The intensity of the infrared radiation emitted by objects is mainly a function of their temperature. In infrared thermography, this feature is used for multiple purposes: as a health indicator in medical applications, as a sign of malfunction in mechanical and electrical maintenance or as an indicator of heat loss in buildings. This paper presents a review of infrared thermography especially focused on two applications: temperature measurement and non-destructive testing, two of the main fields where infrared thermography-based sensors are used. A general introduction to infrared thermography and the common procedures for temperature measurement and non-destructive testing are presented. Furthermore, developments in these fields and recent advances are reviewed.

Accurate temperature measurement in industrial environments is as important as it is challenging. Precise control over temperature measurement is crucial when processing metals, such as iron or steel, where temperature monitoring is critical to productivity and product quality. In the steel manufacturing process, temperature measurement of molten pig iron is particularly important, as it is a required parameter of the physical models used to control operations in steel furnaces. However, measuring the temperature of molten pig iron is not an easy task. Conventional methods using thermocouples or pyrometers present serious drawbacks which limit their applicability and do not provide accurate measurements. In this paper, an infrared computer vision system is proposed to measure the temperature of molten pig iron while it is being poured. The proposed system confronts two challenges: The stream must be detected in the infrared images, and the slag, which can partially cover the stream of molten pig iron, must be detected and removed from the stream. Fast, robust, and accurate methods are proposed. A calibration procedure for the emissivity of the molten pig iron and for the temperature level is also proposed and applied. This procedure makes it possible to differentiate molten pig iron from slag in the stream. Tests indicate that the results meet production needs.

Shape is a key characteristic to determine the quality of outgoing flat-rolled products in the steel industry. It is greatly influenced by flatness, a feature to describe how the surface of a rolled product approaches a plane. Flatness is of the utmost importance in steelmaking, since it is used by most downstream processes and customers for the acceptance or rejection of rolled products. Flatness sensors compute flatness measurements based on comparing the length of several longitudinal fibers of the surface of the product under inspection. Two main different approaches are commonly used. On the one hand, most mechanical sensors measure the tensile stress across the width of the rolled product, while manufacturing and estimating the fiber lengths from this stress. On the other hand, optical sensors measure the length of the fibers by means of light patterns projected onto the product surface. In this paper, we review the techniques and the main sensors used in the steelmaking industry to measure and quantify flatness defects in steel plates, sheets and strips. Most of these techniques and sensors can be used in other industries involving rolling mills or continuous production lines, such as aluminum, copper and paper, to name a few. Encompassed in the special issue, State-of-the-Art Sensors Technology in Spain 2013, this paper also reviews the most important flatness sensors designed and developed for the steelmaking industry in Spain.