Answer is Microbolometer.How does Spike ER achieves long range as no kind of external cooleing, and if it is cooled internally by Stirling engine or thermocouple then why use BCUs for MANPADS, as claimed spike er range suggests it's sensitive enough for acquiring target without use of any BCU. Also doesn't ground targets require more contrast than aircrafts for target acquisition.
But even in that case, spike failed in our summer trials.