HIGH-SPEED IMAGING: Spinning disk records holographic movies

Researchers at North Dancer Labs Inc. (NDL; Shelburne, VT) are taking holographic movies of high-speed ballistic events and receiving a double reward for their efforts. The technique—developed at NDL under a series of Small Business Innovation Research contracts—produces a wealth of precise three-dimensional (3-D) data on the interaction of a projectile with its target, with frame rates reaching 500 kHz and a field of view extending to as much as a meter. At the same time, the resulting series of images retains information on the phase of the recorded wavefronts, allowing shearing interferometry and Schlieren imaging to be performed on the data.

At the heart of the system is a disk coated with a recording medium and spun to speeds of up to 10,000 rpm. The disk, which can range from 100 to 350 mm in diameter, records images of 5–12 mm in size. The first system built by NDL operates at 10 kHz and has enough space on the disk to record 120 images per event. A second version is used in combination with a repetitively pulsed ruby laser to store 2000 holographic images recorded at a 500-kHz rate. A third version uses a copper-vapor laser to record multiple views of an event at a speed of 50 kHz. Although the beam from the copper-vapor laser used in the third version is multimode, NDL has developed proprietary techniques to record high-quality holograms, according to Charles Lysogorski, president.

The ballistic object to be recorded can be illuminated by either specular or diffuse laser light. Diffuse illumination increases the recordable depth at the expense of lateral resolution. Recording media used by NDL include photographic film and bacteriorhodopsin, a light transducer that acts as an erasable holographic material. The highest resolution so far, achieved with specular illumination over a 100-mm field of view, is 25 µm at a 30-kHz data-collection rate. "Assuming you need three pixels to resolve a particle, this is equivalent to a charge-coupled-device (CCD) camera with 12,000 pixels on a side," says Lysogorski. The highest recording rate of 500 kHz is achieved by reference-angle multiplexing, a technique in which several reference beams at different angles allow multiple images to be simultaneously recorded over each other. The individual images can be separately played back later by reilluminating the hologram with the chosen reference beam. Multiple views of the same object at the same instant in time can be recorded simultaneously, with the various angular views imaged onto the hologram by the use of mirrors.

The NDL researchers have built a set of computer-controlled stages containing a CCD camera for digitizing the set of images recorded on a disk. The stage is translated to selected x-y-z positions and the image is digitized. The computer then finds the next image on the disk and digitizes it, and so on. The resulting series can be presented as a movie. Alternatively, images from the disk can be played in real time onto the CCD camera, resulting in a "slide show," Lysogorski explains, adding that it takes a fraction of a second for the disk to be rotated from one hologram to the next.

High-speed events of interest to the researchers include the analysis of composites subjected to impact, the performance of shaped explosive charges, the integrity of liquid-carrying vessels, and shock-wave analysis. Shearing interferometry can be performed on a recorded wavefront of a shock wave to determine its phase; in addition, Schlieren testing—a noninterferometric technique using a knife edge to produce intensity variations from phase variations—can be used to bring out the details of a shock wave.

Along with multiple views of each recorded event, the researchers can vary focus to obtain multiple images along the object's depth. "[The result is] a tremendous amount of information," says Lysogorski. Predicted improvements will result in a resolution of 20 µm over a 300-mm field of view.