Under what circumstances would I use the Flat-PAR sensor instead of the Spherical-PAR sensor?

FAQ - Li-Cor Ambient Light

1. What is PAR? Is this the same as Ambient Light?
2. Why would I want to measure PAR?
3. Q: Under what circumstances would I use the Flat-PAR sensor instead of the Spherical-PAR sensor? A: The flat-faced, or "cosine-corrected" sensor, is used when measurement of sunlight extinction in the water column is important: normally the sensor is always pointed vertically upward so that it measures only unreflected sunlight. A "spherical" sensor that collects light from all directions (not just "up," as does the cosine-corrected sensor) is used when total light energy available for photosynthes is at a certain depth in the water column (including reflected light).
   
   
4. How is the PAR sensor calibrated?
5. Under what circumstances would I use the Dual-PAR sensor instead of the Single-PAR sensor?