Microscopic lens thermography for the observation of features in light cardboard Carlo Santulli Università di Roma – La Sapienza Electrical Engineering Department Via Eudossiana 18 00184 Roma, Italy E-Mail: carlo.santulli@uniroma1.it George Jeronimidis Centre for Biomimetics, School of Construction Management and Engineering University of Reading, Whiteknights, RG6 6AY, United Kingdom E-Mail: g.jeronimidis@rdg.ac.uk ABSTRACT This is a preliminary study for microscopic lens thermography observations on a film-like and non-blackbody material. With this aim, a number of features present on the surface of a cardboard for storage boxes (grammage 300 g/m²) are observed using a Flir SC 3000 thermal imager microscopic lens with 26 mm fixed focal distance. Comments on quality of observation and on probability of false indications depending on on the depth of the features observed are supplied, in particular concentrating on transient time for observation during cooling following illumination and on thermal gradients measured. Keywords: IR thermography, microfocus, cardboard, probability of detection 1. Introduction The recent availability of microscopic lens thermography has considerably extended the field of application of this technique, which was originally aimed more at the detection of defects, such as delaminations in composites, with maximal dimensions exceeding a few millimetres. This implies that the technique can be used more as a microscopical method, therefore allowing individuating surface defects or features which are not revealed by a visual inspection. With respect to optical microscopy, IR thermography would be ideally able to offer some information on the gravity of the defect, e.g. by the measured temperature difference with respect to the background, assumed as “normal” or “undamaged”. Some preliminary studies have confirmed the potential of the application of this technique to very thin, possibly film-like materials. Examples of these studies allowed e.g., the investigation of leakage sites in integrated circuit boards [1] and the measurement of porosities in balsa wood [2], in both cases arriving to spatial accuracies lower than the millimetre. However, some limitation in having a surface response in that range are evident. A first difficulty lies in the fact that what is really obtained and represented in the dynamical thermal map is the global response of the emission from the material in the infrared wavelength range. Less obvious is relating the above response to the presence of defects or abnormal features in the structure. In this regard, the principal issue, where infrared measurements are concerned, is the dependance of emissivity on temperature, wavelength and surface condition [3]. This may conceal the appearance of defects or reduce the time frame available for thermographic observation. In this study, a number of features present on the surface of a cardboard layer for storage boxes are observed, using a microscopic lens thermographic camera, as a preliminary for a study on a film- like and non-blackbody material. The quality of observation is dealt with and the possibility for false indications in dependance on the depth of the features is also discussed. 2. Experimental The material under observation was cardboard for boxes (grammage 300 g/m²), with thickness 0.35 NDT.net - The e-Journal of Nondestructive Testing (May 2008) For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=25