Digital printing and materials
INTRODUCTION
Recent technological advances in digital printing have enabled the high-quality printing of digital images using non-silver-halide media, as well as some high-end printing methods which combine the use of laser technology with silver halide materials. Historically, there have been two main approaches in printing technologies impact methods and non-impact methods. With impact printing the print head comes into contact with the paper. An example of this is the dot-matrix printer, where characters and images are formed from a matrix of dots, however, this is now an obsolete technology. All modern printing technologies are non-impact, where, as the name suggests, there is no contact between paper and printer head. These methods include the ejection of ink through nozzles on to a coated substrate, the use of heat for dye transfer, and the combination of a light source and electrostatic charging to adhere toner particles, all allowing the formation of an image or text on paper. Each method has its advantages and possible limitations, as explained later in this chapter. The physical and chemical properties of the printing media play a significant role in the quality of the printed image as well as the device characteristics. Sound knowledge of the media characteristics, as well as practical experimentation, is important in controlling the printer’s output. The choice of optimal resolution for the input image is also important to avoid large file sizes and slow printing speeds due to excess unnecessary information. Digital printing of images in large volumes as parts of books, brochures, magazines, etc. is carried out using printing presses, which are presented briefly at the end of this chapter.
PRINTING TECHNOLOGIES
Inkjet printers
A desktop inkjet printer is shown in Figure 16.1. Inkjet printing is based on a print head (carriage) with a large number of nozzles which eject ink on the paper. The print head moves at a velocity of over 1 m s−1 backwards and forwards across the width of the paper. The paper advances after each pass (swath), controlled by an encoder which ensures high precision. The diameter of the nozzles is very small, about 10 mm, and the velocity of the ejected ink is around 5–10 m s−1. The method of ink ejection depends on the technology employed, described later in this chapter. The distance between the nozzles is referred to as nozzle pitch and is a parameter that affects the location of the dots on a page. Nozzle pitch is quoted in mm but some manufacturers, such as Canon, quote the nozzle pitch in dots per inch (dpi). This is often the value quoted as ‘print resolution’ and can lead to confusion, if the printer is employing digital half-toning, in which tones are simulated using clusters of dots to represent each pixel. The output resolution in pixels per inch will therefore be significantly lower than this value.
Inkjet printers are non-impact printers. There are several inkjet technologies, with the earliest, electrostatic pull, dating back to the 1960s, in which droplets of ink were drawn through the nozzles using an electrostatic field. Disadvantages of this method include the high voltage necessary to pull the ink and the large nozzles of the jets. Two technologies more commonly used today are continuous inkjet and drop-on-demand inkjet.
Figure 16.1 An inkjet printer.
Continuous inkjet printers are used mainly for industrial applications. Examples include coding of products such as packages, cans, bottles or cables, and printing of logos. Their operation is based on the Rayleigh instability. The Rayleigh instability, named after Lord Rayleigh, who analysed it in the late 1800s, causes a stream of liquid to break into droplets. One of the methods that continuous inkjet printers employ is based on the Sweet method, developed by Dr Richard Sweet, of Stanford University in the 1960s, where the direction of the ink droplets is controlled by deflection. The Sweet method allows two different types of deflection: binary and multilevel. In binary deflection an array of jets is used for ejecting the ink droplets. When a voltage is applied each droplet is charged. An electrostatic field is then used to direct each droplet to the substrate. In multi-level deflection the horizontal movement of the head or the substrate provides the horizontal dimension of the characters. The vertical dimension is provided by deflecting the charged ink droplets vertically, using an electrostatic field. With this method a complete character can be printed by a single pass of the print head. The Hertz method, developed by Professor Hertz of the Lund Institute of Technology, in the late 1960s, is another method used in continuous inkjet printers, in which the density of colour is adjusted by controlling the number of droplets for each pixel (employing digital half-toning – see page 310) using electrostatic methods. Up to around 30 droplets can be ejected for each colour, resulting in an increased number of density levels per pixel. The speed of the ejected droplets in continuous inkjet printers is very high, 50,000–100,000 per second per nozzle, and the droplets are in the range of 15–400 μm, depending on the printer. The ink, however, needs to be recirculated and this demands complex hardware (Figure 16.2).
Drop-on-demand (DOD) inkjet printers mainly comprise two technologies: piezo-electric and thermal (also referred to as bubblejet). Piezo-electric technology utilizes piezo crystals in one of the walls of the ink chamber. Piezo crystals are crystals that produce voltage when pressure is applied to them. With the application of electric current these materials become distorted. This is known as the ‘reverse piezo-electric effect’. The distortion causes pressure to build in the ink chamber and the ink is ejected from the nozzle (Figure 16.3). The type of distortion, elongation or bending, depends on the crystals and the magnitude of distortion depends on the electrical current. It can therefore be controlled, enabling variation of the size of the ink droplets. Two types of ejectors exist: flat ejectors, used in most commercial inkjet printers; and cylindrical ejectors, used for inkjet wide-format printers (Figure 16.4).
Figure 16.2 The process of printing with a continuous inkjet printer.
The ink used for piezo-electric printers is liquid and the droplets produced can be as small as approximately 1–1.5 picolitres (pl). This results in printed dots with a diameter of approximately 10 mm. Another type of piezo-electric printer uses phase-change (or hot melt or solid) ink, which is solid at room temperature. The printing device heats the ink to its melting point, at around 120–140°C, and the ink is then ready for ejection on to the substrate surface. When the ink comes into contact with the substrate, which is at room temperature, it becomes solid, without the need for drying time. An adequate warm-up time, around 10–15 minutes, is necessary before printing so that the ink is allowed to reach its melting point.
In inkjets based on thermal technology, each nozzle corresponds to one ink chamber which is connected to the ink reservoir. A heater resistor is positioned on a wall of the ink chamber (usually behind the nozzle, but this depends on the manufacturer). The chamber is filled with ink. When an electrical current is applied, for around 1 ms, the temperature of the ink that is closest to the resistor reaches around 300°C, causing vaporization of the ink. The bubble created applies pressure to the ink, which is then ejected from the nozzle (Figure 16.5). When the temperature drops, the bubble contracts and the ink returns to its previous state. This method is also based on studies made by Rayleigh on bubble creation and collapse. Inks used for thermal inkjet printers are therefore restricted to formulae which are not affected by heating and vaporization.
Figure 16.3 Drop-on-demand piezo-electric inkjet printing.
Figure 16.4 Wide-format inkjet printers (also known as plotters) can print large-scale images in a variety of substrates. They use roll paper and separate ink cartridges.
In inkjet printers the accuracy with which the ink dots are placed on the paper is controlled with an optical encoder. This is to ensure that any variations in the speed of the print head will not have an adverse effect on the quality of the print. A transparent plastic code strip, consisting of black stripes, is placed between a light-emitting diode (LED) and a photodetector on the moving head. The beam emitted from the LED passes through the code strip but not through the black stripes. The resulting beam is received by the photodetector, which sends a signal that controls the ink ejection.
Figure 16.5 Drop-on-demand thermal inkjet printing.
As mentioned above, the ink nozzles have a very small diameter, therefore posing the possibility of clogging. To avoid clogging due to dried ink on the nozzle when the printer is not in operation, the nozzles are covered. During printing, however, some of the uncovered nozzles may not be used, which means that they will not eject ink (which nozzles eject ink depends on the image or text being printed). The problem is solved by ejecting ink from every nozzle at preset intervals, when the print head is not over the paper. Clogging can also be caused by substances in the ink itself or due to unstable dispersion of ink particles in pigment-based dyes. Filtering the ink during fabrication and in the printer eliminates the presence of these substances. Wiping of the nozzles is also applied in the printer to eliminate clogging. Advances in ink technology have considerably improved the dispersion of pigment-based inks.
Inkjet printing is based on the subtractive method (see Chapter 5) using cyan, magenta, yellow and black (CMYK) inks. With the use of black ink, the effect of any variations of the CMY ink colours on the printed image is reduced. In addition, the image has better contrast. It is also less expensive to print black using black ink instead of using CMY inks. The intensity of colours when printing with an inkjet printer is controlled using digital half-toning and stochastic printing, which are explained in more detail in the section ‘Colour, resolution and output’ later in this chapter. Current inkjet printers designed for printing photographic images use more than the four colours, cyan, magenta, yellow and black, to produce smooth colour tones in the print and to expand the colour gamut obtained by using four inks only. At the time of writing, inkjet printers may use up to 12 inks, depending on the manufacturer. As an example, inks may include ‘light cyan’, ‘light magenta’, ‘light black’, ‘light light black’, ‘photo cyan’ and ‘photo magenta’. More than one black ink is used for high-quality printing of greyscale images, where only the black inks are used. Some printers incorporate red, green and blue inks in addition to the subtractive ones. Inks may also be customized depending on the printing paper. For example, the eight colour printers of one manufacturer (Epson) include a ‘photo black’ ink for printing on glossy paper and a ‘matt black’ ink for printing on matt paper.
Electrophotographic printers
Electrophotographic (EPG) printers are also non-impact printers (see Figures 16.6 and 16.7). Light amplification by stimulated emission of radiation (laser) is the most common light source for EPG printers, but there are other light sources that have been used by manufacturers, as will be described later in this chapter. The introduction of EPG printers that used plain paper instead of the necessary special-purpose paper for earlier EPG printers dates back to the 1970s.
Figure 16.6 An electrophotographic laser printer.
Figure 16.7 Process of printing in an EPG laser printer.
Electrophotographic printers consist of a photoconductor (a surface sensitive to light), which is usually organic or silicon. Early photoconductors were made of cadmium, arsenic or selenium, which had very long life span (over 100,000 pages) but were toxic. Organic photoconductors have a shorter life span (up to 100,000 pages) but are regarded as non-toxic. Silicon photoconductors have a much higher life span, up to 1,000,000 pages. The substrate of the photoreceptor is either an aluminium metal cylinder or a flexible web. The photoconductor surface is uniformly electrostatically negatively charged by corona charging. It is then exposed to an optical image using a light source. The light source can be laser or light-emitting diode (LED) arrays. Other methods include liquid crystal (LC) shutters, which have not been widely used, and optical fibre faceplate cathode ray tubes (CRTs) in early printers. Early laser printers used gas lasers, initially cadmium or argon and later helium–neon. Solid-state laser diodes, which can be controlled directly by the drive current, were introduced in the 1980s, and this reduced the cost for commercial printers. It is important to mention that the spectrum of the laser should match the spectral sensitivity of the photo-conductor. The image data, stored in the buffer memory, is converted bit by bit into on/off signals which modulate the laser beam. The light passes through a lens, which focuses the beam to a spot of approximately 42 μm on a rotating polygon mirror. The rotations per minute (rpm) of the mirror depend on the print speed, the printer’s resolution and the number of facets of the mirror, and it is in the range of 30,000 rpm. Rotation of the mirror is also affected by heat, ambient or generated by the printer, resistance from the air when the mirror turns, vibrations, and any discrepancies in the manufacture of the mirror. The mirror reflects the light and directs it through a lens, which corrects any effects of the abovementioned parameters, to the photoconductor. It should be noted that there is a limitation on the maximum paper width that can be used in a laser printer. This is due to the fact that the laser beam becomes more slanted when its angle with the photoconductor is increased.
The areas of the photoconductor surface which are exposed in consecutive rows to light are discharged and a latent image is created. Development of the latent image is conducted with the use of dry toner. Toner is a polymer mixed with carbon black at approximately 10%. The size of the toner particles is approximately 12 μm. Smaller particles can be produced either mechanically (around 7 μm) or chemically (around 3–5 mm). Although very small toner particles can be produced, there are problems associated with them (see later in this chapter). For this reason there are limitations on the smallest size of particles that can be used in practice.
Positive charge is applied to the toner particles. The toner particles then adhere, with the aid of a material with 3–50 times larger particles, known as a carrier, to the latent image on the photoconductor. This is achieved either by magnetic or by electrostatic forces. The developed image is then transferred from the photoconductor on to paper and fused, usually with a heated roll and high pressure. Another method, cold pressure fusing, is used in some printers. Finally, the excess toner is removed from the photo-conductor so that it is ready for the next cycle.
EPG printers that use LED arrays as a light source work on the same principles as the laser printers described above. Instead of a laser beam and a rotating mirror, they employ an array of LEDs, where each LED provides a spot across the width of the printable area. The number of LEDs in the array is related to the horizontal resolution of the printer and the width of the print line. An array of 600 LEDs per inch, for example, is needed for a 600 dpi printer. For a print line with 8 inches width, the total number of LEDs in that printer would be 4800. Colour LED arrays have four rows of LEDs for cyan, magenta, yellow and black. Although the LED printers have fewer mechanical parts and are therefore less complicated, there is a limitation on how many LEDs can be built into the array because of the technical difficulty of fabricating very small LEDs.
EPG printers use CMY or CMYK toner for colour printing and employ digital half-toning to reproduce continuous tone images. Due to the use of toner particles, however, the dots overlap each other instead of forming distinct half-tone screens. As a result the subtle colours are not well defined or distinguishable from each other.
Thermography
Thermography, printing using application of heat, is divided into two categories: direct thermography and transfer thermography. Direct thermography is used mainly in applications such as printing labels. The substrate has a special coating which, with the application of heat, changes colour. For the scope of this book the second category, transfer thermography, is presented in detail. Transfer thermography is subdivided into two categories: thermal transfer printing (Figure 16.8) and dye diffusion thermal transfer printing (D2T2). Thermal transfer printing, also known as thermal mass transfer, is based on transferring ink (wax or resin) from a donor to a substrate with the application of heat. The ink donor is positioned between a head consisting of heating resistors and a substrate. The donor, usually a sheet or ribbon with thickness of approximately 10 mm, has a layer of cyan, magenta and yellow dyes and a protective layer for the substrate. The arrangement of the dye areas depends on the donor. When a web material is used, the dyes are arranged behind each other so that the image can be printed with one pass of the substrate. With donors using a sheet, several passes of the substrate are necessary for full colour printing. Because the substrate is printed for each colour separately, any slight misregistration will cause a visible effect on the image. The resolution of the printer is determined by the number of resistors. When the resistors are turned on, the ink from the donor is transferred to the substrate with which it is in direct contact. The amount of dyes per pixel deposited on the substrate is modulated by controlling the heating of the resistors. The dot size can therefore vary. Printing with the thermal transfer method is a binary process and half-toning is employed for reproducing continuous tones (varying dot size to reproduce different tones is sometimes called true half-toning, as opposed to digital half-toning, which uses different arrangements of clusters of dots). The quality of the printed images with this method is lower than the quality from the dye diffusion transfer method.
Figure 16.8 Method of thermal transfer printing.
Figure 16.9 Principles of Fuji Pictrography.
Table 16.1 Digital printing technologies and the principles employed
PRINTING TECHNOLOGY |
METHOD |
Inkjet |
Ink injection |
Electrophotographic (EPG) |
Electrostatic toner transfer |
Thermography – Thermal Transfer |
Thermal transfer of ink from a donor to a subtrate |
Thermography – Dye Sublimation |
Thermal transfer of ink from a donor to a subtrate – ink sublimation |
Dye sublimation inkjet |
Dye sublimation ink injection – heat application |
Pictrography |
Thermal development and Dye transfer |
Dye diffusion thermal transfer printers, also known as dye sublimation printers, are based on the sublimation of the ink (the ink is converted from solid to vapour without passing through a liquid stage) and employ the same arrangement as thermal transfer printers. When heat is applied to the donor, the ink evaporates. It then penetrates the substrate by diffusion and becomes solid. A special coating on the substrate is necessary. While in thermal transfer printing the dot size can vary, with dye diffusion thermal transfer printing the ink density can change but the diameter of the dot remains almost the same. With this technology 256 levels can be obtained for each colour, resulting in high-quality prints. The printing speed is slower with thermal transfer printers compared to electrophotographic and inkjet printers. Because the dyes are spread through the coating, the image has very smooth continuous tones. The spreading, however, in combination with the low resolution, results in loss of sharpness in the printed image compared to a photographic print. This is shown more in edges rather than large areas with uniform colour. The longevity of dye sublimation prints is affected by humidity and temperature.
Because both thermal transfer and dye sublimation printers work on the same principle (dyes are transferred to the substrate by a donor with the application of heat), systems can be designed to operate multifunctionally – that is, to operate for both thermal transfer and dye sublimation printing. Suitable donors and inks are necessary for this purpose.
Dye sublimation inkjet printers combine inkjet and dye sublimation technologies. These are large-format printers used to print on flexible or rigid substrates such as fabrics and gadgets. The image is printed with the inkjet technology but these printers use special dye sublimation inks. Heating is applied to the substrate so that the gas inks penetrate it and become solidified. The substrate, however, has to be fully or a high proportion polyester due to the properties of this material with the application of heating. Cotton fabrics cannot be used with this technology.
Pictrography
Fuji Pictrography is a method based on exposing a donor material to light using solid-state laser diodes and silver halide materials (thermal development and dye transfer technology). The latent image is created on the donor, which is in contact with the receiving photographic paper. With the application of heat and a small amount of water, a positive image is created on the paper and then donor and receiving paper are peeled apart (Figure 16.9). Further developments of the system have resulted in materials that combine the donor and receiver. Pictrography does not require inks or chemicals, only water. These high-end printers reproduce images with 256 tonal levels for cyan, magenta and yellow. The printing resolution is standard, 400 dpi. The paper is provided by the manufacturer in different types (glossy, matt, OHP). A list of digital printing technologies is presented in Table 16.1.
PRINTING MEDIA AND THEIR PROPERTIES
The properties of the equipment and materials used for digital printing introduce several factors that can have an effect on the quality of the final prints. These factors include deviations during manufacturing of printer components, and properties of the toner or ink and substrate used for printing the images.
Manufacturers have devised methods to improve the hardware and the printing methods, but also the properties and fabrication of toners, inks and substrates to optimize image quality. It is important to note, however, any physical limitations on improvement to hardware and materials, as is shown later in this section.
In an inkjet printer, for example, a variation in the diameter of nozzles which print the same colour may cause banding in the printed image. High precision in manufacturing the nozzles that will eject ink of the same colour is essential to obtain uniform density of each colour in the print. Usually all the nozzles of one colour are fabricated in one step. In some inkjet printers the density of the printed colour may be modified by varying the size of the ink droplet. The size of the droplet depends on the amount of ink ejected by the nozzle. It can be controlled either by the pulse in piezo-electric printers or by heating in thermal printers. Another method to create large droplets is by ejecting several small droplets in a fast sequence. With this method printers can print several grey levels per pixel by altering the number of droplets, improving the quality of continuous-tone photographic prints. It should be noted that the size of a printed spot on the paper also depends on the properties of the paper, such as its absorption, as described later.
Paper for inkjet printing
High-quality document printing with inkjet printers can be carried out on plain paper without any special coating. For optimal quality when printing photographs, however, coated paper should be used. The coating of the paper is developed taking account of several factors that include the volume of droplets, the thickness of the coating, the rate at which the ink penetrates the paper, and the absorption of the paper, to name a few (Figure 16.10).
Inkjet printers use mainly water, phase-change, solvent or oil inks and some use ultraviolet (UV)-curable or reactive-based inks. With water inks special paper coating is necessary, otherwise the ink is spread on the surface of the paper and penetrates it. The ink spread has an adverse effect on the quality of the printed image due to the increased spot size, which causes loss of sharpness and desaturated colours. Short paper fibres and sizing (where the paper is coated during manufacture to reduce absorption of liquid when dry) are also used to minimize spread of the ink. The paper coating also adds fluorescence that makes the white surface appear brighter. Prints made with phase-change ink are not affected by ink spreading or paper absorption even when the substrate is plain paper or another suitable surface without special coating. The reason is that the ink, ejected as liquid by the nozzle, becomes solid when it comes into contact with the substrate surface, without spreading. This, however, means that the droplets retain a hemispherical shape, which results in light scattering. To eliminate this effect and to increase adhesion, the ink needs to be fused on the substrate. Oil-based inks are used for wide-format printers. UV-curable and solvent-based inks are used for printing on impermeable surfaces such as plastic, glass or metal, and for this reason they are mainly used for industrial applications.
Figure 16.10 Diffusion, adhesion and evaporation in inkjet printing using a coated paper receptor. (a) In-flight droplet. (b) Poor diffusion and adhesion. (c) Good diffusion and adhesion. (d) Excessive diffusion but good adhesion. The dashed arrows represent evaporation.
Papers for inkjet printing are manufactured with different finishes (such as glossy, semi-matt or matt), colour (bright white, white), weights or surfaces. Fillers are used for this purpose such as titanium dioxide, for example, to create a bright white appearance. Colour saturation may be affected by these parameters. For this reason, the printer driver alters the amount of ink deposited on the paper according to the type of paper used, ensuring optimal results. The combination of ink and paper affects the longevity of the print. Differences in print quality may also be observed when using inks and paper from different manufacturers. With inkjet printers that do not use phase-change inks, the print must be allowed to dry before handling and storing it. For creative work, different non-inkjet types of paper, which are uncoated, may be used. In this case careful selection of papers suitable to be used and experimentation on the effects of the different media on image sharpness, colour saturation and contrast can give interesting results.
The lifetime of inkjet papers is affected by humidity, temperature, exposure to UV rays and acidity of the paper (see more on media stability in Chapter 18).
Inks for inkjet printing
Inks used for inkjet printing are dye based or pigment based. In earlier systems, there were differences between the colour gamuts of dye-based and pigment-based inks. The colours of dye-based inks are more vivid than the colours of pigment-based inks. Recent improvements in technology, however, have reduced the gap between the two types of inks. Dye-based inks, however, have a faster rate of fading when exposed to weather conditions and light. Their different properties have led to some manufacturers using both in inkjet printers, for different purposes. For example, dye-based inks may be used for cyan, magenta and yellow, and pigment-based inks for black. Absorption and dispersion of the ink particles on the substrate is another important factor. Dye-based inks are absorbed in the paper while pigment-based inks remain on the surface. Phase-change inks have different properties. They have large colour gamuts and vivid colours and are not affected by ambient humidity or high temperatures, an important property for many applications.
Toner materials
The toner materials used in electrophotographic printers have different properties to those of ink. One of the factors affecting image quality is the size of the toner particles. By application of pressure during fusing the particles are flattened out. When the particles are large, and possibly asymmetrical, the edge definition in an image is affected. Small particles are therefore expected to produce the best image quality. Very small toner particles, however, may also cause problems to the edges of an image, due to scattering. It is also very difficult to control very fine particles inside the printing device. In addition, very fine particles can be a health hazard if breathed in.
In EPG printers the processes of latent image creation, development, fusing and cleaning may cause variability in the results if a large number of prints of the same image is required. This is because the latent image is created and erased for every print. This is a general limitation when printing with non-impact printing technologies compared to press printers, which use a plate.
COLOUR, RESOLUTION AND OUTPUT
Printing photographic images with continuous tones requires around 90 tone levels per colour to minimize the effect of visible banding. Digital printers use a limited number of colour inks, depending on the specific system. Inkjet printers may use up to 12 inks depending on the model, while in EPG printers four colours (CMYK) are used and in transfer thermography three colours (CMY).
The colour gamut of printers is an important factor affecting image quality. The saturation and definition of colours depends on the printing technology. For example, as mentioned previously, saturated colours of an image reproduced by EPG printers are well defined, whereas subtle colours are not. When compared to photographic prints, the saturation of colours produced by a dye sublimation printer is lower and the colour gamut is therefore smaller. Inkjet printers have a wider colour gamut than other technologies such as pictrography. Their colour gamut, however, is affected by the printing media. It is wider, for example, when special photographic quality paper is used compared to the gamut reproduced on plain paper. The gamut is also affected by the type of inks used. Dye-based inks provide wider gamut than pigment-based inks. There is therefore a wide variation in colour gamuts when a combination of different papers and dyes is used.
Half-toning and dithering
Different printing methods have been devised to represent continuous-tone levels per colour using a limited number of inks. Fundamentally, printing using dots of ink or toner is a binary process – in other words, the individual dots are ‘on’ or ‘off’. The printers use various methods of half-toning to give the visual impression of continuous tone. In principle, a greyscale image printed using ‘true’ half-toning on white paper consists of printed black dots spaced at equal distances. The diameter of the dots varies, creating the impression of different levels of grey. This is due to the visual effect of assimilation where, from a suitable viewing distance, mixed black and white dots, for example, are perceived as grey. In digital printers, where the size of an individual ink dot cannot be varied, each pixel is instead printed using a group of dots; this is known as digital half-toning. The pixel is divided into subpixels and each subpixel either receives, or does not receive, ink or toner. This is known as a dot screen. The number of ink dots determines the size of the half-tone dot and therefore the grey level and colour of the pixel (Figure 16.11). Dark tones are reproduced with large half-tone dots which require more ink dots per pixel. Lighter tones with smaller half-tone dots require fewer ink dots per pixel. This method of half-toning, where the size of the half-tone dot varies depending on the grey level of the pixel, is known as amplitude modulated (AM) or clustered dot half-toning. AM half-toning has limitations in reproducing images with smooth tonal transition. Early printers which employed this half-toning method were limited in reproducing basic graphics and were unable to reproduce images with photographic quality.
In the 1970s, frequency modulated (FM) half-toning was introduced and about a decade later affordable colour inkjet printers employing FM half-toning were produced. This method is also used in electrophotography. In FM half-toning the half-tone dot size is fixed while the frequency of the dots varies depending on the grey level of the pixel. This means that in darker tones the dots are in closer proximity than in lighter tones. Hybrid AMeFM half-tone systems are being developed today to overcome the limitations of the FM half-toning due to the high resolution (over 1200 dpi) of today’s inkjet printers. With AMeFM half-tone systems both the size and the frequency of dots varies to represent a grey tone.
Figure 16.11 A half-tone pattern. From a suitable distance the image is viewed with a gradation of grey tones rather than black and white dots.
Inkjet printers today use stochastic printing, a form of FM half-toning, to produce smoother transition of tones. With stochastic printing the shape of the dots is irregular and their placement random (the distance between them is not equal). These systems may also combine a half-toning method with dithering. With dithering, a colour that does not exist in the colour palette of the device can be visually simulated by using information from the neighbouring pixels to mix dots of the existing colours that approximate it. Manufacturers use several different algorithms for dithering. One such example is error diffusion. An example of an error diffusion dithering algorithm is the algorithm created by Floyd and Steinberg. In this method, processing of one pixel depends on its neigh-bouring pixels; a pixel’s output value is determined by a threshold. Figure 16.12 illustrates the steps of the FloydeSteinberg algorithm.
The input pixel is referred to as a and the transformed pixel as b. The indices i and j refer to the row and column of the digital image matrix respectively. When the pixel value of the input pixel is below the threshold, a value of 0 is returned for the output pixel. When it is over the threshold, a value of 1 is returned for the output pixel. The introduced error is the difference between the values of the input and the corresponding output pixels. To obtain an output image with equivalent tonal scale to the input image the introduced error must be cancelled. This is achieved by using four neighbouring pixels with weights equal to 7/16, 1/16, 5/16 and 3/16 of the error (Figure 16.13). The sum of the error of the processed pixel and the error weights of the four neighbouring pixels should be equal to zero. When a photographic image is printed at high resolution using dithering it appears as a continuous-tone image when viewed from a suitable distance (Figure 16.14).
Figure 16.12 The Floyd–Steinberg error diffusion algorithm.
Figure 16.13 The error weights in the neighboring pixels (Floyd–Steinberg error diffusion algorithm).
Resolution
Printing resolution affects the level of detail and tonal gradation that can be recorded in the final printed image. The native resolution of a printer refers to the maximum number of pixels per inch it is capable of printing. The native resolution of inkjet printers may vary depending on the manufacturer and the model, and is a factor that affects the choice of optimal input resolution, as described in more detail in Chapter 14. It is therefore advisable to follow the manufacturer’s recommendation for the required spatial resolution of the input image for best printing results. A practical method to estimate the optimal spatial resolution of an image is described at the end of this section. The addressable resolution (or printer resolution) of a device refers to the total number of dots per inch and can be higher than the value of pixels per inch. It depends on the technology used. In inkjet printers the nozzle size, nozzle pitch, size of droplets and number of droplets are factors that affect the printer output. Small droplets improve the resolution of the printer but there is a practical limit on reducing their size. Below that limit accurate placement of the droplets on the substrate is very difficult due to the effect of aerodynamic forces. Small ink droplets demand higher speed of drop ejection per second compared to ejection of larger droplets to cover the same area. At the time of writing, addressable printer resolutions are in the range 1440–2880 dpi or higher. As mentioned before, however, EPG and inkjet printers are able to print more than one dot per pixel and these dots are overlapping.
Figure 16.14 (a) Original image. (b) Image with reduced number of colours using Floyd–Steinberg error diffusion.
Selecting the optimal input image spatial resolution for output is an important task. Lower resolution may result in printed images appearing ‘pixelated’ while higher resolution will include unnecessary information which will only increase the file size and printing time but will not be used by the printing device. This applies to bitmap images and not vector graphics, which can be enlarged in any dimension without the resolution affecting image quality. The optimal input resolution is described in more detail in Chapters 14 and 24.
In the early years of inkjet printers, their addressable resolution was around three times higher than the spatial resolution of the input image. The optimal spatial resolution of the input image was therefore calculated by dividing the printer’s addressable resolution by 3. With the technological advances of recent years the addressable resolution of inkjet printers increased further, so dividing it by 3 results in images with very high spatial resolution that exceeds optimal resolution.
A general rule for estimating the optimal spatial resolution of an image aimed for printing is based on properties of the human visual system and specifically on the contrast sensitivity function of the human eye (see Chapter 4). It also depends on the viewing distance. The correct viewing distance for a print can be estimated by multiplying the print’s diagonal by 1.5. It has been found that for a viewing distance of 250 mm, which is a typical reading distance, the representative limit of resolution for continuous-tone images is 5 cycles per mm. This corresponds to about 250 ppi at the dimensions of the printed image. Due to the fact that a pixel may be affected by its neighboring pixels, a higher spatial resolution is recommended for printing, typically 300 ppi. However, the manufacturer-recommended printer resolution for printing photo quality images on a particular printer, which is based on the native resolution of the printer (if it has one), may not be 300 ppi, but is likely to be close to it.
Note that in dye diffusion thermal transfer printers, the spatial resolution of the input digital image should match the resolution of the printer. The reason is that this type of printer employs a different printing method without the use of digital half-toning and dithering. For this reason, the resolution of a dye sublimation printer is not comparable to that of an inkjet printer. Typical addressable resolution for this type of printer is 300 dpi, but this depends on the manufacturer and the model.
PRINTING PRESS
Reproduction of photographs in media such as books, brochures, newspapers, etc. is carried out using printing presses. The main categories of printing processes which are carried out by printing presses are relief printing, plano-graphic printing, recess printing and through-printing (Figure 16.15).
Figure 16.15 Categories of printing processes. (a) Relief printing. (b) Recess printing. (c) Planographic printing. (d) Through printing.
Relief printing
The principles of relief printing are based on the letterpress, a type of relief printing, invented by Johann Gutenberg in 1440, where characters are transferred, reversed, by the inked surface to paper or fabric by application of pressure (a form of impact printing). The inked surface is the raised area of the character. Application of the ink on that surface is carried out using rollers. A characteristic of this method is the effect on the edges of the printed area which appear raised with more ink. This is due to the pressure of the inked surface on the paper. The tonal range of an image is reproduced by applying different pressure to areas with highlights compared to shadows. Three types of printing presses exist in relief printing: platen and flatbed cylinder, where the plates are placed on a flat surface; and rotary, where the surface is a cylinder. Use of a web-rotary press with rubber or plastic plates is called flexography. It prints at high speeds and even on tough paper surfaces due to the properties of its plates.
Planographic printing (or lithography)
In lithography the surface of the lithographic plate is flat. It is treated so that the area with the image accepts a greasy ink while the rest of the plate surface accepts water. When a roll applies greasy ink on the surface, only the image area will hold the ink. Because the greasy ink and water do not mix, the image and non-image areas are well defined. Lithography is further divided into direct lithography and offset lithography. Offset lithography uses an intermediate stage; the image from the plate is transferred to a rubber blanket and then to the printing paper. In this case the printing is indirect, with no direct contact between the lith plate and the printed surface (Figure 16.16). Offset lithography is suitable for very large volumes of prints because of the low level of wearing of the plate. For this reason it is the main method used today for printing books, magazines, brochures, newspapers, etc. Also, the images and characters are not reversed.
Figure 16.16 Principle of offset photolithography press. D, dampening roller; G, roller carrying greasy ink; O, large rubber offsetting roller transfers image ink to moving paper; P, printing plate with image.
Recess printing
In this technology the image or characters are engraved on the plate or cylinder. When the plate or cylinder is inked, the ink remains in the recessed areas. By application of pressure it is transferred to the paper. One of the best-known methods is rotary photogravure (or gravure). This method provides a rich tonal range and was used in art photography in the nineteenth and early twentieth centuries. In gravure the inks used are solvent based. Applications of gravure printing include art books and magazines. Intaglio is another method of recess printing, consisting of line engraving, which uses copperplate and steel engraving, and artistic engraving uses drypoint and mezzotint engraving.
Through printing
In through printing or silk screen printing a material such as silk is mounted on a frame. A stencil produced on the silk screen covers the areas that should not be printed. Ink is applied to the uncovered areas and by pressure it is transferred to the substrate. Four screens for cyan, magenta, yellow and black can be used for printing colour photographic images while one screen can be used for black-and-white images.
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