**4. Results and discussion**

The thermogram in **Figure 8** is an example of the methodology followed to determine the sample emissivity.

**Table 1** shows the environmental conditions of the laboratory while the thermograms were produced.

**Tables 2** and **3** show the emissivity and the surface temperatures for each thermogram of both pine samples. The samples are from Leiria (coast) and Serra da Estrela (inland mountains).

**Table 4** shows the average emissivity of both samples of wood of the species *Pinus pinaster* Ait.

The emissivity of each sample results from the average of the two tests performed in each. They are 0.864 for Leiria pine and 0.873 for Serra da Estrela pine. In each case, the emissivity and temperature values were determined by the average area of one square on the software used (100 × 100 pixels). The results are valid for the environmental conditions and the sample conditions described in the previous tables.

It should be highlighted that there is no significant range difference between the two samples: 0.018 for Leiria pine and 0.035 for Serra da Estrela pine. On the other hand, the emissivity values are in accordance with the bibliography consulted, that is, the highest values are around 0.9: 0.855–0.873 for Leiria pine; 0.855–0.890 for Serra da Estrela pine. As reported by Rice [8], manufacturers of IR temperature measuring equipment often recommend values between 0.94 and 0.95 for generic wood and conditions.

The manufacturer FLIR indicates for four samples of pine in the spectral window 8–14 μm values of emissivity, namely between 0.81 and 0.89. The spectral window 2–5 μm indicates values of emissivity between 0.67 and 0.75 [18]. Holst refers 0.85 for the emissivity of wood planed in the spectral window of 8–14 μm [31]. However, it is difficult to compare because emissivity values in the literature

#### **Figure 8.**

*Example of a thermogram obtained from one of the wood samples.*


#### **Table 2.**

*Emissivity values—Leiria Pine sample.*


#### **Table 3.**

*Emissivity values—Serra da Estrela Pine sample.*


#### **Table 4.**

*Samples conditions and emissivity.*

refer to test conditions and experiments not fully described. The environmental characteristics to be described are ambient temperature and the relative humidity. The spectral observation window must be referred to. The conditions that refer to the sample itself, such as surface finishing, water content, density and the wood species itself, must also be mentioned.

**35**

*The Importance of Emissivity on Monitoring and Conservation of Wooden Structures Using…*

The wood has values of emissivity, very dependent on the spectral window

to describe the set of factors conditioning the emissivity: the distance between the machine and the sample, observation angle, spectral window for observation, ambient temperature, and other ambient conditions such as relative humidity, light

intensity, reflected temperature and sample surface temperature.

which the physical properties of the object of study deteriorate.

(parallel or perpendicular direction to the fibre).

zones and had different density values.

spectral band used in this experimental work.

When referring to emissivity of a natural material, such as wood, it is important

The difference between the sample surface temperature and the ambient temperature must be as constant as possible during the determination of the emissivity. According to the ASTM standard [19], the higher the difference the more rigorous will be the emissivity determination. The upper limit is the temperature value at

The type of surface finishing, species identification (by scientific name), colour, sample water content and density should also be described. All these factors condition the emissivity measurement. On the other hand, only this way, by referring all the parameters, make it possible to obtain a comparative notion of the emissivity determined under different conditions. Therefore, more correctly will be the choice, for a real situation of surface temperature analysis with IR thermography technique. Even so, variations are expected in the resulting values because of both sets of environmental and sample parameters. In fact, when referring to sample parameters, there is a large variability among wood species, besides that, there is large variability along the tree itself from which the sample is taken (heartwood or sapwood). Even more, variation occurs depending upon the cutting technique

When reviewing the literature, it was not found systematised emissivity values under the same conditions, that is, for example, ambient temperature, spectral window and species under study. The literature on the subject is scarce and generally does not present all the relevant parameters to be taken into account for

In addition, it is not common the authors to describe the process applied to heat the surface of the sample although this is another factor that conditions the emissivity value [9]. It is relevant because not all types of heating are suitable for this

The aim of this research is to contribute to the obtainment of information relevant to the investigation of wooden building conservation. It was carried out under

Emissivity values were obtained from two different samples of the same wood species. Emissivity values were obtained at an ambient temperature of 21.0°C, at a distance of 0.5 m. The sample dimensions of the observed face were 0.20 × 0.15 m to minimise errors from the surroundings. The spectral band used was the one the FLIR B20 thermographic camera provides, that is 7.5–13 μm. The samples did not have any type of finish. Lab conditions intended to reproduce the *in situ* conditions. The specimens studied, although of the same species, came from different climate

The emissivity values obtained are into the emissivity range suggested in the literature. However, it was not found literature that met the same conditions of observation regarding the species, ambient temperature, relative humidity and the

an experimental approach using pine wood of the *Pinus pinaster* species.

*DOI: http://dx.doi.org/10.5772/intechopen.82847*

used.

measurement.

purpose [18].

**5. Conclusion**

*The Importance of Emissivity on Monitoring and Conservation of Wooden Structures Using… DOI: http://dx.doi.org/10.5772/intechopen.82847*

The wood has values of emissivity, very dependent on the spectral window used.

When referring to emissivity of a natural material, such as wood, it is important to describe the set of factors conditioning the emissivity: the distance between the machine and the sample, observation angle, spectral window for observation, ambient temperature, and other ambient conditions such as relative humidity, light intensity, reflected temperature and sample surface temperature.

The difference between the sample surface temperature and the ambient temperature must be as constant as possible during the determination of the emissivity. According to the ASTM standard [19], the higher the difference the more rigorous will be the emissivity determination. The upper limit is the temperature value at which the physical properties of the object of study deteriorate.

The type of surface finishing, species identification (by scientific name), colour, sample water content and density should also be described. All these factors condition the emissivity measurement. On the other hand, only this way, by referring all the parameters, make it possible to obtain a comparative notion of the emissivity determined under different conditions. Therefore, more correctly will be the choice, for a real situation of surface temperature analysis with IR thermography technique. Even so, variations are expected in the resulting values because of both sets of environmental and sample parameters. In fact, when referring to sample parameters, there is a large variability among wood species, besides that, there is large variability along the tree itself from which the sample is taken (heartwood or sapwood). Even more, variation occurs depending upon the cutting technique (parallel or perpendicular direction to the fibre).

When reviewing the literature, it was not found systematised emissivity values under the same conditions, that is, for example, ambient temperature, spectral window and species under study. The literature on the subject is scarce and generally does not present all the relevant parameters to be taken into account for measurement.

In addition, it is not common the authors to describe the process applied to heat the surface of the sample although this is another factor that conditions the emissivity value [9]. It is relevant because not all types of heating are suitable for this purpose [18].

### **5. Conclusion**

*Advances in Structural Health Monitoring*

**34**

**Table 2.**

**Table 3.**

**Figure 8.**

*Emissivity values—Leiria Pine sample.*

*Emissivity values—Serra da Estrela Pine sample.*

**of tests**

**Samples Number** 

Serra da Estrela

*Samples conditions and emissivity.*

pine

**Table 4.**

species itself, must also be mentioned.

refer to test conditions and experiments not fully described. The environmental characteristics to be described are ambient temperature and the relative humidity. The spectral observation window must be referred to. The conditions that refer to the sample itself, such as surface finishing, water content, density and the wood

**Place Test number Emissivity (ε) Sample surface temperature (°C)**

**Place Test number Emissivity (ε) Sample surface temperature (°C)**

2 0.890 43.0 **Average 0.873 45.3**

> **Dry weight (g)**

Leiria pine 2 4500 2507 2810 12.1 0.624

**Weight (g)**

2 4500 1869 2058 10.1 0.457

**Water content (%)** **Density**

2 0.873 43.4 **Average 0.864 42.7**

IPG 1 0.855 42.0

IPG 1 0.855 47.1

**Volume (cm3 )**

*Example of a thermogram obtained from one of the wood samples.*

The aim of this research is to contribute to the obtainment of information relevant to the investigation of wooden building conservation. It was carried out under an experimental approach using pine wood of the *Pinus pinaster* species.

Emissivity values were obtained from two different samples of the same wood species. Emissivity values were obtained at an ambient temperature of 21.0°C, at a distance of 0.5 m. The sample dimensions of the observed face were 0.20 × 0.15 m to minimise errors from the surroundings. The spectral band used was the one the FLIR B20 thermographic camera provides, that is 7.5–13 μm. The samples did not have any type of finish. Lab conditions intended to reproduce the *in situ* conditions. The specimens studied, although of the same species, came from different climate zones and had different density values.

The emissivity values obtained are into the emissivity range suggested in the literature. However, it was not found literature that met the same conditions of observation regarding the species, ambient temperature, relative humidity and the spectral band used in this experimental work.

Experiments of this type are relevant since an incorrect emissivity measurement can lead to inaccurate results in the interpretation of the thermograms and hence to false conclusions.

The correct way to obtain temperature values at IR thermography systems is to establish the emissivity of the materials to be tested. Nevertheless, it is often not possible in the course of *in situ* investigations. In that case, samples of material should then be collected and tested in the laboratory, reproducing carefully the same environmental conditions as those found *in situ* to avoid distortions that may bias the results.

In the literature, we found few published works on emissivity values for wood materials. Thus, a listing of wood emissivity values at different ambient temperatures for buildings/timber structures is timely and relevant.
