A $2000 piece of equipment to measure the amount of chlorophyll in a sample using the fact that it fluoreses under light. 
Be great to DIY, usefull for tracking algae blooms. 
Bit of discussion here http://forum.arduino.cc/index.php?topic=194982.0 using arduino. 

Related paper:
In situ Measurements of Phytoplankton Fluorescence Using Low Cost Electronics
Thomas Leeuw * , Emmanuel S. Boss and Dana L. Wright [openaccess paper](https://doi.org/10.3390/s130607872)

Who is interested?