The potential for increased ultraviolet (UV) exposure resulting from ozone depletion has helped fuel interest in acquiring accurate in-water measurements of UV irradiance. In the water column, the flux in the UV becomes a vanishingly small signal that must be measured in the presence of a much larger visible component. In addition, the flux of UV in natural waters may be influenced greatly by changes in solar elevation and from focusing/defocusing by surface waves. For these reasons, an instrument that accurately measures the flux of UV in air will not work as well when submerged, and a number of elements must be optimized to produce instruments for use underwater. In response, Biospherical Instruments Inc. has produced the PUV-2500 Profiling Ultraviolet Radiometer. The system is designed to collect time series or vertical profiles of UV (305, 313, 320, 340, 380 and 395nm) and Photosynthetically Active Radiation (PAR: broadband 400-700 nm) irradiance underwater to depths of 350 meters. The PUV-2500 is designed to measure downwelling irradiance with a response capability exceeding nine decades of dynamic range, as well as pressure/depth, and water temperature. The PUV-2510 is a radiometrically-matching surface reference radiometer. Highly versatile because of its small size and lightweight design, the PUV-2500 can be used in traditional vertical profiling mode (surrounded by a lowering frame and supported by its own cable) or, by employing our free-fall design option, in free-descent mode, thereby helping to avoid artifacts associated with ship shadows.
When working with moderate bandwidth radiometers in regions of the solar spectrum with large variations in intensity as a function of wavelength, significant errors can be introduced from the difference between the lamp spectrum and the solar spectrum. Under these conditions, a `perfect' radiometer can grossly over- or under-estimate the solar irradiance, while several otherwise identical radiometers with center bandwidths ranging over +/- 2 nm can report significantly different irradiances. A direct instrument intercomparison experiment was conducted in San Diego, Calif. using 15 five-channel filter radiometers. The instruments were operated for approximately one week alongside a 0.7 nm bandwidth scanning spectroradiometer under a variety of solar conditions. All of these instruments had been calibrated with lamp standards traceable to NIST. In an effort to reduce the uncertainties of transferring calibration made with standard lamps to filter-based radiometers, we suggested that `source-specific' calibration constants, referenced to both lamps and reference solar spectra, be used. This investigation explores how well this procedure can work, and contrasts this result with intercomparison- and regression-based calibrations.
The impending launch of a number of remote sensing platforms such as the NASA SeaWiFS (sea-viewing wide field-of-view sensor) has generated renewed interest in instrumentation for measuring ocean color in situ. The increased number of spectral bands desired and the large dynamic range of the measurements place special challenges on the design of these instruments. Of particular interest in measurements of spectral irradiance and in the calculation of irradiance reflectance are errors introduced by the departure of the instrument response from a true cosine at increasing angles of incident irradiance. Using field data as well as a modelled radiance distribution, this paper presents an evaluation of a commercial cosine collector for measurements of spectral irradiance. Preliminary results form a new program sampling the variability in immersion coefficient are also presented.
As the theoretical understanding of bio-optical relationships increases, new tools for measuring ocean optical properties will be needed, particularly using a spectral resolution approaching 1 nm. Continuing improvements in grating technology for spectrometers coupled with the development of high quality CCD detectors has afforded the opportunity to experiment with a number of innovative instrument designs. However, optical sensors for use in oceanographic instruments must be designed to operate over a wide dynamic range, both spectrally over the region of interest, and radiometrically over a wide range of signal level as flux is lost with increasing depth. This is especially true for work in the ultraviolet, where signal levels are very low compared to the visible region of the spectrum and where attenuation of the flux by water is high. In this paper, the use of optical fibers, imaging spectrographs, and CCD photodetectors in the construction of a multi-channel marine spectroradiometer is presented. Stress induced changes in transmission can result in large artifacts when uncompensated optical fibers are used as part of the light path. Special emphasis is placed on the impact of stray-light characterization on the use of the spectrograph. The design and specification of the fiber optic cable, disperser, and CCD camera are presented, as well as implications to ocean optics of the special considerations that must be taken into account when spectrographs are deployed in the sea.
Optical sensors for use in oceanographic instruments must be designed to operate over a wide dynamic range, both spectrally from the UV through the visible region, and radiometrically as flux is lost with increasing depth. The output from a discrete sensor results from the convolution of the sensor's spectral response with the spectral distribution of irradiance, which changes radically with depth. Thus, a number of factors must be optimized in the instrument design. This is particularly true in the ultraviolet, where spectral leakage from longer wavelengths may significantly influence the output of a sensor at depth. This paper presents a theoretical basis for the evaluation of discrete sensor performance, with special emphasis on sensors for use in the ultraviolet region of the spectrum. The analysis includes a detailed description of the calibration and spectral response function for two channels of a new UV radiometer, the PUV-500, and compares this description with data taken in clear ocean waters in the equatorial Pacific. Finally, a theoretical analysis of two channels of the instrument, 308 and 340 nm, is used to evaluate the potential for measuring total column ozone with this design.
Estimates of the in vivo specific absorption coefficients (m2 mg'; 400-750 nm, 2 nm intervals) for the major algal pigment groups (chlorophylls, carotenoids and phycobilins) are presented. "Unpackaged" absorption coefficients were initially obtained by measuring the absorption properties of pure pigment standards spectrophotometrically and "shifting" their absorption maxima to match in vivo positions. Two approaches for estimating the phytoplankton absorption coefficient (spectral reconstruction and spectral decomposition) are compared by linear regression analysis, incorporating concurrent measurements of particulate absorption and pigmentation performed in the Sargasso Sea. Results suggest that "pigment package" effects are minimal for natural assemblages of open-oceanic phytoplankton and that accessory pigments do not always co-vary with chlorophyll a over depth and time.