FlytrapCare Carnivorous Plant Forums

After wanting to for quite a while, I finally sat down and attempted to figure out how the amount of time grow lights must be kept on varies for every additional inch of distance between the plants & bulb(s). I had 4 t8 bulbs rated at a total of 10000 L in mind so I used 12 hours at 4" as my baseline and then determined how much additional time would be required for each additional inch of distance to achieve the same intensity. Here's what I came up with...hopefully I did it right:
It shows that the first few inches are crucial to your on-time. From 4-5" it required an additional 4.3 hours! After about 10", it approaches an all-day photoperiod and additional distance makes a more minor, decreasing difference in time.

EDIT: I didn't do it right. I decided to edit my original post and remove the original chart just so wrong info doesn't get propagated. Apologies. My original was basically an inverted graph of the light intensity vs. distance but the scale of the Y axis was meaningless. I was looking at it and realized, "wait, this means that even at a mile, the light would only have to be on for 24 hr??" So here's another shot at it:
This was how I originally expected it to look. For each unit of distance added, more time is required exponentially to achieve the same intensity. So according to this, at 2' away, it would need 1728 hours to achieve the same intensity as at 4". Any input/criticism is appreciated.

One thing to consider is this is technically for a single point light source where 4 T8 bulbs is more like 4 arrays of point light sources. I'm really not sure how that might affect it...I think it would either mellow out the rate of change, or not make a difference. Anyway, I'm not saying these numbers are a scale to live by, I just thought it was interesting as a illustration of ratios.


Method:
I started with an output of 10,000 L. I couldn't find exactly how fluorescents outputs are rated. I assumed that's probably at a distance of 0 from the glass tube, but since I didn't know how to work with a 0, I just used 1". This means my intensity numbers are off right off the get go, but since I'm interested only in proportional changes, I think it doesn't matter.

First I calculated the intensity for 1-48 inches using:
which would be (10,000 ⋅ 1²) / d² = 10,000 / d² where d is the distance.

That gives me results of decreasing intensity with increasing distance. Then I wanted to know how long the light has to run to achieve the same intensity as a point receives at a 4" distance. Now here I realized another problem: I couldn't find over what interval a Lumen is actually defined. I'm guessing it is the instantaneous luminous flux. Furthermore, when I looked at how Lumens are measured, I found that it requires an integrating sphere and a calibrating light source which you already know the Lumens output of...how did they measure the calibrating source?? Turns out it doesn't really matter because the output, however its measured, cancels out when I'm solving for time:

T₂ = (I₁ ⋅ T₁) / I₂

Where I is intensity and T is the photo period. In words, I divide the output at 4" over 12 hours by the intensity at the 2nd distance to find the time required at the second distance to achieve the same output.

T₂ = (625 L ⋅ 12 h) / I₂

And plugging all my intensity values into I₂ gives me the graph above.[/size]

I don't understand. Do you mean that bringing the light closer means you can shorten the light hours and viceversa ?

roarke wrote:I don't understand. Do you mean that bringing the light closer means you can shorten the light hours and viceversa ?

Yes. I've figured that if your plant(s) need(s) 12 hr at 4", they would need 19 hr at 5", 27 hr at 6", etc. to receive the same total amount of light. Instead of plants, you could think of a solar panel. In theory, the net power output of that panel over 12 hrs at 4" should be equal to the net power output at 6" over 27 hours.