This circuit is two wire light level detector, we don’t separate wires for power this sensor system and for delivering the output signal. With current loop, we do both function in a single pair of cable, just provide the voltage supply and measure the current as the output signal. The core of this circuit is LM10 voltage reference and op-amp integrated circuit, operating in floating mode. By shorting the supply pin to the output pin, the upper half of the final push-pull pair inside the IC will be disabled, making the output current is not related with any positive input signal, so a fix quiescent current will  be drawn from the loop. At negative signal of the input, the final stage begin working and will produce saturated current at few hundreds micro-volts input.

Light Level Sensor circuit schematic

[Circuit’s schematic diagram source: National Semiconductor Application Note]