# Cavity Detection in Steel-Pipe Culverts Using Infrared Thermography +
## 2 Literature Review  
Infrared thermography (IRT) [28] has received an increasing attention especially for NDT [12] across various industries owing to successful applications and advantages offered by this approach, including three important characteristics: contactless, nondestructive, and nonintrusive. IRT-based inspections are fast, reliable, and among the safest available approaches for personnel (e.g., in comparison with X-rays); the recorded data can be processed later with different image processing methods; and thanks to the mobility of modern IR cameras, in-field IRT-based inspection has become convenient [15; 29]. A downside of IRT is that the required instruments (specifically the IR cameras) are expensive, and the analysis of results demands expertise [15; 29]. However, the training hours for obtaining a first level certification is half of that for the other NDT techniques such as ultrasound and X-rays [15]. Moreover, over the past two decades, the cost of IR cameras has drastically dropped while improving their performance, and this has significantly influenced the popularity of IRT. IRT techniques fall under two general categories: passive and active. The passive approach is used when the objects of interest are naturally at higher (or lower) temperatures than the surrounding area/background without the presence of any external heating (or cooling) source. Passive thermography can be used to detect faulty components/equipment and even for detection of those with high risk of failure which is very useful in preventive maintenance [12]. Fast inspection, low cost, and simplicity are main features of the passive approach. Active thermography [30] requires an external thermal stimulus to generate the necessary heat flow and thermal gradients. It is done in two reflection and transmission modes, depending on the position of the IR camera and stimuli. A number of techniques under this category have been developed. One of the most widely used active-based IRT methods is pulse thermography (PT), in which a short thermal pulse is applied to the object under inspection (specimen) while an IR camera is recording the thermal images. Despite recording thermograms in both heatup and cooldown period, only the latter is usually used for processing. The heat source can be a flash lump (e.g., a xenon tube) or a cold air jet, and the stimulation period is often short (about 2-15 ms) [12]. Another popular IRT method of inspection is lock-in thermography (LT) [31]. This method may also be referred to by other terms such as modulated thermography [32] and phase angle thermography [33]. A periodical (e.g., a sinusoidal) heat source, for instance a halogen lamp modulated with a modulation frequency, is used instead of a thermal pulse stimulation. After reaching a steady state, thermograms are recorded over several modulation cycles. The recording is often carried out with different modulation frequencies for more accurate inspection of the specimen at different depths. Higher frequencies are used to investigate shallower defects while lower frequencies are useful in detecting deeper defective zones. Using recorded thermal images, one can reconstruct the periodical thermal wave, i.e., computing amplitude and phase. Although both can be recovered, phase data are generally preferred as its sensitivity to nonuniform heating and the surface emissivity variations are less significant [34; 35]. Using LT allows the application of thermal excitation of lower power and this feature is specifically important for the investigation of antique art measures [36]. The main drawback of LT is that the process of data acquisition is more time consuming as it requires multiple tests, in each of which the permanent regime should be reached [35].