This is the instantaneous, most-recently-sampled value of the received optical power level measured at this port's input during the current performance-monitoring interval, expressed in the device's optical-power units. A live received-power reading is the first thing an admin checks when a receiver reports a loss-of-signal alarm, to see whether power is actually present right now. For example, if an admin swaps a suspect patch cord and immediately sees inOpticalPowerInstant jump back into the expected range, that confirms the cord, not the transceiver, was the problem.