with a narrow band-pass filter which was centered at 685 nm, was used as the 

 receiver. Results from this field trip are shown in figure 7. Chlorophyll a 

 in vivo concentrations were determined at 1-hour intervals over a 24-hour 

 period. In situ data (data obtained by analyzing samples taken from the water) 

 were supplied by the Environmental Protection Agency (EPA). The fluorescence 

 signals were normalized to EPA findings at 2000 EST on December 29, 1971. The 

 results of this experiment were encouraging, and subsequent helicopter flight 

 experiments were conducted near Wallops Island, Virginia. However, these 

 flight experiments resulted in only limited success. 



Using the same basic laser system which was used in field tests over the 

 Chesapeake Bay, Kim (ref. 12) flew the instrument in a helicopter over Lake 

 Ontario at the request of EPA. A transect was made of Lake Ontario from west 

 of Rochester, New York, to the Canadian shore at an altitude of 30 m. The 

 remotely sensed chlorophyll a in vivo concentrations were found to be 50 per- 

 cent higher along the United States side than in midlake or on the Canadian 

 side. The results of this flight are presented in figure 8. No sea truth 

 measurements were reported for this experiment. One of the primary drawbacks 

 of the single-wavelength laser system was its inability to account for differ- 

 ences in absorption by various pigments contained in different algal color 

 groups. Thus, the laser system would not be able to accurately determine 

 chlorophyll a in vivo concentrations if the mixture of algal types changed 

 over the flight path. Also, any change in the light scattering properties of 

 the water during the flight would also produce errors in the calculated values 

 for chlorophyll a in vivo concentrations. 



MULTIPLE-WAVELENGTH LASER FLU0R0SENS0R SYSTEM 



A four-color airborne fluorosensor (AL0PE - Airborne Lidar Oceanographic 

 Probing Experiment) was developed at NASA Langley Research Center (refs. 8, 

 13, and 14) for determining the distribution and chlorophyll a in vivo concen- 

 trations of the four primary algal color groups. The laser excitation wave- 

 lengths used for the golden-brown, green, red, and blue-green color groups 

 were 454, 539, 598, and 6.1 8 nm, respectively. These were the most optimized 

 laser wavelengths which could be obtained with the low-gain laser design used 

 in the ALOPE system. A single linear flash lamp is used to simultaneously pump 

 four dye cells symmetrically spaced about the flash lamp, and a rotating aper- 

 ture permitted lasing from only one dye laser cavity at a time. The ALOPE sys- 

 tem is shown schematically in figure 9. A system of bending mirrors directs 

 the laser light coaxial with the 25 .4-cm-diameter telescope receiving system. 

 The fluorescence of the chlorophyll a in vivo is collected by the telescope 

 system, which passes only light at 685 nm. 



By assuming uniform vertical distribution of turbidity and chlorophyll a 

 in vivo concentration in the water, a set of four coupled equations results 

 from summing the fluorescence contribution to the power received by the laser 

 fluorosensor system from each algal color group for each laser excitation wave- 

 length. The pertinent equations, derived from equation (6), are 



12 



