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Lock-in thermography refers to the process of repeatedly biasing a circuit at precisely timed intervals. For example, if there is a short between 2 pads on an unpopulated PCB, voltage may be applied to the pads at 1Hz frequency, 0.5 sec on and then 0.5 sec off. The calculations are more complex, but the process can be envisioned by subtracting the "off" image data from the "on" image data to create a differential image displaying localized heating. If only one power cycle is performed, the sensitivity of the detected heating is limited to the sensitivity of a single camera frame (~40mK for the IS640-17). If many cycles are performed however, the sensitivity increases proportionally to the square root of the number of cycles performed. In this way, heating < 0.001°C can be detected within a few minutes using individual image frames that each have ~0.04°C noise level.