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Optics serve two functions—they ensure that the light is directed toward your plants and they increase light intensity. A diode without optics would radiate light in a broad 180° pattern, which would be very inefficient for growing plants since a lot of light would be distributed to the space between the fixture and plants. Optics, whether primary and secondary, are essential for optimal efficiency.
Aside from COBs, high-wattage LED chips are coated with silicon during the manufacturing process. This coating is the primary optic. The shape of the protective coating, or primary optic, determines the angle at which the light will be emitted from the diode.
Different LED grow light companies will select different angles for their diodes. 90° is a common beam angle on LED grow lights, but some companies may use wider angles. Too wide of an angle reduces the intensity over distance, while too narrow of an angle can reduce coverage area.
Even though an LED manufacturer will list a diode’s light distribution at a specific angle—say 150°—it does not mean that the light will be equally intense at all viewing angles. For example, if we use a common relative intensity curve—the LumiLEDs LUXEON SunPlus 20 Line (powered at 350mA and a temperature of 85°C)—we can see that the intensity at 75° is about 50% of the original intensity. Only at an angle of 0-5° can the diode assume 100% light intensity.
Some top brand LED grow light companies only use primary optics but they usually use high-wattage diodes—5-watts or above—to hold the intensity.
Primary optics area ideal for even canopy coverage since they spread the light out more than a narrower beam angle. Furthermore, they assist in maintaining the efficiency of the LED diodes.
Secondary optics include total internal reflection (TIR) lenses, glass lenses and reflectors.
TIR optics are what most growers consider secondary optics. These are cone-shaped, injection-molded, refractive lenses inside a reflector that are placed over the diode.
TIR lenses focus the light in a more collimated distribution, meaning they concentrate the photons that would have been radiating out at a larger degree angle and narrows their beam angle. TIR lenses are more common with smaller chips, such as 1-watt, 3-watt, and sometimes 5-watt diodes, and are used to increase the diode’s intensity. However, this comes at a small cost. TIR optics will decrease a fixtures overall footprint size and optical efficiency. Furthermore, secondary lenses can decrease optical efficiency by about 8% via the reflection and refraction losses that occur in the optic.
Image: TIR optics
Glass lenses are often used to focus the light from COB LEDs. Unlike a TIR lens (that work via reflection and refraction), a glass lens works similar to a magnifying glass in which the convex lens uses only refraction to concentrate the incoming light into a narrower beam.
Some growers consider glass lenses superior to lenses made with polymers since polymers can degrade, yellow or haze overtime due to outgassing and high-energy blue light, respectively. Hazing can reduce optical intensity, while the yellowing can change the color of the emitted wavelengths.
Reflectors are sometimes used instead of TIR lenses to focus the light. A manufacturer might use reflectors because they are simpler and less expensive than TIR lenses. Unlike TIR lenses, reflectors surround the diode and are not placed on top of the diode.
The effectiveness of the reflector depends on the pattern, angle, and finish. Reflectors can be circular (cone-shaped) or panelled (4 flat sides) with varying angles. Steeper angled reflectors increase light intensity, while shallower lenses will throw the light out in a wider pattern. Most (all?) LED grow light reflectors use a film for the reflective surface, which can be optically superior to aluminium or bare plastic.
Image: LED Reflector
Efficiency losses with reflectors can occur in the pattern, angle, or finish. Different patterns may have varying degrees of efficiencies. And if the angles are too steep, this may decrease the optical efficiency. Furthermore, less glossy films or reflective materials reduce reflectivity and, ultimately, the amount of light reaching the plants.
Instead of using a glass lens over the COB to increase light intensity, some manufacturers incorporate reflectors around the COB. The reason of using reflectors instead of a glass lens is because some of the light escapes the sides of the reflector and is not as directed as when a glass lens is used. Some manufacturers might use a reflector design since it increases the light intensity while allowing even blending across the fixture’s footprint.
A well-designed LED grow light will have uniform light intensity and blending around the fixture and at varying distances due to the contribution effect from the other diodes. For example, if you took a PAR reading 6” away from the fixture, the meter should register a similar reading at 18”. This isn’t magic, but is the work of beam angles.
How does this work? Well, when moving the measurement point away from the diodes, the intensity from the diodes above the measurement point decreases, but the nearby diodes will contribute some of their light, keeping the intensity fairly even.
When comparing LED grow lights, it is important to consider the beam angle in relation to the chip wattage, recommended height above the plants, size of the light footprint, and grow environment. Below are some points to that you should keep in mind: