The primary lens of an LED light is mounted directly over top of the components of the LED that actually emit light. These lenses act to improve the efficiency of the photons leaving the semiconductor and direct the light towards your plants. Without a primary lens, the light being produced would just bounce around aimlessly within the LED, occasionally escaping outwards. Primary lenses convert a small, useless LED into a powerful light that can produce a significant amount of light.
However, many LED grow lights also include secondary focusing lenses on their LEDs. Secondary lenses are mounted below the primary lens and function to either focus the light that is first harnessed by the primary lens into either a narrower or broader stream of light – all depending on the angle of the lens. That is why you will see grow lights advertised with varying degrees of secondary lenses. Having an option for the angle of your LEDs is a great feature of LED grow lights that isn’t available in HID or HPS lights because they don’t have the capability of focusing the light further than what is emitted. Those lights spew out light in all directions resulting in a lot of wasted energy. This typically would require you to harness a lot of the light using reflectors which can help, but by then the damage is done and you’ve wasted a lot of light and efficiency by the time the light actually reaches your plants.
Different angles of secondary lenses are useful for different scenarios and is an important factor to consider when purchasing an LED grow light. In general, the larger the angle of secondary lens mounted on the LEDs, the wider area that your PAR light will shine on. If you are purchasing a larger light that you want to cover several plants, then a grow light with 120 degree secondary LED lenses is likely the best option for you such as the Apollo Horticulture line.
If you are limiting yourself to single plants, or can afford to mount a single light above each individual plant you are growing, then perhaps a 60 degree secondary lens LED grow light kit is your best option.
Many people consider 90 degrees the sweet spot as a nice happy medium between 60 and 120.
There are also some generalized LED lights that have a mix of secondary lenses to try and cover the best of both worlds.
Our recommendation is to go wider than you think you need, as you then always have space for more plants, and always have the option to use a few small reflectors to reign in all the stray light that may be spilling past the edges of your plants. But try to think about your growing situation in terms of now and later. What plants do you have now, how many will you want, how much space do you/will have, etc. as everyone’s space is unique.