Hach 2100AN

Manufacturer: Developed by CSIRO Marine Research. Based on 2 Zeiss MMS-VIS spectrometers.

Web address: www.zeiss.de

Nominal wavelength range from 300 to 1100 nm, and an effective range from about 350 to 900 nm. The spectrometers together with tec5 electronics hardware (LOE) are incorporated in a pressure case. Upward-looking (diffuse Teflon) and downward looking (clear acrylic) optical heads are mounted at the ends of 1 m long aluminium tubes, rigidly attached to the pressure case, and connected by quartz optic fibres to the Zeiss spectrometers. A 60m underwater cable and connectors allow RS232 communication between the LOE and a notebook computer at the surface.

The integration time can be varied between a minimum of 8 ms and maximum of 2 secs. This, combined with 15 bit digitisation, provides a very wide dynamic range. In practice, integration times between 8 and 64 ms were used on the cruise, and the electronic noise was very low, with variation in dark counts typically of order a few counts, providing signal to noise ratio of order 1000:1 much of the time.

The Zeiss spectrometers are spectrally calibrated to within 1 nm over the 300 to 1100 nm range (spectral resolution is 10 nm, with a pixel resolution of 3 nm). As they are constructed as monolithic quartz blocks, the spectral calibration should be stable.

Software developed by CSIRO Marine Research (Win 95) allows real-time inspection and capture of incoming data, and control of integration time and sampling interval. The LOE provides an option for accumulation and averaging of up to 10 scans in an internal buffer. This is a very effective way to smooth over instrument and environmental noise, and has been commonly used.

The tec5 LOE electronics operates the two spectrometers in simultaneous mode. In this mode, the readouts are interleaved pixel by pixel, so that integration periods for the two sensors coincide to within about 4 microseconds. This means that the instrument can make accurate reflectance measurements even in strongly fluctuating light environments.

The instrument is usually diver-operated, but it can be used as a remote sensing instrument. The 1 m fibre-optic extensions and small optic head dimensions (2 cm) allow measurements free of diver and instrument shadow even quite close to the substrate. The instrument was weighted so that it was slightly negatively buoyant and, when suspended between top and bottom attachment points, floats horizontally in the water column allowing measurements of vertical profiles.

Problems: instrument is not radiometrically calibrated (at present), cosine response of downwelling sensor is not tested.