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Grow Lights – Effective Concepts LLC
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Apr 162008

Recently I was thinking about Grow Lights again. My mother raises most of her summer flowers from seedlings. Her friends and neighbors in the Garden Society do the same. In the past we string banks of cheap 2-lamp shop lamps in the basement 3-6 inches over the flats of plants. At one time Home Depot had a very nice fixture that would drive either T-12 lamps or T-8 lamps. (T-12 lamps are 12 eighths of an inch or 1.5 inches in diameter. A T-8 is one inch in diameter and requires an modern electronic ballast to drive it.) As a Sylvania distributor I have easy access to high quality T-8 lamps and specialized T-12 Gro-Lux lamps. On one hand the T-8 seem to give the most bang for your buck, but the T-12 Gro-Lux lamps give the plants the kind of light they want. Here is a short list of resources I ran across on the web (note some of these are PDF files) :

  • Another Opinion on Home Depot Shop Lamps ($9 each). I’ve looked at them recently and the boxes state: rated only for T12 lamps only. This is a shame and would limit their usefulness. The fixtures I purchased (2002) were very well built. They weren’t flimsy and had nice rounded edges. I didn’t feel like I had to wear gloves to handle them. I haven’t tried them recently but the line drawing the box looks suspiciously like an inferior shop lamp I ran across from another source.
  • Overdriving Fluorescent Lights by Jim Haworth. This post went into detail about overdriving lamps by using a 4-lamp ballast for two T8 lamps or a 2-lamp ballast for one T8 lamp. I was aware of this- it isn’t at all unusual, in fact it’s in the Sylvania ballast spec sheet. However it is also unnecessary. Sylvania sells high output T-8 ballasts. Every ballast manufacturer has a line of high output ballasts. Known as High Ballast Factor (HBF)- there is no reason you have to jam a bunch of ballast into a fixture to get extra output.
  • mySylvania.com has a good collection of PDF files about all aspects of lighting. This also includes “Lights and Plants“, which explains why Gro-lux lamps work well when they don’t look very bright. And “Photosynthetically Active Radiation Units“, which delves into the science of light and photosynthesis.
  • Indoor Lighting For Plants by Dr. Leonard Perry, Extension Professor of the University of Vermont. A nice quick overview of the subject.
  • Indoor Growing FAQs by RVF Garden Supply. This FAQ went into more detail than most posts on the Internet. There was less conjecture and more science. Their thoughts on using 6500K T-5 lamps sparked my imagination.

RVF Garden Supply had some interesting notes on using a T-5 lamps with a good reflector finished…

T5 lamps provide the ideal spectrum for plant growth. Photosynthesis rates peak at 435 nm and 680 nm. A 6500K T5 lamp has a spectral distribution with relative intensity peaks at 435 nm and 615 nm. This equates to very little wasted light energy in terms of plant growth. T5 lamps promote incredible health and vigor of seedlings and cuttings. Root development is superior relative to other lighting sources.

The problem as I see it is one of cost. Everything about a T-5 system is dramatically more money than a comparable T-8 system. The lamps, ballasts, fixtures are all three times as much money and yet the jump in efficiency between a T-5 system and a T-8 system is minor.  With this in mind I set out to make a decent T-8 grow light fixture.

  1. I looked at loading up a troffer fixture with extra lamps. It had a nice spectral aluminized reflector built in. That means it is almost mirror like. The problem was the reflector made adding more lamps unlikely. I wanted to get 4-6 lamps in the fixtures, but I had no easy way to make room for the sockets.
  2. I also had some reflector kits. Once I set one up I released it was almost a fixture in itself. There is a metal bar, which supports the sockets and mounts to the reflector. All I need is a ballast and some lamps. I showed Ray and he suggested it really needed a channel for support and as a place to mount the ballast(s). My thought was to string a couple of these reflector kits together on a sheet of thin wood. The ballasts could be mounted on the other side of the sheet. This sheet of two reflector kits (8-lamps total) would replace each row of three shop lamps (6-lamps total).
  3. I had some 1-lamp strip fixtures. I was about to take one apart for the channel, when I realized I had some 4′ blank channels laying around. No point in ruining a perfectly good strip fixture.
  4. I also have a box of 2-Lamp HBF Electronic Ballasts. I wired two Sylvania Quicktronic QT2x32/120/PLUS to drive four lamps.
  5. RVF Garden Supply made a good point about using 6500K lamps. This isn’t a lamp I stock. I have 2700K (Incandescent), 3000K (Warm), 3500K (Warm White), 4100K (Cool White), 5000K (Daylight), but no 6500K (Blue). I have only done one job with 6500K lamps. I found them to be very bluish, although the customer loved them. In the future I plan to get some. It seems 6500K may be the new grow lamp as its output spikes at the same frequencies: 435nm and 615nm. In the meantime I used four 5000K lamps (Extended 80 series), which give longer life and higher Color Rendition Index (CRI), which yields better light. I found a cord (The only item I don’t have in abundant supply.) The fixture was very bright.
  6. The sockets weren’t stable enough so I replaced them with four that are screwed into the socket plate. I also ran some zip screws in so it wouldn’t fall apart. It’s a nice looking fixture. The reflector is sharp, but it is also flexible and easy to cut, so I can mount it almost anywhere. The wires running to the sockets aren’t covered, so it would never pass inspection. But for a grow light it’s very good.

 Posted by at 3:07 pm

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