It’s no secret that high wattage DE-HPS lamps produce lots of plant-usable light, with photon efficiency above the “glass ceiling” of 2.0 µmol / J. Since their introduction over a decade ago they have been causing quite a stir in the indoor gardening world as growers know: light and productivity are intrinsically linked! However, many indoor growers—especially medium to large scale commercial cultivators—have adopted practices that negate the potential efficiency gains of DE-HPS technology.
Lights Hoisted Up High
You’ve no doubt seen dozens of similar photos on your Instagram or Facebook feed: row after row of 1000W DE-HPS lights in small, deep-dish “greenhouse style” reflectors raised up 5, 6 or even 8 feet above the plants. These images are so ubiquitous at the moment that new growers could be forgiven for thinking that this is the only way it’s done.
The most commonly cited reason for the “hoist ‘em up high” approach is overall light uniformity—the desired PPFD (light intensity) levels (typically 700 – 900 µmol) can certainly be achieved with 1000W DE-HPS lamps positioned in a 4 / 6 matrix. (Fixtures positioned so there are 4 feet between each column and 6 feet in between each row—with the lamps positioned so they run in line with the rows / perpendicular to the columns).