H
Hermitian
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- Lumens are a measurement of luminous flux – the amount of light perceived by the human eye. Since light bulbs were originally manufactured for human vision alone it became the standard of measure for bulbs. It is still very important in architectural design and in civil building codes. Most governments require that the lumen output of a bulb be included on the label.
- Watts are a measurement of the rate of energy transfer per second, which in science and engineering we refer to as power. Watts / square meter or Watts per square foot in the context of lighting refers to radiant flux; i.e., the amount of energy being transferred per second to a surface area.
- Example. A popular 23 Watt compact-fluorescent bulb states on the label that it has the equivalent output of a 100 Watt incandescent bulb. This means that the compact fluorescent produces the same lumen output as the incandescent. It doesn’t do this by magically creating energy out of nothing! Instead, that particular bulb is more efficient at producing radiation in wavelengths that stimulate the human eye.
- Plants absorb light from the human visible spectrum, but (depending on the plant) in ratios of about 1/3 of the blue sub-spectrum, about 1/3 of the red sub-spectrum, and about 1/3 of the rest combined. Chlorophyll – whose central atom is Magnesium, is key in this process. For most plants, it is an error to grow them with only a single wavelength of blue and a single wavelength of red. It can work, but plant vitality is reduced. It is also an error to overdose your plants the longer wavelengths of red – the so-called infrared. Plants only utilize the infrared for heat content and too much will cause stress if not overheating.
- Effective output refers to the output power of a lamp in the target spectrum. For plants, incandescent bulbs have the least effective output. An HPS or MH bulb has a nominal effective plant light output of 78%, with the lower wattage bulbs (e.g., 400W) achieving less and the highest wattage bulbs achieving near 82%. The remainder goes mostly to long-wave and heat. So for example, a 900W incandescent bulb has an effective plant light output of about 725 Watts. In contrast, fluorescent full-spectrum bulbs in the 5000 Kelvin to 6500 Kelvin range have over 90% effective plant light output, with the T5 HO 6500K bulbs near 92%.
- Diffuse light is preferred by plants as opposed to coherent light. Fluorescent bulbs output diffuse light, where as lasers are extreme examples of coherent light. Incandescent bulbs, spectrum shifters, and polarizers produce a mixture of the two.
- The ideal radiant flux for Cannabis is between 50 Watts per square foot and 75 Watts per square foot of effective output – projected at a distance of 1 meter. If your bulbs are closer then less is needed, if you are farther than more is needed. The distance relationship is not linear. For example, if you half the distance you still need about 70% of the output.
- Projected area is the entire surface area illuminated by a bulb or series of bulbs. If your bulbs are uncovered (no hood) in a room 15’ x 15’ x 9’ high then the projected area is 990 square feet. If instead you have a robust mirrored hood that focuses upward and side output to the floor, then your projected area is about 225 square feet plus about 5% of the remaining 765 square feet for efficiency loss in the reflectance – a total of about 260 square feet.
- Calculate the wattage you need by multiplying the number of square feet your bulbs will project onto by the target number of Watts / square feet, and then dividing by the effective efficiency of the bulb. For example, to achieve 75 Watts / square foot in the 15’ x 15’ x 9’ example room using a reflective hood over T5 HO lamps, we’ll need: 260 s.f. x 75 W/s.f. / 92% which is about 21 kW.
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