The photorefractive materials Bi12Si020(BSO) and its structural isomorph Bi12Ge020 (BGO) have been used as real time holographic materials in a variety of optical storage and information processing applications in the last decade [Ref 1]. Their overall response speed and comparitive ease of crystal growth - and hence availability in relatively large sizes - have frequently made BSO and BGO the materials of choice in these applications despite the supe-rior diffraction efficiencies available with the ferroelectric compounds barium titanate and strontium barium niobate (SBN) [Ref 2]. An understanding of the properties of holographic devices utilizing these materials requires an accurate description of both the holographic formation and read-out processes. The mechanics of the recording process in photorefractive materials, in which photogenerated charge carriers redistribute themselves to form a periodic electric field variation in the medium, have been studied extensively and a number of useful theoretical models now exist [Ref 3, 4]. The holographic read-out process though similarly complex, has been afforded somewhat less attention.