How far to keep lights away from plants?

Alright, I am about to switch from a 400w MH to a 400w HPS, and I was wondering if the HPS ran hotter and thus I might have to keep the light a little farther away from the plants compared to the metal hylide?

i believe hps and mh run about the same temp per watt

400 watt has max lumen output up to 18 inches. then drops off real fast. i keep a 400 about 4 inches from plants in cooled hoods

Close enough that you're not burning them from the heat, but not too close that you suffer from light bleaching. There isn't really a specific answer, it's more trial and error and such.

I will just keep an eye on it. I guess I will leave it where it is now. I'm sure I will be alright. I just didn't want to put the HPS bulb in and after running all day and I get home from work and my plants be fried. I would just rather be safe than sorry.

Put your hand under the light feel for the heat if it is burning your hand you can realize that it is too hot for the plants so just position it accordingly to what you feel is safe enough for the plant to not burn. 4-6 inches depending in my experience.

The hand trick just didn't work for me when I first tried it a long time ago. My leaves sufferend from heat stress and were all curling up. I just keep my lights at about 12" from the top of the tallest plant, and I then let the plants grow into light until I start to notice some heat stress. That's the best way that I found to decide where to keep your HID lights from the tops of your plants. So far it has worked for me! lol