Page 218 - Algae

Working with Light 201

FIGURE 5.13 Example of conversion between lux and lumen.

In terms of collector geometry, a PAR detector is usually equipped with either a planar (cosine)
or a spherical (scalar) collector. A planar or 2p collector measures PAR in a 1808 arc while
a spherical or 4p collector measures the light impinging from all directions on a single point
(Figure 5.14).
The irradiance impinging on the surface of the water or sediment is commonly measured with a
planar detector. Spherical detectors provide a measurement of the amount of light available for
photosynthesis by phytoplankton suspended in the water column. A leaf resembles a small
planar collector whereas algae resemble small spherical collectors.
Direct solar radiation reaching the water surface varies with the angular height of the radiation
and therefore, with the time of day, season, and latitude. The quantity and quality of light also vary

FIGURE 5.14 Examples of PAR detectors equipped with a planar 2p collector or with a spherical 4p
collector.