Indoor grow manuals always mention the importance of using reflectors to help take advantage of every ray of light your growlamp puts out, but most amateur growers don’t realize exactly how much this really matters. Using some simple measurements and mathematical approximations, HIGH TIMES has determined roughly how much light you can save with some cleverly hung reflectors.

Cannabis ferociously devours light on a daily basis making it one of the most important limiting factors for yield. Simply put, the more abundant and better your light source is, the higher your yield. Without needing to hang more electricity-guzzling, heat-generating lamps in your garden, good reflector placement can increase light intensity in the canopy by up to 60%.

What, may you ask, do we use to measure light intensity for the purposes of plant growth? Photosynthetic photon flux, or PPF, is the sum of all the photosynthetically-active photons between wavelengths 400 nm (blue) and 700 nm (red), which roughly coincides with the spectrum of visible light humans can see. PPF is measured in units of micromoles of photons per second, μmol/s. Photosynthetic photon flux density, PPFD, is a measurement of the density of the photon flux falling over a given area and has units of μmol/s/m2. The daily light integral (DLI) is the sum of all the photosynthetically-active photons a given area receives in a day, and has units of moles/day/m2.

For the purpose of measuring plant-usable light that covers your garden’s canopy, PPFD is unit of choice. On the other hand, DLI is a tool that botanists use to compare the light requirements of different plants, and its what greenhouse growers measure to assess how much supplemental lighting they may need on shady days. For example, orchids only require a modest 8 moles/day/m2 for high quality growth, while tomatoes need to bask in up to 30 moles/day/m2. In the cold, dark month of December, Northern California experiences 10-15 moles/day/m2, but in June, this number can go as high as 55 moles/day/m2. Check out this publication for DLI maps of the USA, and this publication for a diagram of the DLI requirements of various common greenhouse plants.

If we knew what the DLI requirement was for cannabis we’d tell you; we’d shout it on the mountain tops from Malibu to Morocco, but the fact is nobody knows for sure. For all intents and purposes, it takes around 25 moles/day/m2 to yield decent harvests, but the more light the better. If a cannabis plant needs 25 moles/day/m2 over an 18 hour light-cycle, this puts the hourly requirement at 1.38 moles/hour/m2. Divide this number by 0.0036 and you get the PPFD your lamps need to deliver: 385 μmol/s/m2.

So how much can reflectors increase the PPFD under an HID growlamp? We put it to the test with HIGH TIMES’ own spectroradiometer. A 400 W cool-tube HPS lamp (with a built-in reflector directly behind the bulb, but not on the sides) produced a PPFD of 148 μmol/s/m2 directly underneath it at three feet away. Simply holding two reflectors on each side of the cool-tube (so that the light projected itself on the floor with a 120° angle) raised the PPFD to 235 μmol/s/m2, a 59% increase. The tighter the angle the reflectors force the light into, the higher the PPFD and the higher your yield will eventually be.

The light from your growlamps should be utterly concentrated to the plants, if any part of it spilling onto the floor or the walls means its not hitting the plants, and they won’t receive their daily dose of light they need for productive growth. In the diagram above you can see a lot of wasted light. As is, those plants under a 400 W HPS light will only receive a DLI of roughly 9.8 moles/day/m2. With just a few reflectors, this DLI could go up to 26 moles/day/m2 without any additional electricity.

Simply coating the walls of the growroom with aluminum foil doesn’t fix this problem, the beams need to reflect directly onto the canopy. Reflectors made using aluminum foil will do the job for cheap, but that foil only reflects around 80% of the light it gets. To really get the 90 -95% reflectance you want, invest in a legitimate reflective surface. Don’t let your yields suffer due to wasted light, get the most out of your lamps and your little garden will be more productive than you ever imagined.

Photo Credit: VortexFarmacy