Patchwork pattern in IR image

In a thermal image of an operating module, the patchwork pattern shows single cells running warmer than others, scattered with no regular arrangement. A patchwork pattern, or warmer cells near the bottom frame, strongly suggests potential induced degradation by shunting (PID-s). Follow up to find the root cause. The case for PID-s is stronger when the affected modules sit near the negative pole of the string, and the number of warm cells tends to match the module power loss. Image the modules in clear-sky sunlight while they operate near maximum power. At open circuit the pattern does not appear, and at short circuit the image is dominated by cells heating in reverse bias. Take outdoor IR images at high irradiance (above 600 W/m2) under stable conditions with low wind and no clouds. Avoid reflections. If current is injected into a string at night or in the dark, PID-s affected cells look cooler than healthy cells, not warmer. A module that is short-circuited as a whole, through shorted bypass diodes or a wiring error, can also show warm single cells without a pattern. In that case module power drops to almost zero and Voc falls sharply, so check the wiring. Treat temperature differences of only 1 to 2 K with care, because differences in emissivity or convective cooling can be larger than that.