Understanding Infrared

Variations in the absorption and the reflection of electromagnetic energy within the visible light portion of the electromagnetic spectrum by materials give rise to what humans detect and perceive as different colors. Similarly, variations in the emission and reflection of electromagnetic energy falling within the thermal IR energy portion of the electromagnetic spectrum gives rise to the science of IR imaging. All imaging cameras, whether they are visual systems or IR, intercept and record electromagnetic radiation as it travels through the atmosphere. Cameras produce an image by intercepting this electromagnetic energy by its’ sensor creating an electronic signal. If enough sensors are available, or one sensor is maneuvered such that it encompasses an entire scene, an image can be generated. A standard video camera intercepts, and then records, the reflection of visible light, whereas IR cameras intercept, and then record, emissions and reflections of thermal “light”.

Since both technologies are passive, the cameras can be used directly by operators in any situation without a health or safety risk; there are no inherent safety concerns from the cameras themselves.

A critical aspect of this “perception” is the way in which objects are distinguished. For any visual system, detection of an object is possible because its visual appearance, or visual signature, is different than that of it surroundings. Differences in IR systems work in the same way only the differences are defined by variations in emitted thermal energy. Various materials emit and reflect different quantities of thermal energy and this thermal difference, as evidenced in the imagery, allows for object detection. We can then identify detected objects based on prior knowledge of what that object should look like. Current IR technology is capable of detecting an object, due to its differing thermal characteristics, or signature, on the order of fraction of degrees Fahrenheit.

IR imaging has been successfully used around the world in a wide variety of industrial, commercial, and environmental applications such as pipelines, roofs, electrical distribution systems, industrial facilities and, specific to this application, bridge decks.  These applications are well documented in trade publications, technical journals, and seminar proceedings. With particular relevance to this discussion, IR has steadily grown over the recent past to become a proven method for bridge deck inspection.  The American Society for Testing and Materials (ASTM) has, in fact, developed a standard procedure for the bridge deck test process. ASTM Standard Test Method for Detecting Delaminations in Bridge Decks Using Infrared Thermography outlines an accepted and proven test method for bridge inspections.

Bridge surfaces are subject to continuously varying thermal inputs due to their exposure to local atmospheric events, such as convection forces, solar radiation, and thermal radiation exchanges. In fact, we choose when an IR scan will occur based on the natural thermal environmental conditions which create thermal anomalies consistent with bridge deck de-laminations (or signature). This change in signature is a predictable pattern resulting from the natural cyclic rise and fall of thermal conditions that occur during the transition from day to night and back again, referred to as the Diurnal Cycle.

If a de-lamination were introduced to an otherwise homogeneous concrete slab, a disruption in thermal properties of the slab would occur at that local site. As a result, the normal flow of energy along the thermal path would be altered relative to its surroundings.This thermal disruption would eventually manifest itself at the slab surface and will be evidenced by a hot or cold spot in the IR imagery.

Corrosion of embedded steel reinforcement within a bridge deck is the main source of this eventual thermal disruption. As the reinforcement material corrodes, it expands and creates a subsurface fracture within the concrete. When this de-lamination occurs in a bridge section, a disruption in thermal conduction properties of the material occurs at that local site. As a result, the normal flow of energy along the thermal path is altered relative to the solid deck structure.During a daytime IR inspection, these de-laminations appear as white or "hot" areas on a gray or "cooler" deck background and conversely for a nighttime scan.

Very generally speaking, and as rule of thumb, there are two windows of opportunity for scanning ambient scenes, both leveraging the diurnal cycle. The first window begins a few hours after sun down. The daytime hours before the test should provide full irradiation by solar loading and the night hours of the test should consist of a clear night sky, which is more appropriately considered to be a cold night sky.

During this window a maximum thermal transfer occurs within the upper concrete layers. The homogeneous material will quickly begin to cool in response to the cooling effect of radiation exchange with the cold night sky, drawing heat from the lower layers of the concrete based on its uninterrupted thermal properties. In contrast, the defect areas can not draw this heat from the lower layers due to the thermal path disruption (a thermal dam). The areas above the defect will effectively respond much faster with a maximum contrast beginning to appear and expand. The normal areas will exhibit whiter (warmer) characteristics and the defected areas will appear darker (cooler) in the thermal imagery.This condition will increase to a peak and then begin to decrease throughout the night and into the morning hours until all surfaces in the scene drive toward thermal equilibrium. The time at which thermal equilibrium occurs is typically referred to as the thermal cross over point.

The second imaging window begins in the morning hours a few hours after the sun has risen. The night hours before the test should consist of a clear night sky, and the morning hours should be fully irradiated with solar energy.Full solar irradiation ensures a maximum transition from cold to warm in the upper concrete layers. Again we take advantage of the thermal transition rate in non-homogeneous content within the concrete. The homogeneous concrete will effectively warm much slower than defected areas due to the inability of the transferring heat to diffuse into the concrete (again, that thermal dam). Consequently we look for whiter areas within the deck for anomaly indications.