5. A POSSIBLE "SINGLE FRINGE" HOLOGRAPHIC CAMERA
Taking yet another cue from Rainbow holography, we can alter a one-step hologram setup [5] and create something which incoherently records white-light holograms without employing coherent optics. In figure 12 the object O is illuminated by a source of structured light SL. SL might be a conventional slide projector with a multi-aperture plate. SL must project tiny spots of illumination onto object O, therefore the depth of field and diffraction limits of the SL device are important. Light from one of these spots on object O is focused by lens L to proudce real image I positioned in front of the hologram recording emulsion H. Aperture plate AP contains a narrow, curved slit with radius 2f; with radius of twice the lens L focal length. As a result, a narrow curved line of light is projected onto the hologram emulsion. The radius of the curve is proportional to the distance between image I and hologram H. Each bright point projected by SL onto object O produces a separate "curved scratch" pattern on H. Note that the illumination might be incoherent white light, and there is no reference beam involved. The resulting "abrasion hologram" pattern could be etched into a metal plate in order to convert the recorded lines into surface scratches (we might paint the etched scratches with gloss black paint, so the paint's meniscus smoothes the square-bottom etched cavities.) Rather than using a single curved slit in plate AP, a series of vertically nested slits could be used. Or instead the slit could be replaced by an aperture pattern resembling the "fresnel-lens-like" scratch-segment pattern mentioned in section 3.4 and depicted in figure 11b. The slit or slits are intended to project a curved-line pattern onto H, so they must not be so narrow that they produce significant interference effects. The aperture plate behaves as a "single-point hologram", and the optical system "rubber stamps" numerous copies of this pattern onto the film plane. The radius of curvature of the recorded patterns is different for different depths of real image point. This last aperture-plate idea is partly inspired by Siemens-Wapniarski and Givens' paper[6], where they justify the practice of producing synthetic holograms by recording single-point holograms on an emulsion; recording one zoneplate at a time. Note that all of the above description of a camera-device is only theory-informed speculation, and this technique hasn't yet been explored empirically.
6. CONCLUSION
At present the main use for "abrasion holograms" is educational. Students can easily produce them at almost no cost. Contrast this with the complexity and expense of even the simplest holographic recording method. In addition, the macroscopic size of the "fringes" in these holograms gives students direct clues as to how Rainbow holograms function. The concepts are no longer buried in the mathematics of 3D wave interference. Also, scratch-holograms communicate some new insights as to the nature of holography, they supply an "alternate mental model" which in some situations might be useful in research. Finally, they provide an interesting hobby where one can sit outdoors on a sunny day with a stack of plastic plates and a set of dividers, while doing simple physics and exploring a strange little niche in optical science.