tom I keep a 400w HPS about 12 inches from our plants. Less then that for a 150 would be fine. I am guessing here, but 6-8 inches would probably be good.

I like to have at least 3500 lumens reaching the plant canopy for seedlings. If your bulb is rated at 15,000 lumens, at two feet away you should get about half that which is more than enough. (My opinion)

Mike

wow, what a divergence of spread on the distance?! So, is is closer to 6-8 inches or is it closer to 2 feet?
~tom

so, I take the first messages ave of 7 inches and the 2nd posters first posters figure of 24 inches and add them together for 31 inches. the average is 15.5 inches.
~tom

Tom, I believe the discrepancy comes from the plant you are growing. For new seedlings wordwiz is probably right in the fact you do not want to over do it. Once a couple sets of true leaves are there etc, 6-8 inches on the light lovers like tomatoes. Don't burn your new delicate seedlings is the answer. Hope that helps,

Larry

Tom,

I just ordered a couple of 150 watt lamps so I should have them by Friday (keeping fingers crossed). Once I get them hooked up I'll give you a light meter reading.

From my experiences, (and if you don't include the heat factor), the only lights you have to worry about getting to close are high power LEDs if the plants are not use it it. I stuck flat of tomato plants that had been under a 14-watt panel ll of their two weeks of life about five inches away from a 45-watt panel - they did not like it at all!

One other consideration - do you need to maximize the coverage area or not? I'm growing a lot of plants and trying not to spend more than necessary on equipment and power while still growing healthy plants. Last year, 3500 lumens at the plant canopy worked great. This year, I'm shooting for 4,500-5,500 if possible.

Mike

Thanks guys. I don't need to maximize the area. I have one place for a flat that I'm using the 150w hps with a side mounted flourescent. My other areas have flourescents only. I look forward to your readings Mike!
~tom

Tom,

They are impressive. At a distance of about 16", I'm getting 30,000 lux directly under the light. If I go to the edge of what I consider the coverage area, I'm still getting 9,000 lux. So your original thesis of 15.5 inches will bathe your plants in light.

For the fun of it, I measured the lux at 8" directly under the light - I was getting about 80,000.

All I have under the light at the present time is a tray of Bhut Jolokia and a tray of Red Savina seedlings that are less than 1.5" tall.

I'll be following the plants' growth to see how they respond. Keep in mind, my goal is to transplant them into small containers to sell so a little little extra bit of stem won't hurt.

HTH,

Mike

that is impressive Mike! Now, if the same hps lamp could be replaced with a 150 watt MH that is also 150 watt, ED17 and a medium base, it would be perfect!

Thank you,
~tom

Tom,

I may have to join a pot forum to get an answer! The light is a different color, something like an yellowish orange. Using my not so reliable spectrometer (a CD laying at an angle with the light reflecting it through a very small slit at the other end, it appears to be mostly yellow, red and green with just a trace of blue. As long as they do not get too leggy, I'm fine.

Mike

Tom,

Finally found the right search terms for Google. Yes, I can use the fixture and switch to a MH bulb. That's the good news. The bad news is I have to use a MH conversion bulb and most of them run at about $90. I did find one place that was having a sale - $40 per bulb.

Mike

Thanks Mike, The price of the conversion bulb is too much for me, so I'll just supplement the hps with flourescents when I need it. or start with flourescents and end with hps. I started leaf lettuces in a flat under flourescents but after 10 days and they weren't doing that great. After moving them under the hps, they took off like weeds and they are not leggy. It's only been 5 days with the hps. I can't wait to try a flat of tomatoes next!
~tom