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Application Notes

Image Intensifiers

Image Intensifiers are light amplifiers that can detect and image objects at extremely low light levels. The largest application for Image Intensifiers is in night-vision devices, used to “see” at night under moonlight or starlight conditions. However their use in many scientific and industrial applications has grown given their low noise, high sensitivity and unique high-speed gating capabilities. While night vision devices are typically viewed directly by eye, in most other applications image intensifiers are readout by digital cameras known as Intensified CCDs (iCCD.) With nanosecond (ns) gating capabilities, Image Intensifiers provide a unique ability to image very high speed phenomena. Applications are as diverse as fluorescence lifetime imaging, astronomy, combustion science, laser induced breakdown spectroscopy and plasma science.

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Fluorescence Lifetime Imaging Microscopy (FLIM)

Fluorescence Lifetime Imaging Microscopy is based on measuring the decay of fluorescence molecules in biological samples and other physical specimens. When a fluorophore is excited by a fast pulsed light source, it de-excites with an average lifetime, τ, typically between a few nanoseconds (ns) and a few milliseconds (ms). The initial intensity of light emitted by these excited fluorophores, decays with time t, as
. A unique property of fluorescence lifetime is its immunity to the local environment. Fluorescence intensity can often be affected by the concentration of fluorophore, photo-bleaching and variations in detection efficiency. Fluorescence lifetime by contrast, is an intrinsic property of the molecule. The lifetime can be changed due to other processes, such as non-radiative energy transfer to local molecules and other physical phenomena, enabling images of these processes. Measurement applications include neural imaging, cancer cell detection, cellular function and FRET.

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Ultra Violet Corona Detection

Corona is an electrical discharge that can occur around objects that have high electric fields, resulting in ionization of the surrounding material. This is a common occurrence in electric power transmission systems where it can occur around transmission lines, transformers, and insulators. Corona has a number of adverse effects including:

  • A loss of power in the transmission lines
  • Radio Frequency (RF) noise generation that can interfere with consumer and industrial electronics
  • The generation of ozone and NOx gases that have adverse effect on people living near transmission lines
  • Damage to the lines, transformers or insulators that can exacerbate the problem, leading to more power loss and eventual arcing conditions

If components of a transmission line system are experiencing corona and the problem is not fixed, the resulting damage can lead to electrical discharge, or arcing, to a nearby conductor. Arcing causes significant damage to the power transmission system and leads to excessive heating, resulting in potential failure.

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Planar Laser Induced Flourescence (PLIF)

Planar Laser Induced Fluorescence (PLIF) is a common analytical technique used to study gas and fluid flows. A laser light sheet is passed through the flowing gas or fluid. The wavelength of the laser is tuned to a specific absorption line in a molecular species within the fluid, or to a molecular tracer that can be added to the fluid. This wavelength is often in the Ultra-violet portion of the spectrum. Molecules that absorb this light transition to an excited state, and can subsequently decay by emitting light at a slightly longer wavelength in a process known as fluorescence. The rate of fluorescence is governed by conditions in the fluid or gas flow including temperature, pressure, velocity and molecular concentration. Carefully calibrated PLIF images can be used to derive the local conditions as a function of location in the flow, with each pixel in the Image Intensified CCD image acting as an individual detector. PLIF can be combined with other flow techniques such as Particle Image Velocimetry (PIV) or Interferometric Mie Imaging (IMI) to provide enhanced measurements.

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Laser Induced Breakdown Spectroscopy (LIBS)

Laser Induced Breakdown Spectroscopy (LIBS) is a powerful atomic emission spectroscopy technique used to determine elemental composition of almost any material. A fast laser pulse, typically nanoseconds or less in pulse width, is focused onto the sample. The high energy density of the laser pulse causes a very small amount of material on the sample surface, as small as nano-grams, to be ablated. The material lifted off the surface is heated to temperatures of tens of thousands of degrees, forming a plasma. A continuum of light is initially emitted from this hot plasma, containing little useful information. As the plasma cools and expands, the ionized atoms begin to recombine with electrons, producing characteristic spectral emissions that are fingerprints of the elemental composition of the surface. For nanosecond laser pulses, useful spectral information can be obtained after a microsecond. For femtosecond laser pulses useful spectral information can be measured after tens of nanoseconds. The optical light emitted by the expanding plume is collected by lenses and/or fiber optics and coupled to a spectrometer. The spectral signature is detected by an Image Intensified CCD (ICCD) which is gated off during the initial continuum emission and then gated on to measure the atomic spectra from the optimal time period for the specific application. The ability to rapidly gate the Image Intensifier on and off makes it the preferred detector for LIBS.

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Remote Sensing

The past 20 years have seen a significant increase in the number of satellite based instruments making measurements of the earth’s atmosphere. These remote sensing missions help to improve our understanding the processes involved in climate change. The same missions can often provide valuable data on the effects of solar activity on our atmosphere and the associated interference with communications and the power grid. The Far Ultra-Violet (FUV) spectrum, covering the wavelengths from 121nm to 200nm, provides a unique window on processes involving N2, O2, O and H in the upper atmosphere. These wavelengths are completely absorbed by the ozone layer and cannot be measured by terrestrial observatories. However, from space, emission features from these chemical species can be detected by FUV instruments leading to a better understanding of their composition and changes in that composition due to atmospheric and solar dynamics.

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Super Resolution Microscopy

The past several decades has seen the development of a number of techniques designed to overcome the diffraction limited spatial resolution of microscopes, typically around 250 nanometers for visible light. These techniques are classified as Super Resolution Microscopy. Single Molecule Localization Microscopy is a wide field sub-set of Super Resolution techniques based on imaging individual molecules many times and then combining these images to obtain resolution as much as 10 times below the diffraction limit. Light from each molecule is localized on a high sensitivity camera, producing an extended disk at the diffraction limit of the microscope. The disk is analyzed by fitting it in two dimensions with a point-spread-function, typically a Gaussian, describing the diffraction limited signal from a point source. The centroid of the disk provides an estimate of the location of the molecule. The precision of the centroid improves roughly as the square-root of the number of photons imaged. For example, the precision of the localization of a molecule imaged with 100 photons would be 10 times better than the diffraction limit. The number of photons can be increased by detecting more photons in a given image and/or by adding multiple images. However, it is critical that each molecule imaged is clearly separated from neighboring molecules; otherwise a diffraction limited image will result.

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