A holographic recording medium is treated to reduce or eliminate unwanted extraneous diffraction in a volume transmission hologram. Surface regions of the medium are desensitized either before or after a holographic recording to reduce the contrast of the holographic fringe pattern along the surfaces, and thereby to eliminate planar phase grating effects. Treatments include applying a burst of gas or vapor to the surface, irradiating the surfaces with highly absorbed incoherent light, thermally treating the recording medium.