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.

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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



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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



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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



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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° ~23 0 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



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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.

investigation on halftoning methods in digital printing technology

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