Optics??

[NanaReefer]

What are 90 degree optics? What do they do?

 

[lucky7]

It's a lens that goes over an led diode to narrow and focus the beam of light essentially. It will
Punch deeper into a aquarium than an led without optics

 

[Reefing Madness]

It focuses the LED into the tank better than it would if it didn't have anything on it. Its just a lens cover, there are many out there being used, but 90 and 120 degree lenses are widely preferred. The AI Hydras use a 80 degree lens that gives the water a very good shimmer, and decent spread.
These units utilize 90 degree lenses.
Full Spectrum 165W Dimmable LED Aquarium Coral Reefs Water Plants Grow Light | eBay