How close should I hang my lights to the plants?

No offense bro, but this has been asked and answered to death.

I live by the "hand rule". My 400W HPS is about 5" from my canopy in one of my cabs (but I use a SunTube to cool the bulb). So there is no hard and fast "distance" rule. It depends on your situation and conditions.

If you have a nice crossflow fan running between your lamp and plants, you can get a lot closer. If you use a SunTube (like me - or a CoolTube), you can get your lamp right on top of them. The only reason my lamp isn't closer, is because I wouldn't be illuminating some of my plants on the perimeter.

On the converse, if you have poor ventilation or if you're growing somehwere hot, like in an attic, you might have to have those lamps 4 feet from your plants. It just depends.

That's why I use the hand rule. I even have a laser temp gauge, but come on. Use the hand rule.

 

You can "light burn" or cause what's commonly called (in the industry) "radiation damage" to your plants if they aren't used to it. For example, if you took a ssedling out of a nursery box that only had some low power fluorescent tubes in it and then put it under your lamp, you'd probably do some serious damage.

If your plants can handle it, give them all the light you can.

 

[quote name='Toke"N"Chill']Wow, 5 inches sounds awfully close for a 400W HPS, but I guess what you are saying is that heat is the only problem. I was just wondering if you can "burn" the plant from having a 400W MH and a 400w HPS light to close to the plant just from the intense light even if using a cool bulb and fans to keep the heat down. I guess I should have been more specific in my first post. I also use the "hand" method to detect if it seems to be too hot. I was just wondering if intense light, even if it is cool can be a problem. Thanks for the reply...[/quote]

It CAN be a problem indeed.
I don't know how much light you to throw at the plants to kill them BUT :

You didn't really think that you can throw infinite energy at the plant at whatever form without causing structural damage now did ya?

 

Usually light bleaching is the most that will happen if there is too much light. The heat is what will burn your plants it will dry them out, like up4 said you need good ventilation, that is the key. I wouldnt go more than a few inches though.

 

heres a light chart i found usefull