George Leslie Clark

Exploding-Wire-Driven Shock Waves

The explosion of fine silver wires by the fast discharge of a low-inductance capacitor has been photographed with an STL Model C Image Converter Camera operated as a streak camera. All of the previously observed shock waves and contact surfaces have been clearly recorded [1–3]. In addition, the large effective aperture, f/0.5, of the camera, due to its fast optics, 50: 1 light gain, and 0.5-µsec phosphor persistence, has allowed the initial shock wave in air at atmospheric pressure to be photographed by its own luminosity. Photographs of both radially and circumferentially propagating shock waves, depending upon dwell duration, have been recorded during the second conduction phase of the discharges.

Light Economics In High-speed Photography

A figure of merit, called the Transfer Efficiency, may be calculated or measured for any camera based upon the exposure and the radiance of the subject required to produce a density of 1.0 on the film. The Transfer Efficiency of a camera serves to calibrate it so that the luminosity of events may be determined from measurements of film density. It also allows one to know in advance which experiments may be instrumented with a particular camera. Basic photometric units and measurements are discussed. The Transfer Efficiency is calculated for several representative cameras, including a Kerr cell camera, a rotating mirror camera, and the STL Image Converter Camera.

Laser-Illuminated Photo-Optical Instrumentation

The characteristics of the laser give it a unique potential value in areas of optical instrumentation which require short light pulses of high intensity. A solid-state laser may be operated in pulses 10 to 20 nanoseconds in duration with intensities far in excess of that required for most photo-optical instrumentation applications. The temporal coherence of the laser eliminates problems due to chromatic aberrations in the optical system and also permits the experimenter to discriminate against light emitted by the experiment itself.

Image Converter Shutters

An image converter tube contains a photoemissive surface which emits electrons from each small area in proportion to the intensity of light striking it. The electrons are pulled away from the surface, or photocathode, by an electric field and accelerate through a vacuum to a fluorescent screen. An electron lens may be used to bring together all of the electrons emitted from any point on the photocathode to a corresponding point on the screen, creating an electron image of the light intensity pattern on the photocathode. The phosphor converts the electron energy to light, recreating the original optical image.