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Find The UVB Photon

Some bugs that ply their trade among vegetable plants are energized by the UVB photon and the greenhouse POLY covering that is designed to block the UVB photon is considered to be a form of insect control as well as lasting longer than a regular POLY covering. But the coverings that block the UVB photon are not suitable for growing marijuana.

A translucent woven ripstop film sold by Shelter Systems transmits a percentage of the UVB radiation. 140 microwatts per cm2 48" beneath the roof cover may not seem like much compared to the 300 microwatts per cm2 of unfiltered sunlight, but compared to the zero UVB photons emitted by HID bulbs or transmitted by corrugated fiberglass and other regular greenhouse coverings, a UVB level of 140 microwatts per cm2 is quite a lot. Installs nicely in an aluminum window screen frame. Rating: excellent hide.
  • Unfiltered sunlight@11:00am = 300 microwatts per cm2
  • 48" beneath ripstop film@11:00am = 140 microwatts per cm2
The following UVB readings were taken beneath new 3 mil. polyethylene translucent plastic sheeting purchased at the local hardware store. Polyethylene plastic sheeting will break down under direct sunlight and may not last much longer than six weeks before it starts to split. Rating: fair hide.
  • Unfiltered sunlight@10am = 260 microwatts per cm2
  • microwatts per cm2 directly beneath plastic sheeting@10am = 155
  • microwatts per cm2 33" beneath plastic sheeting@10am = 105
Even though the actual microwatts per cm2 of UVB photons required to make fully realized THC is not known for sure, four 160-watt Active UVHeat mercury vapor lamps produced enough UVB photons to give the desired effect to the resin spheres on pollinated marijuana grown indoors---in 16 sq ft, under a corrugated fiberglass roof that transmitted zero UVB photons---when compared to other resin spheres that received no UVB photons. The potency of the harvested resin spheres illuminated by the specialized incandescent light bulbs was on average about the same as the harvested resin spheres from plants grown outdoors that were in direct sunlight only from 11am to 4pm in September and October during the six weeks it took the majority of seeds to ripen after the collective ovum were fertilized in mid-September. The reason for the potency seeming to be nearly the same could be explained by the fact that most of the outdoor UVB photons available to the resin spheres were reflected and had an average intensity of only 50 microwatts per cm2 or so based on measurements taken at different times of day on the sides of the plants that were not in direct sunlight.

The main obstacle to conveniently substituting artificial light UVB photons for the UVB photons in sunlight is that UVB intensity falls off according to the Inverse Square Law, which is why the specialized light bulbs had to be between 12" and 8" from the target floral bract clusters for the UVB to have enough intensity to provide the desired effect. The UVB intensity of the GE Daylight ULTRA 6500K fluorescent tubes is also affected by the Inverse Square Law: At 2" the UVB microwatts per cm2 reading is 27; but at 12" it is only 8. All UVB readings were made with the Solartech Model 6.2 meter

Click here for a list of other sources of UVB and UVB transparent coverings.

The views expressed here by the writer are his alone except where indicated.

copyright 2003 George Grass rev 2004 2006

Everyone is permitted to copy and distribute verbatim copies of this license document including translations into other languages.