Shufflebrain

Holography does not require lenses. But lenses may be employed to produce certain special effects. Leith and Upatnieks showed in one of their earliest experiments that when the holographer uses a lens during construction, he must use an identical lens for reconstruction. This fact should (and probably does) interest spies. For not even Gabor or Leith and Upatnieks can read a holographic message directly. It is a code in the most cloak-and-dagger sense of the word. A hologram must be decoded by the appropriate reconstruction beam, under specific conditions. And a lens with an unusual crack in it would create an uncrackable code for all who do not possess that same cracked lens.

We might even come to use a combination of different construction angles and flawed lenses to simulate malfunctions of the mind. Suppose, for instance, a holographer makes a hologram of, say, a bedroom wall, and onto the same film also encodes the image of an elephant, using a lens at this stage. Given the appropriate conditions, he could synthesize the bizarre scene of a pink elephant emerging from the bedroom wall. Humans hallucinate similar scenes during delirium tremens.

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Leith and Upatnieks also made color a part of holography. Physically, a particular hue is the result of a specific energy or wavelength. What we usually think of as light is a range of energies lying in the region of the electromagnetic spectrum visible to humans (and accordingly called the visible spectrum). Specific molecules in our rods and cones make the visible region visible. Red light lies on the weaker end of the spectrum, while violet is on the stronger end. Thus, infra red waves have energies just below red and ultra violet waves are stronger than violet. Physicists often deal with color in conjunction with the subject known as dispersion. For when white light, say a sun ray, passes through a prism, the beam disperses into red, yellow, green, and blue light. (Dispersion also accounts for rainbows.) White light, remember, is a so-called spectral mixture. And full-color illumination of a multicolored scene requires white light.

It is possible to produce white light by mixing red, green, and blue lights. Thus the latter are called the additive primary colors. Not only will they produce white light but varying combinations of them can yield the half-million or more hues we humans can discriminate.

The colors we see depend on which wavelengths reach our retina. The pigment in a swath or red paint looks red in white light because the molecules absorb the other wavelengths and reflect red back to our eyes. The sky looks blue on a clear day because the atmosphere absorbs all but the energetic blue violets. The sea looks black on a moonless night because nearly all the visible wavelengths have been absorbed. Light is energy. Thus tar on a roof heats much more in the sunlight than does a white straw Panama hat; the tar has absorbed more energy than the hat and has therefore reflected less.

Photometrists use the word additive to describe red, green, and blue lights because subtractive primaries also exist: magenta, yellow, and green-blue. When magenta, yellow, and green-blue filters are placed in the path of a beam of white light, no visible light can pass through. The result is sometimes called black light. Black light is a potential product of even the three additive primary colors. For red and blue can produce magenta; green and blue can produce yellow, and, if the algebraic mix is right, some green-blue as well. And a beam of white light--a mixture of the primaries, red, green, and blue--can color a scene white, black, or anything in between, depending on the relative amounts of each primary color.

Leith and Upatnieks described how they would "illuminate a scene with coherent light in each of three primary colors, and the hologram would receive reflected light of each color." Now the hologram plate itself was black and white. For the hologram remembered not color itself but a code for color. Yet when Leith and Upatnieks passed a red-green-blue beam through the hologram, they produced, in their own words: "the object in full color."[2]

Offhand, it might seem as though the reconstruction beam would have to be the same color as the original light source. But Leith's equations said something different: the reconstruction beam's wavelength must be equal to or shorter than that of the original. He and Upatnieks tested the hypothesis. And it worked: they could change the scene from one color to another, if the decoding beam's wavelength did not exceed that of the original beam.

Let's put this property to use in our analogies.

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