what is meant by "color" of lights - Metal Halide

[Weedguru Gooster]

COLOR TEMPERATURE OF LIGHT:

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 of the colors of the spectrum are absorbed by an object, it will appear black. If all of 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 chlorophyll, 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. "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 toward 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 S T-R-E-A-C-H 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.

Because of their color spectrums and wattage outputs, Metal Halide and High Pressure Sodium lamps are the most popular artificial growing lights.

High Pressure Sodium lamps are strong in the orange and red spectrum bands. The HP sodium lamp emits an orange-like glow that is sometimes compared to that of the harvest sun. These colors promote flower production and stem elongation. When using an HP sodium lamp, flower production may increase by 20% or more. Unfortunately, HP sodium lamps lack in the blue portion of the spectrum. Conventional HP sodium lights should not be used alone since their lack of blue light will make plants tall and spindly. When used in a greenhouse setting, however, HP sodium lamps are all that are required for supplemental lighting, since sunlight provides all the blue light necessary.

Metal Halide lamps have a fairly balanced color spectrum, with a higher percentage of blue and violet light rays than High Pressure Sodium lamps. Therefore MH lamps are excellent during the vegetative growth stage. MH lamps help keep plants shorter and stockier, with more compact vegetation. Stockier plants are stronger, better able to resist disease, and generally more productive than spindly plants.

When clear Metal Halide and High Pressure Sodium lamps are used together, a winning combination is achieved. Blue light in the Metal Halide promotes strong vegetative growth, and the red light in the High Pressure Sodium lamp promotes strong fruiting and flowering. Some growers like to use a carousel system, alternating MH and HPS lamps, to provide a balanced light spectrum during the plants complete growing cycle.