1. A system for capturing images of a scene using wide band illumination with known spectral distributions in the presence of ambient illumination having an unknown spectral distribution, comprising: a digital camera; and a wide band illumination source in the form of a multi-stage electronic camera flash unit, comprising, an illumination source comprising multiple flash elements each producing illumination having the same known spectral distribution, and multiple wideband filters each having a different known spectral transmittance distribution, wherein a different one of said multiple wideband filters is disposed in front of each of the multiple flash elements, and wherein the multi-stage electronic camera flash unit is configured so as to, when triggered, cause each of the multiple flash elements to illuminate the scene being captured by the digital camera at a different time.