Light Distance VS Temperature

I've only been growing for about 2 months. I'm not growing Cannabis but veggies. I have a closet I start plants in with a 8 bulb T5 fixture and a larger 8'x15' room with a 600 watt MH air cooled with a 6 inch inline fan venting out of the room.
I'd like to start growing what we all come here for but for now I'm accumulating some experience.

In the closet with the 8 bulb T5 fixture...
I've read that you want to keep a fluorescent fixture around 12" from the tops of the plants but If I keep it there the temp at that distance is over 85 F. I have to keep the light about 16"-18" from the "canopy" to keep the temp around 80 F. I have fans in the closet to vent the heat as well. I've read some people recommend keeping a fluorescent only inches from the plants. What's the deal?
Pics...


My question is, if I keep the light that far from the tops of the plants are they getting an optimal amount of light.

I have to keep the 600w light about 35"-40" from the tops of my plants to keep the temp around 75 F.

I feel like other growers on here keep their air cooled lights and florescents much closer to their plants than I do. Am I wrong?

What kind of exhaust fan do you have? What's the cfm rating? Sounds to me like you might have a crappy duct fan or something that's not really doing the job. You don't really want your 600 further than 18 inches from the top of the canopy. If you air cool your hood properly you can get it as close as 8 inches.

It is a cheap 6" inline exhaust fan so maybe your right. Ambient temps in the room are 65F but 35" under the light its between 72-75F. Could I be getting a false reading from the thermometer getting too much radiant heat? Is it worth buying one of those infrared temp guns?

How about the T5s? The guys at the hydro store told me keeping them 18" from the tops of the plants was good but I don't know I trust his advice. He seemed a bit confused.

OK so I found this a while ago and it took me a while to dig it back up but I'm wondering what everyone else thinks of this. This is what prompted me to raise my lights up so high.

http://www.littlegreenhouse.com/guide3.shtml

Especially the "Grow Light Comparison Chart" about half way down the page. For a T5 it says it should be 1'-3' away. If I had my 8 bulb T5 1' away temps would be about 85-87F. A 4 bulb T5 can be within 6" according to the chart. If you had a 4 bulb T5 and an 8 bulb T5 at the same height and measures the lumens 2' away from the lights, would it be the same? Ive been reading about the Inverse Square Law but I guess I don't fully understand it.

I feel like from my experience with temps and distance this info is pretty accurate. It also says a 600 HPS should be 3'-6' away. For me, three feet away keeps the temps around 75-80F.

What does everyone else think? The general consensus amongst growers around here is to have lights quite a bit closer then this chart recommends. I've been hearing 18" without an air cooled reflector. I have a Blockbuster air cooled reflector with glass.

tripleduece249 said:

If you had a 4 bulb T5 and an 8 bulb T5 at the same height and measures the lumens 2' away from the lights, would it be the same?

Click to expand...

Read the sticky on lumens, lux, and footcandles....
"What this means is that if you put two lights the same distance from a point, and each light provides N photons per unit area at the point, with two lights you will have 2N photons per unit area at the point. Because intensity is a measure of the number of photons per unit area, the light is twice as intense, whatever unit you choose to use. Twice the lumens, twice the lux, twice the footcandles. "

I can't really speak to the T5 distance, but with an AC 600w HPS 18 inches is no problem -- many growers will move it slightly closer, but this can decrease your footprint. My plants are about a foot away from the glass on my air cooled 600w hood.