There are two factors to consider. Temperature is first; monitor the temperature at the average height of your plants. To close and you burn the tips to far away and you give up luminance penetration (PAR). Second is Luminance loss. Most experts are in agreement that light intensity diminishes exponentially fast.
For example:
A 400-watt MH from 0? to 12? is about the same luminance say 55000
Same light at 24? the luminance is now about 30000

Still good

Same light at 36? the luminance is now about 12000

Lumens is a mathematical formula that measures the bulbs intensity at 1 foot. The color or type of bulb is not calculated in the formula I=L/D²
Lumens received are a measured in watts-per-square foot or foot-candles. Calculating foot-candles or lux is a more accurate way to estimate the amount of light plants receive. This still only measures how much light you plants have available but not how much your plants use.
Light diminishes exponentially fast!
The closer your plants are to the light the more lumens they receive the better they will grow, as long as the heat from the light does not burn the foliage.

Peace
Zandor Reviewed by Zandor on . how many plants can i gorw under a 600w hps? hi im going to start my growing this week and i am unshore of how many plants i should start with, i have 2 x PHILLIPS PL-L 36W/840/4P 2900 lm for the first period and one 600w hps for later i am growing magic bud and have 10 seeds and dont know how many of them i should plant to start with any ideas? thank you Rating: 5