Underwater holography - past and future


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Holography is a well-known optical technique which can provide valuable information on the location and distribution of small particles in three-dimensional space. For several years now, we have utilised holography for high-precision subsea inspection and measurement. One specific application which spurred much of our work was the need for high-precision inspection and analysis of plankton sizes, distribution and species identification. To this end we have developed a subsea holographic camera (HoloMar) for recording of plankton and other marine organisms in situ in their natural environment. This camera is unique in that it is able to record simultaneous in-line and off-axis holograms to cover a range of size of marine organisms from a few microns to tens of millimetres and at concentrations from a few particles per cubic centimetre to dense aggregates. Holograms of aquatic systems of up to 50000 cm~3 volume (off-axis) and 9500 cm~3 (in-line), have been recorded in situ, using a pulsed laser (Q-switched, frequency-doubled Nd-YAG, 532 nm). The use of a pulsed laser effectively "freezes" the scene at a given instant. Although the recording of the holograms takes place in water, replay of the image is carried out in the laboratory in air, using the projected (real) image mode of reconstruction. By precision translation of a computer-controlled video-camera through the replayed image volume and performing "optical sectioning" on the image, individual organisms can be isolated and their size, shape and relative location precisely determined. Image processing algorithms, will allow optimisation of the holographic image together with automated identification of individual species and enumeration of concentrations. The local interactions between different organisms and particles can be observed, recorded and quantitatively determined. Following initial laboratory and observation tank testing, the holo-camera was deployed in a sea loch in the West of Scotland to a depth of 100 m and over 300 holograms recorded. However, the HoloMar camera is physically large and heavy and difficult to deploy. It is also based on the use of photographic emulsions to record the holograms. To overcome some of these difficulties we are now developing a new holographic camera (eHoloCam) based on digital holography. In digital or "eHolography", a hologram is directly electronically recorded onto a CCD or CMOS sensor and then numerically reconstructed by simulation of the optical hologram reconstruction. The immediate advantages of this new camera are compactness, ease-of-use and speed of response, but at the expense of restricted off-axis recording angles and reduced recording volume. In this paper we describe both approaches, the use of holography for analysis of marine organisms and the results obtained in the field. We also describe recent work, using both photo and digital holography, to study the behaviour of sediments in river estuaries and outline future applications of underwater holography.
机译:全息术是一种众所周知的光学技术,可以提供有关三维空间中小颗粒的位置和分布的有价值的信息。几年来,我们已将全息术用于高精度海底检查和测量。促使我们开展大量工作的一项特殊应用是对浮游生物大小,分布和物种识别进行高精度检查和分析的需求。为此,我们开发了一种水下全息照相机(HoloMar),用于在自然环境中原位记录浮游生物和其他海洋生物。该摄像机的独特之处在于它可以同时记录在线和离轴全息图,以覆盖从几微米到几十毫米的各种范围的海洋生物,浓度范围从每立方厘米几个颗粒到密集的聚集体。使用脉冲激光(调Q倍频Nd-YAG,波长为50000 cm〜3(离轴)和9500 cm〜3(在线))对水生系统的全息图进行了现场记录。 532 nm)。脉冲激光的使用可以在给定的瞬间有效地“冻结”场景。尽管全息图的记录是在水中进行的,但是图像的重现是在实验室中使用投影的(真实)图像重建模式在空气中进行的。通过重放的图像量对计算机控制的摄像机进行精确转换并在图像上执行“光学切片”,可以隔离单个生物,并精确确定其大小,形状和相对位置。图像处理算法将允许优化全息图像,并自动识别单个物种并进行浓度枚举。可以观察,记录和定量确定不同生物与颗粒之间的局部相互作用。在最初的实验室和观察箱测试之后,全息照相机被部署在苏格兰西部的一个海域中,深度为100 m,并记录了300多幅全息图。但是,HoloMar摄像机体积庞大,笨重,难以部署。它也基于照相乳剂的使用来记录全息图。为了克服其中的一些困难,我们现在正在开发一种基于数字全息术的新型全息照相机(eHoloCam)。在数字或“电子全息图”中,将全息图直接电子记录到CCD或CMOS传感器上,然后通过模拟光学全息图重建进行数字重建。这款新相机的直接优点是紧凑,易用和响应速度快,但以限制离轴记录角度和减小记录量为代价。在本文中,我们描述了两种方法,即全息术在海洋生物分析中的应用以及在现场获得的结果。我们还使用照片和数字全息技术描述最近的工作,以研究河口沉积物的行为并概述水下全息技术的未来应用。



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