UNDERWATER HOLOGRAPHY IN THE FIELD 'I

G Craig, H Nareid & J Watson

Introduction

In this paper we report on the current development of a prototype underwater holographic camera and the associated replay facility for sub-sea inspection and measurement. Although the camera will initially be configured for recording plankton the initial concept was developed as an inspection technique for the offshore industry'.'.

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The advantages of holography over other visual inspection techniques is that it permits high resolution, 3-dimensional recordings which retain parallax and perspective information. Replay then allows the recorded scene to be projected into free space where detailed analysis can be carried out on the image. Although recording underwater introduces some additional complication^'^^ the general principles of holography remain unchanged.

The development of the system will allow holographic visual inspection and measurement, or "hologrammetry" as it has come to be known, to be applied to a variety of underwater applications. It is particularly useful in hazardous situations, as in the offshore industry, where visual inspection and measurements often have to be carried out under conditions of poor visibility and at great depths. Recording a hologram of the underwater scene allows the analysis to be carried out in the safety of a lab as well-as providing a permanent image for archiving.

It has been known for some time that the earth's climate is intimately connected with the chemical and physical processes occurring in our oceans. Monitoring of plankton and other suspended particles within the upper water column can therefore be used to indicate the status of our

Present methods of sampling these marine particles are not well suited to observing precise spatial relationsbps or preserving some of the more delicate organisms. However, holography provides a solution in that it allows non-intrusive and non-destructive recording of the organisms and particles in their natural environment.

Concepts of Holography

From the several different holographic techniques available7 two have been found particulary suitable for field application as high resolution imaging tools, namely in-line and off-axis.

In in-line holography a single coherent beam of coherent light is directed through the subject volume onto a holographic plate. At the plate the light diffracted by the objects in the beam path interferes with the original undiffracted beam to form the hologram. This process relies on transmission of the undiffracted wavefront and can therefore only be used with a high subject transparency (approximately 90%)'. The upper particle size recordable is set by the Fraunhofer7 condition which states:

G Craig, H Nareid & J Watson all presently work at the Department of Engineering, University Of Aberdeen

Q 1999 The Institution of Electrical Engineers. Printed and published by the IEE, Savoy Place, London WC2R OBL, UK. 1

Where 2 is the object to plate distance, and h is the wavelength of the recording that particles down to around 10 pm or IC

In comparison off-axis holography requii reference beam, which’ is directed onto t the subject volume. The subject is then reflected and interferes with the referenc light reflected from the subject much 1 2 only being limited by field of view of th the laser. However, the lower limit of system constraints, to around 30-40 pm7 I

Facility Specification

The holographic camera consists of the fc support rig, a laser and associated power system. A cross sectional diagram of t dimensions of the camera will be approx: system will be capable of either ship d recording down to a depth of 100m.

The off-axis holograms will be recorded line recordings will be made across the t off-axis system will allow recording of v( 500 mm long 90 mm diameter cylinder ol

The two recording geometries have been ,

techniques to give two completely indc additional data will be utilised by the ima

Recordings will be made with a Q-switck nm and with a pulse length of around 8 since it corresponds to the peak transmi: effectively freeze the subject volume dur the maximum particle velocity recordab: external vibrations. Recordings will be r has a capacity of 40 plates for each techni

G Craig, H Nareid & J Watson all presently

is the maximum dimension of the particle to be recorded ight. However the main advantage of in-line recording is [er can be resolved.

s two separate beams to record the image. The first is the 2 plate at an angle (60” in this case) without transversing .luminated with diffuse laser light and a portion of this is beam to form the hologram. Since this process relies on ger objects may be recorded with maximum dimensions camera and the coherence length and available power of

-esolution is restricted, primarily by speckle and optical lepending on geometry and conditions).

lowing components; a watertight pressurised housing with upply, 2 plate holders, holographic optics and the control e internal layout may be seen in figure 1. The overall lately 2 m long by 1 m in diameter. The initial prototype ployment or attachment to a fixed buoy and will allow

]rough the window at the front of the housing and the in- o arms (see figure 1). With the current configuration the umes in the region of 10’ cm3 and the in-line will record a ieawater.

Tanged to allow a common volume to be recorded by both lendent views of the subjects within this region. This : recognition unit to improve species identification.

d, frequency doubled Nd-YAG laser emitting light at 532 5. Light in the green region of the spectrum was chosen ion window for seawater. The short pulse duration will ig exposure. It also gives advantages in that it increases and reduces the necessity of isolating the system from

ide on 100 mm square holographic plates and the camera ue .

/ork at the Department of Engineering, University Of Aberdeen

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Pressurised, housing

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Pulsed f requency-doubled Nd-YAG

Simultaneous recording of

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:.;->.- "D-''..~'. . - ,.-_ _- .:s

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off-axis &

Figure 1 : Plan of Holocam

The full potential of the recorded image can not be realised unless we have an optimised replay arrangement to reconstruct the image. This replay set-up consists of a HeCd continuous wave (c.w.) laser emitting light at 442 nm. Replay is carried out at this wavelength and not that of the recording laser since the holograms are recorded in water and replayed in air. This change in refractive index introduces aberrations to the system which are minimised by replaying at the shorter wavelength'.

To investigate the projected real image a CCD video camera is mounted on computer controlled x-y- z micropositioning stages. Using the CCD camera, images are captured then specially developed image processing techniques are used to enhance the images. The enhanced image is then passed to neural network image recognition software to allow classification and identification of species. Along with this data information about size, relative position and concentration of organisms can be obtained allowing a 3D map of the recorded scene to be generated.

System Performance

Although the holocamera and replay facility are still in development, initial recording using a similar arrangement have been made in our laboratory. These recordings have shown that good resolution and image quality can be achieved under a variety of conditions. The two upper images in figure 2 are from off-axis holograms which were taken whilst the emphasis of our research was directed towards offshore monitoring techniques'. The top left shows the replayed image of a stress induced crack in a titanium block, with an object to film distance of 550 mm, whch has been measured as 40 pm wide. The second is a corroded multi-pass weld bead which shows the detail which can be attained even with a dull low contrast subject.

The other three images are all plankton from recent work carried out by Krantz'and Hobson". From left to right we have two in-line images; a dinoflagellate Cerutium recorded in a field sample of

G Craig, H Nareid & J Watson all presently work at the Department of Engineering, University Of Aberdeen - -

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plankton approximately 200 um long md hstel-ianellcz foimosLz which is around 70 Fm long. Both of these images can easily be seen resolving details in the region of 10 pm. The bottom right image is from an off-axis hologram and sh0n.s a lawer zooplankton copepod approximately 3 mm long.

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- iechnology9: 1987: 97-101

Aberdeen: 1991 4. Kilpatrick: J.M.: "An Evalueiion of Oprica!

Acknowledgements

AbenaIions in Undzrwaier Hoiorammeiry": Phd Thesis: Universip of

The authors wish to thank initiative.

snow aggreyies." Naturc, 362: 1993: 737-739 7. Haiiharan. 1.: "Opdcai Z ~ l o g r ~ p h y " : Cambridge iiniversiiy Press: 1984

We€erences

G Craig, H Narcid 81 J Watson all presently work ;it the Deparrmen: of Engineering, University Of Abcrdecn