A technique has been devised for electronically eliminating background signals in chopped infrared measuring systems by subtracting a reference signal of proper frequency, phase, and amplitude from the background signal. Subsequently acquired point‐source or distant‐small‐source signals are converted to dc voltage levels which are directly related to incident flux. The system, based on operational amplifier principles, permits measurements of weak radiant emissions in the far ir under signal‐to‐noise conditions beyond the capabilities of previous field portable instruments.