Spatial interference: from coherent to incoherent.

We report an optical interference experiment which seems to contradict our common knowledge, in that the formation of the interference pattern originates from a spatially incoherent light source. Our experimental scheme is very similar to Gabor's original proposal of holography [Nature (London) 161, 777 (1948)], except that an incoherent source replaces the coherent one. Though an instantaneous interference pattern between an object wave and reference wave fluctuates irregularly, a well-defined pattern appears in the statistical average, in accord with a hologram in the coherent light case.