Light Intensity

Maximising Your Light Source - Tips From A Pro

Every grower at some point has asked the question “how close do I keep the light to my plant?”. While it is true the closer you can get it the bigger the yield will be, there is one very important consideration, the heat. Any mounted light that is simply too close to the plant can cause the flowers and the leaves to dry significantly, at worst, to the point of burning.

The Importance Of Temperature

The arena of temperature we most need to keep an eye on is between the top of the plant and the lamp. Now many of us will have wall fans installed targeting such areas, some of us may even be hanging hydrometers from lights to monitor temperatures and keep an eye out for hotspots. Generally speaking, this space between the light and the plant should be no hotter than 2 degrees warmer than the surrounding room.

Best Distance For Hanging Lights

The ideal distance to hang the light is 300mm or 0.3m from the top of the plant. However, this is entirely conditional on the temperature. This can only be done if the temperature is within the range you desire. For more customisation and variability to your set up Digital dimmable ballasts are a good investment. It is pretty standard to be able to dim between 250 400 600 and 660w.

How To Measure Light Intensity

The equation used to measure light intensity is the “inverse square law”. Once you get this down, you can work out how to get the most from a light source. Inverse Square law goes something like this:

I = 1/D2

Light intensity (I) = 1 divided by the distance from the lamp squared (D2).

So let’s give the equation a try with a distance of 0.3m

Example 1

I = 1 / 0.3x0.3

0.3 x 0.3 = 0.09

1 / 0.09 = 111 so the light intensity is 111 = 100% this equation proves that with a distance of 300mm from light to plant 100% is being absorbed by the plant.

Let’s do it again this time with a distance of 600mm or 0.6m

Example 2

I = 1 / 0.6x0.6

0.6 x 0.6 = 0.36

1 / 0.36 = 2.7 this gives us a light intensity of 27% so by doubling the distance between the light and plant we have lost 73% of the light intensity

Finally, Remember…

Just because you grow with a 600w lamp doesn’t mean that 600w is being transferred to the plant. If due to excess heat, you are mounting the light 600mm from the top of the plant then you are only transferring around 200w. This is why dimmable ballasts are useful as you can choose to dim down to 400w, meaning, you can mount the light closer to the plant allowing for better light intensity absorption.

It wasn’t long ago I explained the inverse square law to a friend of mine who’d been growing as much as I had. At the time, he was using 1000w HPS lighting for flowering, hanging them 1m above the plant. A bet was made that I could produce a greater yield with 2x 400w HPS lights. He seemed pretty amused at the idea till I later won the bet. As I could mount both the 400w lamps closer to the plant, it could transfer more the light and increase the yield.