In growing plants with artificial illumination, using light's colour temperature is one method. The Kelvin scale is specifically used to measure such temperature, with warm, reddish light coming in under 3,000 degrees, bluish light registering over 3,315 degrees C and natural sunlight falling somewhere around 2760 degrees Celsius. Different plants respond positively to different colour temperatures, and as a result, consumers have access to a number of quality lights at various positions along the Kelvin scale.
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Metal Halide Lamps
Metal halide lamps come in two versions: standard and colour-corrected HQI (mercury quartz iodide). Standard lamps have a lot of yellow light and some blue while HQIs give off a more whitish-blue light similar to sunlight. Metal halide lamps are useful to plant growers who require great amounts of light, with bulbs available in 70, 150, 250, 400 and 1,000 watt outputs. Bulb lives range from 6,000 to 10,000 hours and feature a colour temperature of at least 5,400 Kelvin at 250 watts or greater. Top manufacturers of metal halide lamps include Phoenix, Aquamax and Osram.
Sodium Vapor Lamps
Sodium vapour lamps, which come in both high pressure and low pressure varieties, create a high-intensity discharge through sodium vapours and are often used in conjunction with metal halide lamps or natural sunlight. They produce a pure yellow light that many plants respond to and are a full 10 times more efficient than standard incandescent bulbs. Low pressure lights contain neon and argon to begin the gas discharge, while high pressure ones contain mercury. The average lifespan of a sodium vapour bulb is over 24,000 hours. General Electric is a key manufacturer of quality sodium vapour lights.
Self-Ballasted Mercury Vapor Bulbs
The self-ballasted mercury vapour bulb does not require any special ballast, screwing into an ordinary incandescent fixture base. A wide variety of plants respond to these lights because they have a dual spectrum nature, emitting light from both the blue/white end as well as the red/yellow (due to the type of filaments). The most efficient output for plant growth is around 250 watts. These bulbs have a lifespan of approximately 10,000 hours and are fairly expensive compared to other bulbs. Top-selling brands include Zoo Med's PowerSun UV and Osram's HWL series.
Wide Spectrum Plant Growth Tubes
Wide spectrum plant growth tubes are designed solely for promoting vegetation growth and give off an aesthetically pleasing pink glow, falling in the low end of the Kelvin temperature scale. Such lights are most commonly used on ordinary household plants, providing the type of light required for effective photosynthesis. These bulbs are relatively inexpensive and are found in most nurseries and hardware stores. Sylvania's Gro-Lux tube, General Electric's Gro-N-Sho and Philip's Agro-Lite are the most popular of these lights, with the last demonstrating the greatest proficiency in growing terrestrial food crops.
Actinic Light Tubes
Actinic light tubes are the best for growing algae, seaweed, kelp and coral, with little to no applicable use in growing land-based plants. Such lighting falls at the far end of the Kelvin scale, with colour temperatures usually topping 8,000 degrees, and appears visibly blue. In order to offset colour distortion, it is best to diffuse the light with a daylight bulb in the 5,000 to 6,000 degree Fahrenheit range. Actinic lighting is primarily sold at aquarium stores and is very expensive. Geisemann PowerChrome lights are the top-selling pure actinic lights on the market.
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