A human presence detection system (50) that employs a frame differencing technique for subtracting out background interference from images generated by the system. The system (50) includes an infrared source (28) that generates a beam of infrared radiation, and an infrared detector (72) that receives infrared radiation reflected from objects in the path of the beam. Face recognition software is employed to determine the presence of a person (16) from the reflected radiation. The infrared source (28) is pulsed on and off and the detector (72) is synchronously shuttered to the pulses so that image frames are generated at different times, where one frame includes reflected radiation and background radiation and another frame includes only background radiation. The frames are subtracted to separate out the background radiation. In one embodiment, the detector (72) includes a pixel array of photodiodes (90) and first and second capacitor storage sites (94, 98) for storing the image frames on a single CMOS chip (70). The storage sites (94, 98) are subtracted in a summation device (102) that is also on the CMOS chip (70). In an alternate embodiment, the frames are stored and subtracted at an off-chip site.