Also..........................
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(http://www.yahooka.com/forum/faq.php?faq=gfaqtext23)

High Pressure Sodium (HPS)

The HPS lamp is often regarded as the best all round lamp to have. Not as effective as the MH in vegetative growth, more effective in flowering and generally a higher par and lumen output per W than MH. HPS lamps work in a similar way to MH lamps except the inner tube is ceramic rather than quartz (withstands higher temperature) and this tube is filled with Sodium, Mercury and sealed with Xenon gas.

HPS lamps come in ranges from 70 to 1,000W and produce upto 145,000 lumens. They produce light in a redder spectrum (2,200K) that is very efficient for flowering. These lamps do not emit any UV light.

Some newer HPS (son Agro) have an extra coil to produce a small amount of blue light these bulbs are available in 250, 400 and 100W and, for example, the 400W bulb would be 430W, these bulbs can be used in a standard HPS lamp holder and ballast.
This allows HPS users to reap the benefits of MH (tighter internodal space, stronger stems etc.) without giving up the benefits of HPS.
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Yeah definatly going for it!