and some more
METAL HALIDE LAMPS

Metal halide, or MH, lamps emit a white light that looks slightly bluish. They are used to light stadiums, convention centers, and other large areas where a natural-looking light is desired. They are used for rooting and vegetative growth by many gardeners. Some gardeners use them in conjunction with high-pressure sodium lamps during flowering.
Aside from the low-wattage lamps sold in houseware stores, MH lamps come in 175-,250-.400-, and 1000-watt sizes. Each lamp comes with its own ballast.
High-wattage systems are more efficient than low-wattage ones. MH lamps have an efficiency of 35 to 50%. The higher the wattage, the more efficient the bulb. Moving the lamp and reflector is easy, since they are fairly light.

HIGH-PRESSURE SODIUM LAMPS

High pressure sodium, or HPS, lamps emit a pink or amber light. They are used to illuminate parking lots and other areas where the color of the light is not important. HPS lamps are more efficient than MH lamps. They can be used by themselves and will promote faster growth than MH bulbs during both vegetative growth and flowering. Combinations of bulbs are not required, because the HPS has all the light spectrums necessary for healthy growth.
Under HPS lamps, some varieties of indoor plants grow flowers while in the vegetative state. This will not hurt the plant, and it will start flowering more quickly once it is forced to flower.
HPS lamps come in 150-,400-, and 1000-watt sizes and have an efficiency of 50 to 55%. They also come with their own ballasts.

HPS vs. MH LAMPS

HPS lamps emit more light in the red spectrum than MH lamps. The red spectrum is used more efficiently than the blue by plants for photosynthesis. However, both lamps produce high levels of light in the critical wave lengths.
HPS lamps produce more growth because they emit more light as well as a higher percentage in the red spectrum, resulting in more energy that the plants can use to power photosynthesis.
MH lamps do have their uses. The high percentage of blue light they emit promotes rooting and stocky plants. Many growers use these lights for the early stages of growth and switch to HPS lamps during the later stages. However, in an informal comparison of the Chroma 5000 lamps and MH, the fluorescents produced more successful rootings and stockier, healthier plants during early growth.
Some gardeners and garden-store salespeople maintain that combinations of MH and HPS lamps produce the fastest growth. My observation is that HPS lamps by themselves produce the fastest growth. Plants grown under HPS lamps exhibit a little more stem etiolation (stretching) and ripen up to a week later. This is more than compensated for by a considerably larger crop.
Because of the ease and convenience of operating HID lamps and because of their terrific efficiency, they are recommended for most indoor gardens.

LIGHT INTENSITY

Gardens should receive between 3000 and 5000 lumens (per square foot), although plants will grow under as little as 1000 lumens. The brighter the light, the faster and the higher the yield of the garden. When plants receive high light levels, they grow stocky with profuse flowering and dense colas. Given less light, the plants stretch more and produce looser flowers.

LIGHTS AND REFLECTORS

Sunlight comes from a distant source, so light rays striking a small portion of planet earth (say, a garden ten feet wide) are virtually parallel. Also, their intensity does not decrease over the length of a plant three feet tall.
Light emitted from tubes or lamps travels in all directions. As the distance from the lamp increases, intensity of the light decreases. It is not that light is lost, just that the same amount of light is spread over a larger area.
HID lamps and reflectors come in two configurations. The lamps are held either vertically or horizontally.
Horizontally held lamps direct most of the light downward because the light is emitted along the lenght of the lamp. Only a small reflector is required to beam the rest of the light downward.
Vertical lights emit most of their light horizontally. To reach the garden, the light must be reflected downward using a large, bulky reflector. Manufacturers have developed elaborate and innovative hoods, but they do not have the light-delivery efficiency of a horizontal lamp.
Horizontally held lamps have several other advantages over verticals. They take less vertical space, and the reflectors are much less bulky. All in all, horizontally held lamps are the best configuration for an indoor garden.
Aluminum reflectors deliver the most light, more than white ones. Stainless steel reflctors absorb some spectrums of light and should not be used.