Clearing the Image

On a flight from Amsterdam to Paris in 1965, French Astrophysicist Jacques Blamont had a sad tale to tell his traveling companion. Because of faulty equipment, every photographic plate of groups of stars exposed during a complex and expensive balloon-borne telescope experiment had been hopelessly blurred. The companion, University of Michigan Electrical Engineer George Stroke, was less discouraged. "Don't throw anything away," he urged Blamont. "Give me time and I'll get pictures out of your ruined film."

Stroke, who is now on the faculty of the State University of New York at Stony Brook, is nearly ready to make good on his promise. Next week, in The Netherlands' prestigious Physics Letters, he tells how he has used holography (TIME, March 18, 1966) to produce relatively clear images from badly blurred photographs.

In a perfectly formed image, Stroke explains, every geometrical point on the photographed object should produce a corresponding point on the film. But existing optical systems, instead of producing points, project tiny spots, each of which consists of alternating light and dark concentric circles, or diffraction patterns. If the optical system is in focus, the circles are small. But if it is out of focus, the circles are large and overlap each other, causing blurring.

Noticeably Clearer. Nonetheless, Stroke says, all of the details of the photographed object are contained in the picture. The overlapping of spots, no matter how blurred the image, can be expressed in complex mathematical terms called Fourier transforms. Applying mathematical theory to holography, which also produces interference patterns that can be expressed by Fourier transforms, Stroke set up the optical equivalent of an equation. Using laser light, he made two transparencies --one of the blurred photograph of a microscope, the other of a purposely blurred picture of a spot of light shot by the same camera. Then he produced a hologram by projecting laser light through the blurred light-spot transparency onto unexposed film while simultaneously shining a beam split off from the same laser directly onto the film, thus producing the holographic interference patterns.

Having set up the optical equivalents of Fourier transforms, Stroke beamed laser light first through the transparency of the blurred microscope photograph and then through a "dividing" filter that consisted of both the hologram and the transparency of the blurred spot of light; in mathematical terms, he had thus divided one transform by another. Projected onto film the beam produced a crude but noticeably clearer picture of the microscope. Stroke had solved his optical equation.

As soon as he has perfected his technique, Stroke says, holographic deblurring should become invaluable in salvaging out-of-focus pictures from deep space probes, balloon flights, aerial reconnaissance and other photographic missions that are difficult to repeat. Eventually, Stroke's process may be used to bring a professional sharpness to the pictures of even the most inept amateur photographer.

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