Any photon with wavelength from 400 to 700 nm is in principle available for photosynthesis; however, the probability of a given photon being captured by the photosynthetic biomass within a water body will vary markedly with wavelength in accordance with the absorption spectrum of
the array of photosynthetic pigments present. It is quite possible for two different light fields to have the same irradiance for PAR but, because of their differing spectral distributions, to differ significantly in their usefulness to a particular type of alga. Thus, a better assessment of the value of a particular light field for photosynthesis can be made if the way in which irradiance or scalar irradiance varies across the spectrum is known. An instrument that measures this is known as a spectroradiometer. A spectroradiometer is an irradiance or scalar irradiance meter in which a variable monochromator, typically a diffraction grating,79,1386 is interposed between the collector and the photodetector, and such instruments are commercially available. To achieve monochromaticity the spectrum can be dispersed onto a silicon photodiode array, as in the Satlantic HyperOCR instrument606 or onto the pixels of a charge-coupled device (CCD) as in the HOBI Labs HydroRad-E1.605 The typical spectral range of such instruments is 300 to 800 or 850 nm.
A highly specialized spectroradiometer has been developed , , 5 for measuring scalar irradiance at a very fine vertical scale within microbenthic communities such as microbial mats or coastal sediments. This has a 70 mm diffusing sphere attached to the tip of an optical fibre (125 mm diameter, tapered to 15-20 mm diameter, and coated with opaque enamel772). Light from the fibre-optic microprobe is focused onto the entrance slit of a spectrograph, within which a diffraction grating disperses the light (400-880, or 440-920 nm) over a diode array (1024 photodiodes). In another version of this instrument the light probe is an untapered 125 mm fibre with a flat-cut end.754,755 This acts as a spectral radiance meter with an acceptance half-angle in water of 8.6°. A microprobe for approximate measurements of scalar irradiance of PAR (isolated with a broad-band filter, 420-730 nm) within microbial mats has also been developed.312
As we shall discuss later (Chapter 7), in the context of remote sensing, the ratio of nadir radiance, Lu (the radiance of upwelling light in an angular interval centred on the vertical), to the downward irradiance, Ed, is of particular significance. The Biospherical PRR-800 Profiling Reflectance Radiometer is an underwater spectroradiometer that measures both quantities simultaneously in 15 wavebands (selectable within the range 305 to 875 nm) achieved with interference filters. To collect reflectance data over large areas of ocean, Robinson et al. (1995b) have developed an instrument - the Lightfish - which measures near-surface irradiance reflectance simultaneously in six wavebands (410, 443, 490, 520, 550, 670 nm; nominally 20 nm wide) while it is towed from a research vessel at a depth of a few metres. The irradiance sensors are mounted with six on the upper, and six on the lower, surface of a cylinder which, to minimize the self-shading problem (see below), has a diameter of only 63.5 mm. The appropriate sampling period depends on the speed of the vessel: at 9 knots, for example, a 10 s interval was adopted.
As a result of the world-wide depletion of stratospheric ozone, and the development of a hole in the ozone layer over Antarctica, there is greatly increased interest in the penetration of ultraviolet radiation into the sea, but this is difficult to measure both because of the low levels of energy in the UV under water, and of the very rapid change of irradiance with wavelength in this spectral region. To specifically address this problem, Smith et al. (1992) developed an underwater spectroradiometer that measures irradiance from 250 to 350 nm with 0.2 nm resolution, and from 350 to 700 nm with 0.8 nm resolution. A commercially available underwater radiometer, designed for the UV region, measures irradiance in six wavebands from 305 to 395 nm, using interference filters with 10 nm FWHM.1095
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