Do CFL Bulbs Have Poor Light Penetration When Growing Weed?

cfls poor light penetration  

The “fact” that CFL bulbs have low “light penetration”…is false!

Moreover, it’s but a myth passed down and spewed by the likes of uniformed souls who may have missed a physics class or two.

Take note of a very, very important rule which affects how light diminishes; it’s called the Inverse Square Law.

Inverse Square Law   –  I=L/D2  (Intensity = light output/distance squared)



So what does the inverse square law recommend?

That, at some point, ALL light has poor penetration, depending on what is hindering the light’s path in the first place, particularly: distance. Now, light diminishes at the square of the distance, as above. So, if you have 100,000 lumens at one foot away, you don’t any longer at two feet away. You NOW have roughly 25,000 lumens to work with.

So, if you are starting with 10,000 lumens of CFLs, you’ve already lost most USABLE light at a meager one foot away; so two feet away is virtually wasted light as far as your plants are concerned.


That scrawny little 40-watt CFL bulb is going to give your plant close to 0 lumens at one foot away. Thus the reason CFLs are known to have “poor penetration.”

Light is light, and plants don’t discern what brand or kind of bulb it’s receiving light from; they just utilize the spectrum and PAR (Photosythetically Available Radiation). PAR is measured in (µmol/s), micro-mols per second, and is an indicator of how many photons in the 400nm to 700nm range shine onto the plant per second.

The PAR rating more or less determines how much light will “theoretically” be used by the plant, for any particular type of grow light. So the myth can now be quashed: CFLs do NOT have “poor” penetration! They are just victims of the Inverse Square Law–that determines how much light actually “penetrates” your foliage.

It’s not penetrating anything at all, it’s simply providing enough intensity to provide more light to the plant and reduce the PERCEPTION of shaded areas. Remember, plants don’t see nor use light the way we humans do.


Still need more anecdotes?

  • Go buy 44 – 6,500K bulbs that are each 23 watts.
  • Connect all of these bulbs in a tight bundle so the bulbs are all bunched closely together.
  • You now have 1,012 watts of super-bright CFL lighting….
  • Grab a light intensity meter and measure those CFLs against your 1,000 watt HPS or MH and see which has the “worst penetration.”
  • Send us your results and we will publish them here on the blog!


By Robert Narley