Abstract: A system for sequentially measuring the current generated by a plurality of individual current sources such as photodiodes monitoring the light output of individual LEDs representing discrete colors arranged in an array of pixels constituting an area illumination source such as an electronic billboard includes (1) a current measuring circuit connected to each current source/photodiode; (2) a separate switch such as a switching diode connected in series with each current source/photodiode and (3) sequential control means for sequentially enabling each switch/switching diode to conduct current from the current source/photodiode to the current measuring circuit while maintaining all of the remaining switches in the disable state.