Few inventions have inspired more fantasy and achieved less of a purported potential  than holography. Predicted and partially demonstrated by 1950, it took the invention of the laser in 1960 to create the first practical visible light holograms. The decade-long wait merely increased anticipation. Many futurists believed that holography was the analogue to the invention of photography itself, a twentieth century expansion of the medium into the third dimension. But there were problems...

In order to make its complex image, the hologram had to be recorded on emulsions that were extraordinarily insensitive. During an historical period when ordinary photographic films pushed to ASA ratings above 1000, holographic emulsions went well below ASA 1. Exposure times were in the order of tens of minutes. As if that were not bad enough, subjects had to remain stationary to fractions of a wavelength of light. Any movement above these dimensions blurred the interference pattern and made the expensive plates useless. Then the plate itself was required for playback. There was no such thing as a holographic projector. So the plates had to be rather large compared to 35 mm film, the common photographic format that can be enlarged from a negative to a projection positive or to paper for display. As a consequence some holographic plates were a meter square. These holograms were inordinately expensive and hard to handle. Recording required lasers with coherence lengths equal to the depths of the 3D object, and playback required monochromatic light, also from lasers. Even with the invention of Benton's rainbow hologram in 1969 which included a reproduction step from a holographic master to a display plate and could be played back under ordinary incandescent light, the hologram remained an impractical curiosity. It was pursued by technical wizards and aficionados, but it never saw a popular dissemination into the general public.

As originally conceived, a hologram was the detailed interference pattern of every point radiating light from the surface of a target subject. As a result the hologram could be cut into smaller pieces, and the subject could still be seen nearly in its entirety, albeit from some inconvenient viewing angles, much like looking through a keyhole. This surplus of information in the hologram coupled with its playback through the original holographic plate were two aspects of the invention that worked against its practical implementation, notwithstanding that these features were a contributing source of wonderment.

A hologram is a diffraction grating, and conversely a diffraction grating is a type of hologram. The difference between a plane grating and a conventional image hologram is in complexity. However, there is a simple case. Call it the vanilla flavored hologram. If a hologram is made by the interference of two plane waves, the hologram is a plane grating like those made by ruling machines. If the hologram is made by the interference of a plane wave and a spherical wave, the resulting diffraction grating has regular features that are typically conical sections.

Gratings of this type have features that vary in curvature and spatial frequency. The curves are parabolic in appearance. The frequencies are modulated, a feature that is sometimes referred to as a chirp. Such gratings have never been produced by the ruling machines which traditionally manufactured scientific grating masters. The properties of this type of grating are not widely discussed in the literature, but there are performance advantages. They behave like a parabolic reflector, a surface often used as a substrate in spectrometry gratings.

Chirped holographic gratings may be deemed complex in the context of plane gratings, but they are simple holograms. When fabricated by the intersection of a plane wave and a spherical wave, they are no more than the hologram of a single point. Existing in a world between conventional holography and conventional machine ruled gratings, the chirped grating allows for an unprecedented method for taking 3D pictures.

The key is making the recording not directly on a holographic plate but through one. When the grating creates the image it has regular features that can be digitized and stored on a computer for image processing. The acquired image can then be played back by any process that reproduces the 3D surface. The flavor of the hologram is vanilla, but the wave front striking the grating is complex. The resulting image is a profile of the surface which can be acquired directly, not in coherent illumination as is required for conventional holography, but in incoherent and broad band illumination. The surface does not need to be held stationary to the wave length of light. The process tolerates motion of target and apparatus as well as conventional photography. It comes down to this. Rather than making a separate complex grating for each recorded surface as in holography, a single grating is used to record any surface. That one hologram can be made on a lab bench in controlled settings where exposure times are relatively short and laser light is used at its greatest efficiency. The masters lend themselves to duplication by embossing and other mass replication techniques, because the groove structures are highly organized, especially when compared with conventional holograms.