HID 400W Distance Advice
 

It will soon be time for me to move my collection indoors for the winter. I have a 400 W Warm Deluxe HID bulb that I can adjust from three to five feet above the plant table. I usually put my Blc varieties directly under it, with Phals, Paphs and such off to the side. The room stays cool, so I'd like to heat the plants as much as possible from the light without burning the leaves. The tallest plants are 22" from table to leaf tips. Do you think I dare lower the light to 36"? (for clarity, that would be 14" above the leaf tips) When new, hung four feet above the table, this light was putting out 5000 foot-candles at leaf level. (that was 26" above the leaf tips)

I'm guessing that the positioning of the plants is based on light intensity, the center being higher than the edges. If you want to consider lowering the light, measure the temperature at leaf level (and if you do it, feel the leaves to be sure they aren't too hot) - you do risk toasting leaves. In addition, you would be lighting a significantly smaller area. Perhaps, leave the light where it is, and add a small space heater (with a thermostat and fan) in the area of the Phals to give them a bit more warmth. A seedling heat mat might accomplish the same thing.

I agree. Plus - if you measured 5000 fc from the higher light elevation, it would be significantly greater if lowered. Intensity is a square function, not linear. - half the distance being 4x the intensity.

Also, keep in mind that a plant that can take a 5000 fc noontime intensity in sunlight only needs half that if grown under the continuous light intensity of artificial lighting.