For my COB Led Cree: Lens, reflector, or nothing at all ?
In general, you will avoid using a Cree COB without an optical accessory.
The 90 ° reflector (Ledil Angelina in general) will be used for normal cultivation conditions (4 minium spots per m²). For the growth period (in the case of a reduced number of spots per m²), 120 ° lenses will be preferred. Ditto for the scrog / sog methods where one will seek an optimal cover of the canopy rather than the luminous penetration. The lenses also make it possible to protect COB chips against splashes, and dust in suspension (the COB Cree is covered with a layer of silicone that will trap any dust that will pass within range ...). Make sure that the lenses are made of optical glass (less than 5% loss). The cheap lenses that you will find on Chinese merchant sites will generally be of poor quality (> 20% loss) and undersized.
In the case of using reflectors, if your budget allows, add a RZ diffuser. It will protect the COB chip and help to diffuse the luminous flux.