LED grow lights come in a wide range of shapes. Some are round, some are square, but about half of the lights reviewed here are rectangular. There's nothing wrong with a rectangular light; depending on your particular setup, rectangular lights may even be ideal–the key is to find a light with a footprint that matches the shape of the area you're trying to grow in.
Of course you can get 2 or more lights to cover a rectangular area, or even get a light mover to change the shape of the footprint (not so much change the total coverage area, but the shape–and if you get the right one, light movers are awesome!).
But whether you're getting one light, multiple lights or using a light mover, you still need to know the shape of the lighting footprint that the LED panel will put out to properly plan coverage for your growing area. Most of the LED vendors helpfully provide the suggested lighting footprint for their light.
However, after getting burned by a light that didn't cover the advertised footprint at any reasonable height, I started noticing just how many advertised footprints are wrong based on the shape of the light itself. Figuring out the shape of a light's footprint is fairly easy; light spreads out evenly in all directions from an LED no matter what the lens angle is; the lens angle only controls how fast it spreads out as you increase the light's height. The shape of the footprint is dictated by the shape of the light itself.
In the table above, companies that advertise footprint shapes for all of their lights which are within 90%-100% of the actual footprint shape are placed in "The Good" column. Companies advertising footprint shapes for all of their lights within 75% of the actual footprint shape are placed in "The Bad"; those with any lights less than 75% are in "The Ugly". Apache Tech does not provide information on the size of their lights, and Truth Lighting provides no suggested footprint sizes, so they were placed in "The Unknown".
Calculating an LED Grow Light's Footprint Shape
For an example of how to figure out the lighting footprint shape, Hydro Grow's 168X-PRO 270W light is extremely long (48 inches) and narrow (6 inches); if the light is raised to whatever height it takes to spread the light from each LED out to a circle 12" across (6" in radius), the total length for the footprint would be 48" (light length) + 6" on one end of light spread + 6" on the other end for light spread, which is 60". At the same height, the footprint width would be 6" (light width) + 6" light spread + 6" light spread = 18" footprint width. And indeed, the 168X-PRO 270W light is advertised as having a 60"x18" footprint, so if you hang it at the right height, you're getting 100% coverage of the advertised footprint shape. (I chose the LED light spread of 12" in this example because it demonstrates that this advertised footprint is the correct shape. I find it interesting that Hydro Grow can get some of their footprint shapes completely correct, and yet be horribly wrong on others.)
Lush Lighting's Dominator light is also extremely long (35") and narrow (8.5") and is advertised as having a 48"x48" footprint. To get one side of the actual lighting footprint to be 48", we would need to raise it to whatever height it needs to be to get light spreading out in a 13" diameter circle from each LED (6.5" in radius, but 13" in total from the ends and sides of the light) so the actual footprint would be 48"x21.5". So the actual footprint is (48 * 21.5 = ) 1032 square inches, but the advertised 48"x48" footprint is 2304 square inches. Covering only 1032 of the advertised 2304 square inches means that only (1032/2304 = ) 44.8% of the advertised footprint is being covered at whatever height it takes to get a 48" long footprint.
But wait– we can raise the light higher to spread the light out more and get it to cover the full 48" advertised footprint width. The light's width of 8.5" means that the light would need to spread out 39.5" total (19.75" on each side) to get it 48" wide. At this same height and light spread, the footprint's length is now (35" + 39.5" = ) 74.5", so the total footprint area is (48 * 74.5 = ) 3576 square inches. The original advertised 48"x48" footprint is 2304 inches, meaning that (3576 – 2304 = ) 1272 square inches of light are now falling outside of the advertised footprint and are wasted if you were planning on the 48"x48" footprint. 1272 wasted square inches compared to the originally advertised 2304 inches means that (1272/2304 = ) 55.2% of the light is wasted, and that you're only using (100% – 55.2% = ) 44.8% of the light being produced if you hang it at that height.
So if you hang Lush's Dominator high enough to match the advertised footprint length, you're only getting 44.8% of the advertised area to grow in, and if you hang it high enough to cover the advertised footprint width, you're only using 44.8% of the light you're paying for. If you split the height difference in half, you'll get half of your wasted footprint area back, but you'll still be wasting half of the wasted light off the ends. In other words, it doesn't matter what height you hang the light at, you're always only getting 44.8% use out of your Dominator light compared to the advertised footprint– less than half of what you were expecting.
Calculating this "coverage score" for all of the lights compared here, it's shocking how badly most of the LED sellers have done at giving accurate coverage footprint shapes for all of their lights. It isn't just rectangular lights either– some square lights have rectangular advertised footprints!
A coverage score less than 100% but higher than 90% is pretty good; you probably would never notice that the lighting footprint isn't perfect, and I'm not going to say that a light with a score above 90% is bad. In some cases, rounding off numbers may be the reason a score isn't 100%. For example, if I were an LED vendor and my light footprint was 19.6" long, I'd just call it 20".
In California Light Works' Solar Storm series of lights, the coverage score is somewhat unfairly skewed by having the UV fluorescent reptile bulbs hanging off the side of the LEDs. In the Solar Storm 880W's case, the LEDs themselves are almost square and the light's shape is made even more rectangular by the fluorescent tubes. It still scores above 90%, so this isn't being unfair to it. For the Solar Storm 440, the LEDs are spaced out at least twice as long as they are wide, and the fluorescent tubes make the light fixture itself more square, giving it a score above 90% when it really doesn't deserve it. Without having an accurate measurement of the LED dimensions, I can't say what the score should be, but it would likely be around 50%.
I should note that some of the LED vendors such as G8LED only gave coverage footprints for their lights in square feet and did not give a shape; for these I calculated the perfect footprint shape to cover the given area so they would get a 100% score. It's easy to see in the table which lights these are, as they have fractional inches in the footprint dimensions.
Footprint Shape Comparison Graph
To summarize again, scores from 90-100% below are good, 75-90% are questionable, 50-75% is really bad, and those less than 50% are shockingly bad.
Less than 90%: Poor Performers
For lights that have less than 90% but more than 75% of the advertised footprint adequately covered, you'll likely notice that plant growth would be uneven in the advertised footprint. At the low end of 75%, you're not getting ¼ of what you paid for.
Less than 75%: Have they Actually Tested These?
For lights with less than a 75% coverage score, it means that at least ¼ of the advertised footprint isn't getting direct light, or that if you raise the light to fix that, more than ¼ of the light is now wasted shining outside the footprint.
Human eyes are actually quite bad at discerning differences in brightness, so you may not notice obvious uneven coverage for a light with a score of 75%, though your plants would. It makes me wonder where they come up with some of these footprint sizes, because it probably wasn't from a growing test, and clearly wasn't from scientific measurements or even basic math. This makes me nervous about accepting any of their other claims.
Less than 50%: Unshapely Lies
Even to a human eye, it would be obvious if you plugged in a light with a 50% or lower score that the shape of the light footprint isn't the same as the advertised footprint shape. To me this indicates that they've never even plugged these lights in to examine their claims, or that they are lying to try and sell you lights– either way I wouldn't recommend buying them.
Lush Lighting's coverage scores go all the way down to 26% for the Dominator lights, meaning that almost ¾ of the advertised "intense coverage" footprint is unusable or that ¾ of the light is wasted if you pull the light up high enough to cover the full footprint width. If they are giving such grossly misleading information about footprint shapes, it really makes you wonder how else they're trying to mislead you.
The Illumitex NeoSol DS light has an innovative feature that allows you to change the angle of each of the individual bars that make up the light so that you can aim them out to get more even coverage. This is a very neat feature, but even if you angle all the bars out perfectly, it still won't give you even light coverage for a square footprint. This is because the light has to travel further to reach the outside of the footprint, and due to the inverse square law of light, the further it travels, the less intense it becomes. So even with all the bars properly aimed, right under the center of the light fixture the light will be intense, but on the sides of a square footprint parallel to the long dimension of the light, where it has to travel the furthest, it will be much less intense.
If they had put a lot more LEDs on those outer bars than they did on the middle bars, that could start to even things out, but with the current design each bar has exactly the same number of LEDs. Even if they do put more LEDs on the outside bar, the footprint shape still won't be square; the light that has to travel the farthest to the corners of the footprint will spread out more as it travels, so the footprint shape would be something like a butterfly's wings, with lots of light wasted off the corners. It's just really, really hard to get a square lighting footprint out of a rectangular light.
Figuring out ideal footprint shapes when you only have the size of the light and an area for coverage gave me my first-ever real-world need to use algebra's quadratic equation; it may have taken 30 years, but I guess Mr. Richardson was finally right about it being worth learning! If you don't feel like deriving the equation yourself, and you're trying to figure out the footprint shape for a light of length L and width W, whose footprint is given only as an area A, it's as easy as figuring out the light spread S needed to get the total coverage area, given by the equation:
Given the light spread S, the footprint length is then L + S, and the footprint width is W + S. It's important to convert L, W and A all to the same units (feet, inches, centimeters, whatever) for this to work.