: for Staccato
DX FM screening and for traditional AM screening with screen ruling of 300 lpi.
The real printing job has been implemented on Kodak NexPress2100 Plus digital
production color press, using Dry toner technology, main features are shown in Table 2.
Further, in an aim of this research has been used wood free coated glossy paper
(WFC) for digital printing, grade 130 gsm.
Table2. Kodak NexPress2100 Plus – technical characteristics
Features Possibilities Extra Possibilities
Feeder Sheet fed printer Expanded feeder
Modular option 5 printing sections, CMYK+L, with
Clear Dry Ink coating or RGB; Clear
Dry Ink system apply a clear layer of
dry ink which diffuses light to
improve quality.
Clear Dry Ink coating provides
superior smoothness to the prints
by decreasing any screen noise
which may be visible
Print rate: single sided 4/0 or 5/0 – 2100 A3
sheets/h;
double sided printed speed, 4/4 or 5/5,
is half of single sided
or 4200 А4 sheets/ h
Type of papers Coated, grade 80-350 g/m2
,
matte coated, glossy coated, cast
coated and textured, wood free,
recycled paper, including a wide
selection of standard offset papers
uncoated, 60-350 g/m2
,
special substrates: uncoated, matte
coated and glossy coated labels,
paper-back transparencies and
select opaque foils
Size of paper sh max: 356 x 520 mm min: 279 x 200 mm
Imaging
technology
ROS system , 600 dpi
Dry toner EP, non-toxic and easily
recycled
multi-bit (up to 8-bit with 256
levels of exposure through the
complete data path)
Screen Classic HD, Classic, Line, Optimum,
Supra
Kodak Staccato DX
Air t° ~230
C air humidity - 55%

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4. EXPERIMENTAL RESULTS AND DISCUSSION
The reproduction colour results achievable on the examined conditions are shown in
Fig.3 and Table 3. The results show that when printing on wood free coated paper, the
reproducing ability of Kodak NexPress2100 Plus is enough high, without large deviation
regarding screening technology.
Fig. 3 Comparison between Tonal Value Increase in the field of process colours (Cyan,
Magenta, Yellow, Black) for AM and FM screening in digital EP printing
What happened in digital dry toner printing? To make comparison of the results in
traditional AM and stochastic FM screening is applied high screen ruling of 300 lpi which is
almost compatible with minute dots of Staccato FM screening, where the small dot size of
around 20 µm are applied. On the base of the results can be conclude: the continuous tones
and respectively TVI are too closed for all process colours (Cyan, Magenta, Yellow), without
large deviation. This can be explained by the specific electrophotographic way of printing
and toner quality inking. (Table 2) EP as a so called non impact digital printing method is
applied not high pressure (impact) when transfer ink on the paper. The advantage of the
examined Kodak NexPress2100 Plus digital printing over the quality reproduction ability can
be attributed to the large colour strength of CMY fine dry toners and the stable toner transfer,
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9 10 11
AM, Cyan
FM, Cyan
0
5
10
15
20
25
1 2 3 4 5 6 7 8 9 10 11
AM, Magenta
FM, Magenta
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9 10 11
AM, Yellow
FM, Yellow
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9 10 11
AM, Black
FM, Black

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fusing and fixing to the paper, that provides permanently high saturation of solid inks
densities including excellent darkness for black.
For black half-tones can be said that with AM screen are obtained a higher values in
the whole tone range except middle tones (50%), where a very small (~1%) increase of FM is
available. TVI for black FM is changed in a narrow interval of values, while the results of
TVI in the highlights, up to 40% with AM, are visible higher and the tonal interval is larger.
(Fig. 3)
For different process colours the results are more or less fluctuated. (Fig.3, CMY)
Middle tone values for cyan and yellow with AM screen are definitely higher compared to
the same with FM screen, while for the very highlights in magenta and particularly in yellow
TVI for FM screen are above those of AM screen. The same can be said for the darkness in
magenta (80%), but middle tone values in magenta are almost equal with two screening
technologies. For the rest of tone range some deviations are occurred.
To make comparison with conventional sheet-fed offset, depending on the paper
category various screening rulings can be applied to obtain an optimum between the highest
possible screening resolution and the minimal dot gain during the printing process. Generally
FM screening in traditional offset leads to much higher TVI in print than AM screening
because of very minute dots. (Fig.4) Dot gain here is dependent on the summarized length of
screen dot edges, and thus dot gain is larger with smaller screen dots. Some way is, to
optimise CTP compensation curves in order to achieve results that are as close as possible to
AM. [7]
a.) b.)
Fig.4 Dot Gain in sheet-fed offset for Black with different screening methods, LWC gloss
paper, 60 gsm, DV=1,65: a.) without CTP compensation; b.) with CTP compensation
according to ISO
On the Fig.4 are shown results of different AM screening resolution applied in black
printing and compared to FM screening in the same type of paper. This is result of applying
tools and targets for a standardized offset print production for different screening according
to FOGRA/PSO. (FOGRA and ECI provide targets and tools for standardized printing with
periodical and non-periodical screening for coated and wood free paper types.) [7] Evidently
in order to minimize TVI should be apply compensation in the plate making process to reach
the same ISO/PSO target.
0
5
10
15
20
25
30
35
40
1 2 3 4 5 6 7 8 9 10 11
AM, 120 lpi
AM, 150 lpi
FM, 30 mkm
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9 10 11
AM
FM

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Dot gain in halftone prints encompasses two fundamentally different phenomena:
Physical dot gain (mechanical dot gain) and Optical dot gain (known as the Yule-Nielsen
effect).[1] Physical dot gain closely relates to the printing process, including the ink-transfer
and ink-setting processes and the print cylinder pressure. Optical dot gain originates from
light scattering inside the substrate, causing light exchanges between different chromatic
areas. Optical dot gain is due to different paths for a photon entering a halftone print on
paper. The reason for optical dot gain is illustrated by those photon portion which entering
the bare paper and then is scattered within the paper under a halftone dot, and partially
absorbed by the ink on its way back.
When the dot size becomes small in relation to the lateral scattering length, the optical
dot gain will increase. That is why the higher the screen ruling the higher the optical dot gain
including minute dot in FM. The Effective dot area is an estimated value, including the effect
of both physical and optical dot gain. Evidently in digital EP with Kodak NexPress2100 Plus
optical dot gain is predominant compared to physical, because this is non-impact printing.
That is why dot gains are so close for both, FM and AM screening.
Towards solid densities (SID) values are almost the same for AM and FM screening
what is the right approach to standardized printing. In details we see a little higher value for
black and yellow fields for the case of FM and for cyan and magenta in AM, solid densities
are higher.(Table 3)
Table3. Solid Inks Densities for both way of screening
AM FM
Cyan 1,56 Cyan 1,47
Magenta 1,59 Magenta 1,56
Yellow 1,44 Yellow 1,46
Black 1,52 Black 1,55
In digital printing in order to print solid areas, the diameter of an individual image dot
must be greater than the width of the pixel cell. The smallest pixel screen ruling corresponds
to the distance between the pixels (pitch). Also results for black solid densities for both
screening methods are little under to corresponding values in sheet-fed offset, which can be
explained by toner quality printing.
All dry toner prints with FM screening exhibited overall excellence smoothness with a
finer screening, with lack of artefacts, such as additional moiré pattern. The print quality has
shown good realism on images and colours were rendered well balanced with no particular
colour bias.
5. CONCLUSION
FM screening, also referred to as non-periodic, or stochastic, can be successfully
applied in digital EP printing with dry toner process by Kodak NexPress2100 Plus. This is the
best technology for appreciating half-toning process by implementing the main screening
methods AM and FM.
All evaluated parameters have shown high capability, described high print quality and
thus has the potential to make substantial improvement in digital EP dry toner printing. The
important finding is that the visual quality and print reproducing of the whole prints made on

10
the coated wood free paper is very satisfied and compatible to those specified as the highest
printing quality. Printed results are without compromising of density, TVI and printed colors
are trustworthy and visual perception was very satisfied. Frequency modulated Kodak
STACCATO screening in digital EP printing enables high fidelity, consistent presswork that
exhibits fine details, without gray level limitations, or abrupt jumps in tone.
Moreover, FM screening is hardly suitable for high quality, full colored illustrative
printing products. However the widespread of FM screen requires precise developments in
prepress technologies and a different way of thinking about the whole digital workflow, from
prepress to press processes.
Kodak NexPress2100 Plus is an example for digital computer-to-print technology
targeted to short runs in commercial, publication, and even packaging environmental printing
today.
REFERENCES
1. Kipphan H., Handbook of Print Media, 1.4 (94-96, 354), Springer-Verlag, ISBN 3-540-
67326-1, 2001
2. Bennett P., Romano Frank, Levenson H.R., The Handbook for Digital Printing and
Variable-Data Printing, ISBN 978- 5-98951-020-7, 113-126, PIA/GATF Press,
Pitsburgh, NPES, 2007
3. www.kodakcorporation.com
4. Sardjeva R., Mollov T., Stochastic screening for improving printing quality in sheet fed
offset, International Journal on Information Technologies and Security, ISSN 1313-
8251,1, 2012, 63-74
5. Sardjeva R., Mollov T., Application of Frequency Modulated Screening in Digital EP,
Journal of the Technical University – Sofia, Plovdiv branch, Bulgaria, Fundamental
Sciences and Applications, Vol. 19, 2013, ISSN 1310-8271, International Conference
Engineering, Technologies and System TECHSYS 2013, Proceedings, 253-258
6. ISO 12647-2:2004, Graphic Technology – Process control for the production of half-
tone color separation, proof and production prints, Part 2: Offset lithographic processes
7. Hamann J.P., Gerd Carl, Leppanen T., Recommendations for preperess and Printers,
Paper vs. Screening+Non Periodic screening, UPM, Helsinki, Finland, 2012, pp. 20-22
8. Patange V.V and Prof. Deshmukh B.T, “Visual Acknowledgement [O.C.R.] – A
Method to Identify the Printed Characters”, International Journal of Computer
Engineering & Technology (IJCET), Volume 3, Issue 2, 2012, pp. 108 - 114,
ISSN Print: 0976 – 6367, ISSN Online: 0976 – 6375.
INFORMATION ABOUT THE AUTHOR
Dr. Rossitza Sardjeva is working for Technical University Sofia, br. Plovdiv, as an
associated professor of Graphic Arts Printing Specialty, where she teaches students for
bachelor and master educational degrees. Also she is a lecturer for many years in Book
Publishing Department of Journalism Faculty in Sofia “St. Kl. Ohridski” University.

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