Multiple Coherence Volumes
In the illustration below the laser beam exits the laser in the upper right. It is then split by a beam splitter. Following the red path (red only for distiction and no correlation to laser wavelength) you can see it travels 3+14+15 = 32inches to the film plane. Now following each of the black paths from the first beam splitter you can see that the beam that illuminates the back of the object travels 14+2+8+8 = 32inches, as does each of the other paths 14+2+2+8+6 = 32inches and 14+2+2+2+8+4 = 32inches respectively to the film plane.
The benefit of this technique is as follows. If your laser only has a coherence length of 3 inches and it can be deduced that the object is 7 inches deep and only the center object beam is used, only 3 inches of depth will be visible in the final hologram, 1 1/2 inches in front of the center of the central object beam and 1 1/2 inches in back of the center of the central laser beam.
With the configuration below the front beam not only reaches to the front of the object which is only 1 inch away from the center of the front beam but also reaches 1 1/2 inches toward the back of the object at which point 1 inch back the second beam is reaching and the coherentness overlaps. Thus coherence limitations are maintained for each of the three "Volumes" of the object.
The expanded reference beam is not shown for clarity.
The coherence "Volume" is show with the object beams by only showing that part of the expanded object beam.