Benton's 1969 patent enabled holograms viewable without a laser

What you see

A viewer of a finished rainbow hologram, per MIT's own obituary of its inventor, "Holography pioneer Stephen Benton dies at 61," sees a hologram reconstructed under ordinary white light rather than a laser, most familiarly in the small embossed foil holograms later used on credit cards, with a band of colour that shifts as the viewer moves vertically. The original patent behind this is less glamorous: it describes a laboratory method for making a "low bandwidth stereoscopic hologram" meant to survive television transmission, not a consumer product.

How it works

The patent record itself, US Patent 3,633,989, "Method for Making Reduced Bandwidth Holograms," filed by Stephen A. Benton and assigned to Polaroid Corporation, sets out a two-stage process. First, a conventional two-beam hologram of a subject is made with laser light. Second, that hologram is masked except for a narrow horizontal aperture, illuminated to form a real image through only that slit, and a new photosensitive surface placed in that image records a second hologram, together with a reference beam. This second hologram has had nearly all of its vertical parallax discarded. The patent states directly that if the light source used for reconstruction is incoherent, the speckled appearance typical of laser-lit holograms is minimized, the property that permits reconstruction under an ordinary extended white-light source rather than a laser, dispersing colour vertically into a spectrum instead of blurring the image.

Viewing conditions and limits

Because vertical parallax has been deliberately removed, a viewer keeps horizontal parallax, so moving side to side still reveals different perspectives, but loses vertical parallax, so colour shifts with vertical position rather than staying fixed. That trade-off, not a flaw, is what makes single-axis white-light viewing possible at all. The patent's own stated purpose was reducing a hologram's information density enough for television broadcast; MIT's obituary is the source that connects this mechanism to the visible "rainbow" viewing behaviour and to the credit-card-scale holograms Benton later became known for.

What it is not

A rainbow hologram is not a full-parallax hologram: tilting it vertically does not reveal a genuinely different view of the recorded scene the way tilting it horizontally does, and colour here is an artifact of viewing angle rather than a recorded property of the object. It is also a different mechanism from Yuri Denisyuk's single-beam reflection hologram, which reaches white-light viewability through Bragg-selective layering rather than by discarding parallax.

• Does moving the hologram vertically reveal a new view of the scene, or only a colour shift?
• Is the described white-light viewing achieved by discarding vertical parallax, as in Benton's patent, or by a different mechanism such as Denisyuk's layered emulsion?
• Does the source distinguish the patent's stated bandwidth-reduction purpose from the white-light viewing property it enabled?

Benton's patent was filed to solve a television bandwidth problem, and the white-light viewing that made it famous reads, in the document itself, as a secondary property of an incoherent reconstruction source rather than the invention's stated goal. MIT's own obituary is where the connection to "rainbow holography" as a named, credited invention is made explicit.