Characterization of spatial light distribution of flash lamp systems

Abstract

In this work results of an experimental activity are reported, where we evaluated the spatial distribution of light intensity for two different flash lamp systems. Two different methods have been experimented; in the first one a map of the energy distribution was obtained using a lab energy-meter while in the second one the temperature distribution of a homogeneous flat surface few instants after being flashed was imaged using an IR camera. A comparison between the spatial features of the two flash systems and between the two methods is done. 1. Forward NDE active thermographic techniques have found wide applications in the detection of defects within materials close to the surface [1]. The main applications concern the assessment of the structural integrity of coated components. These techniques are based on the IR detection (by using a thermographic system) of the inhomogeneities (related to the presence of a sub-surface defects) of surface temperature distribution produced by a uniform heating due to a thermal radiant source. This source may be pulsed or modulated. In dependence of the type of the radiating source the thermographic techniques are defined as follows: Video Pulsed Thermography (VPT)[2,3] and Lock-in Thermography (L T) [4]. VPT performances in detecting sub-surface defects can be influenced by the characteristics of the heating source, typically a flash lamp system. In particular, both flash duration and light spatial distribution playa keyrole in the surface thermal gradient (contrast) across a defect. To optimise the detection of defects the spatial distribution of the lighting thermal source is required as uniform and intense as possible. In order to investigate the influence of the above parameters on the induced thermal field a characterization of the spatial distribution of two different systems has been carried out. 2. Experimental 2.1 Experimental set-up For the characterization, we used a commercial photographic flash lamp (A) and a prototype flash lamp system (8). The two systems differ in the flash tube colour temperature, in the geometry of tubes, in energy, and in the surface finish of the parabolic reflectors. In table 1 the main features of both systems are reported. Two different measuring methods have been used; in the first one a direct measurement of the emitted radiation has been performed by detecting the incident energy in different points of a defined grid laying on a plane perpendicular to the direction of lamp emission. The features of the energy meter (a thermopile) used for the tests are the followin~: range 400nm-1100nm; minimum energy resolution 1mJ ; effective sensitive area 25mm ; collimation angle 22°. The measurement plane was chosen at one meter