How do I measure LED light levels?

Hey guys huge newb when it comes to this stuff so I'm sure ill get a lot of great information. i know some things but i am trying to understand a little bit more of the scientific portion of the light spectrum and whatnot. i do not grow yet but would like to in the future at some point i just want to be well established before i get caught with my hand down my pants and a thumb up my ass lmao.

Anyways I have been researching LED's and it seems as if those are most interesting to me. My questions is, is there any special way to measure the light output for LED's? i know there are Lux meters/par meters and other gadgets but which is the best and how do i use it? also if anyone knows a great guide how to go about doing this and learning about this particular topic please send me in the right direction. I have found some things but not specifically what i want. grow weed easy.com has been great but they do not show how to measure LED output just other types of lights. thanks guys i look forward to your responses!

 

also i see videos with these square sections in their grow room could anyone explain more about that to me?