1. A system for performing three-dimensional, three-component displacement or velocity field detection from a single observation direction defining an optical axis, said system comprising: a double-reference beam, off-axis holographic recording system disposed on said optical axis and including means for independently recording two three-dimensional images of a displacing particle or image-element field corresponding to two distinct moments on a single holographic plate, and means for reconstructing said images with separate beams; an image acquisition system disposed on said optical axis and including a large aperture image relay optics system to re-image said reconstructed images, a magnifying optical element for magnifying said re-imaged images, and a solid state detector for receiving said magnified images and digitizing them; and a control and processing means for receiving said digitized images and processing them according to an algorithm: EQU C(u,v w)=.intg..intg.I.sub.w.sbsb.1 (x-u,y-v,0)I.sub.w.sbsb.2 (x,y,w)dxdy EQU I.sub.w.sbsb.1 (x,y,0)=I.sub.1 (x,y,0)W.sub.1 (x,y) EQU I.sub.w.sbsb.2 (x,y,z)=I.sub.2 (x,y,z)W.sub.2 (x-.DELTA.x.sub.p y-.DELTA.y.sub.p z-.DELTA.z.sub.i where C equals correlation value, I.sub.1 equals initial particle-field image, W.sub.1 equals interrogation plane window centered at an origin, I.sub.2 equals displaced particle-field image, and W.sub.2 equals transplaced search volume window centered at a displacement estimate (.DELTA.x.sub.i, .DELTA.y.sub.i, .DELTA.z.sub.i).