Editor’s Note: This article uses a lot of technical terms! Be ye forewarned.
Regardless of the type of lighting system you choose (CMH, HPS, MH or CMH/HPS hybrid, LED), it is important to understand before purchase and installation how well the reflectors and luminaires will deliver a uniform PPFD (photosynthetic photon flux density) measured as µmol/m2/sec over the plant canopy as it changes over time. The uniformity of the target minimum PPFD intensity over the full plant canopy is critical to optimizing the dry weight yield per plant, which translates into increased profits per harvest.
Think of it this way. If each of the lamps you are evaluating produce the same PAR (micromoles/second) output, then the average PPFD light uniformity over the full canopy throughout the veg or flower cycles will be determined by the interaction the beam patterns from all the reflectors in the room. This is especially true for hybrid systems where the uniform homogenization of the CMH spectrum with the dominant HPS is key to optimizing the yield per plant1.
The efficiency of any prescribed nutrient regiment is also directly affected by light uniformity. Over the course of a year, plants receiving low light compared to the targeted PPFD average will consume more nutrients and that will unnecessarily impact profits.
Beam Patterns of HID Lights
Reflectors use horizontal or vertically mounted lamps. The beam pattern of a typical 1000W DE HPS, 630W DE CMH, or 945W DE CMH lamp reflector is rectangular, designed to provide approximately 4×6 foot coverage at 3 feet. These systems depend on overlapping beams from adjacent reflectors to fill in around the edges of the hot-spot directly below each light. Hot spots are the result of light emitted by the lamps arc tube exiting directly downwards without interacting with the reflector. Hot spots create uneven plant growth that negatively impacts dry weight yields and increased nutrient demand from the under lit plants.
Light that lands on diffuse surfaces, like white paint or powder-coated aluminum, reflects in every direction, including the ceiling. The reflectivity of high-quality flat, white paint is approximately 90% reflectivity with a drop off in the 600-700nm (red) range. By comparison, diffuse white ORCA® grow film is 94% reflective over the full spectrum.
Light landing on a specular surface is reflected at an angle opposite to the angle of incidence. This means the light will reflect downward onto the canopy in the case of indoor plant production lighting. Overall, this reduces light wasted on the ceiling from diffuse white walls. Examples of specular surfaces are polished aluminum or stainless-steel metal that are typically 85% reflective, and RadiantGuard® film at 95% reflectance. When designing a room with suspended lights you may want to consider applying RadiantGuard® to the walls as an inexpensive and easy way to improve energy efficiency.
- Comparison of Luminaires: Efficacies and System Design. L.D. Albright, A.J. Both, Cornell University, and Retrofitting Growth Chambers to Improve PPF Intensity, Efficiency and Uniformity. B. Bugbee, Utah State Crop Physiology Laboratory.
- Next-Generation, Energy-Efficient Uniform Supplemental Lighting for Closed-System Plant Production. D.S. de Villiers et al, Cornell University.
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