Both passive and active thermography offer several advantages over other nondestructive testing techniques. Infrared thermography inspection is fast, and contact with the object being inspected is unnecessary. Large components can be inspected easily with the proper thermography equipment.
In passive thermography, the component is inspected during or directly after a thermal cycle. The thermal cycle is the component’s normal operations. When in use, the component will heat; testing takes place during or directly after use.
An example of a time where passive thermography is used is in inspecting aircraft for water ingress immediately after landing. The aircraft will be hot from use, but the water will be cooler then the aircraft. The infrared camera will be able to detect these differences in temperature. Once the aircraft cools down, the camera will no longer be able to detect the temperature variations.
A benefit of passive thermography is components do not need to be taken out of use in order to be inspected. It is most effective when inspecting for strong thermal indications, such as water ingress.
In active thermography, the component being inspected is heated and cooled in a controlled environment. The component is continuously monitored while it is heated and cooled.
There are a variety of ways to create a controlled heating and cooling environment for the component. Heat lamps, heated blankets, and hot air guns are heat sources that can be controlled by the operator. Another way is to set the component in the sun for a specified period of time to heat, then move it into the shade to cool.
A benefit of active thermography is the operator has control over the thermal cycle. This control also allows for the component to be consistently heated and cooled multiple times. The operator has control over the thermal variables in this method of infrared thermography inspection
There are a few different types of active thermography testing, including flash thermography, lock-in thermography, and vibrothermography.