We get asked this question, just about every day.   The answer is enough to saturate photosynthesis.  

• Too much light will damage the algae's pigments.
• Too little light and it will not grow as fast as it possibly could.

Saturating Intensity

This is the intensity of light where the plant is using all of it's pigments and is not wasting energy as heat and fluorescence.  OVER saturation, and the plant has to work to get rid of the energy.   UNDER saturation, and the plant will re-arrange the pigments to be more efficient with limited light.   

120uE/cm2 is a good general saturation intensity to maximize growth.  

What is too much light?  

• For a not very dense culture, (secchi disk depth <15cm) keep intensity <800uE/m2s
• For dense cultures (Secchi Disk Depth <4cm), you can go up to full sun, if the culture can stay under 32C (90F).   Light plus heat kills algae.  

This image is from the above experiment:

Water temperature was 38C with only glass protecting the culture, and 32C with the glass and the window screen.

-Matthew Huber