Rotating mirror cameras have been the mainstay of mega-frame per second imaging for decades. There is still no
electronic camera that can match a film based rotary mirror camera for the combination of frame count, speed, resolution
and dynamic range. The rotary mirror cameras are predominantly used in the range of 0.1 to 100 micro-seconds per
frame, for 25 to more than a hundred frames. Electron tube gated cameras dominate the sub microsecond regime but are
frame count limited. Video cameras are pushing into the microsecond regime but are resolution limited by the high data
rates. An all solid state architecture, dubbed 'In-situ Storage Image Sensor' or 'ISIS', by Prof. Goji Etoh has made its
first appearance into the market and its evaluation is discussed.
Recent work at Lawrence Livermore National Laboratory has concentrated both on evaluation of the presently available
technologies and exploring the capabilities of the ISIS architecture. It is clear though there is presently no single chip
camera that can simultaneously match the rotary mirror cameras, the ISIS architecture has the potential to approach their
The advent of CCD cameras and computerized data recording has spurred the development of several new cameras and techniques for recording nanosecond images. We have made a side by side comparison of three nanosecond frame cameras, examining them for both performance and operational characteristics. The cameras include; Micro-Channel Plate/CCD. Image Diode/CCD and Image Diode/Film; combinations of gating/data recording. The advantages and disadvantages of each device will be discussed.
A novel technique to multiplex high energy laser pulses from a Nd:YLF laser into an array of fibers at energies near the bulk damage limit of fused silica is presented with the object of delivering N equal high energy laser pulses with a minimal time dispersion. Characteristics of the multiple fiber system, diffractive grating splitter, and spatial mode structure of the laser to minimize fiber damage are presented along with preliminary results in scaling the system to larger fiber numbers (N approximately equals 200) with a high energy 10-Joule Nd:Glass laser system. Fiber array alignment techniques and morphologies of fiber damage will also be presented and discussed.
We have previously reported our observations of the dynamic behavior of laser driven plates. Recent improvements and modification of the imaging techniques have identified and provided measurements of Raleigh-Taylor (R-T) instabilities that occur in these events. The microscope system in the LLNL Micro Detonics Facility, was converted to an epi- illuminated polarization configuration. A double pulse nanosecond illuminator and a second independently focusable frame camera were also added to the system. A laser driven plate, that is a dense solid driven by a laser heated, lower density plasma, is inherently R-T unstable. The characteristics and growth of the instability determine whether or not the plate remains intact. In earlier reports we correlated the surface patterning of thin plates with the fiber-optical transmission modes. In subsequent experiments we noted that the plasma burn through patterning in thin plates and the surface patterning of thicker plates did not correspond to the thin plate early time patterning. These observations led to the suspicion of R-T instability. A series of experiments correlating plate thickness and pattern spatial frequency has verified the instability. The plates are aluminum, deposited on the ends of optical fibers. They are launched by a YAG laser pulse traveling down the fiber. Plate velocities are several kilometers per second and characteristic dimensions of the instabilities are a few to tens of microns. Several techniques were used to examine the plates, the most successful being specularly reflecting polarization microscopy looking directly at the plate as it flies toward the camera. These images gave data on the spatial frequencies of the instabilities but could not give the amplitudes. To measure the amplitude of the instability a semi-transparent witness plate was placed a known distance from the plate. As above, the plate was observed using the polarization microscope but using the streak camera as the detector. Both the launch of the plate and its impact into the witness plate are observed on the streak record. Knowing the plate velocity function from earlier velocimetry measurements and observing the variations in the arrival time across the plate, the amplitude of the instability can be calculated.
Computer-generated, ion-milled, holographic structures and gratings are being used for multiple splitting of high-power pulsed laser beams. The structures were manufactured by Teledyne Brown Engineering in Huntsville Alabama and both a linear grating and a 2D structure were tested. Damage thresholds were measured in single-shot exposure using a 300 mj, 1 micrometers wavelength laser with a nominal 15 ns gaussian pulse. Energy densities on the gratings were adjusted with a focusing lens and the energy distribution was mapped with an imaging profiler. The gratings were examined dynamically for sparking and breakdown. They were examined after exposure using phase contrast microscopy. Initial results indicate the grating surface is always damaged before the smooth surface independent of the beam direction. The initial indication is that the average energy density threshold for single-shot damage is in the excess of 12 j/cm2.
Laser driven plates have been used for several years for high velocity shock wave and impact studies. Recent questions about the integrity and ablation rates of these plates coupled with an improved capability for microscopic stop motion photography led to this study. For these experiments, the plates were aluminum, coated on the ends of optical fibers. A high power laser pulse in the fiber ionizes the aluminum at the fiber/coating interface. The plasma thus created accelerates the remaining aluminum to high velocities, several kilometers per second. We defined `thick' or `thin' coatings as those where a flying plate (flyer) was launched vs. the material being completely ionized. Here we were specifically interested in the thick/thin boundary to develop data for the numerical models attempting to predict flyer behavior.
Cameras and laser illuminators have been upgraded and combined into a coaxial streak and frame system. The argon laser illuminating the streak camera over the duration of the event is combined with the short pulsed dye laser for stop motion framing, using a polarization splitter. The combined beams are sent to the target, viewed by the same objective lens, then split by a second polarizing splitter and relayed to the appropriate cameras. A half-wave plate before the second splitter allows adjustable levels of light of each laser to be leaked to the opposite camera for fiducial purposes. A particular advantage of this arrangement is that both the laser beams are tailored and aimed separately. The dye laser is spatially filtered and focused to cover the frame. The argon laser is focused to a line on the target which is then imaged to the streak camera slit. The ability to see the focal line of the argon laser on the frame camera allows precise collocation of the fields of view of the two cameras.
Recent advances in high-speed microscopy allow successful streak photography of nonreactive shock waves passing through individual explosive crystals. Measurement of the shock velocity both in the crystal and in the water surrounding the crystal allows us to determine the shock pressures. These diagnostic tools have the required temporal and spatial resolutions to enable us to study the microscopic processes in nonreactive and possibly reactive shock environments.
In high explosives designed for air blast cratering fragmentation and underwater applications metallic additives chemically react with the oxidizer and are used to tailor the rate of energy delivery by the expansion medium. Although the specific mechanism for sustained metal combustion in the dense detonation medium remains in question it is generally accepted that the fragmentation of the molten particle and disruption of its oxide layer are a necessity. In this study we use high speed microphotography to examine the ignition and combustion of small 25-76 jim diameter and 23 mm long aluminum wires rapidly heated by a capacitor discharge system in water. Streak and framing photographs detailing the combustion phenomenon and the fragmentation of the molten aluminum were obtained over periods of 100 nsec - 100 j. tsec with a spatial resolution of 2 . im. The wire temperature was determined as a function of time by integrating the circuit equation together with the energy equation for an adiabatic wire and incorporating known aluminum electrical resistivity and temperature functions of energy density in the integration. In order for the aluminum to sustain a rapid chemical reaction with the water we found that the wire temperature has to be raised above the melting temperature of aluminum oxide. The triggering mechanism for this rapid reaction appears to be the fragmentation of the molten aluminum from the collapse of a vapor blanket about
SC658: Challenges in Imaging for Data, Evidence & National Security
Imaging, the use of photographic techniques, has become an important tool for scientific data, legal evidence, and national security. Every phase of the photographic process alters the information carried within the image. In a time when image manipulation has become commonplace, it is a challenge to make an image record information that is scientifically and legally defensible. This course will explore the technical issues of imaging where accuracy, validity, and credibility are the key issues. This is a system survey, touching on all aspects of the process for both film and digital technologies including subject, lighting, camera, image analysis, and archiving. An overview of techniques for choosing and adjusting equipment appropriate for given tasks will be given. We will cover the visualization and recording of diverse subjects ranging from microscopic biological samples and crime scenes to issues in surveillance, reconnaissance, ordinance evaluation and target designation. Finally, we will discuss the challenges and techniques involved in establishing a chain of evidence essential for defensible information.