Emissivity considerations in building thermography

Energy and Buildings 35 (2003) 663–667

Emissivity considerations in building thermography N.P. Avdelidisa,*, A. Moropouloub a

b

Materials Research Centre, University of Bath, Bath BA2 7AY, UK Section of Materials Science and Engineering, School of Chemical Engineering, National Technical University of Athens, Iroon Polytechniou 9, 15780 Zografou, Athens, Greece Received 30 August 2002; received in revised form 14 October 2002; accepted 14 October 2002

Abstract In the last 25 years, there have been considerable efforts put into the use and development of infrared thermography on buildings and large structures. As a result, nowadays, there are systems that can be used effectively in outdoor and/or indoor building surveys; indication and monitoring of problems such as voids, detached areas, deposits of humidity, etc. However, the principal problem where infrared thermographic measurements are concerned is the emissivity—emittance of the material(s). Given that an infrared camera detects the radiation emitted by a material under investigation and renders this energy to a temperature—thermal image, the feature that describes the relation between the emitted radiation and the material’s temperature, is termed as emissivity. Emissivity is actually a surface property that states the ability of the investigated material to emit energy. Correct emissivity values could provide valuable information concerning the interpretation of thermal images obtained from thermographic surveys. There is a considerable amount of work that has been published on emissivity of different materials and under various circumstances (i.e. temperature, surface condition, wavelength). In this work, a review on emissivity measurement techniques and the importance of emissivity values on building diagnostics was materialised. Furthermore, the emissivity of selected building materials were determined at a variety of temperatures, in the mid and long wavelength regions of the infrared spectrum, using different approaches and were discussed and explained in terms of the approach used, the wavelength and temperature effects, as well as the materials surface state. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Infrared thermography; Emittance; Temperature; Wavelength; Surface condition; Buildings

1. Introduction At temperatures above absolute zero all bodies emit electromagnetic radiation. Infrared thermography is a measurement technique based on the detection of radiation in the infrared spectrum (usually in the 2–5.6 and 8–14 mm regions). These two spectral bands are commonly used, because of their low atmosphere absorption [1]. The use of infrared thermography in building monitoring is found to be extensive. It has been used successfully for more than 25 years in building diagnostics concerning, historical buildings and sites, monuments, modern structures. Passive thermography, commonly used in the investigation of buildings, is generally a qualitative form of assessment; the objective is to detect irregularities of the kind function—malfunction. However, correct temperature readings and so quantitative results are typically not possible *

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to attain without knowledge of the materials’ emissivity values. There is no infrared camera that can read temperature directly. All cameras interpret the infrared radiation coming from the investigated surface, which involves emitted, reflected and occasionally transmitted infrared radiation [2]. Emissivity is an expression used to characterise the optical properties of materials in sense of the amount of energy emitted in comparison with an ideal black body [3]. Values for emissivity (e) can be between 0 (perfect reflector—mirror) and 1 (perfect emitter—black body). As a result, emissivity plays an important role in building thermographic surveys and is dependent on temperature, wavelength and surface condition. A surface with a low emissivity value (i.e. aluminium, steel, etc.) acts as a mirror (high reflectance). However, such problem is usually solved [4] using high emittance flat paints (i.e. black colour waterbased paints) for painting the investigated surface(s). Some materials transmit energy at some wavelengths, whilst absorbing energy at others. Thus, emissivity is affected by wavelength. The transmission—absorption of

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Table 1 Description of samples examined in the laboratory Sample no.

Sample

Description/source of the sample

P-EM P-MP P-RL M-PNLG M-KBG S-KBG S-MCR S-VFH

Plaster Plaster Plaster Pentelic marble Kokkinaras marble Kapandritis stone Porous stone Porous stone

Consisted of a mixture of binders, additives, aggregates of suitable grain size distribution and fibres Cement-free mortar based on a special hydraulic binder, natural sand and fibres Consisted of a mixture of hydraulic binders and special aggregates and additives White micro-crystalline: from the National Library of Greece historical building in Athens Grey cohesive macro-crystalline: from the Bank of Greece historical building in Piraeus Fine-grained calcite: from the Bank of Greece historical building in Piraeus Biocalcarenite: from the medieval city of Rhodes Bioclastic limestone: from the Venetian fortifications of Heraklion, Crete

energy will also vary, according to the temperature and the surface condition of the investigated material—structure. Most common building materials (i.e. plaster, stone, concrete, etc.) have high emissivity values (usually higher than 0.8). Grey bodies with emissivity values less than 0.5 (i.e. polished metal surfaces) are usually difficult to obtain (low emittance of radiation energy). In view of this, one can easily understand that building thermographic surveys are quite complicated subjects, provided that they are consisted of an assortment of materials. Despite this, infrared thermography in building diagnostics can be used efficiently in the detection of heat losses, missing or damaged thermal insulation in walls and roofs, thermal bridges, air leakage and moisture sources [5]. Substantial amount of work has been published on emissivity of different materials and under different conditions. Hansman Jr. [6] proposed emissivity determination techniques with the intention of avoiding errors that are due to reflection and absorption matters. Moreover, Marshall [7] calculated the emissivity values of semi-transparent materials at cold temperatures, Ravindra et al. [8] modelled the wavelength dependence of emissivity, whilst Moropoulou and Avdelidis [9] determined the emissivity values of materials collected from historical buildings, at different temperatures, employing mid and long wave thermography. As far as emissivity measurement techniques are concerned, the ASTM standard E1933-97 [10] provides a measure guide for determining the emissivity of a variety of materials using infrared imaging systems. Madding [11] though, suggested alternative ways of calculating the emissivity of materials. His empirical approach can be used efficiently in the calculation of emissivity values for a variety of materials. Usually though, a reference emitter—material with a known emissivity value must be used in order to attain the emissivities of the investigated material(s). In this research work, emissivity measurements in the laboratory, for a variety of building materials, were attempted, employing two different approaches. The emissivity values of the investigated materials were measured at numerous temperatures, employing mid (3–5.4 mm) and long (8–12 mm) wavelength infrared thermography. The results were discussed in terms of the approach used, as well as the temperature, wavelength, and surface condition

effects. Conclusively, the importance of emissivity in infrared thermographic building surveys was seemingly acknowledged.

2. Experimental procedures and techniques Two different approaches were used for the calculation of the emissivity values on various building materials, such as plasters, marbles, and porous stones. The first approach was in accordance with the ASTM standard E1933-97, whereas the second one was completed according to an empirical procedure. All samples (Table 1) were measured at both wavelength infrared thermographic regions (3–5.4 and 8– 12 mm), at a variety of temperatures. A non-contact thermometer (Gann IR40) was also utilised for merely confirming that the examined specimens were in a temperature equilibrium state while inside the chamber. The infrared thermographic systems (Avio TVS 2000 Mk II series) were then adjusted in order to determine the materials’ emissivity values. It is also worth mentioning that in order to overcome the problem of moisture (porous materials), the samples were first heated at a temperature of 105 8C for a period of 24 h and then were stored in a desiccator for at least 2 h. 2.1. Approach using the ASTM standard Batches of three specimens from the investigated samples were measured in the laboratory with the intention of determining their emissivity values at 0, 48.8, and 100 8C. For the 0 8C the specimens were positioned into an environmental chamber (Heraeus Votsch CTC-E), whilst for the other two temperatures into a convection oven (Binder FD-115). Exposure was for 24 h intervals. 2.2. Approach using an empirical procedure The same specimens were also investigated in the laboratory using an empirical developed approach. The samples were placed into an oven (Binder FD-115) for 24 h intervals at the desirable temperature, attaching as a reference emitter a piece of electrical tape (3M Scotch Super 88 Vinyl) with a known emissivity of 0.95, at 48.8 8C.

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3. Results and discussion The emissivity values for the 3–5.4 and 8–12 mm wavelengths are presented in Figs. 1 and 2, respectively. Comparing the results according to the wavelength used (mid or long), it can be seen that when measured in the 3–5.4 mm wavelength region the emittance is higher at the higher temperatures (i.e. 100 8C), whilst in the 8–12 mm, for most materials, the emissivities are higher at the low temperatures (i.e. 0 8C). Presumably, this is due to the fact that short or mid wavelength systems are more sensitive to high temperatures—above ambient, whereas long wavelength systems are more sensitive to lower temperatures—ambient and below [12]. The temperatures of 0 and 48.8 8C, where the materials were tested in order to obtain their emissivities, can be considered as rather substantial temperatures when applying outdoor thermography on buildings and structures. As far as the 100 8C temperature is concerned, this was used in order to look into if emittance is greater at high temperatures. In other words if building thermography can operate better (provide better quality results) at high temperature applications. In addition, a straightforward comparison of the emissivity values that were determined at the 48.8 8C using the two diverse approaches can be carried out. The emissivity values of the samples presented variations, in reference to the approach used. This is mostly in the case of the mid

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wavelength region (3–5.4 mm). Moreover, due to the materials’ different textures (materials with different composition), the emissivities were also affected. Since temperature has also an effect on the emissivity of a material, it can be concluded that emissivity values vary for different wavelengths and they are directly related to the materials’ surface condition and temperature. Such is validated by the results obtained in this research work. Correction of emissivity is the way to obtain the correct temperature data of the investigated material(s) and so to properly interpret the attained thermographs. It is evidently that if during a building thermographic survey random emissivity values are selected, there will be lack of information and so an incomplete building assessment. So, the role of emissivity in building thermography is unquestionably substantial. In point of fact, several thermographs from in field applications using correct and/or accidental emissivity values are presented in Figs. 3 and 4. Specifically, in Fig. 3a an outdoor plastered surface of a building is presented using a random emissivity value, whilst in Fig. 3b the same surface is examined, this time using the corrected emissivity value. As it can be clearly seen, an incorrect emissivity could lead to inaccurate results, and so to faulty conclusions. A similar situation, this time concerned with a porous stone masonry, is also presented in Fig. 4a and b. Once more, an improper emissivity value was selected randomly (Fig. 4a) and the proper emissivity (calculated from the laboratory results) was used (Fig. 4b).

Fig. 1. Emissivity values for the 3–5.4 mm wavelength.

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Fig. 2. Emissivity values for the 8–12 mm wavelength.

Fig. 3. Thermal image of an outdoor plastered surface of a building using: (a) random emissivity value; (b) corrected emissivity value.

Fig. 4. Thermal image of a porous stone masonry using: random emissivity value; (b) corrected emissivity value.

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4. Conclusions The emissivity values of numerous building materials, such as plasters, marbles, and porous stones, were determined in the laboratory, at a variety of temperatures. The samples were investigated in the mid and long wavelength regions of the infrared spectrum, using different approaches. The examined materials presented different emissivities that were attributed to their texture and composition, to the temperature used, as well as to the wavelength effect. The way to obtain correct emissivity values for the infrared thermographic systems and applications is to establish the emissivity of the materials to be tested. While this approach is not always achievable in the course of in situ investigations, samples of the materials can be collected and assessed, as in this work, in the laboratory. Since in literature there is little work that has been published on emissivity values for building materials (mostly historical buildings) at a range of temperatures, the results that arise from this research work, provide valuable information to the thermographer whose investigation is concerned with buildings and historic complexes. Nonetheless, the emissivity of such materials for a range of temperatures should be further looked into, in an attempt to provide a complete referenced emissivity values manual for the building thermographer.

References [1] X.P.V. Maldague, Nondestructive Evaluation of Materials by Infrared Thermography, Springer-Verlag, Berlin, 1993.

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[2] A. Moropoulou, N.P. Avdelidis, The role of emissivity in infrared thermographic imaging and testing of building and structural materials, in: X.P.V. Maldague, A.E. Rozlosnik (Eds.), Thermosense XXIV, SPIE Press, Orlando, Florida, USA, 2002, pp. 281–287. [3] A. Moropoulou, N.P. Avdelidis, M. Koui, I. Tzevelekos, Determination of emissivity for building materials using infrared thermography, Journal of Thermology International 10 (3) (2000) 115–118. [4] N.P. Avdelidis, A. Moropoulou, Infrared thermography. Philosophy, history, approaches, applications and standards, in: Proceedings of the Fourth National Conference of HSNT and the Second Balkan Conference of BSNDT, Athens, Greece, 2002. [5] C.A. Balaras, A.A. Argiriou, Infrared thermography for building diagnostics, Journal of Energy and Buildings 34 (2) (2002) 171–183. [6] J.R. Hansman Jr., Infrared temperature measurement, in: MIT Video Series, Massachusetts Institute of Technology, USA, 1995. [7] S.J. Marshall, The infrared emittance of semi-transparent materials measured at cold temperatures, in: G.J. Burrer (Ed.), Thermosense VI, SPIE Press, Orlando, Florida, USA, 1983, pp. 188–192. [8] N.M. Ravindra, F.M. Tong, S. Amin, J. Shah, W.F. Kosonocky, N.J. McCaffrey, C.N. Manikopoulos, B. Singh, R. Soydan, L.K. White, P. Zanzucchi, D. Hoffman, J.R. Markham, S. Liu, K. Kinsella, R.T. Lareau, L.M. Casas, T. Monahan, D.W. Eckart, Development of emissivity models and induced transmission filters for multiwavelength imaging pyrometry, in: J.R. Snell (Ed.), Thermosense XVI, SPIE Press, Orlando, Florida, USA, 1994, pp. 304–318. [9] A. Moropoulou, N.P. Avdelidis, Emissivity measurements on historic building materials using dual wavelength infrared thermography, in: A.E. Rozlosnik, R.B. Dinwiddie (Eds.), Thermosense XXIII, SPIE Press, Orlando, Florida, USA, 2001, pp. 224–228. [10] ASTM E1933-97, Standard Test Methods for Measuring and Compensating for Emissivity Using Infrared Imaging Radiometers, American Society for Testing and Materials, Pennsylvania, USA, 1997. [11] R.P. Madding, Emissivity measurement and temperature correction accuracy considerations, in: D.H. LeMieux, J.R. Snell (Eds.), Thermosense XXI, SPIE Press, Orlando, Florida, USA, 1999, pp. 39–47. [12] R.A. Thomas, Thermography Monitoring Handbook, Coxmoor Publishing Company, Oxford, UK, 1999.