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Resolution abilities and measuring depth of High-Frequency densitometry on wood samples
Measurement 45 (2012) 1913–1921
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Resolution abilities and measuring depth of High-Frequency densitometry on wood samples Simon Boden ⇑, Martin G. Schinker, Philipp Duncker, Heinrich Spiecker Albert-Ludwigs-Universität Freiburg, Institut für Waldwachstum, Tennenbacher Straße 4, 79106 Freiburg, Germany
a r t i c l e
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Article history: Received 3 September 2011 Received in revised form 17 January 2012 Accepted 17 March 2012 Available online 16 April 2012 Keywords: Dielectric property High-Frequency (HF) densitometry Penetration depth Resolution ability Surface preparation Wood density
a b s t r a c t The High-Frequency (HF) densitometry measures relative density variations on wood samples utilizing the dielectric properties in wood. This method is based on the propagation of an electric stray field through the surface-near region of a wood sample. We studied experimentally the penetration depth and the differentiation abilities of the HF-densitometry method on wood samples. Two experimental approaches showed that penetration depth is related to the geometrical dimensions of the micro-electrode measuring system. Characteristic patterns of the HF-output signal were used to determine the resolution abilities of each HF-probe. Due to a very small integration area geometric structures of earlywood cells of Norway spruce were indicated by stepwise profiles in the HF-output signal pattern. With the High-Frequency densitometry it was possible to distinguish different cell structures due to their variations in wood density up to a resolution of 1 lm. In addition it was possible to determine wood density variations at various resolution levels. Based on the respective resolution abilities and penetration depths of five different purpose-built HFprobes we show the optimal operating conditions for measurements on wood of this indirect densitometry method. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Wood density has a direct or indirect influence on various physical and mechanical wood properties and is a well known estimator of wood quality [1–4]. Wood density may be determined in a variety of ways [5]. Relatively easy and without intensive sample preparation are ‘basic’ wood density, calculated as oven-dry weight of a wood sample divided by the volume in a water saturated state, and ‘apparent’ wood density, calculated as weight divided by volume at a given moisture content [3,5–7]. Variations in wood density at high resolution are however not so easy to determine. Such measurements require a considerable experimental effort in suitable preparation and measurement methods due to specific growth-related, nonhomogeneous anatomical cell structures of the sample and the underlying level of abstraction in the study. ⇑ Corresponding author. Tel.: +49 761 2033737; fax: +49 761 2033740. E-mail address: [email protected] (S. Boden). 0263-2241/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.measurement.2012.03.013
Various methods to measure variations in wood density for example such as X-ray densitometry [8–11], transmission measurements in the far-infrared range [12] and near infrared reflectance spectroscopy [13], microwave scanning [14] or scanning of colour images using flatbed scanner [15] have been developed over the last decades. Those methods are characterised by different resolution abilities [16–18]. An alternative method is the fast and non-destructive High-Frequency (HF) densitometry. This method relies on the dielectric properties of wood, the close relationship between wood density (q) and the relative dielectric constant of wood (er). The method is based on the propagation of a continuous electric stray field in a high-frequency transmitter–receiver link of an extremely small electrode system, which is in direct contact with the investigated wood surface [19]. The relative dielectric constant (er), also described as permittivity of a material, determines the interaction between a material such as wood and an alternating electric field [20]. For measurements on wood the
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relative dielectric constant depends on species, excitation frequency of the electric field, temperature, moisture content, density and the orientation of wood fibres with respect to the direction of the electric field [21–23]. At given air temperature, moisture content, orientation of wood fibres and excitation frequency the wood density is almost linear related to the dielectric constant [22,24]. An important aspect for the optimal use of HFdensitometry for measurements on wood samples is the ability of differently structured HF-probes to identify small geometric cell structures that are characterised by a considerable deviation in wood density. This differentiation ability is referred to as resolution ability. An indication of the resolution ability requires knowledge of the penetration depth of the respective electric stray field. The penetration depth is defined as the measure of how deep the electric field can penetrate into a certain material. Although for measurements on wood samples the length of a tracheid cell is potentially infinite relatively to the penetration depth, knowledge of the penetration depth is important for the determination of the optimal operating conditions for the HF-densitometry method and a previously required surface preparation of the wood sample. In this study we systematically analyse the differentiation abilities of the HF-densitometry method on wood samples. Schinker et al. [19] showed that it is possible to determine variations of wood density with the HF-densitometry. The first objective of our study is to investigate if it is possible to determine wood density variations at various resolution levels. The second objective is if it is possible to identify geometric structures of individual cells with the HF-densitometry. Several authors mentioned that climate fluctuations were reflected in intra-annual wood density profiles [25–28]. Taking into account the respective study material and the aim of a potential study, the ability to determine wood density at various resolution levels is indispensable to get wood density values given in appropriate resolution for estimating climate variability within a growing season on intra-annual wood density patterns. Based on the respective resolution abilities and penetration depths we show the optimal operating conditions for measurements on wood of this indirect densitometry method.
2. Materials and methods 2.1. Adjustment and calibration The micro-electrode measuring system is the main part of the slit-shaped HF-probe. This consists of two very closely placed parallel electrodes: a transmitting electrode and a receiving electrode [19]. Except for the probe tip area, the electrodes are insulated from each other by a thin layer of metallic foil to prevent interference. The technical apparatus and set-up of the micro-electrode system are described in detail in Schinker et al. [19]. In this study HFdensitometry patterns are obtained using five different purpose-built HF-probes. Length (L) of the electrodes range from 150 lm to 4900 lm and width (d) between the electrodes from 34 lm to 300 lm (Table 1).
The product of the length and width of the electrode system defines the integration area (A) of a HF-probe, and is therefore a quantitative expression of a respective lateral resolution. HF-probes are mounted to a spring loaded device on a special tube which fits in the holders for the objective lenses of the revolving nosepiece of a CARL-ZEISS-microscope (Fig. 1). The wood sample is placed on the microscope stage which includes actuators to position the sample under the microscope optical axis. These holding the sample with a defined preload in a close contact with the measuring slit of the HF-probe. The transmitting electrode is connected to the high-frequency oscillator and induces a continuous electric stray field with an excitation frequency of 10 MHz. Depending on integration area (A) of the HF-probe, the field propagates through a certain volume of the wood sample. The wood structure produces a backscatter effect on the electric stray field, which is detected by the receiving electrode. This gives an indication of the variations in permittivity of the material and thus of the variations in wood density. With an assumed constant cell wall density the ratio of cell wall to cell lumina determines the relative dielectric constant (er) and thus the strength of the measured signal. The variation of the relative dielectric constant in wood structures is correlated with density variations in such a way that the input voltage at the receiving electrode increases with increasing backscatter effect due to greater wood density. The receiving electrode is connected to a highfrequency receiver with precise linear amplifying and rectifying characteristics to transfer the very low input signal (lV) to a higher HF-output signal (V). Iterative adjustment of the amplification factor is required for accurate measurements. For an appropriately calibrated measuring system the amplified HF-output signal varies depending on the HF-probe used, in a range of 1–10 V. By trimming the offset of the linear amplifier the´ zero point´ of the received amplified output voltage (1.001 V) is positioned in such a way that this value corresponds to the relative dielectric constant of air (eair = 1.00056). Accordingly the gain of the linear amplifier is set so that for close contact of the micro-electrode system with a reference medium the HF-output signal corresponds to the dielectric constant of that material. A polished ultrapure crystal of calcium fluoride with a known dielectric constant ðCaF2 ¼ 6:76Þ is used for this calibration step. For efficient description of the HF-probes concerning the respective resolution ability, it is important that the natural variability of the system (the noise component) is known. The noise component in this case is the sum of unwanted or disturbing random fluctuations in the HF-output signal generated by the electric devices of the system. Knowledge of the noise component is a necessary precondition for the separation of the actual signal from the noise level. Measurements on air for a time period of 1 s showed a normal distribution (Kolmogorof-Smirnoff test; P < 0.1) for the HF-output signal for all five probes. The maximum peak-to-peak amplitudes varied between 15 mV (type 5) and 37 mV (type 1), the standard deviation
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S. Boden et al. / Measurement 45 (2012) 1913–1921 Table 1 Set-up of different structured HF-probes and characteristic values concerning penetration depth.
*
HF-probe
Type 1
Type 2
Type 3
Type 4
Type 5
Length (L) Width (d) Integration area (A) d50% (Air) dMinimum (Air) d50% (PET) dMinimum (PET) Spatial resolution (PMMA) Spatial resolution (wood)
150 lm 34 lm 1.8 103 mm2 5.7 lm 39.1 lm 8.6 lm 24 lm 22 lm 1 lm
560 lm 34 lm 6.72 103 mm2 9.2 lm 38.7 lm 12.1 lm 49 lm 25 lm <10 lm*
680 lm 65 lm 13.6 103 mm2 11.9 lm 72.7 lm 15.4 lm 61.2 lm 40 lm <20 lm*
1320 lm 91 lm 43.6 103 mm2 14.6 lm 83.7 lm 19.8 lm 72 lm 69 lm <30 lm*
4920 lm 300 lm 492 103 mm2 28.2 lm 251.1 lm 67.2 lm 198 lm 87 lm <60 lm*
For HF-probe types 2–5 spatial resolution abilities for wood could only be estimated.
Fig. 1. HF-probe type 1 mounted to spring loaded device and linked to a Carl-Zeiss-microscope.
of the mean (1 V) varied between 2 mV (type 5) and 5 mV (type 1). The differences between the HF-probes are due to changing amplifying conditions according to the dimensions of the HF-probes. With fine adjustment of the high-frequency oscillator the noise component of the signal contributes only between 0.2% of the full scale range of the HF-output signal (1–10 V) for HF-probe type 5 and 0.4 % for HF-probe type 1. 2.2. Penetration depth We hypothesised that HF-probes vary in their specific penetration depth due to the differing dimensions of the micro-electrode measuring systems and that this was dependent on the integration area of the HF-probes. To test this hypothesis and potentially quantify the penetration depth of HF-probes two experimental approaches were established. In the first experiment a sharp-edged crack in a glass plate, which is covered by a stack of polyester films (Polyethylenterephthalate PET), was used as indicator to measure the penetration depth of the different HF-probes. This experiment is based on the question of what extent the respective HF-output signals still contain a sufficient signal based on the backscatter effect of the crack in the glass plate. In the context of wood measurements the crack under the layer of PET films may simulate a tree-ring
boundary hidden under a subsurface damage zone. With a relative dielectric constant of ePET = 3.2, PET-films are equivalent to a mid-range value of dielectric constant values for wood (ewood = 1 – 7) [22,29]. The measurements were conducted with identical calibration of the respective HF-probe, and the preload of the spring loaded device of the HF-probes was kept constant in order to exclude a possible influence by a changing contact pressure. The measuring slits were precisely oriented parallel to the edges of the crack during a measuring scan. After each measurement a new PET-film with precisely determined thickness was added to progressively increase the stack-height of the PET-films on top of the crack. The aim of this experiment was to measure the stack height of PET-films at which the HF-probes are still able to detect a considerable change in the relative dielectric constant regime. If the tip of a HF-probe is not in close contact with the medium to be investigated, the dielectric constant value of the surrounding air will affect the HF-output signal. To describe the influence of air a second experiment was conducted where the measuring slit of the HF-probe was kept in close contact with a polished glass cylinder. The glass cylinder had a surface roughness of about 0.001 lm. Due to the extremely thin gap between the surface of the glass cylinder and the measuring slit, consequently just the relative dielectric constant of glass (eglass = 7) was registered. Following this the distance (d) between the glass cylinder and the measuring slit was progressively extended by steps of 1 lm from close contact up to an air gap of several millimetres. The received amplified output voltage was then registered against the respective distance.
2.3. Resolution ability To determine the resolution ability of the different HF-probes regular spaces with known differences in their dielectric properties are needed. To create such a sequence, parallel V-grooves were milled into a smooth and flat plate of polymethylmetacrylat (PMMA) (Fig. 2). A variable spacing between the grooves of 3–200 lm allowed for comparison of the resolution abilities of the five HF-probes. The geometric shape of the V-grooves resulted from the tip geometry of the milling tool which was a sharp diamond with a 90° angle between the lateral flanks. The groove flanks were thus inclined to the plate
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Fig. 2. Experimental setup to determine resolution ability of HF-probes on diamond-machined PMMA-plate with 90°-shaped V-grooves. V-groove spacing (s): 3–200 lm; V-groove depth: s/2.
surfaces at 45°. The V-groove depth was set by tilting the PMMA plate on the milling slide and the depth was half of the width of the V-groove. A depth of more than half of the width of a V-groove would have been resulted in unusable V-grooves due to brittle removal of the material. The lifetime of the milling tool was sufficiently high to avoid a tearing effect on the flank-surfaces due to a decreasing sharpness of the diamond. The PMMA plate material had homogeneous dielectric properties with a relative dielectric constant of ePMMA = 2.9 (measured at 10 MHz). The dielectric contrast was determined between the air-filled V-grooves (relative dielectric constant eair = 1.00056) and the ridges between the V-grooves of the PMMA-plate. Scanning the V-grooved patterns perpendicular to the grooves obtained a sufficiently high dielectric contrast (dependent on groove depth and width) for the measurement of the resolution abilities. To test the resolution abilities of the HF-probes on an inhomogeneous material such as wood, measurements were conducted on radial profiles of Norway spruce (Picea abies (L.) Karst). The spruce samples were air-dried for several months under ambient laboratory conditions (wood moisture content 12%). Surface preparation with an ultra-precise diamond flycutter produced smooth surfaces with low roughness defects (deviations <1 lm) and low subsurface damage zones (thickness 2 lm) [30]. For repeated measurements with identical and different HF-probes the same track on the wood sample was strictly followed to ensure the comparability of several scans. To avoid increased deformation of the fragile wooden structure by shear forces the measuring speed was limited to 250 lm per second. The distance between each measuring point was set to 1 lm, which is the smallest step size of the control system of the microscope stage table. 3. Results 3.1. Penetration depth Patterns of the HF-output signal obtained by measurements of a crack in a glass plate hidden under layers of PET-films followed similar patterns of curve progression for the respective HF-probes: near the edge of the crack
Fig. 3. HF-probe type 1 output signals for measurements scans of a crack in a glass plate covered with different thicknesses of PET-films.
Fig. 4. Relationship between peak-to-peak amplitude of HF-output signals and stack heights of PET-films for HF-probe type 1.
the signal increased slightly and then fell steeply to a value lower than the beginning. Reaching the opposing edge of the crack the signal increased in a reverse of the prior pattern (Fig. 3). In all cases the signal dropped steeply to a global minimum and passed through a more or less distinctive area of low values. However, with increasing stack height of PET-films the range of the HF-output signal was reduced and appeared to asymptotically approach a final value. For the HF probe type 1 the influence of the crack (or more precisely, the air in the crack) was barely apparent with a 24 lm thick PET film, only the slight over- and undershooting of the signal on the edges of the crack gave an indication on the crack position (Fig. 3). The over- and
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undershooting of the HF-output signal near the edges of the crack was more or less pronounced for all HF-probes and all combinations of PET-films. Fig. 4 shows that the peak-to-peak amplitude, a measure for the change between the highest and lowest amplitude values, for the HF-probe 1 output signal decreased to a value of almost zero with increasing PET-stack height. For each of the HF-probes it was possible to define the characteristic point (Uminimum), where a spatial limit (dMinimum in PET) for the penetration depth is reached. From this point onward the backscatter effect and thus the input signal at the receiving electrode was not sufficient to determine changes in the dielectric constant regime in the HF-output signal. With increasing dimensions of the measuring system (width and length of the HF-probes) Uminimum was reached at respective higher values fordMinimum (Table 1). With respect to the reliability of the HF-densitometry measuring results, a further characteristic point was defined. The mid-range value for the measured peak-to peak-amplitudes (U50% at d50%) was determined through linear interpolation of the measuring points. This mid-range value was useful for setting the optimal operating conditions for the different HF-probes. For this halfpoint of the penetration depth the respective backscatter effect of the various HF-probes was strong enough to distinguish between variations in wood density. Values for d50% increased with increasing measuring system dimensions of the HF-probes (Table 1). For measurements on the influence of air the HF-output signals for the different HF-probes were also similar in their curve progression. Beginning from an initial maximum value with close contact of the probe tip with the glass cylinder, the curves fell steeply with an increasing air gap (near field zone), passed through a shallow minimum and rose towards a constant value (far field zone), which equals the relative dielectric constant of air (Fig. 5). The lowest signal (Uminimum) was obtained for a different air gap (dMinimum) for different HF-probes. The midrange value (U50%) of the HF-output signal was reached simultaneously for respective HF-probes at different wide air gaps (d50%). Values for dMinimum and d50% in air for the different HF-probes increased with increasing dimensions of the measuring system, except for the dMinimum for HFprobes types 1 and 2 (Table 1). The measuring system of these two HF-probes has the same width (34 lm), but differ in their length (150 lm and 340 lm respectively). It appears that this difference in dimensions is linked to a more rapid decrease of the HF-output signal for HF-probe type 1 and a respective lower value for d50%. 3.2. Resolution ability Characteristic patterns of the HF-output signal were used to determine the resolution ability of each HF-probe on scans parallel to the V-grooves on the PMMA-plate with increasing groove spacing and groove depth. The peak-topeak amplitudes of the HF-output signal increased continuously with increasing measurement duration, due to the increasing width and depth of the V-grooves (Fig. 6).
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Fig. 5. HF-output signal for measurements on glass and air for HF-probe type 4.
Fig. 6. HF-output signal for measurements of V-grooves on a PMMA-plate for HF-probe type 1.
Fig. 7. Detail of HF-output signal for measurements of V-grooves on a PMMA-plate for HF-probe type 1.
In the early stage up to a measuring length of about 4000 lm the peak-to-peak amplitudes showed a uniform pattern and were in the range of the natural noise component (10–20 mV). From this point on, which was in the area of 20 lm wide V-grooves, the peak-to-peak amplitude started to change beyond the natural noise component. In Fig. 7, which illustrates better this area of changing peak-to-peak amplitudes, it is clearly visible that between each major peak in the HF-output signal there was an average distance of about 22 lm and 23 lm. It seems that for a width of V-grooves which were below the resolution for HF-probe type 1 only the uniform pattern of the natural noise component was visible. For widths of V-grooves within the resolution abilities, which
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Fig. 8. Comparison of HF-output signal pattern of the first two scans and the third scan for HF-probe type 1. Measurements were conducted on same track on Norway spruce sample, figure shows first earlywood cells of an annual growth ring.
starts for HF-probe 1 at about 20 lm, the arrangement of the V-grooves was mapped in increasing peak-to-peak amplitudes (>20 mV). This experimental approach was conducted for all HF-probes, Table 1 includes the resolution limits for the HF-probes on the material PMMA. For testing the resolution abilities on an inhomogeneous material such as wood, consecutive replicate scans with identical HF-probes were conducted on a single Norway spruce sample. The HF-output signals showed a systematic shift of the curve progression for the HF-probes types 1–5. Fig. 8 shows an example of the first three measurement scans conducted on the same track over a section of earlywood cells of Norway spruce. The pattern for the first was smoother in its curve progression, while the pattern for scans two and three showed more stepwise curves. The displacement of the pattern in the radial direction took place primarily after the first scan. The shifts between scan two to three however were less pronounced and in the range of a few micrometer. Due to the very small integration area of the HF-probe type 1 the geometric structures of earlywood cells of Norway spruce were indicated by stepwise profiles in the HF-output signal pattern (Fig. 9). The flanks of these rectangular curves rose or fell within the 1 lm step size of the control system of the microscope stage table. This ‘slope‘ characterises the ability to distinguish different cell structures up to a resolution of 1 lm. Some parts of the pattern showed however deviations
Fig. 9. HF-output signal for scan 5 with HF-probe type 1. Same track was followed as in the previous figure (Fig. 7).
Fig. 10. Detail of Norway spruce sample in the area of earlywood tracheids after the growth ring boundary. Red bar indicates the integration area of the HF-probe type 1. The integration of HF-probe type 1 over 3–4 cell rows explains the stairs in the output signal (Fig. 9). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
from rectangular curves towards curves with distinct peaks. This can be explained by considering that due to cell dimensions and the dimensions of the HF-probe a number of cell rows are sampled simultaneously (Fig. 10). Sometimes cells are non-collinear due to cell growth differences, which results in cliffs or stairs in the HF-output signal. Comparison of measurements derived from the same growth ring of a Norway spruce sample with various HFprobes revealed some differences in the curve progression of the respective HF-output signals. Common for all types of HF-probes was that on an intra-annual scale the signal rose more or less asymptotical to a peak value near the year ring boundary with a following decline to a minimum. The patterns varied however in terms of signal variability and the magnitude of the HF-output signal. HF-probe types 1 and 2 showed highly oscillating signals, whereas with increasing length (L) and width (d) of the measuring slit (increasing integration area and lateral resolution), the patterns tended to be smoothed (Fig. 11). The greater the lateral resolution, the less pronounced are changes in the ratio of cell wall to lumen for a particular integration area and thus the precision of the identification of geometric cell structures.
Fig. 11. HF-output signals for HF-probes types 1, 3, and 5. Measurements were conducted on the same track on a Norway spruce sample. Output signals are normalised to simplify the comparison.
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4. Discussion 4.1. Penetration depth The assumption that penetration depth is related to the geometrical dimensions of the micro-electrode measuring system was confirmed by the fact that the characteristic points (Uminimum and U50%) in the curve progression of the HF-output signals obtained by measurements of a crack in a glass plate hidden under PET-films and of further experiments on the influence of air were found at different spatial distances (dMinimum and d50%). However, if the penetration depth and thus those spatial limits are completely stable along the scanned track, or if the penetration depth varies in areas with low and high density could not be conclusively answered by the experiments. The propagation and the penetration depth of the electric stray field in a wood sample are comparable to those in the material PET. The relative dielectric constant of PET (ePET = 3.2) is a mid-range value of various wood species [22,29], therefore it can be assumed that, depending on the type of HF-probe and the investigated species, more or less comparable spatial limits concerning the penetration depth can be found for wood. HF-probes are not able to optimally reproduce variations in the dielectric properties in a wood sample beyond those limits. For measurements on wood the HF-output signal is strongly influenced by the anatomical wood conditions in contrast to the geometric structure of the respective electrode system. Depending on the HF-probe used the relatively small size of the penetration depth, indicated by the very small values for dMinimum and d50% in PET, implies that wood surfaces with the smallest possible destruction zone are favourable. For accurate measurements the thickness of the subsurface destruction zone due to sample preparation should be small enough that the electric field may extend far into the undamaged wood structure. For optimal operating conditions therefore we suggest that the half-point of the penetration depth (d50%) should be considered as the practical limit of measurement. An advantage of the HF-probes with larger lateral resolution (types 4 and 5) is their larger penetration depth, which even for an assumed thicker subsurface damage zone makes intra-annual density alterations in the wood sample visible. The over- and undershooting of the HF-output signal obtained by measurements of a crack in a glass plate were probably due to a partial reflection of the lobe side of the electric field on the geometrically sharp interface between glass (eglass = 7) and air (eair = 0.00056). For wood samples the over- and undershooting was lower due to less sharp interface between latewood and earlywood. However, for the extreme values of the HF-output signal the over- and undershooting represented a source of error. For determination of such parameters a correction factor has to be considered. In some cases a graphical smoothing may be sufficient. Measurements on air showed furthermore the importance of keeping the probe tip in close contact to the medium under investigation. The midrange values of the
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HF-output signal were reached for the different HF-probes at even lower values for d50%, which indicates the risk of an increasing weakening of the backscatter effect of a particular medium due to the influence of air, which results from a poor setup of the HF-probe tip on the sample. 4.2. Resolution ability HF-densitometry scans along the surface of isotropic materials like glass or PET with homogeneous microscopic structures result in constant HF-output signals. The signal is solely dependent on the relative dielectric constant of the material. For structured, inhomogeneous and anisotropic materials however the signal is also affected by small-scale physical and chemical composition differences within the material [29]. V-grooves on a PMMA-plate gave a sufficiently fine sequence of dielectric varieties for the determination of the resolution abilities of the HF-probes. For each HF-probe the resolution abilities could be defined in a more or less conservative way according to their respective passing of the starting point of the increase in the peak-to-peak amplitudes in the HF-output signal on PMMA. For measurements on the inhomogeneous material wood, HFprobe type 1 showed higher resolution abilities with the ability to represent density variations within the control table step size of 1 lm. For this type of HF-probe cells of Norway spruce were well suited to the analysis of the resolution abilities as they have more or less continuous regular, sharp-edged dielectric structures. For the other HFprobe types the representation of cell structures is below their resolution abilities, therefore their resolution abilities for measurements on wood could only be estimated (Table 1). The clear signal changes associated with the flanks of non-collinearity cell rows compared to the changes associated with the flanks of the V-grooves at 45° allows a better determination of the actual resolution abilities, at least for the smallest HF-probe type 1. It can be assumed that the resolution abilities of the HFprobes are determined by size and shape of the induced electric field in front of the transmitting electrode, the sensitivity of the receiving electrode, and by the signal-tonoise ratio of the detector system. For determination of the resolution abilities of the HF-probes, the natural noise component of the measured HF-output signal must be known in order to separate the noise from the signal. One method of improving the resolution of HF-densitometry is to increase the signal-to-noise ratio of the system. With fine adjustment of the high-frequency oscillator a very high signal-to-noise ratio can be attained, the natural noise component contributes less than 0.5% of the full scale range of the HF-output signal (1–10 V) for all HF-probes. For determination of ring boundaries or transition points between earlywood and latewood such a low noise component is irrelevant, but for a possible representation of cellular structures this noise component must be considered. If the differences in represented deviations in dielectric properties are in a range of a few microvolt at the receiving electrode, the noise may prohibit the determination of cell structure parameters.
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Another possibility for improving the resolution ability is an effective reduction in the sample-to-probe distance. Results of measurements on wood samples presented here were obtained on samples prepared with an ultra-precise diamond flycutter [30]. Such surfaces are characterised by an extremely high flatness and smoothness, as well as a very shallow depth of subsurface destruction. The importance of surface preparation for the most precise measurements of HF-densitometry were illustrated by the different HF-output signal characteristics of successive scans on the same track on a Norway spruce sample. Systematic shifts of the HF-patterns due to cut-off of the tip of the HF-probes were in particular investigated for the first scan. For further scans the differences were negligible. The sensitivity to air gaps of the HF-probes ranged less than 1 lm, therefore the first scan on a wood sample was used for conditioning of the measuring track for the later scans. Even on samples prepared with the diamond flycutter at least two scans should be conducted for very high contact accuracy due to an effective reduction of sample-to-probe distance. A combination of surface preparation conducted with a diamond flycutter and preconditioning of the measuring track gave an optimal sample-to-probe distance. Compared to the required sample preparation steps for measurements with other densitometry methods [18,31,32], sample preparation procedure for HF-densitometry requires special equipment (fly-cutter), but is less time consuming and not limited to sample dimensions. The spatial extent of the electric field, the quantity of the electric field and the resulting detection of the signal along the propagation directions (x-, y-, z-direction) are critical parameters for the resolution abilities of the HFprobes. For questions concerning cell structure analysis on wood samples or precise localisation of tree-ring boundaries HF-probes with a high resolution are preferable. A high resolution is a prerequisite for the discrete representation of natural structures up to the cellular level. The results of the measurements on Norway spruce with the HF-probe type 1 showed that with this probe it is possible to distinguish different structures up to a resolution of 1 lm. Brändström [33] evaluated various studies on tracheid dimensions of Norway spruce and gives a mean radial tracheid wall thickness of 3.52 lm (earlywood) to 6.23 lm (latewood). Mean radial tracheid diameter varies from 39.3 lm (earlywood) to 13.1 lm (latewood). Thus, the HF-output signal pattern of HF-probe type 1 gave a good representation of the tracheid microstructure of Norway spruce. However, the resolution ability of the HF-probes is only adequate if the data distance (in this study 1 lm) is smaller than the smallest dissolved structure. The accuracy of the representation of the cell structure is considerably affected by the collinearity of the cell structures. For the possible replacement of currently used optical measurement techniques for cell structure analysis, especially for species with strong cell collinearity, further investigations will be necessary. HF-probes with a larger penetration depth and spatial extent of the measuring range lose some resolution potentials and are less able to accurately reproduce cell structures. However, with these HF-probes more robust intra-
annual density variations may be made visible without complex mathematical post-processing. 5. Conclusions The High-Frequency densitometry allows non-destructive measurements of wood density variations at various resolution levels. With this method it is possible to determine wood density values at a desired resolution level to get more robust signals of intra-annual density variation which gives great potential for further investigations on the estimation of climate variability within a growing season on intra-annual wood density patterns. Moreover it is possible to distinguish different cell structures due to their variations in wood density up to a resolution of 1 lm. The method allows a cost and time effective measurement procedure of large data sets at any resolution level. The penetration depth of the electric stray field is related to the geometrical dimensions of the micro-electrode measuring system. For accurate measurements the thickness of the subsurface destruction zone of a wood sample due to sample preparation should be small enough that the electric field may extend far into the undamaged wood structure. Therefore an intensive sample preparation is needed, which may be an application limit in praxis for interested researchers. Acknowledgements The authors are grateful for language revision by Christopher Eastaugh and technical comments from two anonymous reviewers, all of which served to greatly improve the paper. References [1] E. Mork, Die Qualität des Fichtenholzes unter besonderer Rücksichtnahme auf Schleif- und Papierholz, Der Papier-Fabrikant 26 (1928) 741–747. [2] F. Kollmann, Technologie des Holzes und der Holzwerkstoffe, Band 2, Springer, Berlin, Heidelberg, 1955. [3] R. Trendelenburg, H. Mayer-Wegelin, Das Holz als Roh- und Werkstoff, Carl Hanser Verlag, München, 1955. [4] B.J. Zobel, J.P. Van Buijtenen, Wood Variation – Its Causes and Control, Springer, Berlin, 1989. [5] P. Fearnside, Wood density for estimating forest biomass in Brazilian Amazonia, Forest. Ecol. Manag. 90 (1997) 59–87. [6] D. Fengel, G. Wegener, Wood: Chemistry, Ultrastructure, Reactions, Reprint Kessel, Remagen, 2003. [7] E.M. Nogueira, B.W. Nelson, P.M. Fearnside, Wood density in dense forest in central Amazonia, Brazil, Forest Ecol. Manag. 208 (1995) 261–286. [8] H. Polge, Une nouvelle méthode de détermination de la texture du bois: l’analyse densitométrique de clichés radiographiques, Ann. Sci. Forest. 20 (1963) 533–580. [9] H. Polge, Etablissement des courbes de variation de la densité du bois par exploration densitométrique de radiographies d’échantillons prélèves a la tarière sur des arbres vivants. Applications dans les domaines Technologique et Physiologique, Ann. Sci. Forest. 23 (1966) 1–187. [10] F.H. Schweingruber, H.C. Fritts, O.U. Bräker, L.G. Drew, E. Schär, The X-ray technique as applied to dendroclimatology, Tree-Ring Bull. 38 (1978) 61–91. [11] D.J. Cown, B.C. Clement, A wood densitometer using direct scanning with X-rays, Wood Sci. Technol. 17 (1983) 91–99. [12] M. Koch, S. Hunsche, P. Schuacher, M.C. Nuss, J. Feldmann, J. Fromm, THz-imaging: a new method for density mapping of wood, Wood Sci. Technol. 32 (1998) 421–427.
S. Boden et al. / Measurement 45 (2012) 1913–1921 [13] P. Hoffmeyer, J.G. Pedersen, Evaluation of density and strength of Norway spruce wood by near infrared reflectance spectroscopy, Holz als Roh- und Werkstoff 53 (1995) 165–170. [14] J. Johansson, O. Hagman, B.A. Fjellner, Predicting moisture content and density distribution of Scots pine by microwave scanning of sawn timber, J. Wood Sci. 49 (2003) 312–316. [15] D. McCarroll, E. Pettigrew, A. Luckman, Blue reflectance provides a surrogate for latewood density of high-latitude pine tree-rings, Arct. Antarct. Alp. Res. 34 (2002) 450–453. [16] F.H. Schweingruber, Der Jahrring. Standort, Methodik, Zeit und Klima in der Dendrochronologie, Paul Haupt, Bern, 1983. [17] S. Hunsche, M. Koch, I. Brener, M.C. Nuss, THz near field imaging, Opt. Comm. 150 (1998) 22–26. [18] F. Babst, D. Frank, U. Büntgen, D. Nievergelt, J. Esper, Effect of sample preparation and scanning resolution on the Blue Reflectance of Picea abies, in: R. Kaczka, I. Malik, P. Owczarek, H. Gärtner, G. Helle, I. Heinrich (Eds.), TRACE – Tree Rings in Archeology, Climatology and Ecology, Scientific Technical Report STR 09/03, vol. 7, GFZ Postdam, Potsdam, 2009, pp. 188–195. [19] M.G. Schinker, N. Hansen, H. Spiecker, High-frequency densitometry – a new method for the rapid evaluation of wood density variations, IAWA J. 24 (3) (2003) 231–239. [20] E. Peyskens, M. De Pourcq, M. Stevens, J. Schalck, Dielectric properties of softwood species at microwave frequencies, Wood Sci. Technol. 18 (1984) 267–280. [21] W. Trapp, L. Pungs, Einfluß von Temperatur und Feuchte auf das dielektrische Verhalten von Naturholz im großen Frequenzbereich, Holzforschung 5 (1956) 144–150. [22] G.I. Torgovnikov, Dielektrische Eigenschaften von absolut trockenem Holz und Holzstoff, Holztechnologie 30 (1990) 9–11. [23] L. Hansson, N. Lundgren, A.L. Antti, O. Hagman, Finite element modeling (FEM) simulation of interactions between wood and microwaves, J. Wood Sci. 52 (2006) 406–410.
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[24] G.I. Torgovnikov, Dielectric properties of wood and wood-based materials, Springer, Berlin, Heidelberg, New York, 1993. [25] V. de Micco, M. Saurer, G. Aronne, R. Tognetti, P. Cherubini, Variations of wood anatomy and d13C within-tree rings of coastal Pinus Pinaster showing intra-annual density fluctuations, IAWA J. 28 (2007) 61–74. [26] R. Wimmer, G. Strumia, F. Holawe, Use of false rings in Austrian pine to reconstruct early growing season precipitation, Can. J. For. Res. 30 (2000) 1691–1697. [27] A. Rigling, P.O. Waldner, T. Forster, O.U. Bräker, A. Pouttu, Ecological interpretation of tree-ring width and intraannual density fluctuations in Pinus sylvestris on dry sites in the central Alps and Siberia, Can. J. For. Res. 31 (2001) 18–31. [28] O. Bouriaud, J.-M. Leban, D. Bert, C. Deleuze, Intra-annual variations in climate influence growth and wood density of Norway spruce, Tree Physiol. 25 (2005) 651–660. [29] W. Simpson, A. TenWolde, Physical properties and moisture relations of wood, In: Wood Handbook – Wood as an engineering material, Gen. Tech. Rep., US. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, Wisconsin, 1999, p. 463. [30] H. Spiecker, M.G. Schinker, J. Hansen, Y.I. Park, T. Ebding, W. Döll, Cell structure in tree-rings: novel methods for preparation and image analysis of large cross sections, IAWA J. 21 (3) (2000) 361– 373. [31] F.H. Schweingruber, Tree Rings: Basics and Applications of Dendrochronolgy, D. Reidel Publishing Company, Dordrecht, The Netherlands, 1988. [32] F.H. Schweingruber, Tree-Ring Morphology, Paul Haupt Publishers, Bern, Switzerland, 1996. [33] J. Brändström, Micro- and ultrastructural aspects of Norway spruce tracheids: a review, IAWA J. 22 (4) (2001) 333–353.
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Resolution abilities and measuring depth of High-Frequency densitometry on wood samples Simon Boden ⇑, Martin G. Schinker, Philipp Duncker, Heinrich Spiecker Albert-Ludwigs-Universität Freiburg, Institut für Waldwachstum, Tennenbacher Straße 4, 79106 Freiburg, Germany
a r t i c l e
i n f o
Article history: Received 3 September 2011 Received in revised form 17 January 2012 Accepted 17 March 2012 Available online 16 April 2012 Keywords: Dielectric property High-Frequency (HF) densitometry Penetration depth Resolution ability Surface preparation Wood density
a b s t r a c t The High-Frequency (HF) densitometry measures relative density variations on wood samples utilizing the dielectric properties in wood. This method is based on the propagation of an electric stray field through the surface-near region of a wood sample. We studied experimentally the penetration depth and the differentiation abilities of the HF-densitometry method on wood samples. Two experimental approaches showed that penetration depth is related to the geometrical dimensions of the micro-electrode measuring system. Characteristic patterns of the HF-output signal were used to determine the resolution abilities of each HF-probe. Due to a very small integration area geometric structures of earlywood cells of Norway spruce were indicated by stepwise profiles in the HF-output signal pattern. With the High-Frequency densitometry it was possible to distinguish different cell structures due to their variations in wood density up to a resolution of 1 lm. In addition it was possible to determine wood density variations at various resolution levels. Based on the respective resolution abilities and penetration depths of five different purpose-built HFprobes we show the optimal operating conditions for measurements on wood of this indirect densitometry method. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Wood density has a direct or indirect influence on various physical and mechanical wood properties and is a well known estimator of wood quality [1–4]. Wood density may be determined in a variety of ways [5]. Relatively easy and without intensive sample preparation are ‘basic’ wood density, calculated as oven-dry weight of a wood sample divided by the volume in a water saturated state, and ‘apparent’ wood density, calculated as weight divided by volume at a given moisture content [3,5–7]. Variations in wood density at high resolution are however not so easy to determine. Such measurements require a considerable experimental effort in suitable preparation and measurement methods due to specific growth-related, nonhomogeneous anatomical cell structures of the sample and the underlying level of abstraction in the study. ⇑ Corresponding author. Tel.: +49 761 2033737; fax: +49 761 2033740. E-mail address: [email protected] (S. Boden). 0263-2241/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.measurement.2012.03.013
Various methods to measure variations in wood density for example such as X-ray densitometry [8–11], transmission measurements in the far-infrared range [12] and near infrared reflectance spectroscopy [13], microwave scanning [14] or scanning of colour images using flatbed scanner [15] have been developed over the last decades. Those methods are characterised by different resolution abilities [16–18]. An alternative method is the fast and non-destructive High-Frequency (HF) densitometry. This method relies on the dielectric properties of wood, the close relationship between wood density (q) and the relative dielectric constant of wood (er). The method is based on the propagation of a continuous electric stray field in a high-frequency transmitter–receiver link of an extremely small electrode system, which is in direct contact with the investigated wood surface [19]. The relative dielectric constant (er), also described as permittivity of a material, determines the interaction between a material such as wood and an alternating electric field [20]. For measurements on wood the
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relative dielectric constant depends on species, excitation frequency of the electric field, temperature, moisture content, density and the orientation of wood fibres with respect to the direction of the electric field [21–23]. At given air temperature, moisture content, orientation of wood fibres and excitation frequency the wood density is almost linear related to the dielectric constant [22,24]. An important aspect for the optimal use of HFdensitometry for measurements on wood samples is the ability of differently structured HF-probes to identify small geometric cell structures that are characterised by a considerable deviation in wood density. This differentiation ability is referred to as resolution ability. An indication of the resolution ability requires knowledge of the penetration depth of the respective electric stray field. The penetration depth is defined as the measure of how deep the electric field can penetrate into a certain material. Although for measurements on wood samples the length of a tracheid cell is potentially infinite relatively to the penetration depth, knowledge of the penetration depth is important for the determination of the optimal operating conditions for the HF-densitometry method and a previously required surface preparation of the wood sample. In this study we systematically analyse the differentiation abilities of the HF-densitometry method on wood samples. Schinker et al. [19] showed that it is possible to determine variations of wood density with the HF-densitometry. The first objective of our study is to investigate if it is possible to determine wood density variations at various resolution levels. The second objective is if it is possible to identify geometric structures of individual cells with the HF-densitometry. Several authors mentioned that climate fluctuations were reflected in intra-annual wood density profiles [25–28]. Taking into account the respective study material and the aim of a potential study, the ability to determine wood density at various resolution levels is indispensable to get wood density values given in appropriate resolution for estimating climate variability within a growing season on intra-annual wood density patterns. Based on the respective resolution abilities and penetration depths we show the optimal operating conditions for measurements on wood of this indirect densitometry method.
2. Materials and methods 2.1. Adjustment and calibration The micro-electrode measuring system is the main part of the slit-shaped HF-probe. This consists of two very closely placed parallel electrodes: a transmitting electrode and a receiving electrode [19]. Except for the probe tip area, the electrodes are insulated from each other by a thin layer of metallic foil to prevent interference. The technical apparatus and set-up of the micro-electrode system are described in detail in Schinker et al. [19]. In this study HFdensitometry patterns are obtained using five different purpose-built HF-probes. Length (L) of the electrodes range from 150 lm to 4900 lm and width (d) between the electrodes from 34 lm to 300 lm (Table 1).
The product of the length and width of the electrode system defines the integration area (A) of a HF-probe, and is therefore a quantitative expression of a respective lateral resolution. HF-probes are mounted to a spring loaded device on a special tube which fits in the holders for the objective lenses of the revolving nosepiece of a CARL-ZEISS-microscope (Fig. 1). The wood sample is placed on the microscope stage which includes actuators to position the sample under the microscope optical axis. These holding the sample with a defined preload in a close contact with the measuring slit of the HF-probe. The transmitting electrode is connected to the high-frequency oscillator and induces a continuous electric stray field with an excitation frequency of 10 MHz. Depending on integration area (A) of the HF-probe, the field propagates through a certain volume of the wood sample. The wood structure produces a backscatter effect on the electric stray field, which is detected by the receiving electrode. This gives an indication of the variations in permittivity of the material and thus of the variations in wood density. With an assumed constant cell wall density the ratio of cell wall to cell lumina determines the relative dielectric constant (er) and thus the strength of the measured signal. The variation of the relative dielectric constant in wood structures is correlated with density variations in such a way that the input voltage at the receiving electrode increases with increasing backscatter effect due to greater wood density. The receiving electrode is connected to a highfrequency receiver with precise linear amplifying and rectifying characteristics to transfer the very low input signal (lV) to a higher HF-output signal (V). Iterative adjustment of the amplification factor is required for accurate measurements. For an appropriately calibrated measuring system the amplified HF-output signal varies depending on the HF-probe used, in a range of 1–10 V. By trimming the offset of the linear amplifier the´ zero point´ of the received amplified output voltage (1.001 V) is positioned in such a way that this value corresponds to the relative dielectric constant of air (eair = 1.00056). Accordingly the gain of the linear amplifier is set so that for close contact of the micro-electrode system with a reference medium the HF-output signal corresponds to the dielectric constant of that material. A polished ultrapure crystal of calcium fluoride with a known dielectric constant ðCaF2 ¼ 6:76Þ is used for this calibration step. For efficient description of the HF-probes concerning the respective resolution ability, it is important that the natural variability of the system (the noise component) is known. The noise component in this case is the sum of unwanted or disturbing random fluctuations in the HF-output signal generated by the electric devices of the system. Knowledge of the noise component is a necessary precondition for the separation of the actual signal from the noise level. Measurements on air for a time period of 1 s showed a normal distribution (Kolmogorof-Smirnoff test; P < 0.1) for the HF-output signal for all five probes. The maximum peak-to-peak amplitudes varied between 15 mV (type 5) and 37 mV (type 1), the standard deviation
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S. Boden et al. / Measurement 45 (2012) 1913–1921 Table 1 Set-up of different structured HF-probes and characteristic values concerning penetration depth.
*
HF-probe
Type 1
Type 2
Type 3
Type 4
Type 5
Length (L) Width (d) Integration area (A) d50% (Air) dMinimum (Air) d50% (PET) dMinimum (PET) Spatial resolution (PMMA) Spatial resolution (wood)
150 lm 34 lm 1.8 103 mm2 5.7 lm 39.1 lm 8.6 lm 24 lm 22 lm 1 lm
560 lm 34 lm 6.72 103 mm2 9.2 lm 38.7 lm 12.1 lm 49 lm 25 lm <10 lm*
680 lm 65 lm 13.6 103 mm2 11.9 lm 72.7 lm 15.4 lm 61.2 lm 40 lm <20 lm*
1320 lm 91 lm 43.6 103 mm2 14.6 lm 83.7 lm 19.8 lm 72 lm 69 lm <30 lm*
4920 lm 300 lm 492 103 mm2 28.2 lm 251.1 lm 67.2 lm 198 lm 87 lm <60 lm*
For HF-probe types 2–5 spatial resolution abilities for wood could only be estimated.
Fig. 1. HF-probe type 1 mounted to spring loaded device and linked to a Carl-Zeiss-microscope.
of the mean (1 V) varied between 2 mV (type 5) and 5 mV (type 1). The differences between the HF-probes are due to changing amplifying conditions according to the dimensions of the HF-probes. With fine adjustment of the high-frequency oscillator the noise component of the signal contributes only between 0.2% of the full scale range of the HF-output signal (1–10 V) for HF-probe type 5 and 0.4 % for HF-probe type 1. 2.2. Penetration depth We hypothesised that HF-probes vary in their specific penetration depth due to the differing dimensions of the micro-electrode measuring systems and that this was dependent on the integration area of the HF-probes. To test this hypothesis and potentially quantify the penetration depth of HF-probes two experimental approaches were established. In the first experiment a sharp-edged crack in a glass plate, which is covered by a stack of polyester films (Polyethylenterephthalate PET), was used as indicator to measure the penetration depth of the different HF-probes. This experiment is based on the question of what extent the respective HF-output signals still contain a sufficient signal based on the backscatter effect of the crack in the glass plate. In the context of wood measurements the crack under the layer of PET films may simulate a tree-ring
boundary hidden under a subsurface damage zone. With a relative dielectric constant of ePET = 3.2, PET-films are equivalent to a mid-range value of dielectric constant values for wood (ewood = 1 – 7) [22,29]. The measurements were conducted with identical calibration of the respective HF-probe, and the preload of the spring loaded device of the HF-probes was kept constant in order to exclude a possible influence by a changing contact pressure. The measuring slits were precisely oriented parallel to the edges of the crack during a measuring scan. After each measurement a new PET-film with precisely determined thickness was added to progressively increase the stack-height of the PET-films on top of the crack. The aim of this experiment was to measure the stack height of PET-films at which the HF-probes are still able to detect a considerable change in the relative dielectric constant regime. If the tip of a HF-probe is not in close contact with the medium to be investigated, the dielectric constant value of the surrounding air will affect the HF-output signal. To describe the influence of air a second experiment was conducted where the measuring slit of the HF-probe was kept in close contact with a polished glass cylinder. The glass cylinder had a surface roughness of about 0.001 lm. Due to the extremely thin gap between the surface of the glass cylinder and the measuring slit, consequently just the relative dielectric constant of glass (eglass = 7) was registered. Following this the distance (d) between the glass cylinder and the measuring slit was progressively extended by steps of 1 lm from close contact up to an air gap of several millimetres. The received amplified output voltage was then registered against the respective distance.
2.3. Resolution ability To determine the resolution ability of the different HF-probes regular spaces with known differences in their dielectric properties are needed. To create such a sequence, parallel V-grooves were milled into a smooth and flat plate of polymethylmetacrylat (PMMA) (Fig. 2). A variable spacing between the grooves of 3–200 lm allowed for comparison of the resolution abilities of the five HF-probes. The geometric shape of the V-grooves resulted from the tip geometry of the milling tool which was a sharp diamond with a 90° angle between the lateral flanks. The groove flanks were thus inclined to the plate
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Fig. 2. Experimental setup to determine resolution ability of HF-probes on diamond-machined PMMA-plate with 90°-shaped V-grooves. V-groove spacing (s): 3–200 lm; V-groove depth: s/2.
surfaces at 45°. The V-groove depth was set by tilting the PMMA plate on the milling slide and the depth was half of the width of the V-groove. A depth of more than half of the width of a V-groove would have been resulted in unusable V-grooves due to brittle removal of the material. The lifetime of the milling tool was sufficiently high to avoid a tearing effect on the flank-surfaces due to a decreasing sharpness of the diamond. The PMMA plate material had homogeneous dielectric properties with a relative dielectric constant of ePMMA = 2.9 (measured at 10 MHz). The dielectric contrast was determined between the air-filled V-grooves (relative dielectric constant eair = 1.00056) and the ridges between the V-grooves of the PMMA-plate. Scanning the V-grooved patterns perpendicular to the grooves obtained a sufficiently high dielectric contrast (dependent on groove depth and width) for the measurement of the resolution abilities. To test the resolution abilities of the HF-probes on an inhomogeneous material such as wood, measurements were conducted on radial profiles of Norway spruce (Picea abies (L.) Karst). The spruce samples were air-dried for several months under ambient laboratory conditions (wood moisture content 12%). Surface preparation with an ultra-precise diamond flycutter produced smooth surfaces with low roughness defects (deviations <1 lm) and low subsurface damage zones (thickness 2 lm) [30]. For repeated measurements with identical and different HF-probes the same track on the wood sample was strictly followed to ensure the comparability of several scans. To avoid increased deformation of the fragile wooden structure by shear forces the measuring speed was limited to 250 lm per second. The distance between each measuring point was set to 1 lm, which is the smallest step size of the control system of the microscope stage table. 3. Results 3.1. Penetration depth Patterns of the HF-output signal obtained by measurements of a crack in a glass plate hidden under layers of PET-films followed similar patterns of curve progression for the respective HF-probes: near the edge of the crack
Fig. 3. HF-probe type 1 output signals for measurements scans of a crack in a glass plate covered with different thicknesses of PET-films.
Fig. 4. Relationship between peak-to-peak amplitude of HF-output signals and stack heights of PET-films for HF-probe type 1.
the signal increased slightly and then fell steeply to a value lower than the beginning. Reaching the opposing edge of the crack the signal increased in a reverse of the prior pattern (Fig. 3). In all cases the signal dropped steeply to a global minimum and passed through a more or less distinctive area of low values. However, with increasing stack height of PET-films the range of the HF-output signal was reduced and appeared to asymptotically approach a final value. For the HF probe type 1 the influence of the crack (or more precisely, the air in the crack) was barely apparent with a 24 lm thick PET film, only the slight over- and undershooting of the signal on the edges of the crack gave an indication on the crack position (Fig. 3). The over- and
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undershooting of the HF-output signal near the edges of the crack was more or less pronounced for all HF-probes and all combinations of PET-films. Fig. 4 shows that the peak-to-peak amplitude, a measure for the change between the highest and lowest amplitude values, for the HF-probe 1 output signal decreased to a value of almost zero with increasing PET-stack height. For each of the HF-probes it was possible to define the characteristic point (Uminimum), where a spatial limit (dMinimum in PET) for the penetration depth is reached. From this point onward the backscatter effect and thus the input signal at the receiving electrode was not sufficient to determine changes in the dielectric constant regime in the HF-output signal. With increasing dimensions of the measuring system (width and length of the HF-probes) Uminimum was reached at respective higher values fordMinimum (Table 1). With respect to the reliability of the HF-densitometry measuring results, a further characteristic point was defined. The mid-range value for the measured peak-to peak-amplitudes (U50% at d50%) was determined through linear interpolation of the measuring points. This mid-range value was useful for setting the optimal operating conditions for the different HF-probes. For this halfpoint of the penetration depth the respective backscatter effect of the various HF-probes was strong enough to distinguish between variations in wood density. Values for d50% increased with increasing measuring system dimensions of the HF-probes (Table 1). For measurements on the influence of air the HF-output signals for the different HF-probes were also similar in their curve progression. Beginning from an initial maximum value with close contact of the probe tip with the glass cylinder, the curves fell steeply with an increasing air gap (near field zone), passed through a shallow minimum and rose towards a constant value (far field zone), which equals the relative dielectric constant of air (Fig. 5). The lowest signal (Uminimum) was obtained for a different air gap (dMinimum) for different HF-probes. The midrange value (U50%) of the HF-output signal was reached simultaneously for respective HF-probes at different wide air gaps (d50%). Values for dMinimum and d50% in air for the different HF-probes increased with increasing dimensions of the measuring system, except for the dMinimum for HFprobes types 1 and 2 (Table 1). The measuring system of these two HF-probes has the same width (34 lm), but differ in their length (150 lm and 340 lm respectively). It appears that this difference in dimensions is linked to a more rapid decrease of the HF-output signal for HF-probe type 1 and a respective lower value for d50%. 3.2. Resolution ability Characteristic patterns of the HF-output signal were used to determine the resolution ability of each HF-probe on scans parallel to the V-grooves on the PMMA-plate with increasing groove spacing and groove depth. The peak-topeak amplitudes of the HF-output signal increased continuously with increasing measurement duration, due to the increasing width and depth of the V-grooves (Fig. 6).
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Fig. 5. HF-output signal for measurements on glass and air for HF-probe type 4.
Fig. 6. HF-output signal for measurements of V-grooves on a PMMA-plate for HF-probe type 1.
Fig. 7. Detail of HF-output signal for measurements of V-grooves on a PMMA-plate for HF-probe type 1.
In the early stage up to a measuring length of about 4000 lm the peak-to-peak amplitudes showed a uniform pattern and were in the range of the natural noise component (10–20 mV). From this point on, which was in the area of 20 lm wide V-grooves, the peak-to-peak amplitude started to change beyond the natural noise component. In Fig. 7, which illustrates better this area of changing peak-to-peak amplitudes, it is clearly visible that between each major peak in the HF-output signal there was an average distance of about 22 lm and 23 lm. It seems that for a width of V-grooves which were below the resolution for HF-probe type 1 only the uniform pattern of the natural noise component was visible. For widths of V-grooves within the resolution abilities, which
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Fig. 8. Comparison of HF-output signal pattern of the first two scans and the third scan for HF-probe type 1. Measurements were conducted on same track on Norway spruce sample, figure shows first earlywood cells of an annual growth ring.
starts for HF-probe 1 at about 20 lm, the arrangement of the V-grooves was mapped in increasing peak-to-peak amplitudes (>20 mV). This experimental approach was conducted for all HF-probes, Table 1 includes the resolution limits for the HF-probes on the material PMMA. For testing the resolution abilities on an inhomogeneous material such as wood, consecutive replicate scans with identical HF-probes were conducted on a single Norway spruce sample. The HF-output signals showed a systematic shift of the curve progression for the HF-probes types 1–5. Fig. 8 shows an example of the first three measurement scans conducted on the same track over a section of earlywood cells of Norway spruce. The pattern for the first was smoother in its curve progression, while the pattern for scans two and three showed more stepwise curves. The displacement of the pattern in the radial direction took place primarily after the first scan. The shifts between scan two to three however were less pronounced and in the range of a few micrometer. Due to the very small integration area of the HF-probe type 1 the geometric structures of earlywood cells of Norway spruce were indicated by stepwise profiles in the HF-output signal pattern (Fig. 9). The flanks of these rectangular curves rose or fell within the 1 lm step size of the control system of the microscope stage table. This ‘slope‘ characterises the ability to distinguish different cell structures up to a resolution of 1 lm. Some parts of the pattern showed however deviations
Fig. 9. HF-output signal for scan 5 with HF-probe type 1. Same track was followed as in the previous figure (Fig. 7).
Fig. 10. Detail of Norway spruce sample in the area of earlywood tracheids after the growth ring boundary. Red bar indicates the integration area of the HF-probe type 1. The integration of HF-probe type 1 over 3–4 cell rows explains the stairs in the output signal (Fig. 9). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
from rectangular curves towards curves with distinct peaks. This can be explained by considering that due to cell dimensions and the dimensions of the HF-probe a number of cell rows are sampled simultaneously (Fig. 10). Sometimes cells are non-collinear due to cell growth differences, which results in cliffs or stairs in the HF-output signal. Comparison of measurements derived from the same growth ring of a Norway spruce sample with various HFprobes revealed some differences in the curve progression of the respective HF-output signals. Common for all types of HF-probes was that on an intra-annual scale the signal rose more or less asymptotical to a peak value near the year ring boundary with a following decline to a minimum. The patterns varied however in terms of signal variability and the magnitude of the HF-output signal. HF-probe types 1 and 2 showed highly oscillating signals, whereas with increasing length (L) and width (d) of the measuring slit (increasing integration area and lateral resolution), the patterns tended to be smoothed (Fig. 11). The greater the lateral resolution, the less pronounced are changes in the ratio of cell wall to lumen for a particular integration area and thus the precision of the identification of geometric cell structures.
Fig. 11. HF-output signals for HF-probes types 1, 3, and 5. Measurements were conducted on the same track on a Norway spruce sample. Output signals are normalised to simplify the comparison.
S. Boden et al. / Measurement 45 (2012) 1913–1921
4. Discussion 4.1. Penetration depth The assumption that penetration depth is related to the geometrical dimensions of the micro-electrode measuring system was confirmed by the fact that the characteristic points (Uminimum and U50%) in the curve progression of the HF-output signals obtained by measurements of a crack in a glass plate hidden under PET-films and of further experiments on the influence of air were found at different spatial distances (dMinimum and d50%). However, if the penetration depth and thus those spatial limits are completely stable along the scanned track, or if the penetration depth varies in areas with low and high density could not be conclusively answered by the experiments. The propagation and the penetration depth of the electric stray field in a wood sample are comparable to those in the material PET. The relative dielectric constant of PET (ePET = 3.2) is a mid-range value of various wood species [22,29], therefore it can be assumed that, depending on the type of HF-probe and the investigated species, more or less comparable spatial limits concerning the penetration depth can be found for wood. HF-probes are not able to optimally reproduce variations in the dielectric properties in a wood sample beyond those limits. For measurements on wood the HF-output signal is strongly influenced by the anatomical wood conditions in contrast to the geometric structure of the respective electrode system. Depending on the HF-probe used the relatively small size of the penetration depth, indicated by the very small values for dMinimum and d50% in PET, implies that wood surfaces with the smallest possible destruction zone are favourable. For accurate measurements the thickness of the subsurface destruction zone due to sample preparation should be small enough that the electric field may extend far into the undamaged wood structure. For optimal operating conditions therefore we suggest that the half-point of the penetration depth (d50%) should be considered as the practical limit of measurement. An advantage of the HF-probes with larger lateral resolution (types 4 and 5) is their larger penetration depth, which even for an assumed thicker subsurface damage zone makes intra-annual density alterations in the wood sample visible. The over- and undershooting of the HF-output signal obtained by measurements of a crack in a glass plate were probably due to a partial reflection of the lobe side of the electric field on the geometrically sharp interface between glass (eglass = 7) and air (eair = 0.00056). For wood samples the over- and undershooting was lower due to less sharp interface between latewood and earlywood. However, for the extreme values of the HF-output signal the over- and undershooting represented a source of error. For determination of such parameters a correction factor has to be considered. In some cases a graphical smoothing may be sufficient. Measurements on air showed furthermore the importance of keeping the probe tip in close contact to the medium under investigation. The midrange values of the
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HF-output signal were reached for the different HF-probes at even lower values for d50%, which indicates the risk of an increasing weakening of the backscatter effect of a particular medium due to the influence of air, which results from a poor setup of the HF-probe tip on the sample. 4.2. Resolution ability HF-densitometry scans along the surface of isotropic materials like glass or PET with homogeneous microscopic structures result in constant HF-output signals. The signal is solely dependent on the relative dielectric constant of the material. For structured, inhomogeneous and anisotropic materials however the signal is also affected by small-scale physical and chemical composition differences within the material [29]. V-grooves on a PMMA-plate gave a sufficiently fine sequence of dielectric varieties for the determination of the resolution abilities of the HF-probes. For each HF-probe the resolution abilities could be defined in a more or less conservative way according to their respective passing of the starting point of the increase in the peak-to-peak amplitudes in the HF-output signal on PMMA. For measurements on the inhomogeneous material wood, HFprobe type 1 showed higher resolution abilities with the ability to represent density variations within the control table step size of 1 lm. For this type of HF-probe cells of Norway spruce were well suited to the analysis of the resolution abilities as they have more or less continuous regular, sharp-edged dielectric structures. For the other HFprobe types the representation of cell structures is below their resolution abilities, therefore their resolution abilities for measurements on wood could only be estimated (Table 1). The clear signal changes associated with the flanks of non-collinearity cell rows compared to the changes associated with the flanks of the V-grooves at 45° allows a better determination of the actual resolution abilities, at least for the smallest HF-probe type 1. It can be assumed that the resolution abilities of the HFprobes are determined by size and shape of the induced electric field in front of the transmitting electrode, the sensitivity of the receiving electrode, and by the signal-tonoise ratio of the detector system. For determination of the resolution abilities of the HF-probes, the natural noise component of the measured HF-output signal must be known in order to separate the noise from the signal. One method of improving the resolution of HF-densitometry is to increase the signal-to-noise ratio of the system. With fine adjustment of the high-frequency oscillator a very high signal-to-noise ratio can be attained, the natural noise component contributes less than 0.5% of the full scale range of the HF-output signal (1–10 V) for all HF-probes. For determination of ring boundaries or transition points between earlywood and latewood such a low noise component is irrelevant, but for a possible representation of cellular structures this noise component must be considered. If the differences in represented deviations in dielectric properties are in a range of a few microvolt at the receiving electrode, the noise may prohibit the determination of cell structure parameters.
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Another possibility for improving the resolution ability is an effective reduction in the sample-to-probe distance. Results of measurements on wood samples presented here were obtained on samples prepared with an ultra-precise diamond flycutter [30]. Such surfaces are characterised by an extremely high flatness and smoothness, as well as a very shallow depth of subsurface destruction. The importance of surface preparation for the most precise measurements of HF-densitometry were illustrated by the different HF-output signal characteristics of successive scans on the same track on a Norway spruce sample. Systematic shifts of the HF-patterns due to cut-off of the tip of the HF-probes were in particular investigated for the first scan. For further scans the differences were negligible. The sensitivity to air gaps of the HF-probes ranged less than 1 lm, therefore the first scan on a wood sample was used for conditioning of the measuring track for the later scans. Even on samples prepared with the diamond flycutter at least two scans should be conducted for very high contact accuracy due to an effective reduction of sample-to-probe distance. A combination of surface preparation conducted with a diamond flycutter and preconditioning of the measuring track gave an optimal sample-to-probe distance. Compared to the required sample preparation steps for measurements with other densitometry methods [18,31,32], sample preparation procedure for HF-densitometry requires special equipment (fly-cutter), but is less time consuming and not limited to sample dimensions. The spatial extent of the electric field, the quantity of the electric field and the resulting detection of the signal along the propagation directions (x-, y-, z-direction) are critical parameters for the resolution abilities of the HFprobes. For questions concerning cell structure analysis on wood samples or precise localisation of tree-ring boundaries HF-probes with a high resolution are preferable. A high resolution is a prerequisite for the discrete representation of natural structures up to the cellular level. The results of the measurements on Norway spruce with the HF-probe type 1 showed that with this probe it is possible to distinguish different structures up to a resolution of 1 lm. Brändström [33] evaluated various studies on tracheid dimensions of Norway spruce and gives a mean radial tracheid wall thickness of 3.52 lm (earlywood) to 6.23 lm (latewood). Mean radial tracheid diameter varies from 39.3 lm (earlywood) to 13.1 lm (latewood). Thus, the HF-output signal pattern of HF-probe type 1 gave a good representation of the tracheid microstructure of Norway spruce. However, the resolution ability of the HF-probes is only adequate if the data distance (in this study 1 lm) is smaller than the smallest dissolved structure. The accuracy of the representation of the cell structure is considerably affected by the collinearity of the cell structures. For the possible replacement of currently used optical measurement techniques for cell structure analysis, especially for species with strong cell collinearity, further investigations will be necessary. HF-probes with a larger penetration depth and spatial extent of the measuring range lose some resolution potentials and are less able to accurately reproduce cell structures. However, with these HF-probes more robust intra-
annual density variations may be made visible without complex mathematical post-processing. 5. Conclusions The High-Frequency densitometry allows non-destructive measurements of wood density variations at various resolution levels. With this method it is possible to determine wood density values at a desired resolution level to get more robust signals of intra-annual density variation which gives great potential for further investigations on the estimation of climate variability within a growing season on intra-annual wood density patterns. Moreover it is possible to distinguish different cell structures due to their variations in wood density up to a resolution of 1 lm. The method allows a cost and time effective measurement procedure of large data sets at any resolution level. The penetration depth of the electric stray field is related to the geometrical dimensions of the micro-electrode measuring system. For accurate measurements the thickness of the subsurface destruction zone of a wood sample due to sample preparation should be small enough that the electric field may extend far into the undamaged wood structure. Therefore an intensive sample preparation is needed, which may be an application limit in praxis for interested researchers. Acknowledgements The authors are grateful for language revision by Christopher Eastaugh and technical comments from two anonymous reviewers, all of which served to greatly improve the paper. References [1] E. Mork, Die Qualität des Fichtenholzes unter besonderer Rücksichtnahme auf Schleif- und Papierholz, Der Papier-Fabrikant 26 (1928) 741–747. [2] F. Kollmann, Technologie des Holzes und der Holzwerkstoffe, Band 2, Springer, Berlin, Heidelberg, 1955. [3] R. Trendelenburg, H. Mayer-Wegelin, Das Holz als Roh- und Werkstoff, Carl Hanser Verlag, München, 1955. [4] B.J. Zobel, J.P. Van Buijtenen, Wood Variation – Its Causes and Control, Springer, Berlin, 1989. [5] P. Fearnside, Wood density for estimating forest biomass in Brazilian Amazonia, Forest. Ecol. Manag. 90 (1997) 59–87. [6] D. Fengel, G. Wegener, Wood: Chemistry, Ultrastructure, Reactions, Reprint Kessel, Remagen, 2003. [7] E.M. Nogueira, B.W. Nelson, P.M. Fearnside, Wood density in dense forest in central Amazonia, Brazil, Forest Ecol. Manag. 208 (1995) 261–286. [8] H. Polge, Une nouvelle méthode de détermination de la texture du bois: l’analyse densitométrique de clichés radiographiques, Ann. Sci. Forest. 20 (1963) 533–580. [9] H. Polge, Etablissement des courbes de variation de la densité du bois par exploration densitométrique de radiographies d’échantillons prélèves a la tarière sur des arbres vivants. Applications dans les domaines Technologique et Physiologique, Ann. Sci. Forest. 23 (1966) 1–187. [10] F.H. Schweingruber, H.C. Fritts, O.U. Bräker, L.G. Drew, E. Schär, The X-ray technique as applied to dendroclimatology, Tree-Ring Bull. 38 (1978) 61–91. [11] D.J. Cown, B.C. Clement, A wood densitometer using direct scanning with X-rays, Wood Sci. Technol. 17 (1983) 91–99. [12] M. Koch, S. Hunsche, P. Schuacher, M.C. Nuss, J. Feldmann, J. Fromm, THz-imaging: a new method for density mapping of wood, Wood Sci. Technol. 32 (1998) 421–427.
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