ANALYSIS OF LASER FLUOROSENSOR SYSTEMS FOR REMOTE ALGAE 



DETECTION AND QUANTIFICATION 



Edward V. Browell 

 Langley Research Center 



SUMMARY 



The development and performance of single- and multiple-wavelength 

 laser fluorosensor systems for use in the remote detection and quantification 

 of algae are discussed in this paper. The appropriate equation for the fluo- 

 rescence power received by a laser fluorosensor system is derived in detail, 

 and the result is found to differ by as much as a factor of 10 from those 

 previously reported in the literature. Experimental development of a single- 

 wavelength system and a four-wavelength system, which selectively excites the 

 algae contained in the four primary algal color groups (green, golden-brown, 

 red, and blue-green), is reviewed, and test results are shown. A comprehen- 

 sive error analysis is reported which evaluates the uncertainty in the remote 

 determination of the chlorophyll a concentration contained in algae by single- 

 and multiple-wavelength laser fluorosensor systems. Environmental parameters 

 which can greatly affect the fluorescence cross section of algae include long- 

 and short-term light history and nutrient and age effects. Results of the 

 error analysis indicate that the remote quantification of chlorophyll a by a 

 laser fluorosensor system requires optimum excitation wavelength ( s) , remote 

 measurement of marine attenuation coefficients, and supplemental instrumenta- 

 tion to reduce uncertainties in the algal fluorescence cross sections. With- 

 out these additional measurements, both the single- and multiple-wavelength 

 laser fluorosensor systems can only provide qualitative information about 

 chlorophyll a concentration contained in the algae, and the single-wavelength 

 system yields results which are comparable to a multiple-wavelength system 

 which has optimum excitation wavelengths. 



INTRODUCTION 



An algae laser fluorosensor system consists of a laser, which is used to 

 directly or indirectly excite the chlorophyll a pigment contained in algae, and 

 a collocated telescope receiver system, which detects the emitted fluorescence 

 from chlorophyll a at 685 nm. The quantification of chlorophyll a in vivo 

 (chlorophyll a contained in living algal cells) represents a measure of the 

 concentration and distribution of algae. These measurements are of interest 

 because they are indicative of primary productivity and chemical, thermal, and 

 mechanical conditions in the marine environment. 



Passive techniques have been developed to remotely evaluate chlorophyll a 

 in vivo concentration from aircraft and satellites (refs. 1 to 5). These tech- 

 niques rely on the spectral characteristics of reflected sunlight for determi- 

 nation of the chlorophyll a concentration contained in the algae and, thus, are 



