Off-axis transmission holographic system for recording aquatic particles

We describe a holographic system for recording particles suspended in water. The hologram plate is located in air, separated from the test tank by an air/glass/water boundary. The holographic emulsion is therefore unaffected by adverse aquatic conditions within the tank (i.e. surface contamination, non-uniform swelling). The design geometry is intended to minimise the aberrations that arise from recording subjects located in water and replaying their hologram image in air. Third order aberrations, most crucially spherical aberration and astigmatism, are suppressed to give an experimental resolution of 7 lp/mm using USAF 1951 target in water 600mm from the boundary. Particles (plankton species) in the sub-millimeter to several millimeters size range are observed at planar sections within the recording volume by visual inspection of the hologram replayed in real image mode

Holographic mensuration of suspended particles in aquatic systems

The distribution and dynamics of aggregates in the aquatic environment play an important role in the modelling of biogeochemical processes. Previous work on aggregates in the ocean (e.g. sedimentary 'marine snow' particles), which vary in size from tens of microns to several millimetres, has used electronic counting or conventional photography coupled with image analysis. Here we describe a non-destructive in situ approach by use of holographic mensuration, hologrammetry, that affords greater scope and higher accuracy for the enumeration, sizing, and spatial distribution determination of aggregate particles. By means of two complimentary techniques, in-line and offaxis transmission holography, we present the initial experiments conducted in our laboratory and discuss the preliminaiy results from real image analysis

In situ off-axis holography of marine plankton

We present an off-axis transmission holographic technique for recording marine plankton in situ within a test tank of 36,000 ml with a pulsed laser in a 40 ns interval. The holographic plate is located in air and is therefore unaffected by aquatic conditions that may cause emulsion degradation (e.g. non-uniform swelling and surface contamination). The reference beam traverses a path in air only, and thus remains unaffected by dense concentrations of plankton. Third order aberrations, notably spherical aberration and astigmatism, are suppressed to yield an experimental resolution of 7 lp/mm (70 micrometres) with a USAF 1951 target located 600 mm in water from the observation window. Plankton particle counts examined by real image reconstruction show a strong correlation with duplicate samples examined under a microscope

A data extraction system for underwater particle holography

Pulsed laser holography is an extremely powerful technique for the study of particle fields as it allows instantaneous, noninvasive high-resolution recording of substantial volumes. By replaying the real image one can obtain the size, shape, position and - if multiple exposures are made - velocity of every object in the recorded field. Manual analysis of large volumes containing thousands of particles is, however, an enormous and time-consuming task, with operator fatigue an unpredictable source of errors. Clearly the value of holographic measurements also depends crucially on the quality of the reconstructed image: not only will poor resolution degrade size and shape measurements, but aberrations such as coma and astigmatism can change the perceived centroid of a particle, affecting position and velocity measurements. For large-scale applications of particle field holography, specifically the in situ recording of marine plankton with 'HoloCam,' we have developed an automated data extraction system that can be readily switched between the in-line and off-axis geometries and provides optimised reconstruction from holograms recorded underwater. As a videocamera is automatically stepped through the 200 by 200 by 1000mm sample volume, image processing and object tracking routines locate and extract particle images for further classification by a separate software module

The development and sea trials of a subsea holographic camera for large volume in-situ recording of marine organisms

We describe the development, construction and sea testing of an underwater holographic camera (HoloCam) for in situ recording of marine organisms and particles in large volumes of sea water. HoloCam comprises a laser, power supply, holographic recording optics and plate holders, a water-tight housing and a support frame. Added to this are control electronics such that the entire camera is remotely operable and controllable from ship or dock-side. Uniquely the camera can simultaneously record both in-line and off-axis holograms using a pulsed frequency doubled Nd-YAG laser. In-line holography is capable of producing images of organisms with a resolution of better than 10 Pm (at concentrations up to a few thousand per cubic centimetre at the smallest sizes). Off-axis holograms of aquatic systems of up to 50,000 cm3 volume, have been recorded. Following initial laboratory testing, the holo-camera was evaluated in an observation tank and ultimately was tested in Loch Etive, Scotland. In-line and off-axis holograms were recorded to a depth of 100 m. We will present results on the test dives and evaluation of the camera performance

Ambient humidity control for maximising replay intensity and resolution in aberration-compensated off-axis holograms of underwater objects

In hologrammetry it is usually more desirable to reconstruct the real image than the virtual image, since the latter must be viewed at a distance through the window of the holographic plate itself. In applications where the recorded scene was in water but with replay into air it is necessary to correct for the refractive index difference. This can be done by reconstructing the image with shorter wavelength illumination combined with a change in beam angle to satisfy the grating equation, but these changes mean that the Bragg condition may no longer be satisfied during replay, reducing the diffraction efficiency and making the reconstructed images difficult to see. Changing the replay beam angle to better satisfy the Bragg condition makes the images brighter, but also renders them unrecognizable by introducing severe optical aberrations. A possible solution is to alter the Bragg properties of the hologram. In particular, the emulsion thickness can be conveniently controlled by altering the humidity of the atmosphere surrounding the hologram without causing any long-term changes or damage to the holographic plate. The validity of using humidity change to tune the Bragg properties of emulsions during replay has been demonstrated by measuring the brightness and perceived resolution of a reconstructed real image from a hologram over a wide range of humidities. The results have been compared with a simple model based on the Flory-Huggins theory of polymer swelling

HoloCam: A subsea holographic camera for recording marine organisms and particles

The HoloCam system is a major component of a multi-national multi-discipline project known as HoloMar (funded by the European Commission under the MAST III initiative). The project is concerned with the development of pulsed laser holography to analyse and monitor the populations of living organisms and inanimate particles within the world's oceans. We describe here the development, construction and evaluation of a prototype underwater camera, the purpose of which is to record marine organisms and particles, in-situ. Recording using holography provides several advantages over conventional sampling methods in that it allows non-intrusive, non-destructive, high-resolution imaging of large volumes (up to 10^5 cm^3) in three dimensions. The camera incorporates both in-line and off-axis holographic techniques, which allows particles from a few micrometres to tens of centimetres to be captured. In tandem with development of the HoloCam, a dedicated holographic replay system and an automated data extraction and image processing facility are being developed. These will allow, optimisation of the images recorded by the camera, identification of species and particle concentration plotting

Optimising replay intensity and resolution in aberration-compensated

In hologrammetry it is desirable to reconstruct the real image rather than the virtual image as the latter must be viewed at a distance through the window of the holographic plate itself. When a scene is located in water but the image is replayed in air, it is necessary to correct for the refractive index difference by reconstructing the image with shorter wavelength illumination and changing the beam angle to satisfy the grating equation. However this means that the Bragg condition may no longer be satisfied during replay, reducing the diffraction efficiency and decreasing the signal-to-noise ratio of the reconstructed images. Changing the replay beam angle to satisfy better the Bragg condition makes the images brighter but also renders them unusable by increasing the optical aberrations. Our solution is to alter the Bragg properties of the hologram by altering the humidity of the surrounding atmosphere. This approach has been experimentally demonstrated for Agfa 8E56HD emulsions by measuring the brightness and resolution of a reconstructed real image from an off-axis hologram over a humidity range from 6 to 93 percent. The emulsion swelling and its effect on the Bragg properties of the hologram were modelled using the Flory-Huggins theory of polymer swelling

A holographic system for subsea recording and analysis of plankton and other marine particles

We report here details of the design, development, initial testing and field-deployment of the HOLOMAR system for in-situ subsea holography and analysis of marine plankton and nonliving particles. HOLOMAR comprises a submersible holographic camera ("HoloCam") able to record in-line and off-axis holograms at depths down to 100 m, together with specialised reconstruction hardware ("HoloScan") linked to custom image processing and classification software. The HoloCam consists of a laser and power supply, holographic recording optics and holographic plate holders, a water-tight housing and a support frame. It utilises two basic holographic geometries, in-line and off-axis such that a wide range of species, sizes and concentrations can be recorded. After holograms have been recorded and processed they are reconstructed in full three-dimensional detail in air in a dedicated replay facility. A computer-controlled microscope, using video cameras to record the image at a given depth, is used to digitise the scene. Specially written software extracts a binarised image of an object in its true focal plane and is classified using a neural network. The HoloCam was deployed on two separate cruises in a Scottish sea loch (Loch Etive) to a depth of 100 m and over 300 holograms were recorded

InGaN nano-ring structures for high-efficiency light emitting diodes

A technique based on the Fresnel diffraction effect for the fabrication of nano-scale site-controlled ring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ring structures have an internal diameter of 500 nm and a wall width of 300 nm. A 1 cm-1 Raman shift has been measured, signifying substantial strain relaxation from the fabricated structure. The 9 nm blueshift observed in the cathodoluminescence spectra can be attributed to band filling and/or screening of the piezoelectric field. A light emitting diode based on this geometry has been demonstrated. © 2005 American Institute of Physics.published_or_final_versio