Canopy Penetration and Beam Angle

by /u/SuperAngryGuy of /r/HandsOnComplexity

Plant canopy penetration is a function of both optical power output and how focused that light source is. This is a huge consideration when building or buying a LED grow light.

Growing lettuce without much vertical space? Get a LED grow light with wider beam angles. Growing a 3 foot tall plant where you want the bottom leaves receiving a lager amount of light? Get a LED grow light with a narrow beam angle but have the light higher (further away from the plant) than a light with a wider beam angle.

Lighting beam angle concepts (source)

Linear lights sources such as fluorescent tubes have such a wide beam angle which is distributed along the bulb, are great for lettuce which can then be stacked on shelves. In most grow situations, the closer your light source is to the plant, the wider the beam angle you want:

  • Short plants = wider beam angle
  • Tall plants = narrow beam angle and have the light up a little higher so that you're not light saturating the upper leaves

Practical examples

A 3 watt LED does not necessarily have better penetration than a 1 watt LED. I've seen such discussions in multiple forums multiple times.

Let's say we have a single LED that is a (theoretical) point light source. It's light output will follow the square of distance law of light drop off in this case. At one foot we have one unit of light which covers one square feet, at two feet from the LED we have ¼ unit of light which is 4 square feet, at 3 feet from the LED we have ⅑ unit of light which covers 9 square feet etc.

But, what if we had a 1 watt LED with a 60 degree light beam output and the 3 watt LED with a 120 degree output. What LED penetrates the plant canopy better? The one watt LED is going to penetrate better since its light is 4 times more focused (simplifying here).

I have a 100 watt LED at 120 degrees and a typical 0.005 watt laser pointer (lasers are rated on their true optical power output unlike LEDs) at what ever degrees it is. The tiny laser has better penetration. I can focus it to put tiny burn holes on the bottom leaves of a typical indoor plant in a few seconds with better optics than a cheap laser pointer.


Beware that some LED grow light manufacturers/importers might make a comparison and have the ruler (or whatever) say 12 inches from their light (A) and a competitors light (B). So A with a light meter puts out at that 12 inches so much more light than B, maybe even twice as much as their competitor! No, check the beam angle of both the lights.

I've seen this trick so many times by people in the trade. In most forums it's not under standing beam angle and light drop off. You just need to put the light closer if using shorter plants or low stress training and a screen of green.

A lot of distributors are coming out with multi-beam angle LEDs. The ratio of wide angle to short angle would he a handy piece of information to know as well as their general spectrum (red, far red, blue, etc).