A question from a fellow grower:
How high should I place a 350 W Metal Halide and then High Pressure Sodium light about the plants? I don’t want to burn them (obviously) but want to maximize the light? Also would it differ for ages of the plants 1 week verses 5 verses 12?
Yes it does differ for the age of the plant. Seedlings and cuttings will require less intense light. And any plant should be acclimated kinda slowly, bringing the HID light inch by inch closer maybe over a few days. Also this depends on a few things that are specific to how you have your setup. For example you can get your light closer if it is inside a well ventilated hood/air-cooled reflector (a housing shaped to reflect the light toward your plants and enclosed with a glass face that has an air circulation system that pulls or pushes all the heat of of the light’s housing out of the grow area) without burning your plant. But even with extremely well ventilated hoods when the plant gets too close, the light can literally “sun bleach” the color and potency out of whatever gets too close. In general the proper hanging height of an H.I.D. lamp would be 12˝ – 48˝ depending on wattage.
So as a loose general HID guide:
350w - 400w 12" - 48" but maybe as close as 6"
600w 18" - 60" and maybe as close as 8"
1000w 48" - 72" and maybe as close as 12"
I’m sure many gardeners will say they get their lights even closer, and it is entirely possible so you have to take it as stated a loose guide. Start off slow and maybe start experimenting with getting your lights closer and closer slowly and see what works best in your garden.
Good stuff.
Here is an old grower tip: Most will say average your height approximately 32" above your canopy. Once your lamp is fully hot. Place your hand “Palm down” right at the canopy. If the heat is too hot for the back of your hand; It is too hot for the plants! Adjust up or down 1-2" at a time until you find a soft warmth that you would be comfortable laying under. Perfect 
xTip: Always try to place a fan that blows between the lamp, and the canopy in order to easily remove unwanted radiant heat.
Great, simply, easy to remember. Perfect way to teach.