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Book: Analytical methods for printing material structure characterization of printing material (Editor: Diana Gregor-Svetec)

Optical Modern methods for determination of paper surface topography

Aleš. Hladnik, Department of Textiles, Faculty of Natural Sciences and Engineering, University of Ljubljana, Slovenia

Gary. Chinga-Carrasco, Paper and Fibre Research Institute (PFI), Trondheim, Norway

Alojz. Suhadolnik, Faculty of Mechanical Engineering, University of Ljubljana, Slovenia

1 Introduction

The quantification of surface topography is of major importance for several industry sectors. For the paper industry, the surface topography is essentially important for printing paper grades. The topography affects several paper and print properties like gloss, missing dots, ink distribution, ink transfer, mottling and picking. It is thus important to have a detailed and standardised assessment of surface topography for understanding its influence on the paper and print characteristic details.

The quality of a given surface structure is commonly characterised by the quantification of its roughness. Although there are several roughness parameters (see Lipshitz et al. 1990; Peltonen et al., 2004; Chinga et al., 2007b), the root-mean-square is the most used roughness descriptor. The surface roughness may be divided into several scales, each scale affecting a given paper or print property. Apart from using traditional air-leak based and mechanical stylus profiling methods, surface topography of printing materials can effectively be assessed and analyzed by applying one of the numerous optical techniques that became available relatively recently; their overview is given elsewhere in the book (p. ?). In this chapter four of these methods are presented in a greater detail: laser profilometry, photometric stereo method, scanning electron microscopy and atomic force microscopy.

A proper description of a given surface structure requires reliable image acquisition devices. This is most important as the quality of a given surface representation will determine or limit the extraction of valuable numerical data, affecting a given surface property. During the last two decades several methods have been proposed for assessing the surface structure of paper material. This includes stylus profilometry (see e.g. Wagberg and Johansson, 1993; Enomae and LePoutre, 1995), laser profilometry (Chinga 2004), photometric stereo method (Hansson and Johansson, 1999), CLSM (Béland XXXX; Dickson, 2005), SEM (Enomae et al. XXXX; Reme and Kure, 2004), AFM (Niemi et al., 2002). This chapter will focus on i) laser profilometry, ii) photometric stereo methods, iii) SEM and iv) AFM. The mentioned assessmentcharacterization methods are complementary and are complementary to each other and cover a wide range of roughness scales, from the micron-level (laser profilometry, photometric stereo method) to the nano-level (SEM and AFM).

Hladnik, Chinga, Suhadolnik: Optical Modern methods for determination of paper surface topography 2/31

2 Laser profilometry

In optical profilometry a sensing head scanns across the surface of a given specimen to create profiles and 3D topographiessurface representations of topography structure. Unlike Contrary toin mechanical stylus profilometry, laser profilometry there ishasapplies no physical contact between the sensor and the substrate surface, so the latter can not be thus avoinding surface damageddamaging, and resulting in a faster scanning. Laser profilometry is also characterized by high vertical and lateral resolution.

2.1 Optical triangulation

Laser profilometry is often based on a principle referred to as optical triangulation (Fig. 1). It comprises three basic elements: a light source, imaging optics, and a photodetector (sensor). The light source and imaging optics are used to generate a focused beam of light that is projected onto a target surface. Although any light source can be used for basic optical triangulation, the most frequently applied are diode lasers, due to their high brightness, narrow-band wavelength, and phase coherence [1]. An imaging lens captures the scattered light and focuses it onto a photodetector, which generates a signal that is proportional to the position of the spot in its image plane. It is important for Tthe incident light beam from the laser source has to be well collimated to produce a uniformly small spot size over the entire measuring range thus resulting in good spatial resolution of the image. As the distance to the specimen surface changes, the imaged spot shifts due to parallax. To generate a three-dimensional image of the part surface, the sensor is scanned in two dimensions, thus generating a set of distance data that represents the local surface topography of the part.

The photodetector may be either a single-element position-sensing device (PSD) or a charge-coupled device (CCD). CCD arrays can be used to accurately determine the shape and intensity distribution of the light spot on the detector, whereas PSDs only determine the position of the spot’s centroid and total intensity. On the other hand, CCD arrays have slower signal response and require more signal processing circuitry than PSDs.

In addition to the above mentioned components, a dedicated software is required to coordinate probe motion, data acquisition, analysis, post-processing and reporting. Data may also be exported for further analysis and processing with commercial appropriate scientific software packages.


Fig. 1. Optical triangulation principle. Reproduced from[2].….

2.12 Other Alternative measurement principles

Along with a high-ly precisione computer controlled X-Y movable stage to which the specimen is attached, commercially available instruments can be equipped with one of various point sensors depending on the specific application. Laser profilometers by Solarius [3] utilize autofocus or confocal point sensors (see Table 1 for specifications), among others. In autofocus measurement (Fig. 2), condensed light is focused from a laser diode onto the specimen surface. The reflected light is directed to a focus detector, which measures deviations from the ideal focus to within a few nanometers. The deviation in focus generates


an error, which is used to re-focus the objective. The position of the objective represents an absolute measurement of the height.

In confocal principle, a point light source and detector pinhole are used by the sensor to discriminate depth. The point light source emitts the laser beam that is focused on a specimen through an objective moving rapidly up and down. The maximum light intensity occurs when the specimen lies within the focal plane of the objective. As the objective moves closer to or farther from the specimen, however, the reflected light reaching the pinhole is defocused and does not pass through it. As a result, the quantity of light received by a detector behind the pinhole decreases rapidly. A detection signal is only generated when the maximum of light goes through the pinhole. Continuous scanning along the z-axis ensuresA precise height measurement of the illuminated point is achieved by continuously scanning along the z-axis.

Other sensor types can be applied as well. The chromatic white light sensor [4] is also based on the confocal technique. The function of the pin-hole diaphragm is assumed by an objective lens with high chromatic aberation, while a spectrophotometer is used to measure the heights from colour differences. Due to its compact design, this type of sensor is especially suited for measuring in inaccessible places. The holographic sensor can be applied when form and geometry with large differences in height are of interestshould be assessed.

Fig. 2. Autofocus (left) and confocal (right) measurement principle [Error: Reference source not found].

Reproduced from…

.


Table 1. Sensor measurement specifications [Error: Reference source not found].

2.3 Paper and print applications

Chinga!

3 Photometric stereo method

Stereoscopy implies the use of stereo, or binocular, images to derive height information. Either oOptical or and electron microscope images can be used [5]. Thus, the range of resolution is quite wide, from perhaps several tens of nanometers up to centimeters. This technique requires significant computational resources, however these are now commonly available. Modern analysis algorithms have significantly improved both the speed and accuracy of this technique, making it a much more viable option then it was in past decades.

Suhadolnik !

Photometric stereo method is based on the utilization of stereo or binocular images to derive height information. Optical and electron microscope images can be used [6]. The range of resolutions is quite wide, from perhaps several tens of nanometers up to centimeters. Both the speed and accuracy have improved significantly over the last few years so the technique has been used successfully in a number of applications.

A method which determines the shape of an object from a single image is known as shape from shading method [7] and relies on the intensity variation in the image. Unfortunately only intensity can be determined from a single image point so to be able to describe sample surface orientation, another variable is required. Another drawback of the shape from shading method is susceptibility to intensity noise in the image. In general the shading of the surface depends on the reflectance properties of the surface and on the light source illumination properties. By contrast, with a photometric stereo method several images at different angles are acquired and there is no dependency on the reflectance of the surface.

3.1 Irradiance

Let us denote by the object surface function in Euclidian coordinate system. We presume orthogonal projection of the image surface onto the image plane over the compact region . The surface and the image have the same coordinate system. The surface gradient (p, q) is defined by equations:

(1)

. (2)

In order to recover the object surface, gradients must be integrated for each . The

integration is performed along some path and starting at point . The image irradiance is then equal to the so called reflectance map :

(3)

which determines image intensity as a function of surface orientation. The reflectance map combines information about the surface illumination, viewing geometry and surface material by mapping from the surface orientation to image intensity. If the surface is assumed to be Lambertian illuminated with diffused light than the irradiance on the camera can be written as:


(4)

where is the reflectance of the surface, is the irradiance of the light source, S is the direction to the light source and is the surface unit length normal at point (x,y) defined as follows (Fig. 3):

(5)

Fig.3. Illumination conditions with two light sources S1 and S2 at angle between source and detector.

The unit surface normal is denoted by N, and are angles of incidence for both illuminations and

the object surface function.

With photometric stereo method which was introduced by Woodham [8] the estimation of the local surface orientation is performed by several simultaneous images acquiring the same surface at different illumination directions.. Number of the light sources depends on the prior knowledge of the surface reflectance. In most cases the number of light sources is two, three, four or even more. In case of two light illuminations (Fig. 1) the light source directions are:

(6)

and

(7)

If both light sources have equal and the incident angles are and , the sum of two irradiances is:

, (8)

where scalar multiplication of vectors N and S gives the value of . By putting equations 5, 6, 7 into equation 8 the final irradiance sum is equal:

(9)


Last approximation is valid if the gradients p and q are small. From this equation the reflectance R is then equal:

(10)

The difference of the light illuminations is:

(11)

The surface gradient in x direction is finally calculated from the intensity ratio:

(12)

If Lambertian irradiation equation (4) is corrected for the experimentally determined correction factor , then the irradiation is:

(13)

and the surface gradient calculated in equation 13 becomes:

(14)

The correction factor is determined empirically and has value of 0.07 in case of unpolarized illumination and value of 0.04 in case of p-polarized illumination and s-polarized detection.

3.2 Integration

The surface function f(x,y) can be determined from gradients (p,q) by one of the integration techniques. The local integration technique enables the surface normal vector determination at two or eight adjacent points to a given point by calculating the average tangent trough the given point and than interpolating the height and the surface normal. This technique uses the following curve integral:

(15)

Variable represents arbitrary integration path from point to point . In this calculation the initial height value must be known. The errors propagate along the integration paths and the results depend strongly on data accuracy.

The equations 1 and 2 link together the surface height and gradients. The global integration technique minimizes the following functional which is also called the cost function:

(16)


In this integral is gradient field of the surface and given non-

integrable gradient field. Furthermore the gradient field can be expressed as

where expression denotes the correction of

the gradient field which inverts non-integrable field to integrable one.

Integration in the frequency domain with the Fourier transform is also possible. In case of two

light sources the partial derivative is unknown and we assume that the mean value of

the profile function with constant y is zero. If the noise in the acquired image

and light spreading in the material is taking into account, the partial derivative

becomes:

, (17)

where PSF denotes the point spread function and * convolution. This equation in frequency domain with the spatial frequencies u and v is expressed as

(18)

The Fourier transform of the PSF is the optical transfer function OTF. The estimated surface function can be calculated by the inverse Fourier transform:

(19)

Before the inversion process is convolved with a Wiener filter :

, (20)

where , denotes complex conjugate, is power spectrum and

is the signal-to-noise ratio in frequency domain. This

calculation is performed if is assumed to be a stochastic process with known spectral density.

In case of paper sample the OTF can be approximated by:

(21)

where u and v are in mm-1 and signal-to-noise ratio (SNR) by:

(22)

where is the resolution in millimeters in the x direction.

4 Scanning electron microscopy

Resolution of the conventional optical – light – microscope is limited by the visible light wavelength (λ = 400-700 nm) to approximately 0.2 m. In addition, out of focus light from points outside the focal plane reduces image clarity. An important progress in the investigation of material surfaces came about with the development of an electron microscope – starting with the transmission electron microscope (TEM) and continuing with the scanning electron microscope (SEM). Principles of SEM were developed in the early 1950s at the University of Cambridge, U.K., but the technique did not become commercially available until 1965. Since then, numerous improvements have been made on the instrument in terms of lens design, electron sources, detectors, and electronic signal processing. Today, SEM is one the most widely used analytical techniques providing means to study both the morphology and composition of various materials. Its main advantages [9] are the high lateral resolution (1-10 nm), large depth of focus (100 m at 1000 x magnification), wide range of magnifications (20-300.000 x) and numerous types of electron-specimen interactions that can be used for further material examination or processing. The technique is being implemented in a vast range of applications, such as semiconductor research and manufacturing, metalurgy, biology, geology and also for paper and print investigations.

A detailed discussion on various SEM components, functions and capabilities has been provided by numerous authors – see, e.g. [10, 11] – so only fundamentals of SEM and its modes of operation are presented here. More emphasis will be given to the implementation of this technique for paper- and print-related surface analysis.

4.1 Instrument design

SEM basically contains two main components (Fig. 43) [Error: Reference source not found]: a vacuum chamber and an electron-optical column and a vacuum system. An electron gun is placed on top of the column and its cathode filament can be made of various materials: in conventional SEM, it is produced of tungsten or LaB6 – usually with an additional electrode (Wehnelt) placed between the cathode and anode – while in the modern field-emmission gun SEM (FEGSEM) it is made of an extremely thin tungsten monocrystal needle or a ZrO2

monocrystal attached to a tungsten wire. Upon heating the cathode, a thermoionic (or field-emission in FEGSEM) emission of electrons takes place. Primary electrons are accelerated using anode voltage typically ranging from 500 to 30.000 V.

A system of magnetic lenses – sometimes combined with electrostatic ones – and apertures directs and focuses electron beam onto the specimen surface. The condenser lenses and spray apertures are responsible for changing the beam divergence angle and therefore the probe current, thus affecting the probe diameter. Purpose of the objective lens is to focus the electron beam into an extremely fine spot on the surface of the sample: in modern conventional SEM using tungsten cathode its diameter is as low as 10 nm. Scanning (rastering) of the focused electron beam across the specimen surface is accomplished by special coils located in the bottom part of the electron-optical column. The magnification and scan velocity are varied by changing scan coil excitation. Rastering together with the signals generated in the sample by the incident electron beam (see below) are monitored


simultaneously. The signals are collected by special detectors, amplified, displayed and stored in a computer memory (or on a photographic film) for further image processing.

Vacuum in the SEM specimen chamber is 10-3 – 10-4 Pa. Some modifications of this instrument operate at a much weaker lower vacuum – low vacuum SEM (LVSEM) – or at a controlled low pressure of certain gases – environmental SEM (ESEM) and are especially suitable for investigation of nonconductive samples containing water, oils, organic solvents, etc.


Fig. 43. SEM schematic cross section [12]. Reproduced from….

4.2 Electron beam-specimen interaction

When primary electrons hit the solid sample in a vacuum, numerous signals are generated as a result of electrostatic interactions with the nuclei and electrons of the target atoms (Fig. 54). These interactions, i.e. scattering, can be either elastic or inelastic. Elastic scattering is a result of primary electron interactions with the specimen atomic nuclei. After multiple scattering events and changes of direction, a portion – about 30% – of the incident electrons orient themselves towards the surface eventually leaving the sample. These electrons are called back-scattered electrons (BSEs); all electrons having energy E > 50 eV belong to this category. The number of BSEs increases with an increase in target's atomic number, Z.

In an inelastic scattering, electrons are losing their kinetic energy due to their contact with the core and valence electrons of the sample. This results in the emission of secondary electrons, Auger electrons, characteristic X-rays, continuum X-rays (Bremsstrahlung), etc. The excited valence electrons, called secondary electrons (SEs), have low energy (E < 50 eV) and are quickly absorbed by the sample, so only those generated close to the surface – typical depth 50 nm – can be detected.


Conventional SEMs are normally equipped with BSE and SE detectors, which provide topographical information about the specimen surface, while generation of characteristic X-rays and Auger electrons can, for example, be used in qualitative and quantitative chemical analysis of sample composition (EDX, WDX, AES).

Sample

Auger electrons

Cathodoluminescence

Primary electron beam

Brehmsstrahlung

Secondary electrons (SE)

Back-scattered electrons (BSE)

Characteristic X-rays

Absorbed current

Fig. 54. Signals generated upon interaction of primary electron beam with a specimen in SEM. Reproduced from…

4.3 Preparation of paper samples

Samples to be analyzed by the conventional SEM need to be electrically conductive, stable under vacuum and insensitive to the local heat generated during electron beam-specimen interaction. Pretreatment of nonconductive materials involves coating the specimen surface with a thin metal or carbon layer of 5-40 nm thickness. For the preparation of paper samples the following procedure has been recommended [13]:


1. Cut a paper sample the size of the microscope stub with a razor blade while taking care not to touch the surface with either fingers or the instrument to avoid deforming and compressing the paper.

2. Stick the paper sample onto the stub with double-sided tape, preferably a conductive carbon tape. For thick or rough samples a conductive pathway of silver paste or colloidal silver paint in an ethanol base should be made between the paper surface and the metal stub.

3. Leave the sample overnight in a desiccator to allow the silver paint solvent to evaporate and to remove any possibly present residual moisture.

4. Use a sputter coater or an evaporation coater to coat the sample with a thin film (not exceeding 30 nm in thickness) of a heavy metal, e.g. gold, palladium or a mixture of both. The film thickness should be such that the surface details are not obscured; for smooth samples, such as coated papers, 10 nm should be appropriate. Papers with a rougher surface need thicker metal layers to bridge small gaps and create a continuous conducting layer.

By preparing cross-sections of the paper sheets, information about paper structure in z-direction can be obtained using SEM (Williams and Drummond, 1995?; Allem, 1998; Chinga and Helle, 2002). For such purposes, the paper sample should first be embedded under vacuum in a polymer followed by cutting and polishing the resulting stub. To improve the z-direction contrast, the sample should be coated with a thin film of carbon to prevent charging.

4.4 Paper and print applications

Chinga !

5 Atomic force microscopy

Together with scanning tunneling microscopy (STM), atomic force microscopy (AFM) belongs to a family of scanning probe microscopy techniques, which offer the highest resolution available for studying surfaces of various materials. AFM was invented in 1986 by Binning, Quate and Gerber [14] and only three years later the first commercial instrument was produced by Digital Instruments (USA). Resolution of the surface features is typically on the nanometers scale laterally and on the angstrom scale vertically, although atomic resolution can be achieved under certain conditions [15]. Sample preparation is quick and relatively simple, requiring no stains, contrast agents, or conductive coatings (unlike e.g. in SEM, see above). Other advantages of the technique include its nondestructiveness, potential to use a broad range of environmental conditions – ambient air, various gases, humidity levels, temperatures – and ability to study short- as well as long-range molecular and atomic forces. AFM was implemented to investigate a large variety of materials (thin and thick film coatings, ceramics, composites, glasses, synthetic and biological membranes, metals, polymers, and semiconductors) and phenomena (abrasion, adhesion, cleaning, corrosion, etching, friction, lubrication, plating, and polishing) [16].

Together with scanning tunneling microscopy (STM), atomic force microscopy (AFM) belongs to a family of scanning probe microscopy techniques, which offer the highest resolution available for studying surfaces of various materials. AFM was invented in 1986 by Binning, Quate and Gerber [17] and only three years later the first commercial instrument was produced by Digital Instruments (USA). Resolution of the surface features is typically on the nanometers scale laterally and on the angstrom scale vertically, although atomic resolution can be achieved under certain conditions [18]. Sample preparation is quick and relatively simple, requiring no stains, contrast agents, or conductive coatings (unlike e.g. in SEM, see above). Other advantages of the technique include nondestructiveness, potential to use a large variety of materials – nonconductive, magnetic, elastic, etc. – and environmental conditions – ambient air, various gases, humidity levels, temperatures – and ability to study short- as well as long-range molecular and atomic forces.

5.1 Operating principle

The probe used in AFM is an extremely sharp tip – typically less than 5 μm tall and less than 10 nm in diameter at the apex [19] – integrated at the end of a 100-500 m long cantilever. The cantilever bends or deflects due to the forces between the tip and the sample surface while either the tip is scanned over the sample, or the sample is scanned under the tip. Changes in the angle of the cantilever caused by changes in sample topography result in different voltage levels out of the detector. These voltages are sent to a computer for processing and display of the topographic image.

Most commercial instruments use optical techniques to detect the position of the cantilever (Fig. 6). Here a light beam from a laser bounces off the back of the cantilever and onto a position-sensitive photo-detector (PSPD) or a photodiode. Bending of the cantilever causes a change in the laser beam position on the detector.


Fig. 6. Basic operational principle of AFM with an optical detection system [20].

5.2 Probe, scanner and detector

The probe, i.e. cantilever with the tip, is made of a solid and inert material, such as silicon nitride (or silicon for intermittent mode, see below) to prevent mechanical deformations of the tip and tip/substrate chemical reactions. To get an atomic-scale resolution the tip has to be extremely sharp with its radius of curvature ranging from 5 nm to 60 nm. Tips for some AFM applications may be coated with metals (e.g. gold) or other materials (colloid particles, monolayers, magnetic materials) or can have globular shape where nanometric resolution is not crucial. Important cantilever parameters include its geometry (triangular or rectangular), flexibility (spring constant) and resonance characteristics (frequency).

AFM scanner is made of piezoelectric (PZT) ceramics that expands and contracts in response to the applied electric field. It allows for an extremely precise sample movement in a defined – raster – pattern consisting of a series of rows in a zigzag pattern. As mentioned above, the sample can be mounted directly onto the scanner and rastered underneath the cantilever tip, or the cantilever can be mounted to a scanner tube and rastered over a sample fixed below it. The former case is preferential when imaging larger samples, it also increases the imaging speed.

In modern AFMs the cantilever deflection is usually detected with a photodiode consisting of four quadrants, i.e. separate diodes. Interferometry can also be used as a detection system. Its phase change is compared with that of a standard to determine the shift in cantilever position. Another option is integration of the detector into the cantilever using the piezoresistive properties of silica [Error: Reference source not found].

5.3 Operational modes

In order to monitor topographical images with AFM, investigate different surface sample properties, or modify its surface, several modes of operation are available (Fig. 7). In contact mode the tip is in constant contact with the substrate surface so short-range ionic repulsive forces dominate. The tip follows the surface topography pattern. Here two possibilities for generating topography data exist: Z feedback (loop) can be either turned on or off. Most


applications are based on Z feedback being turned on – so called constant-force mode. Here vertical (Z) position of the sample changes so that the force between the tip and the sample (and therefore cantilever deflection) remains constant. By contrast, in the constant-height mode Z feedback is turned off so the sample vertical position during the measurement remains unchanged while force between the tip and the sample changes. The constant-height mode is often used to obtain atomic-scale images of very flat surfaces and for recording real-time images of changing surfaces where high scan speed is necessary.

In noncontact mode the tip oscillates up to approx. 10 nm above the surface with a low amplitude and resonance frequency by a piezoactuator, longer-range attractive – mainly van der Waals – forces prevail. The cantilever is stiffer than that that used in the contact mode. As the tip does not touch the substrate, it can not be contaminated. This mode is applicable for measuring very delicate samples, but due to a higher distance to the sample it is difficult to achieve an atomic-scale resolution.

Intermittent contact (tappingTM ) mode works by lightly tapping the surface with an oscillating probe tip. Surface topography causes the cantilever oscillation amplitude to change, the topography image is obtained by monitoring these changes and closing the Z feedback loop to minimize them [Error: Reference source not found]. This mode is useful for measuring soft samples or molecules that are not strongly attached to the surface, but due to the danger of tip contamination and change in resonance frequency of the cantilever the latter has to be recalibrated. Intermittent contact mode also overcomes problems that can occur when using AFM in an ambient environment – i.e. in air or some other gas – where a liquid layer often covers the substrate surface.

Fig. 7. Regions of operations for AFM contact, non-contact and tapping mode [21].

Apart from the above described primary AFM imaging modes, there is a large variety of secondary imaging modes available that are derived from the primary ones, such as lateral force microscopy, phase imaging, magnetic force microscopy, conductive AFM, tunneling AFM, electric force microscopy, and others [Error: Reference source not found].


Hladnik !

6 Paper and print applications

The techniques described in this chapter are complementary, thus enabling assessment of assessing several surface characteristics and scale of roughnesses. A photometric stereo method is a relatively simple, yet powerful technique for assessing surface structures (Hansson and Johansson, 1999). The method is fast and is capable of assessing large areas. However the maximum resolution of 5 m may be considered low for some purposes such as the assessment of coated paper sub-micron roughness and how this is affected by mineral pigment particles. On the other side, an optical stereo method may be suitable for assessing missing dots in rotogravure printing as the surface cavities inducing missing dots seem in the range of 80 m in diameter, thus reducing resolution requirements.

A clear example of the applicability of a photometric stereo method is presented in Fig. XX. Surface depressions below -1 mm can be quantified and related to the occurrence of missing dots. The method seems to give reasonable results, detecting the surface depressions causing a poor contact between the paper surface and the gravure printing cylinder. A similar approach has been applied in flexographic printing, where surface depressions may induce the occurrence of unprinted areas (see Hansson and Johansson, 1999; Barros and Johansson, 2006). In addition, Barros and Johansson, 2008 described the applicability of this instrumentation in combination with bandpass filtering for finding the relationship between surface topography and the reflectance of flexography printed samples. The authors concluded that small differences in surface roughness (wavelengths 0.8-1.6 mm) influenced ink distribution and consequently print mottle in full-tone flexography printing.

Fig. XX Photometric stereo method about missing dots.

During the last years laser profilometry (LP) has been used extensively for assessing the surface topography of several paper grades, including newsprints, SC and LWC paper. The method is fast, fully automated, non-contact, non-destructive and capable of assessing large areas (see Chinga-Carrasco et al., 2008). However, LP has some limitations with respect to the detection and description of steep gradients. Such limitation causes noise on the digital images, especially along coarse surface fibres. Recently Chinga et al. (2007), proposed to cover the surface with a layer of gold before the image acquisition. The gold layer seems to reduce internal reflections and reduces the amount of error height values.

Suonstausta (2002) applied LP for assessing the effect of coating and calendering on the surface structure of paper. The study demonstrated one of the major advantages of profilometer devices, i.e. a topographical map may be decomposed into several scale of roughness. This gives the opportunity of assessing the effect of a given scale of roughness on a specific respons. Wavelength analysis was thus used for exploring the development of the surface structure depending on a given coating composition and calendering configuration


(Suontausta 2002). Using LP, Holmstad et al. (2004) studied the effect of temperature gradient calendering on the surface structure of pilot calendered paper. According to the authors, the calendering conditions used in the study had a major effect on the uppermost surface structure, being the calendering temperature the variable having the major impact on the reduction of the surface roughness and thus the increasing of the paper gloss.

LP has been applied for studying the effect of the surface structure of SC and LWC paper on gloss (Suontausta 2002; Chinga 2004; Holmstad et al., 2004; Chinga-Carrasco et al., 2008). Gloss is one of the most important paper and print quality parameters of printing paper. Gloss may be affected by the surface roughness and the mineral pigment particles, used as fillers or in a coating layer. For the same roughness, clay yields usually higher gloss compared to ground calcium carbonate (GCC) (see e.g. Stanislawska and LePoutre, 1996). In addition, it has been demonstrated that the amount of fillers in the surface layers explains most of the gloss development of commercial SC papers (Chinga et al., 2007a). Compared to SC paper, LWC paper has commonly higher gloss levels due to the smoothing effect of the coating layer. In addition, when SC paper is in contact with water a roughening phenomenon occurs due to the swelling of the fibre material. The roughening is less in coated paper (see e.g. Chinga et al., 2004). Contrary to water application that causes roughening, a layer of printing ink usually smoothens a given surface structure (Fig. XX).


Fig. XX. Upper row) Laser profilometry surface representations of an unprinted (left) and printed (right) LWC sample from the same local area. The calibration bars are given in micrometers. Lower row) The corresponding 3D surface plot. The 3D plots are generated with the Interactive 3D surface plot v.2.3 ImageJ plugin, by Kai Uwe Barthel, Internationale Medieninformatik, Berlin, Germany.

A surface structure can be decomposed into gradients. A gradient is a small facet having a given angle relative to the paper plane (Fig. XX). For comparison purposes, such analysis has been performed on the images from Fig. XX. Note that the gradient image of the printed sample has a lower amount of high angles compared to the gradient image of the unprinted sample. A lower amount of high angles indicates that the surface has been smoothed due to the printing ink (see also Chinga et al., 2004). Consequently, the printed surface (Figs. XX and XX, right) will thus have a higher gloss than the unprinted surface. This exemplifies one the benefits of using a non-destructive surface assessment device in combination with the appropriate image analysis for quantifying a given response upon a finishing variable.


Fig. XX Gradient analysis of LWC samples. Upper row) Fig. XX converted into gradients of the unprinted (left) and printed (right) samples.Each gradient corresponds to an angle relative to the paper plane. The calibration bar is in degrees. Lower row) the corresponding histograms. Images generated by G. Chinga Carrasco, PFI, Norway.

Based on a LP concept, Sung and Keller (2008) reported a new method defined as a twin laser profilometer (TLP). The method consists on two laser sensors, placed to each side of a sample holder. This configuration makes it possible to acquire surface profiles from each side of a paper sample. The profiles are thus combined to generate a thickness map with a resolution of 1 m. According to the authors, the TLP method yields the intrinsic thickness of a given sample, thus eliminating the overestimation of thickness that is characteristic of standard caliper methods (Sung and Keller, 2008).

A photometric stereo method is a relatively simple, yet powerful technique for assessing surface structures (Hansson and Johansson, 1999). The method is fast and is capable of assessing large areas. However the maximum resolution of 5 m may be considered low for some purposes such as the assessment of coated paper sub-micron roughness and how this is affected by mineral pigment particles. On the other side, an optical stereo method may be suitable for assessing missing dots in rotogravure printing as the surface cavities inducing missing dots seem in the range of 80 m in diameter, thus reducing resolution requirements.

A clear example of the applicability of a photometric stereo method is presented in Fig. XX. Surface depressions below -1 mm can be quantified and related to the occurrence of missing dots. The method seems to give reasonable results, detecting the surface depressions causing a poor contact between the paper surface and the gravure printing cylinder. A similar approach has been applied in flexographic printing, where surface depressions may induce the occurrence of unprinted areas (see Hansson and Johansson, 1999; Barros and Johansson, 2006). In addition, Barros and Johansson, 2008 described the applicability of this instrumentation in combination with bandpass filtering for finding the relationship between surface topography and the reflectance of flexography printed samples. The authors concluded that small differences in surface roughness (wavelengths 0.8-1.6 mm) influenced ink distribution and consequently print mottle in full-tone flexography printing.


Fig. XX Photometric stereo method about missing dots.

Fig. XX presents a clear example of the capabilities of the scanning electron microscope (SEM) for exploring the surface structure of printing paper. SEM is a versatile device for acquiring structural information. Images can be acquired at several magnifications and with a resolution unattainable by other techniques. Images can be acquired in secondary electron (SE) (Fig. XX) or backscatter electron mode (BSE) (Fig. XX). Due to its extensive capabilities, SEM has been a most used device for assessing the structure of paper and prints (see e.g. Helle and Johnsen, 1994?; Enomae et al., 1995?; Forseth and Helle, 1997; Allem, 1998; Reme and Kure, 2003?; Chinga and Helle 2003; Zou et al. XXXX; Eriksen and Gregersen, XXXX).

Fig. XX SEM-SE and SEM-BEI images from the same area.

The SEM is a powerful tool for assessing different characteristics of the surface structure of paper. In SE-mode the SEM gives a clear 3D impression of the topography of a given surface. This capability has been used extensively for exploring e.g. the surface development due to calendering (Holmstad et al., 2004), the consolidation of coating layers (Enomae and LePoutre XXXX) and the smoothening effect of printing inks (Chinga et al., 2004), to name a few. Common for the mentioned studies is that the SEM was used for exemplifying a given phenomenon, however no quantification was performed. The SEM can also be used for reconstructing surface structures (Fig. XX). The method is based on stereo imaging and parallax (see e.g. Hein, 2001). Such surface reconstruction makes it possible to perform a quantitative assessment of the surface topography (see Helle and Johnsen, 1994; Gregersen et al., 1995; Reme and Kure, 2004). Another approach has been presented by Enomae et al. (1995??). The authors used a SEM having two SE-detectors. Images were acquired from each side of the vacuum chamber and a topographical height map was reconstructed.


Fig. XX SEM stereo imaging of a newsprint sample. Note the coarse surface fibre on the left side of the image. Use red/green stereo glasses for better visualization. The SEM stereo image has been acquired by G. Chinga Carrasco, PFI, Norway.

Surface coverage by a layer of fillers is an important characteristic of printing paper. Coverage may determine some paper and print quality properties. SEM in backscatter mode yields contrast depending on the average atomic number of a given local area. Mineral fillers such as clay and CaCO3 carbonate may appear lighter that the matrix of darker fibres (Fig. XX). SEM-BEI mode images with appropriate image segmentation and analysis procedures make this technique suitable for quantification of coverage and related characteristics. Such capability has been used for quantifying the coverage of the coating layer on coated papers (see e.g. Kaartovara 1989; Dickson et al., 2002; Forsström et al., 2003;) and the coverage of fillers on SC paper surfaces (Chinga et al., 2007). Coverage may be given in percentage and may be defined as the ratio of areas covered with a coating layer to the whole imaged area. Kaartovara (1989) quantified several statistics of uncovered areas, such as percentage, average size and number of uncoated areas. The author found a reduction of uncoated areas as the coat weight was increased from approximately 8 to 20 g/m2. However, care must be taken when using the SEM-BEI mode for quantification of coverage, as the amount of uncovered areas will depend on the accelerating voltage used during image acquisition in the SEM–BEI mode. It is recommended to use low accelerating voltage in order to assess only the uppermost layers of the paper surface. The effect of accelerating voltage on the quantification of coverage is depicted in Fig. XX.


Fig. XX Coverage and accelerating voltage. A) Secondary electron image showing surface variations in a LWC sample. B)-F) Images showing the same area, taken in BEI-mode with raising accelerating voltage: 5, 10, 15, 20 and 25 kV resp. Note the gradually emerging fibres when raising the accelerating voltage. Bar: 200 m. The SEM images have been acquired by G. Chinga Carrasco, PFI, Norway.

In addition to surface assessment, the SEM is a powerful tool for quantification of cross-sectional details of paper structure (Allem, 1998; Chinga and Helle, 2002; Holmstad et al., 2004; Zou et al., XXXX). Such quantification may be used for describing the porosity (Allem 1998; Zhou XXXX?), fibre and pore cross-sectional dimensions (Chinga et al., 2007), filler distribution (Holmstad et al., 2004), fines distribution (Eriksen et al., 2006) and the structural details of mineral pigment layers on paper (Chinga and Helle, 2002). However, SEM has the limitation of yielding 2D images of 3D structures. As an attempt to circumvent this limitation, serial sectioning and serial grinding has been applied for making 3D reconstructions of paper structure (Aronsson et al., 2002?; Chinga et al., 2004). This methods yield detailed information about a given paper structure, though the method is time-consuming and difficult to use as a standard analytical method for paper structure assessment.

Atomic force microscope (AFM) is a most suitable scientific device for quantifying the micro (1-100 m), sub-micron (0.1 – 1 m) and nano-structure (1-100 nm) of paper and print


surfaces. According to Niemi et al. (2002) AFM have tree major advantages compared to other microscopy techniques, i.e. i) no or little preparation, ii) high resolution and three-dimensional surface information and iii) the microscope can be used in environments inaccessible with other techniques.

Fig. XX AFM image of a LWC paper sample and the corresponding 3D surface representation. The calibration bar is given in micrometers. The 3D plots are generated with the Interactive 3D surface plot v.2.3 by Kai Uwe Barthel, Internationale Medieninformatik, Berlin, Germany. The AFM image has been acquired by B. Wang. Dept. of Physics, NTNU. Norway.

In addition to extracting 3D topographic information (see Peltonen et al., 2004; Chinga-Carrasco et al., 2008), AFM can be used in phase mode to distinguish different components in a coating structure, such as pigment particles and latex (see Niemi et al., 2002). The AFM is thus a suitable method for assessing the structure of calendered coated paper. Rougher surfaces have to be analysed with care, as the maximum movement in the z-direction (height) is only a few micrometers. Local areas of fibre surfaces may also be assessed thus giving a detailed description of pulp fibres, including the different layers of the fibre walls with their characteristic arrangements of microfibrils (see e.g. Niemi et al., 2002).

Using AFM the structure of coating structures has been studied in detail (Ström et al., 2003; Larsson et al., 2007; Järnström et al., 2007; Chinga-Carrasco et al., 2008). Ström et al. (2003) applied an AFM analysis for assessing the structure of XXXX. The study proved the suitability of AFM for assessing the structure of coated paper and prints. The topography was related to the print gloss. A similar approach was applied by Järström et al. (2007) for relating the surface topography of model calcium carbonate –based coating layers to the corresponding gloss levels. The authors described a set of roughness descriptors that can be applied for exploring a surface structure in detail. The applicability of a surface skewness parameter for exploring the topography development at different scales was discussed. A


coating layer structure is also affected by the coating formulation. Calcium carbonate and clay pigment particles have different shapes, i.e. ground calcium carbonates particles are blocky, while clays are platey. They affect the surface development and pore structure in coating layers in different ways. Larson et al. (2007) performed a study to reveal the effect of several blends on the surface and bulk structure of coating layers. The behaviour of the coatings upon calendering was also assessed. The results showed that increasing the amount of clay caused a smoother surface, a more compact coating bulk structure and consequently higher gloss levels. Most recently, Chinga-Carrasco et al. (2008) showed the relative relationship between several scales of roughness and gloss. Roughness below a wavelength of approximately 1 m did not affect the gloss of LWC paper significantly. The complementary capabilities of LP, AFM and X-ray microtomography for assessing surface structures was also demonstrated, thus giving valuable insight into the structure of coating layers.

6 Conclusions

Modern methods for determination of paper surface topography 30

7 References

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