Rainbow hologram

The rainbow or Benton hologram is a type of hologram invented in 1968 by Dr. Stephen A. Benton at Polaroid Corporation (later MIT). Rainbow holograms are designed to be viewed under white light illumination, rather than laser light which was required before this. The rainbow holography recording process uses a horizontal slit to eliminate vertical parallax in the output image, greatly reducing spectral blur while preserving three-dimensionality for most observers. A viewer moving up or down in front of a rainbow hologram sees changing spectral colors rather than different vertical perspectives. Because perspective effects are reproduced along one axis only, the subject will appear variously stretched or squashed when the hologram is not viewed at an optimum distance; this distortion may go unnoticed when there is not much depth, but can be severe when the distance of the subject from the plane of the hologram is very substantial. Stereopsis and horizontal motion parallax, two relatively powerful cues to depth, are preserved. The rainbow or Benton hologram is a type of hologram invented in 1968 by Dr. Stephen A. Benton at Polaroid Corporation (later MIT). Rainbow holograms are designed to be viewed under white light illumination, rather than laser light which was required before this. The rainbow holography recording process uses a horizontal slit to eliminate vertical parallax in the output image, greatly reducing spectral blur while preserving three-dimensionality for most observers. A viewer moving up or down in front of a rainbow hologram sees changing spectral colors rather than different vertical perspectives. Because perspective effects are reproduced along one axis only, the subject will appear variously stretched or squashed when the hologram is not viewed at an optimum distance; this distortion may go unnoticed when there is not much depth, but can be severe when the distance of the subject from the plane of the hologram is very substantial. Stereopsis and horizontal motion parallax, two relatively powerful cues to depth, are preserved. The holograms found on credit cards are examples of rainbow holograms. Figure 2 shows an optical arrangement for making a rainbow hologram. The object is illuminated with laser light (not shown in the diagram), and an image is formed in the plane of the hologram plate used to record the hologram. A narrow horizontal slit is placed between the object and the lens. The hologram plate is also illuminated with a reference beam derived from the same laser (not shown in the diagram), and the interference pattern between object and reference beams is recorded.