Commonly referred to as "HID lights", these horticultural lights produce 5-6 times as much light compared to incandescent lights of the same wattage and last 10-30 times as long.

These are most commonly (almost solely) used for indoor gardening, especially with hydroponics. They're very commonly used as a replacement for direct sunlight when growing marijuana in your closet. If you try this, I recommend you also get mylar sheeting for the walls, to keep the heat in as well as help the plants.

An average fluorescent bulb produces 2300 lumens. 41,000-46,000 lumens are needed to grow tomatoes. That means you would need 20 fluorescent light tubes to equal one 400 watt HID light.

There's two different kinds of HID lights, blue spectrum, produced by metal halide bulbs, and the orange-red spectrum created by high pressure sodium bulbs. The blue spectrum promotes vegetative growth. If you wanted to grow only leafy plants such as lettuce and herbs, then a Metal Halide light is all you need. There is enough Orange-Red spectrum for fruiting and flowering. Orange-red lights speed up plant production, and promote fruiting & flowering and over all production of the plant.

When growing cannabis plants, it is best to start the life cycle with metal halide bulbs, then change to sodium ones when they start budding. This will produce optimum results. Alternatively, you can use both kinds of bulbs side-by-side for the full lifespan of the plant, but this will run up your electric bill a bit more. Most closet growers want about 400 watts of power behind their lighting (per bulb), though you can get away with less.

I recommend using HID bulbs and fixtures from Diamond Lights.

In addition to its use in 'horticulture', HID lighting is finding increasing use as in scuba diving lighting systems. (Water absorbs light towards the red end of the spectrum, leaving everything underwater a green / black colour, making lighting essential in order to appreciate the natural colours).

This is due to the lower power consumption per lumen (approximately 5 times more efficient than conventional halogen), which means longer durations and battery life and the closer approximation to sunlight, which renders underwater objects closer to their natural colours.

The disadvantage of these systems is cost (approximately 400ukp for a system) and the bulk of the light head, which must incorporate the ballast necessary to strike the initial arc.

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