In this case of thermography the increase of surface temperature is monitored during the application of a stepped heating pulse (a “long pulse”). The sample is continuously heated, at low power. Variations of surface temperature with time are related to specimen features as in pulse thermography. This technique of step heating (SH) is sometimes referred to as time-resolved infrared radiometry or TRIR. The time-resolved part means the temperature is monitored as it evolves during and after the heating process.

SH finds various applications such as for coating thickness evaluation (including multilayered coatings, ceramics), integrity of the coating-substrate bond determination or evaluation of composite structures, characterization of airframe hidden corrosion among others.

In a typical set-up for SH experiments, an Argon laser is used to point-heat or line-heat the specimen and is time-gated using an acousto-optic modulator allowing a variety of pulse lengths. If the specimen is opaque at the Argon wavelength (= 0.514 µm) then surface heating is obtained. Dedicated electronics allow synchronization of the laser heating with respect to IR camera frame rate (notice that the IR camera operates here in the line mode). Three types of measurement are possible:

• temperature line scan at a specific time after heating,
• collection of temperature line scans as function of time (showing the temperature development in time over a location of the specimen) and
• reconstructed image at a specific time (reconstruction proceeds on a line by line basis with the specimen mounted on a positioning stage moved step by step).

One of the main advantages of TRIR is the temporal resolution which is faster than in full-field imaging since only one image line is acquired per thermal event (instead of recording a complete image for each thermal event). Such observation of quick thermal events enables inspection of thin coatings as explained below. A corresponding disadvantage is that the line must be scanned over the sample, thus increasing the time to analyze a given surface completely.