D 

 z tan 9 + R tan 9 ' ^ - 



2 



where is the angle of the fluorescence emitted from the volume A z dz mea- 

 sured from the vertical, 6' is the angle of the fluorescence after refra ction 

 at the water surface, and D, the telescope diameter, is equal to ^MAp/fi where 

 A r is the area of the receiving telescope. By using Snell's Law and assuming 

 that 9 and 1 are small angles, the previous criterion becomes 



2(z + mR) 



where m is the index of refraction for water. The fluorescence which is 

 emitted from the volume A z dz into the solid angle tt(z9) 2 /z 2 is collected 

 by the telescope receiver system. Based on the limiting values for 9 , the 

 maximum value of the collection solid angle is A r /(z + mR) 2 . Since the fluo- 

 rescence from the volume A z dz is emitted into 4tt steradians, the fractional 

 proportion of emitted fluorescence to be collected by the telescope is 

 A r /4iT(z + mR) 2 . Thus, the differential power received at the detector in 

 the laser fluorosensor system with the field of view of the receiver large 

 enough to accept the fluorescence emitted from the algae contained in the 

 volume A z dz is 



dP r = dF(X f ) e" YfZ (l - p f )e~ Bf \ AX d (3) 



Mtt(z + mR) 2 



where 



Yf effective attenuation coefficient of water at 685 nm, m -1 



pf water surface reflectivity at 685 nm 



Bf atmospheric extinction coefficient at 685 nm, m -1 



5 optical efficiency of receiver system 



AX(j receiver wavelength bandwidth, nm 



Since the absorption coefficient a is in general larger than the scat- 

 tering coefficient b at 685 nm for a wide range of water turbidity (ref. 17), 

 the effective attenuation coefficient for the fluorescence Yf is dominated 

 by a. The form of Yf is 



b f 

 Y f = af + — 



