detector array configurations. Chl a absorption of red light occurs in a rather narrow
band. Optimal detection of the chlorophyll red absorption peak in laboratory
spectrophotometers requires a spectral bandwidth of 2 nm or less (Rowan, 1989). The
blue, Soret band of chl a has a comparably narrow absorption peak. Broader bandwidth
not only reduces the ability to resolve these peaks, but also allows greater overlap with
the absorption regions of accessory chlorophylls (especially chl b) and other pigments.
These wavelength resolution issues may have similar importance in measurements with
field radiometers. However, signal-to-noise constraints in field spectroradiometers and
remote platform sensors may necessitate broader bandwidth (Dekker, 1993).
Wavelength selection for pigment detection is affected by a well known
wavelength shift (roughly 5-10 nm) for the wavelength of maximum pigment
absorption in extracted versus in vivo conditions. For example, chl a and b in 90%
acetone have maximum absorption values at 664 nm and 647 nm (consensus averages
from Rowan, 1989) whereas Bidigare et al. (1990) estimated the maximum red
absorption in vivo at the longer wavelengths of 674 nm and 652 nm (see Figure 1).
Also, the wavelengths of maximum red absorption for chl a can vary by about 5 nm,
depending on the extractant and pH conditions (Rowan, 1989). Chl a has a prominent
left “shoulder” of absorption associated with the blue, Soret band and a secondary red
absorption peak broadly centered at 628 nm (Figure 1). Ratios of in vivo, weight
specific absorption coefficients for the blue and red primary peaks and the blue and red
secondary peaks are about 1.3 and 4.95, respectively (Bidigare et al., 1990). Global
ocean comparisons reveal that chl a, total carotenoids, chl c and chl b account for about
47%, 43%, 8%, and 4% of total phytoplankton pigment, although a range of 35% to
53% for chl a was reported for different bio-optical provinces (Aiken et al., 1995).
Maintaining proper instrument calibration is an important but sometimes
challenging issue. Wavelength to channel registration and radiometric energy
calibration may not be stable over time (Evans and Gordon, 1994; Starks et al., 1995).
Instruments should be checked periodically for gradual drift or erratic changes that may
be related to operating temperature or to degradation or damage to the optical
components or their light path alignments. Wavelength specific lasers, emission lines
from lamps, and narrow pass filters are commonly used to establish a regression fit
between known wavelengths and the instrument channels with maximum response to
these signals (Starks et al., 1995). Radiometric responses of individual channels are
calibrated against well characterized lamp emission responses. Proper radiometric
calibration can be more difficult when foreoptics and/or fiber optic guides deliver light
to the optical slit entrance of the instrument. Researchers performing close range
measurements with spectroradiometers usually convert their water spectra
measurements to fractional or percent reflectance, a normalization procedure in which
upwelling and downwelling estimates are ratioed for each channel of an instrument or
pair of instruments.
Bottom substrate reflectance contributes to and can cause bias in the composite
upwelling irrandiance and reflectance signals in optically shallow waters (Akleson and
Klemas, 1986; Maritorena et al., 1994; Lee et al., 1998; Odhe and Sigel, 2001; Albert
and Mobley, 2003), and may interfere with interpretation of water column signals and
chlorophyll retrieval. Rundquist et al. (1995) measured reflectance from black and
white panels suspended at 10 cm depth intervals between 2 and 82 cm during dilution
of an algal bloom in an outdoor tank mesocosm. With the black panel at 82 cm, a ratio
of the NIR peak reflectance to chlorophyll absorption band reflectance near 674 nm
(Mittenzwey et al., 1992) had a linear relationship to chl a over a range of 30 to 336
µg/l (r
2 = -0.98, n = 15). The white panel had an amplifying effect on spectral
37
Optical Remote Sensing Techniques
band. Optimal detection of the chlorophyll red absorption peak in laboratory
spectrophotometers requires a spectral bandwidth of 2 nm or less (Rowan, 1989). The
blue, Soret band of chl a has a comparably narrow absorption peak. Broader bandwidth
not only reduces the ability to resolve these peaks, but also allows greater overlap with
the absorption regions of accessory chlorophylls (especially chl b) and other pigments.
These wavelength resolution issues may have similar importance in measurements with
field radiometers. However, signal-to-noise constraints in field spectroradiometers and
remote platform sensors may necessitate broader bandwidth (Dekker, 1993).
Wavelength selection for pigment detection is affected by a well known
wavelength shift (roughly 5-10 nm) for the wavelength of maximum pigment
absorption in extracted versus in vivo conditions. For example, chl a and b in 90%
acetone have maximum absorption values at 664 nm and 647 nm (consensus averages
from Rowan, 1989) whereas Bidigare et al. (1990) estimated the maximum red
absorption in vivo at the longer wavelengths of 674 nm and 652 nm (see Figure 1).
Also, the wavelengths of maximum red absorption for chl a can vary by about 5 nm,
depending on the extractant and pH conditions (Rowan, 1989). Chl a has a prominent
left “shoulder” of absorption associated with the blue, Soret band and a secondary red
absorption peak broadly centered at 628 nm (Figure 1). Ratios of in vivo, weight
specific absorption coefficients for the blue and red primary peaks and the blue and red
secondary peaks are about 1.3 and 4.95, respectively (Bidigare et al., 1990). Global
ocean comparisons reveal that chl a, total carotenoids, chl c and chl b account for about
47%, 43%, 8%, and 4% of total phytoplankton pigment, although a range of 35% to
53% for chl a was reported for different bio-optical provinces (Aiken et al., 1995).
Maintaining proper instrument calibration is an important but sometimes
challenging issue. Wavelength to channel registration and radiometric energy
calibration may not be stable over time (Evans and Gordon, 1994; Starks et al., 1995).
Instruments should be checked periodically for gradual drift or erratic changes that may
be related to operating temperature or to degradation or damage to the optical
components or their light path alignments. Wavelength specific lasers, emission lines
from lamps, and narrow pass filters are commonly used to establish a regression fit
between known wavelengths and the instrument channels with maximum response to
these signals (Starks et al., 1995). Radiometric responses of individual channels are
calibrated against well characterized lamp emission responses. Proper radiometric
calibration can be more difficult when foreoptics and/or fiber optic guides deliver light
to the optical slit entrance of the instrument. Researchers performing close range
measurements with spectroradiometers usually convert their water spectra
measurements to fractional or percent reflectance, a normalization procedure in which
upwelling and downwelling estimates are ratioed for each channel of an instrument or
pair of instruments.
Bottom substrate reflectance contributes to and can cause bias in the composite
upwelling irrandiance and reflectance signals in optically shallow waters (Akleson and
Klemas, 1986; Maritorena et al., 1994; Lee et al., 1998; Odhe and Sigel, 2001; Albert
and Mobley, 2003), and may interfere with interpretation of water column signals and
chlorophyll retrieval. Rundquist et al. (1995) measured reflectance from black and
white panels suspended at 10 cm depth intervals between 2 and 82 cm during dilution
of an algal bloom in an outdoor tank mesocosm. With the black panel at 82 cm, a ratio
of the NIR peak reflectance to chlorophyll absorption band reflectance near 674 nm
(Mittenzwey et al., 1992) had a linear relationship to chl a over a range of 30 to 336
µg/l (r
2 = -0.98, n = 15). The white panel had an amplifying effect on spectral
37
Optical Remote Sensing Techniques