T ECHNICAL DEVELOPMENTS

3.5.1. Light management

Using in situ imaging of tiny objects, like zooplankton species, for taxonomic identification one is faced with several problems. At first it requires high mag-nifications, whereas available light de-creases approximately by the factor four when magnification is doubled (Figure 3.2). Consequently, the luminous flux provided by the used light source is an important factor. As a second point the distance between the camera lens and the imaged volume needs to be short, as

particles and dissolved matter can detract image quality. High magnifications at short dis-tances also result in a small depth-of-field (DOF). Points that lie in the object plane are correctly imaged as a point on a photosensitive sensor. With greater distance from the ob-ject plane the so called circles of confusion get larger. The DOF is defined as the range within the circles of confusion that remains small enough that a point in the object plane appears to the human eye as a single point (Figure 3.3). In digital imaging it is desirable to

keep the diameter small enough to reduce the number of pixels excited simultaneously by the same point. Thus taxonomic features, important for the identification of species, can only be obtained within a observed volume. While the first two are physically defined by the size of the photosensi-tive sensor, the principal axis is infinite and needs to be constrained. As the depth of field is narrow, illumination is only necessary within this range. The approach introduced here is based on an illumination technique that projects a light frame of high luminous flux into the water. The camera aims with an angle of 90° at this light frame, whose depth is in the range of the DOF (Figure 3.4). Particles within this frame are illuminated, while not di-rectly illuminated ones are nearly invisible. Thus, the required clipping along the principal axis is obtained for the depth axis. Consequently, the development of illumination devices with a high light flux and precise targeting is a pivotal precondition for in situ imaging of small planktonic species.

For this, two different approaches were developed (Chapter 4.3). The first was designed as a combination of a linear light source and three cylindrical lenses of different focal lengths (Patent T1, Paper T2). It creates a light frame constrained in one dimension that is used for depth limitation. In the other directions the light beam fans out and intensity decreases with distance from the source. To improve efficiency for the use of rod like high voltage dis-charge lamps a special reflector was designed (Paper T3, Paper T4). Although it resulted in an intense light beam the system was prone to cast shadows from objects due to the unidi-rectional illumination. As a further advancement of this device a second one was designed

Figure 3.3: Circles of confusion on the photosensitive area (o’) for a point a) at the near end (af) of the depth of field, b) in the object plane (o) and c) at the rear end (a_b) of the depth of field. d) The use of a diaphragm improves the image quality, enlarges the depth of field but less light reaches the photosen-sitive area.

vice, surrounding the observation volume (Paper T5). With high efficiency Light Emitting Diodes (LED) and cylindrical Fresnel-lenses a homogeneous illumination was achieved without casting shadows. As the observed volume is rather small, low abundant species have a higher probability to remain undetected. Thus the developed system was named Light-frame On-sight Key species Illumination (LOKI).

3.5.2. System specifications

Based on the circular illumination device a prototype was designed, where a camera looks over a short distance into the light frame. With a frequency of 15 frames per second a digi-tal camera with four million pixels (JAI Pulnix^® TM4000) takes images of objects within the light frame with shutter times below 1 ms. The illumination device creates a light frame with an extension in the depth of field range and is operated in flash mode. In towed opera-tion mode a relative water movement replaces the water and entrained objects between two

Figure 3.4: a) Schematic overview of the LOKI application. b) View from the front through the circular LOKI illumination and into the lens of the camera. c) The prototype during an initial test in the North Sea, showing the different components. From left to right the circular illumination device, the camera housing and the main housing, bearing the embedded PC can be seen.

frames. Thus, objects or species appear just once and allow quanti-fying the abundance of different objects. The minimum object size for detection can be manually ad-justed and reduces required data storage space. Every Area-Of-Interest (AOI) can be assigned to sensor readings of the ambient environmental parameters within a time frame of one second. Initial problems with the transmission of HF (high frequency) signals between camera housing and the main underwater unit were due to interactions with ambient sea water. Additional insulation techniques were used to avoid interferences. To date the initial laboratory phase is completed and field tests are in progress (Figure 3.5). The work for the evaluation of the hydrodynamic design has been completed. The camera is connected via CameraLink^® in-terface with a PCI-X frame grabber board (Matrox^® Helios^® XCL) that processes the data stream of approximately 60 MB s^-1. Image frames are pre-processed in real time in the un-derwater unit and only parts that contain objects are stored as AOI. AOI extraction is exe-cuted by an Intel^® Dual Xeon^® board (clock rate 3.8 GHz, 2 GB central memory), that communicates with several microcontroller subunits by an internal Ethernet network.

These subunits gather environmental information from various sensors, perform prelimi-nary calculations and assist in the communication with the surface. The communication between surface and the underwater unit is achieved by an internet protocol signal (TCP/IP) modulated onto the power supply. This enables the use on ships with hawsers bearing just two-conductor cables for connection. A multi frequency modem allows com-munication between the underwater unit and the surface over more than 8 km of copper coax cable with a maximum speed of 1.5 MBit. The use of the TCP/IP also allows remote operation and data acquisition via the inter- or an intranet. Thus the gear can be operated on unmanned stations, while configuration and data access is accomplished by a remote operator.

Figure 3.5: Initial test of the prototype at Helgoland Roads in Summer 2006. Photo by courtesy of Carsten Wanke, Helgoland.