Photo refractive crystals are useful for spatial-temporal information processing of coherent optically modulated signals due to the real-time, read-write capabilities of the holographic medium. When used to write volumetric holographic gratings for real-time applications, it is necessary to isolate the recording, or writing beams, from the diffracted read-out beam. This isolation is an extremely important factor in adaptive feedback systems because it determines the amount of feedback gain available to the processor. This talk therefore provides a brief overview of the photo refractive effect and an introduction to momentum space before discussing the intricacies of photo refractive volume holography and potential read-write multiplexing geometries.