ISSN (Online) Volume 4, Issue 2, May - August 2013, IAEME MULTIMEDIA (IJGM)

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1 INTERNATIONAL International Journal of Graphics JOURNAL and Multimedia OF (IJGM), GRAPHICS ISSN (Print), AND MULTIMEDIA (IJGM) ISSN (Print) ISSN (Online) Volume 4, Issue 2, May - August 13, pp. 1- IAEME: Journal Impact Factor (13): 4.89 (Calculated by GISI) IJGM I A E M E 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 contone 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. 1

2 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 7, 1, and 4 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 2

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-toprint 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 NexPress Plus. (Fig.2) Fig. 2 Kodak NexPress 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 3

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 computerto-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, nonperiodical. 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 NexPress Plus.(Fig.2) This is a -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 3 lpi. For screen performance is used Kodak square spot 4

5 Imaging technology devices, where depending upon its capabilities and screening algorithm, for FM we have used 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.[] 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 Raster options, min light dot in resolution of 24 dpi Workflow Controlling (on the subject level) Redefinition of Calibration curves Suitable for different printing applications Sheet fed or Web fed offset printing machines, Digital printing Color proof Independent raster for 4 and 7 color separations Staccato raster screening is rendering, converted and exposed on the full speed, without negative effect on the software and the productivity,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) Kodak FM Staccato can be integrated in Kodak Prinergy workflow Screening process can be applied by using Adobe Acrobat plug in on the level of the separation, image, page or document. 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. Staccato screening can be used with different types of papers (coated, uncoated), cardboard, recycled, newsprint, folia, films Staccato screening assures high qualitative printouts and improving an effectiveness of commercial printing, packaging, direct mailings and newspapers as well. Staccato screening can be applied on Kodak Matchprint Inkjet and Matchprint Virtual system for color proof. 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.

6 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 ). 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 :4, using special control test-form and Deep Eye (Xrite) 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 3 lpi. The real printing job has been implemented on Kodak NexPress 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 13 gsm. Table2. Kodak NexPress Plus technical characteristics Features Possibilities Extra Possibilities Feeder Sheet fed printer Expanded feeder Modular option 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 Clear Dry Ink coating provides superior smoothness to the prints by decreasing any screen noise which may be visible improve quality. Print rate: single sided 4/ or / A 3 sheets/h; double sided printed speed, 4/4 or /, or 4 А 4 sheets/ h is half of single sided Type of papers Coated, grade 8-3 g/m 2, matte coated, glossy coated, cast coated and textured, wood free, recycled paper, including a wide selection of standard offset papers Size of paper sh max: 36 x mm Imaging technology ROS system, 6 dpi Dry toner EP, non-toxic and easily recycled uncoated, 6-3 g/m 2, special substrates: uncoated, matte coated and glossy coated labels, paper-back transparencies and select opaque foils min: 279 x mm multi-bit (up to 8-bit with 26 levels of exposure through the complete data path) Kodak Staccato DX Screen Classic HD, Classic, Line, Optimum, Supra Air t ~23 C air humidity - % 6

7 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 NexPress Plus is enough high, without large deviation regarding screening technology AM, Cyan FM, Cyan 2 1 AM, Magenta FM, Magenta AM, Yellow FM, Yellow 3 2 AM, Black FM, Black 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 3 lpi which is almost compatible with minute dots of Staccato FM screening, where the small dot size of around 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 NexPress 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, 7

8 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 (%), 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 4% 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 (8%), 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.) AM, 1 lpi AM, lpi FM, 3 mkm AM FM b.) Fig.4 Dot Gain in sheet-fed offset for Black with different screening methods, LWC gloss paper, 6 gsm, D V =1,6: 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. 8

9 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 NexPress 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 1,6 Cyan 1,47 Magenta 1,9 Magenta 1,6 Yellow 1,44 Yellow 1,46 Black 1,2 Black 1, 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.. 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 NexPress 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 9

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 NexPress 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, 34), Springer-Verlag, ISBN , 1 2. Bennett P., Romano Frank, Levenson H.R., The Handbook for Digital Printing and Variable-Data Printing, ISBN , , PIA/GATF Press, Pitsburgh, NPES, Sardjeva R., Mollov T., Stochastic screening for improving printing quality in sheet fed offset, International Journal on Information Technologies and Security, ISSN ,1, 12, 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, 13, ISSN , International Conference Engineering, Technologies and System TECHSYS 13, Proceedings, ISO :4, Graphic Technology Process control for the production of halftone 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, 12, pp 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, 12, pp , ISSN Print: , ISSN Online: 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.