Grow lights

[Aberdeenplotter]

The answer is as close as possible to the plants without the possibility of heat damage to the plants. WIth an led growlamp there will be very little risk of heat damage so you can get the light almost touching

[Jimmy]

As a guide natural light from the sun comes from appox 93,000,000 miles away.
Perhaps not the most useful reply, but an interesting fact.
Jimmy

[toomanytommytoes]

The answer is as close as possible to the plants without the possibility of heat damage to the plants. WIth an led growlamp there will be very little risk of heat damage so you can get the light almost touching

There is still the possibility of the light intensity being too high and causing damage if the light is positioned too close. Without knowing how much light is being put out, it's difficult to say how close the light should be. Plants generally require between 200-600 PPFD for vegetative growth (100-300 PPFD for seedlings). Reputable companies are usually happy to publish light intensity maps for their products, which show the PPFD values over an area at a given hanging height. But yes, if the light is low powered like the one from Twowests or IKEA, it can be kept very close to the plant canopy.

[monkeyboy]

I made a frame from plastic waste pipes that had an adjustable column. It was a pretty crappy effort due to lack of proper tools (trying to cut pipe square with a hacksaw and my eye was not good) but it worked well.

I kept the lowest distance about 12" from top of the 4" plant pots to the bottom of the fluorescent tube up to 24". So I could adjust it as the plants grew. Yes, as pointed out, some plants grow faster than others so I would rearrange plants to increase light to smaller plants.