Although some multicellular aquatic plants have their photosynthetic tissue so arranged as to achieve maximum interception of light incident from above, to the randomly oriented cells of the phytoplankton all light, from whatever direction it comes, is equally useful. Scalar irradiance E0, as we saw earlier (§1.3), is defined as the integral of radiance distribution at a point over all directions about the point: it is equivalent to the total radiant flux per m2 from all directions at a given point in the medium. In the context of photosynthesis, information on the scalar irradiance is therefore somewhat to be preferred to data on irradiance.

The technology of measurement of scalar irradiance is essentially the same as that of irradiance with the exception of the collector. Since this must respond equally to light from all directions, it has to be spherical. Figure 5.4 illustrates the principles involved in collectors used for scalar irradiance and for its downwelling and upwelling components. Like the collector of a normal irradiance meter, the spherical collector is made of

Fig. 5.4 Principles involved in collection of light for determination of (a) scalar irradiance, (b) downward scalar irradiance and (c) upward scalar irradiance. In the cases of downward and upward scalar irradiance, the collector must have a shield wide enough relative to the collector to ensure that very little upward and downward light, respectively, reaches the collector.

Fig. 5.4 Principles involved in collection of light for determination of (a) scalar irradiance, (b) downward scalar irradiance and (c) upward scalar irradiance. In the cases of downward and upward scalar irradiance, the collector must have a shield wide enough relative to the collector to ensure that very little upward and downward light, respectively, reaches the collector.

diffusing plastic or opal glass so that it collects all incident photons with equal efficiency, regardless of the angle at which they encounter its surface. The light that penetrates the spherical collector can be transmitted to the photodetector via another, flat collector in the bottom of the sphere570 or, in the case of a solid diffusing sphere, can be transmitted from the centre of the sphere by a fibre-optic bundle or quartz light-conducting rod.133 Scalar irradiance meters can use a filter to confine the measured light to a narrow waveband,570 or can be designed as quanta meters covering the whole (400-700 nm) photosynthetic range.133,1282 Figure 5.5 shows two commercial quantum (PAR) scalar sensors.

Karelin and Pelevin (1970) developed a meter with three collectors: upward and downward irradiance collectors and a spherical collector for scalar irradiance. Any one of five colour filters could, by remote control, be placed in front of the photodetector. From measurements of E above and below a given depth, KE, the vertical attenuation coefficient for net downward irradiance is obtained. The absorption coefficient is then calculated from a = KEE/Eo (§1.7) using the values of E and E0 at the depth in question. The data obtained with this meter were used to determine the absorption coefficients of the medium, as well as to characterize the underwater light field.

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