Ron,
There are high pressure sodium lamps (SON-T) which have a spectrum designed
specifically for growing plants. Together with Metal Halide this is the
main lamp used by commercial growers. Metal Halide has a stronger UV
content and so is used for green growth. SON-T has a higher red spectrum
which encourages plants to flower. In commercial set ups the two lamps are
used for different stages of growth and sometimes together.
For the amateur grower discharge lighting has several advantages.
Principally the running costs are considerably cheaper since the light
output is far greater than with conventional fluorescents. This is off set
somewhat by the higher costs of the lamps. Also since the light output is
so much greater taler growing plants such as Sarracenias and climbing
tuberous Drosera can be grown under the lamp. I use a Mercury discharge
which is has a lower output than a Sodium or Metal Halide lamp but has a
slightly better spectrum and is a bit cheaper to buy.
-- Phil Wilson (cp@pwilson.demon.co.uk)