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 Metal Halide (MH) Bulbs
Provided by Advanced Nutrients
Approximately 40 years ago, General Electric began to experiment with more complicated mixtures inside of mercury arc lamps, in order to get better efficiency, and a more pleasing, white color. Starting with mercury metal vapor, they added iodine salts of other metals (indium, scandium, sodium, and thallium). Iodine is one of the halogen elements, and thus a compound of metal and iodine is called a "metal halide" salt. By 1962, they had applied for patents on this variant arc lamp, which they called a "Multi Vapor Metal Halide" lamp. The "Fountain of the Planets" at the 1964 New York World' Fair was illuminated by GE's new metal halide lamps. Metal Halide bulbs are more efficient than their parent, the mercury vapor arc lamp. The chemical compounds in the new lamp produce a more natural color balance, than it's predecessor. To the naked eye it looks like white light, but just like sunlight, it favors the blue end of the spectrum. As such, they are very useful in indoor horticulture, where the replication of sunlight is highly desirable, especially during the growth stage of the plants.
As we've stated before, the sun's spectrum contains visible light in every color of the rainbow. Blue and violet rays appear at one end of the visible spectrum, at about a 380 nanometer wavelength, and red light appears at the other end of the spectrum, at about 760 nanometers. If all the colors of the spectrum are absorbed by an object, it will appear black. If all the colors are reflected by an object, it will appear white.
Plants appear green because they reflect green light, while absorbing the other colors of the spectrum. The green pigment in plants is chrorophyll, essential for photosynthesis. The green portion of the light spectrum is relatively unimportant to photosynthesis, since it is mostly reflected by the plant. Chlorophyll absorbs light energy from the blue and red portions of the spectrum, to convert carbon dioxide and water into sugars to be used by the plant. Without red and blue light rays, the plant will weaken or die prematurely.
The orange-red portion of the light spectrum is very important to plant reproduction. Pigments called "photochromes" absorb the red and far red portions of the light spectrum. Photochromes regulate seed germination, root development, tuber and bulb formation, dormancy, flowering and fruit production. Therefore, the red portion of the light spectrum is essential to plant growth, particularly during the flowering, fruiting, and reproductive stage.
Plants need light from the blue end of the spectrum, as well. Blue light stimulates chlorophyll production more than any other color, promoting thick leaves, stocky stems and strong vegetative growth. Carotenoids, the yellow-orange pigment in plants, absorb blue light and control leaf fall and fruit ripening. Riboflavin, containing another pigment, absorbs violet light and influences "phototropism," the movement of plant foliage in response to a light source.
"Positive tropism" is the movement of plant foliage toward a light source, while "negative tropism" is the movement of plant foliage away from a light source. Positive tropism is greatest in the blue end of the spectrum at about 450 nanometers. At this level, plants lean towards the light, spreading their leaves out horizontally to absorb the most light possible. Lack of blue light will make plants tall and spindly, since the plants will stretch out in search of blue rays. Adequate blue light helps ensure each plant's natural shape and prevents "legginess," or unnatural elongation of the stems.
Let's look more closely at a Metal Halide lamp. The center of the MH lamp contains a quartz arc tube. Light is produced by arcing electricity through combined vapors of mercury and argon, with other metals, introduced into the arc tube as compounds of iodine: thorium iodide, sodium iodide, and scandium iodide. Spring supports in the dome and neck of the outer bulb hold the arc tube in place. The outer bulb acts as a protective jacket. It protects the arc tube, while absorbing harmful ultraviolet radiation. If you spend a lot of time in the grow room, protective goggles that filter out UV are also recommended.
Phosphor coatings are also an option. Phosphor provides more diffuse light, which is easier on the eyes. Plus, the phosphor coating tends to tip the color spectrum towards the red end of the spectrum, providing slightly more red light for fruiting and flowering. A phosphor coated bulb is recommended if a "single source" Metal Halide lamp is required.
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