Wow awesome answer my man, really appreciate that! I'm learning as I go here but clearly I still have alot to learn.thats a good question. many different factors can influence the distance of your bulb from your plants. some things to consider the size of the bulb. is the bulb in an air cooled hood or not. is the room air conditioned or not? are you using those new LEC bulbs/ballasts? also the age of the plants you don't want to burn your seedlings in solo cups. i think the best thing to do is either get a laser that tells you the temperature when pointed at your plants or a sensor probe like you see in my photo that will give you a rough idea of your canopy temperature if you don't have either of those then i would put the back of my hand under the light and if its to hot for my hand then its to hot for you plants. the idea is to get the bulb as close to your plants as possible without burning them. when the plants leaves start to cup or the edges of the leaves curl the bulb is too close. in my case i'm using a high pressure sodium bulb that is 600watts thats been converted to emit the spectrum of light that you would normally get from a metal halide bulb since metal halide bulbs only come in 250,400,1000 wattages. so the bulb burns a bit hotter than a metal halide normally would. my hood is air cooled but i've been having some hot weather lately and i haven't been able to bring the bulb to close to the plants yet i don't wish to exceed a canopy temperature of 75 degrees Fahrenheit. also i found this photo on the internet i might have saved it from this forum or maby a website i saw it has an excellent chart for lumens and bulb distance based on size.
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