Quantitative study of the temperature dependence of normal LWIR apparent emissivity Olivier Riou ⇑ , Pierre-Olivier Logerais 1 , Jean-Félix Durastanti 2 CERTES, IUT de Sénart-Fontainebleau, Université Paris-Est Créteil, rue Georges Charpak, 77567 Lieusaint, France highlights  Characterization of normal LWIR apparent emissivity.  Implementation of the classical model of apparent emissivity.  Connection between spectral and apparent emissivity.  Agreement between the model and the measurements over a large temperature range.  Application for quantitative thermography using a commercial camera. article info Article history: Received 22 January 2013 Available online 6 June 2013 Keywords: Apparent emissivity measurement Normal LWIR apparent emissivity Spectral emissivity Quantitative thermography abstract In a previous work, a method of measurement of apparent emissivity in situ was implemented. This approach has the decisive advantage of being suitable for any commercial infrared systems. It was tested successfully to characterize the normal LWIR apparent emissivity of an aluminium nitride plate in the temperature range [40–550 °C]. Apparent emissivity exhibits a tight temperature dependence. By using the classical model of apparent emissivity and taking into account the spectral emissivity of aluminium nitride ceramic and the spectral response of the IR sensor, we modelled our apparent emissivity measure- ment with 5% of accuracy and with dispersion better than 1% within the overall temperature range. The effect on the apparent emissivity of both the detection window and the temperature dependence of the spectral emissivity are highlighted. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction In quantitative thermography, the apparent emissivity and the reflected temperature have a determining role in the accuracy of the non-contact temperature measurement. According to the typ- ical procedure of temperature measurement, any operator is re- quired to find out and enter only these two values in the microcomputer system. The apparent emissivity and the reflected temperature are gen- erally difficult to evaluate. The reflected temperature is defined as ‘‘the temperature of the energy incident upon and reflected from the measurement surface of a specimen’’ and its determination is at present standardized in the case of specular reflection. The method consists in using an infrared reflector positioned on the specimen and in recording the apparent temperature which is identified to the reflected temperature by the specimen itself. This method is not suitable if the specimen has a diffuse reflection. To overcome this limit, a second approach consists in pointing the radiometer at a variety of locations within a cone of ±45° centred on the angle of incidence. The average of the recorded tempera- tures leads to the definition of the reflected temperature [1]. Quan- tifying the apparent emissivity is also a difficult problem. Its value depends not only on the emission properties of the target (spectral emissivity, state surface, thickness...), but also on the spectral char- acteristics of the IR system (bandwidth detection, spectral re- sponse of the detectors, optical transmittance...) and on the view angle. The challenge consists thus in adjusting the value of apparent emissivity. Due to the integration of infrared systems and their current use, operators are constraint to enter an emissivity value in a purely functional goal assuming that it correctly represents all the characteristics mentioned above. Therefore, it is likely that most operators of thermography do not exactly know the rele- vance of the emissivity that they use. The terminology in use in the literature is often ambiguous. As it was pointed out in [2], the classical formula which calculates the apparent emissivity as the mean value of spectral emissivity weighted by the blackbody 1350-4495/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.infrared.2013.05.012 ⇑ Corresponding author. Tel.: +33 1 64 13 46 86; fax: +33 1 64 13 45 01. E-mail addresses: olivier.riou@u-pec.fr (O. Riou), pierre-olivier.logerais@u-pec.fr (P.-O. Logerais), durastanti@u-pec.fr (J.-F. Durastanti). 1 Tel.: +33 1 64 13 46 86; fax: +33 1 64 13 45 01. 2 Tel.: +33 1 64 13 42 33; fax: +33 1 64 13 45 01. Infrared Physics & Technology 60 (2013) 244–250 Contents lists available at SciVerse ScienceDirect Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared