What is PAR and how much do we need?

simple enuff for me to follow and understand
detailed enuff to consider similar action

Nice short article, friend

Small correction:
"Photosynthesis (not photosynthetic) is the process by which (not just "these") organisms transform light energy into food. Corals have tiny plants called zooxanthellae, living within their tissue and so together they are called symbiotic (not photosynthetic?)".

Could be more precise and specify that the "food" produced in photosynthesis is in fact sugar. For aminoacids and fatty acids, corals need to capture prey with their polyps.

Good and simple approach to lighting intensity.

The article is great at describing the basics when it comes to PAR measurements. I would also mention (unless I missed it) - rather than buying a meter, one can often rent or borrow one from an LFS - or local reef club.