It is well known that rainbow holograms can be reconstructed by a spatially extened (rather than a point) white light source. But this fact coule not be explained by available theories [1-6]. In this paper, we propose a new concept "the effective width of extended source" that can explain the above fact successfully. The effective width omes from two limiting apertures, the skut and the viewer s pupil. These two apeurertsc make a rainbow hologram different from other holograms. The effects of spattal and spectral extensions 011 the image point of the rainbow hologram were studied and proved to be approximately equivalent whether they exist separately or simultaneously. The resolution of the rainbow holographic image is derived for the spatial and spectral extended reconStrnctibn fiotirce. Some reasonable conclusions axe giveis.. Obyiotisly, -these analyses are useftil for ftirfclier trnderstanding the physical meaning of rainbow holograms, explaining some experimental phenomena^ reducing temperal and spatial coherence requiments, and chosing reasonable optical parameters in practice.