Abstract: Accurate measurement of algae concentration is important for prevention and treatment of algal bloom, which may seriously affect the fishery production and residents' health and safety.Measurement of chlorophyll a concentration using chlorophyll fluorescence is an effective means to reflect the algae concentration. The fluorescence is very weak and is susceptible to ambient light, so a lock-in amplifier was employed to measure the fluorescence. LED was used as excitation light source, PIN detector was employed for fluorescence detection, and a lock-in amplifier was designed based on AD630. Anabaena of 10 concentration gradients between 0-100 μg/L was used for system testing, and the measurement results showed that the correlation coefficient of the measured value and the actual concentration of chlorophyll a is 0.99815.

Key words: algae, chlorophyll a fluorescence, lock-in amplifier