Reprinted from Science, J_67, (1970) 



Copyright 1970 by the American Association 

 for the Advancement of Science 



Reports 



Spectra of Backscattered Light from the Sea Obtained 

 from Aircraft as a Measure of Chlorophyll Concentration 



Abstract. Spectra of sun and skylight backscattered from the sea were obtained 

 from a low-flying aircraft and were compared with measurements of chlorophyll 

 concenlralion made from shipboard at the same localities and at nearly the same 

 times. Increasing amounts of chlorophyll were foimd to be associated with a rela- 

 tive decrease in the blue portion of the spectra and an increase in the green. 

 Anomalies in the spectra show that factors other than chlorophyll also affect the 

 water color in some instances; these factors include other biochromes. suspended 

 sediment, surface reflection, polarization, and air light. 



The penetration of daylight into the 

 sea is of fundamental significance in 

 the oceanic ecosystem because it con- 

 trols the growth of the primary plant 

 producers and the behavior patterns of 

 many marine animals. Previous investi- 

 gations have revealed great variation 

 in the rates of light penetration due to 

 differences in amounts and kinds of 

 materials in the water. In addition, the 

 spectral composition of the light be- 

 neath the surface is altered by differ- 

 ential absorption and scattering due to 

 the water itself, and also to whatever 

 dissolved and particulate matter (both 

 living and nonliving) may be present 

 (/). Because chlorophyll affects the 

 spectrum in a characteristic way and 

 because it is associated with living 

 plants, spectral measurements of chlo- 

 rophyll concentration may be used as 

 an index of the amount of phytoplank- 

 ton present. Regions with high phyto- 

 plankton abundance can support large 

 populations of herbivores and of suc- 

 cessive links in the animal food chain, 

 many of which are of economic impor- 

 tance to man. Thus, abundant chloro- 

 phyll indicates the presence of a po- 

 tentially productive area (2). 



The spectral changes imposed on the 

 downwelling daylight by natural waters 

 and by the materials in them have been 

 measured by lowering an upward- 

 directed spectrometer in a watertight 

 case to various depths (3). The up- 

 welling, or backscattered, light that can 

 be measured by employing the spec- 

 trometer in the inverted position is 



20 FEBRUARY 1970 



found to have its spectrum similarly 

 modified by its passage through the 

 water. A portion of the backscattered 

 light escapes upward through the sur- 

 face, where It has been recorded by an 

 inverted spectrometer suspended above 

 the water from a ship and from air- 

 craft {4). Allowance must be made for 

 light reflected from the ocean surface 



1 - 600 ft 



2 - 1000 ft 



3 - 2000 ft 



4 - 5000 ft 



5 - 10,000 ft 



450 500 550 600 



Wavelength (nm) 



650 



Fig. 1. Upwelling light as received at the 

 indicated altitudes at Station S (Fig. 2) 

 east of Cape Cod, 26 August 1968 be- 

 tween 1345 and 1512 hours, E.D.T. 



5-2 



itself or scattered by the stratum of air 

 above the water. 



The possibility thus exists that spec- 

 tral measurements of backscattered 

 light can be used to delineate water 

 masses, to trace currents, and to deter- 

 mine the abundance of chlorophyll, 

 pollutants, or other significant materials 

 in the water. Because measurements 

 from aircraft or spacecraft can be 

 made over extensive areas much more 

 rapidly than from ships, they are 

 especially suited to the study of small- 

 scale, rapidly varying distributions of 

 oceanic properties (5). Tests of some 

 of these possibilities are reported here 

 for water masses of widely different 

 known chlorophyll concentrations off 

 the New England coast. 



During the summers of 1967 and 

 1968, records of the spectrum of back- 

 scattered light from the ocean have been 

 made from our research vessel Craw- 

 ford and our C-54-Q research aircraft. 

 The spectrometer used was designed 

 by Peter White of TRW Systems, Inc., 

 and described by L. A. Gore (6). R. C. 

 Ramsey of TRW operated the instru- 

 ment and took part in the reduction of 

 the data and in the interpretation of the 

 results. 



The TRW spectrometer is an electro- 

 optical sensor of the off-plane Ebert 

 type with an RCA 7265 (S-20 response) 

 photomultiplier. The spectral range is 

 400 to 700 nm with a spectral reso- 

 lution of 5 to 7.5 nm, a scan time of 1.2 

 seconds, and a field of view of 3° by 

 0.5°. A continuous curve of the spec- 

 trum is provided by a Sanborn re- 

 corder for each scan. The spectrum of 

 the incident light from the sun and sky 

 was determined before and after each 

 series of measurements by recording 

 the light reflected from a horizontally 

 placed Eastman Kodak "gray card" 

 with a nonselective reflectivity of 18 

 percent. A series of tests was made to 

 detect changes in the spectral distribu- 

 tion of incident light during the 3 

 hours before and after noon due to 

 changes in the sun's altitude and to 

 changes in sky conditions from clear 

 to light cloudiness. Changes found were 

 not great enough to affect significantly 

 our investigation of the differences in 

 backscattered light from the ocean. By 

 taking advantage of the fact that light 

 reflected from a plane surface at Brew- 

 ster's angle (approximately 53° from 

 vertical incidence for normal sea 

 water) is plane polarized with its vibra- 

 tion plane perpendicular to the plane 

 of incidence, we could reduce the light 



