A rainbow hologram trades vertical parallax for daylight viewing

What you see

In a museum case, a rainbow hologram shows a picture with real depth that stays sharp under an ordinary lamp or sunlight, no laser required. Move your head side to side and the scene's parts shift the way a real object would. Move up and down instead, and the picture does not reveal a new view; it changes colour, sweeping through the spectrum. The MIT Museum's own glossary calls this a white light transmission hologram, 'often called rainbow holograms,' and states they are 'illuminated with incandescent light (white light)' rather than laser light.

How it works

The trade-off has a specific cause. A US patent for making 'reduced bandwidth holograms,' assigned to Polaroid and filed in October 1969, describes the method: a narrow horizontal aperture eliminates vertical parallax during recording, and on playback 'a real image of the aperture is formed' that the viewer looks through 'as if it were a window' onto the scene. Discarding vertical information reduces what must be reconstructed, allowing playback under an ordinary point-like white light instead of a laser's narrow-wavelength beam.

MIT Museum credits the best-known version to a specific inventor: its own collection page states it holds 'the first white light transmission hologram, created by... MIT Professor Stephen Benton.' The glossary's entry for reflection holograms describes a different family, 'lit from the front... like a painting,' underlining that this aperture-based, white-light-viewable trick belongs only to the transmission branch.

Viewing conditions and limits

Because vertical parallax was never recorded, moving vertically cannot show a new angle the way moving horizontally can; the glossary states only that 'the colours change as the viewer moves up and down.' The horizontal axis keeps its depth cues, the entire point of this trade. Both sources describe incandescent, point-like illumination rather than diffuse room light, though neither gives an exact recommended lamp distance, so any specific exhibit's lighting instructions are that exhibitor's own setup, not a figure from these sources.

What it is not

This is not an ordinary laser transmission hologram, which the glossary notes was 'the earliest type of hologram, developed by Leith and Upatnieks in 1962' and still needs coherent laser light to reconstruct. Nor is it a reflection hologram, the front-lit type the glossary compares to a hung painting. A rainbow hologram is a third category, trading one axis of parallax for everyday-light viewing, not a lesser hologram than the other two.

• Can it be seen under an ordinary lamp, or does it need a laser, distinguishing a white-light hologram from the 1962-style laser transmission kind?
• Does the image change colour moving vertically rather than revealing a new angle, confirming the parallax trade-off in the patent record?
• What light source and distance does the exhibitor recommend, since the aperture-based method is built around a specific illumination geometry?

The rainbow hologram is a documented engineering trade, recorded in a Polaroid-era patent and described plainly by the museum holding Benton's own example, not an unexplained curiosity.