34 
Spectral Band Selection 
The CZCS demonstrated the feasibility of de- 
termining ocean chlorophyll concentrations and 
diffuse attenuation coefficients from multispectral, 
visible, satellite-acquired observations. The goal 
of the SeaWiFS Working Group's band-selection 
process was to recommend a baseline set of 
bands that would be capable of providing the 
data required to resolve chlorophyll concentra- 
tions to within 50 percent over a range of con- 
centrations from 0.05 ug/l in the open ocean to 
10.0 ug/l in outer continental shelf areas. This is 
a major goal for a follow-on ocean-color sensor 
to the CZCS (JO! 1984, 1985). 
Visible and Near-Infrared Bands 
The topic of spectral band selection for Sea- 
WiFS was introduced by the instrument design 
experts by proposing the following baseline band 
centers in the visible and near-infrared regions: 
443, 500, 565, and 765 nm. The band at 443 nm 
is near the absorption maximum of chlorophyll at 
435 nm, but its location minimizes interference 
from a Fraunhoffer absorption line (G) that oc- 
curs near 435 nm. The 500 nm band is between 
the maximum and minimum regions of pigment 
absorption, so it can be used to estimate pigment 
concentration when the concentration is so large 
that useful signals cannot be derived from meas- 
urements at the absorption peak. The 565 nm 
band is near the absorption minimum of phyto- 
plankton pigments, and the 765 nm band is in the 
near-infrared region where water can be consid- 
ered black, enabling use of data from these 
bands in atmospheric-correction algorithms. 
Since the instrument designers indicated that 
two bands could be added in the visible/near- 
infrared (VNIR) rather easily with only a minor in- 
crease in cost, in the ensuing discussion several 
other bands were suggested. For example, a 
band centered at 665 nm was requested for at- 
mospheric correction instead of the CZCS band 
at 670 nm to avoid the strong overlap of the 670 
nm band with the in-vivo sunlight-induced fluores- 
cence feature of chlorophyll, centered at 685 nm. 
Although the SeaWiFS band center would be 
5 nm lower than the CZCS band center, it would 
still permit existing CZCS algorithms for atmo- 
spheric correction to be used. This was consid- 
ered essential, since time will be required to de- 
termine the optimum techniques for using the new 
765 nm band for estimating aerosol radiance, and 
because it is important to have direct compari- 
sons between historical CZCS data and SeaWiFS 
data to provide continuity in the study of long- 
term trends in marine productivity. After the 
atmospheric-correction algorithms for the 765 nm 
band have been validated, the 665 nm band will 
be useful in developing new algorithms for esti- 
mating pigment concentration (Clark 1981) and in 
assisting in the effort to extrapolate Angstrom ex- 
ponents from the near-infrared region to the visi- 
ble. 
As a result of these discussions, both panels 
agreed that the following five VNIR bands should 
be included in SeaWiFS as a minimum set: 443 
+10 nm, 500 +10 nm, 565 £10 nm, 665 +10 nm, 
and 765 +20 nm. Data from these bands would 
be used for the purposes identified in Table 2. 
After agreement on adding a band at 665 nm 
was reached, the possibility of adding a sixth 
band was explored. It was felt that this opportu- 
nity should be exploited to improve accuracy and 
the number of optical properties of the ocean that 
could be derived in the presence of multiple con- 
stituents. The spectral position of this band was 
debated at length, using as a guideline that the 
position of the selected band should not have a 
significant influence on the cost or complexity of 
the instrument. 
