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Digital analysis of thermal images

By S. Rogotis, V. Chatziathanasiou and S. Kadi

Dept. of Electrical & Computer Engineering
Aristotle University of Thessaloniki (AUTH)
P.O. Box 486, GR-54124, Thessaloniki, GREECE
Tel: +302310996295, Fax: +302310996302
e-mail: hatziath@auth.gr

Abstract:

The aim of this paper is the digital analysis of thermal images of a material with abnormalities lying beneath its surface focusing on the search of the image’s feature that gives the best information about the abnormalities. The process followed observes the change of thermal images with time and by the use of certain algorithms, information concerning the abnormalities, like the depth from the surface is gathered.

1. Introduction
Thermography is a well known non-destructive technique used to reveal the invisible thermal radiation. All objects emit and absorb thermal radiation. The wavelength of the emitted radiation depends on the objects surface temperature and for most objects it extends in the infrared spectrum. Thermography is used in applications of various orientations such as medicine, military, constructions, electronics etc. Thermal cameras detect thermal radiation and with proper transformations surface temperatures can be calculated. In pulse active thermography the experimental material is thermally stimulated from an external source and the thermal camera records the descending surface temperature. Active thermography requires powerful image processing algorithms. Passive thermography is used only when controlled heating of the material is practically impossible. The great disadvantage of this approach is that the thermal images might indicate non existing subsurface abnormalities due to the non homogeneous heating.

2. Experimental material

The experimental material must have high emissivity in order to present significant thermal changes during the recording time. These thermal changes will be captured by the thermal camera and will result in a continously changing thermal image. Additionally it should have high thermal conductivity and thermal diffusivity in order to facilitate the homogeneous heating and cooling process. In this work the material chosen for the experiment is marble. Marble properties meet sufficiently the above mentioned requirements and it has the following features: 1. It is a known construction material, widely available and of low cost. 2. Its emissivity ε at 20 οC is 0.93.

3. Its thermal conductivity k ranges from 2.07 W·m-1·C-1 to 2.94 W·m-1·oC-1. 4. Its thermal diffusion α ranges from 10∙10-7m2·sec-1 to 13.6∙10-7 m2·sec-1. The chosen piece of marble has the following dimensions:

Length: 35 cm
Width: 28 cm
Thickness: 3 cm

At the back side of the marble piece six holes of different diameter and depth were drilled simulating subsurface abnormalities. Moreover, one piercing hole was made.

Fig. 1. The back side of the marble.

The holes have the following dimensions:

Hole 1: diameter 5.3 cm, depth 1.5 cm.
Hole 2: diameter 3.5 cm, depth 2.1 cm.
Hole 3: diameter 3.5 cm, depth 1.6 cm.
Hole 4,5,6: diameter 0.6 cm, depth 2 cm.
Piercing hole: diameter 0.6 cm.

The front side of the marble was painted with black mat color, in an attempt to increase the absorbability.

3. System set-up

The heated marble and the thermal camera were 120 cm apart, in a dark room,. The air renewal and the constant temperature were achieved by an air-conditioning system. In order to ensure that the marble’s heat losses are only due to radiation and convection, a metal grate with minimum contact surface was used for the test piece support.

4. Data processing

The first step of the experiment was the recording of the descending surface temperature of the marble. After that special software was used in order to isolate a number of selected frames. These n selected frames are the thermal images to be further processed.

For the image processing, dedicated...
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