Below we have a convenient list for growers to use in checking how far apart they want their plants and lights to be positioned from one another. Make sure temperature never exceed 80 degrees.
Fluorescent light 2 to 4 inch (5-10 cm)
Low-energy light bulb 6 to 6 inch (10-15 cm)
Metal halide lights 250-Watt 8 to 12 inch (20-30 cm)
Metal halide lights 400-Watt 12 to 16 inch (30-40 cm)
HPS standard 250-Watt 8 to 12 inch (20-30 cm)
HPS 400-Watt 12 to 16 inch (30-40 cm)
HPS 600-Watt 20 to 24 inch (50-60 cm)
HPS Agro light 400-Watt 12 to 16 inch (30-40 cm)
HPS Agro light 600-Watt 20 to 24 inch (50-60 cm)
One thing to remember in regards to light and positioning is that you want each plant to receive the maximum benefit possible from the lights. That means smaller or shorter plants should be put closer to a light source than big ones so that they receive equal amounts of light. Although this seems simple, it’s a common oversight for new growers (and sometimes old ones.)
Utilizing this technique also offers a grower the advantage of keeping smaller plants out of the shadows of the larger ones, an issue which would compound the lack of light. If your plants are of similar sizes, it’s not a bad idea to move them or the lights around periodically to help ensure that they are all getting equal shares of the bright stuff.