Light intensity and horticultural grow lamps

If you have no other information regarding the latitudes that are most favorable for your target species then 1000 W/m^2 is a good first guess.

A question that sometimes comes up is "How close should I put my grow lamps to my plants?"

That is a tricky question and is going to be specific to the species, clone, grow lamps and grow-room.  Rules-of-thumb that worked for Metal Halide lamps might be too conservative for LED lamps that produce far less heat.

One way to generate a starting point for LED lamps is to look at natural sunlight at various locations and extrapolate.

Data collected at Argonne National Labs, 41.7 Latitude.  Multiply by 3.16 to convert to W/m^2

Looking at June light levels.  Remember that June is the best month for growth rates in the Northern Hemisphere because it has the most hours of light.

This is the June light intensity "probability density" graph blown up for easier viewing.  Note that the horizontal axis represents hours and the vertical axis represents light intensity.

This is the same graph with the 50th percentile day filled in with orange.  This graphic is a little bit optimistic because most sites will have some shading in the morning and evening due to surrounding vegetation or structures.

Same as above but the yellow box represents what 1000W/m^2 LED lighting looks like when on from "sunup-till-sundown".  The fact that horticultural grow lights can provide twice as much light over the same time period is a major reason why growers can see such fantastic growth rates.

Please note that the 1000W/m^2 is the power level that corresponds to noon on June 21 in the Chicago, Illinois area on a clear day.  If the species or clones you are trying to grow are not stressed by this power level then you should be fairly safe at this power level.

When you actually go through the math and translate this into the number of lamps in the grow room you will see that this looks like a lot, lot more light than you would expect.

How to use these numbers
Choose the latitude where your crop grows best.  For example, leaf lettuce grows fantastically well in Bangor, Maine (45th Latitude).  Pick the peak power from the chart at the top (980W/m^2).  Then set your lamps so the brightest region is providing about 80% of that power (flux).

Why 80%?  Because the light intensity will increase as the tops of the plants grow!  The leaves get closer to the lights.  The plan is to let the plants tell you when they are getting too much light.

MONITOR at least daily.  When you see scorching or signs of excessive light then record the distance between the tops of the plants and the lights.  Then increase the distance between the lights and the plants.

The distance you recorded is specific to your set-up, your species and your clones.  Any change to your set-up reduces the precision of that number.  For example, increasing ventilation (more fans) might allow you to grow closer to the lights.  Increasing the fertilizer concentration will increase your growth rate and you might find it necessary to increase the distance between the plants and the lights because they close the distance so quickly.