? High Performance Lighting
High pressure sodium lights are part of the high intensity discharge family (HID)of lighting family, along with metal halide. HPS come out of the advancements of street lighting, where their most commonly used.
? Horticultural lighting
Horticultural lighting can be divided into to families; high intensity discharge and low intensity discharge-fluorescent tube technology High pressure sodium lights emit an orange-red glow. This band of light triggers hormones in plants to increase flowering/budding within plants. They?re the best grow lights available for secondary or supplemental lighting (especially if used in conjunction with natural sunlight). This is ideal for greenhouse growing and grow rooms.
? A more rounded light spectrum
High pressure sodium son t plus bulbs for a more all round bulb, these light have more blue light output, which have been designed pacifically for growing than street lighing, like the Metal halide bulbs although their a lot more efficient and with almost double the life span of Metal halide bulbs at 18,000 hours, although its recommend replacing after half this time if being used for grow lighting.
? So how many lumen's for my watts?
High pressure sodium HPS bulbs are real efficient. They produce up to 140 lumen's per watt. Although that lessons the further away the bulb is to the plant, but typically you?d expect 40,000 to 50,000 lumen's from a HPS.
? Can my plants absorb all the lumen's I throw at them?
Well short answer is no, lumen is a measurement of light for us humans, of what we pro sieve as bright light, but its most defiantly not a measurement that you?d use for plants as a large part of HPS spectrum is in the green, the same part that plants largely don?t use. 
? Which parts of the spectrum do plants use then.
The lions share of the light spectrum for plants lay in the blue, like Metal halide and some fluorescents, and red like standard HPS bulbs, this is why son t plus is such a good all rounder for growing
? magnetic Ballasts
The most popular type of ballast is magnetic ballasts, these types are still popular due to there price tag and are technically referred to as CWAT (constant wattage auto transformers) which are large (due to the massive transformer inside) and can also become very noisy too,also shortens the life of high pressure sodium lights and sounds like a power sub station-BUZ BUZ
. Digital ballasts
Digital ballasts are a fairly new arrival, they?re more expensive than the traditional magnetic ballasts and some offer automatic detection of HPS and MH bulb as the two types cant be ran on the same type of ballast, inn less a switch able ballast is used, there also fairly silent, light and have an extend bulb life due to more accurate control over current, the appearance of a "flicker-free" light output due to the electronic ballasts high-switching frequency generally 15,000 Hz as opposed to 60Hz for the standard magnetic(transformer ballast)
? Bulbs for growing (vegetative growth cycle) The most common bulbs used for growing are Metal halide and horticultural fluorescents and LED lights as the predominant light is blue spectrum that provides very healthy, lush, green growth making it ideal for vegetative and cloning applications.
? Bulbs for flowering/Bloom
The light emission from High pressure sodium lights bulbs tends to be predominantly orange in spectrum and HPS lamps produce the most lumen's per watt of visible light power and the highest PAR watt plant grow power of all classes of common discharge lights. Due to their high quantity of near-red and relatively high ultraviolet (UV) light output, these lights are well suited for mimicking the summer and winter seasons. Crops grown under these lights tend to be stockier with tight inter nodes and solid well-developed fruit sites, showing increased yields.
? Switching relays
Switching relays are used when a grow light needs switching off remotely, by a segment timer or digital timer, the relay is used to bypass the need for the light current to go though the timer, as domestic timers are not build for such high currents, and normally break down under these circumstances.
AlthoughHigh pressure sodium lights others mentioned above are at present the most common bulbs for horticulture growing see why LED lights are catching up?
LED grow lights page
? Safety
As with anything, when involving electricity, heat and water, please be careful, and remember to read all instructions and take all precautions, when dealing with water and electrics
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