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If we assume the circuit was copied from a commercial design then the test pulses ought to be longer than the C1R3= 2.2 s  time constant and with a slow rise time to mimic nature. 

However if this detector is looking a pulse modulated IR light during sunlight with attenuation of fog attenuating both optical events then a higher frequency pulse can be blocked by a higher current pulse as the R2 22Meg gets shunted by the C1 coupled 1 MegR1.  So the high frequency sensitivity is blocked by both too low a current pulse and too high a current pulse at too high a frequency limited by R1C1..

The purpose is getting clearer but still foggy.