A computer includes a processor and a memory, the memory stores instructions executable by the processor to project a first specified time-varying illumination pattern to an area outside of an object undergoing authentication, wherein the area outside of the object undergoing authentication excludes pixels corresponding to the object undergoing authentication; capture images of the object and a portion of the area outside of the object; and authenticate a scene including the images prior to authenticating the object, based on a comparison between the first specified time-varying illumination pattern with a second time-varying illumination pattern extracted from the area outside of the object in the images.