Letters

What Is Holography?

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Science  28 Jun 1996:
Vol. 272, Issue 5270, pp. 1857e-1861e
DOI: 10.1126/science.272.5270.1857e

The Research News article “Two versions of holography vie to show atoms in 3D” by Steve Nadis (3 May, p. 650) discusses exciting new developments in x-ray analysis at atomic resolution (1). Is it accurate, however, to describe these methods as holography? Coherent illumination is not required, and the methods described allow one to reconstruct a representative unit cell when many unit cells are rotationally (although not necessarily translationally) aligned, rather than a point-to-point image of an object in the usual sense. Can it be applied to a single unit cell (that is, a noncrystalline specimen)? Issues which must be dealt with include fundamental considerations of radiation damage (2), even for materials science specimens, and of the desired condition |a| << |r| in holography between a reference wave r and an object wave a (diffraction analysis considers |a|**2).

The Research News article states, “x-ray holography, like crystallography, cannot illuminate highly disoriented samples such as living biological tissue.” This statement is not in agreement with findings (3) about the use of x-ray holography to image subcellular structures and microfabricated test objects at sub-100-nanometer resolution; holography with x-ray lasers has been demonstrated (4) as a step toward flash imaging of initially living specimens. Other publications report biological imaging at a resolution of less than 60 nanometers by x-ray holography and plans for extension of the method to frozen hydrated specimens and 3D reconstruction by means of holographic tomography (5). These experiments involve holography in the usual sense of the word: A nonrepetitive object is illuminated by a coherent beam, and a classical image is reconstructed by propagation of a reconstruction wave through the processed hologram.

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