Okay see if this helps - I've been able to figure out the following. A 400 watt HID light produces the following foot candles at these distances - 3200 @ 7.5", 1600 @ 15.5", 800 @ 26". Low light orchids need 1000-1500, mid light 1500-3000 and high light 3000-4500. Keep in mind this is a NEW HID light bulb. You might be surprised how much they decay over time - usually replaced at least every year even though your eye will not see the difference. To read more try this link
http://www.krischanphoto.com/gardens...ng/hid/hid.htm
Good luck - mike