Submitted by Teo Graca and Stephen Adler
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Aquaponics - Lighting Considerations - Part 2
Metal Halide: This is what we have been looking for. Metal halide lights have an efficiency range of 80 - 120 lumens per watt. This let's us use a lower input wattage, generates more light and less heat than all of the previous systems. The following table shows the different wattage bulbs, their efficiency, total light output, and the light intensity over a 4 foot x 4 foot garden.
Many people choke over the initial cost of a High Intensity Discharge fixture, but in the long run they are much cheaper. For the same amount of light output M.H. uses 2-20 times less power than other light sources. Sodium Vapor: The king of efficiency! Sodium vapor lights output from 90-150 lumens per watt. Sodium bulbs put out more light than metal halides but the spectrum is severely shifted towards the red end of the spectrum. The effects of the reddish light are supposed to produce more flowering and fruiting than more balanced lights.
As you can see by the table, sodium vapor wins the light intensity contest hands down.
This diagram shows the full range of light and where each type of lighting system falls within that range. Artificial lights produce just a slice of the full range. This leads to much discussion and experimentation to determine which, or which combination of lighting is best for a particular crop. Lets establish a reference point to work from, examine several types of lighting and put this information to practical use.
Reference point: For most of the daylight hours, the outside daylight peak is centered on 5500 degrees Kelvin (refer to the above chart).
Metal Halide: These lights emit a light on the bluish side of the spectrum. They are considered a grow light and it is considered that they produce a more stalky vegetative type of growth in plants. These lights are commonly used throughout all phases of plant growth and produce excellent results.
Agro Sun Halide: Agro Sun is a hybrid halide bulb that generates extra red light for flower and fruit production. This is considered to be the best choice for artificial lighting of plants.
Sodium Vapor: Sodium vapor lighting is way down in the red. There is some indication as well as a lot of marketing hype that the spectrum produced by these lights promote flowering. Personally, I'd like to see a scientific study to verify this.
The bottom line on spectrum: Spectrum is secondary to the over all intensity. Remember, in any artificial lighting situation, we are able to provide only a fraction of natural lighting. Therefore, it is more important to provide intensity than any other lighting factor. For example if you have to choose between a 70 watt sodium vapor and a 400 watt metal halide the only choice is the 400 watt system. The over all performance will be much greater, even if you favor a certain spectrum.
Click for Details --> Lighting Part 1 <--