Fast repetition Rate Fluorimeter (FRRF II)
Introduction
The FRRF is a fluorimeter designed to probe the photosynthetic activity
of phytoplankton in marine environments. It works by irradiating
a small volume of water containing plankton with multiple short pulses
of blue light. Any plankton which is ready to photosynthesise
will use thi slight to drive its internal chemistry, but if the planton
is already "saturated" with light - either because it is in bright
sunlight or because its photosynthetic machinery is compromised
(perhaps by a toxin) will not be able to "use" the light energy.
In this case the light re-emrges as deep red / infrared fluorescence
which can be detected. By applying multiple flashes and looking
to see how the fluorescence builds up, it it possible to study the
photosynthetic state and well-being of the plankton. More
details of the science can be found in the links below.The technique can be used for marine scientific research where one wants to know the distribution of plankton, and its ability to photosynthesise. It is also useful to water companies as a way of detecting potenrtially toxic build-ups of algae in reservoirs.
It is also possible to detect if water has been contaminated by a toxin by monitoring natural algal responses.
The technique was first originated at the Brookhaven National Laboratory in the USA.
Instrumental Design
The FRRF Mark II was intended to be a smaller, lighter ,cheaper and more effective replacement for the Mark I instrument. I originated and refined a novel "one-sided" coaxial optical design in which all the optical parts are kept safely on one side of a single pressure window: this is important because the instrument may have to operate at depths of up to 1000m where the ambient pressure is about 100 atmospheres! The image above shows the illumination system being tried out for the first time using a ring of high-power blue LED sources. Two plastic aspheric condenser lenses are used to concentrate the light into the target area just above the second lens: the fluorescein in the tank acts as a marker showing where the light is going. In the final instrument, the upper lens acts as a pressure window and interaces directly with the ambient sea or lake water.
The two lenses and the ring of LED light sources are
mounted into a titanium pipe and end cap so that the top lens acts both
as an optic and a pressure window. The sample zone, which sits
just outside the window, is viewed by an optical system that is built
down the centre of the illumination system (in the part which is not
used by the LEDs). This system of lenses and apertures carefully
collects as much light as possible from within the brightest-lit part
of the sample area and guides it to a miniature photomultiplier tube
detector (another amazing component by Hamamatsu!). The detection
system has a carefully controlled field of view and is able to operate
in relatively bright ambient conditions (i.e shallow seas).The one-sided arrangement allows for minimal shading of the sample volume by the instrument: this allows the measurement to be made under un-disturbed ambient illumination conditions.
The rear of the instrument houses the power supplies, instrument control and LED drivers. This apsect of the design was handled by Andy Rawkins and Dr David Griffiths. The detailed mechanical design was undertaken by Alan Boother and an excellent job was done by all!
The "window" is some 3 inches in diameter, and must therefore withstand an enormous load without failure. There was a tense period whilst the first prototype was placed in the pressure chamber and pumped up ready to be left overnight. Luckily the seals held and there was no leak the next morning! The final instrument, equipped with a suspension frame, and an optional "dark chamber" is very compact and is dwarfed by its forebear, the Mark I instrument. It is also much more efficient and capable.
The instrument can operate on 1000m of marine cable and can either store data locally or transmit it to the surface in real time. The illumination can be controlled very finely (using some very clever drivers designed by David Griffiths) to extract differnt types of information from the plankton. Thanks for this are due to Kevin Oxborough.
Further Infomation 
The FRRF II is marketed by Chelsea Technologies from whence further information can be had. More information is available in the instrument data sheet and environmental moniroring application note.