The dependence on the source location of photothermal bistabilities of a thin CdS:Cu film is demonstrated by the investigation of bistability in luminescence in reflection geometry. Particularly, it is shown that the infrared portion in the reflected beam clearly exhibits contrasting bistable loops in contrast to the reflection itself. The unexpected effect is explained by the fact that bistability in luminescence is a bulk feature which is not affected by the surface region. ©1995 American Institute of Physics.