Why do we use primary lenses?
We use grow lights with primary lenses because they allow the light produced from an LED to most efficiently reach your plants. To understand why, some background information about the refraction of light is important.
Refraction happens when light traveling through one medium (air, water, glass, etc.) enters a different material and is bent (refracted). This occurs because light travels through different materials at different speeds-- the speed of light in a vacuum is constant, but it varies in air, water, glass or any other matter. Refraction is responsible for how prisms "split" white light into its constituent colors. Objects appear to bend when you partially dip them in water due to the different degree of refraction between air and water. Refraction is also responsible for rainbows and liquid crystal (LCD) displays.
A material's index of refraction measures the degree to which light is bent when entering or exiting the material. The greater the difference between two different materials' index of refraction, the more likely the light is to be reflected back into the first material.
The light-emitting portion of an LED (called a die or chip) is primarily made of silicon, with minuscule amounts of various other elements added to affect the color of light produced. Bare, uncoated silicon has a refractive index of 3.4-3.9, while air has a refractive index of 1.0003. The large difference between the refractive index of silicon and air means that light leaving the LED chip exposed directly to air is often just reflected back into the silicon, as if it were a mirror. Light produced in the LEDs is useless if it never manages to hit your plants!
An LED grow light lens placed directly on the silicon die actually helps to harvest more light from the LED. Glass, silicone and acrylic have a refractive index of about 1.5, intermediate between that of silicon and air. This intermediate step allows more photons out of the silicon and into the air, actually increasing the amount of light the LED emits. "COB" or "integrated" LEDs are often much less expensive because they don't use primary lenses, but are much less efficient in terms of photons-per-watt.
Glass and acrylic lenses cause some of the light to be lost, which is one reason we don't use secondary lenses with our lights or even have a protective glass lens under our LEDs, but as a primary lens it actually gets more light out of the silicon LED than any losses it incurs, so primary lenses produce a net gain in light and make LEDs more efficient.
Black Dog LED always uses primary lenses but never secondary lenses for our LEDs-- we maximize efficiency and evenly cover the entire footprint to keep all your plants happy, rather than just being bright in the center to look good on paper.