Light and power theory

So, I had this random thought this morning about lighting and adjustable ballasts. Of course, the person I knew to go to with this question is @dbrn32 but anyone well versed in lighting technology is welcome to chime in.

So, I was thinking… let’s say you have (for example) an HPS light set to 500 watts and 20 inches above the canopy. Then, you change it to 1000 watts and adjust the height so that the temperature at the canopy remains the same. In a very poor example, we’ll just say the height doubled to 40 inches.

Is the net effect on the plant the same because the power and distance offset each other, or is the net effect that there is still more gain at 1000 watts because the photons have more energy?


I think there’s a lot of different ways to look at this. Limiting the conversation to dimming hid ballasts narrows conversation a little, but still leaves some doors open for interpretation.

First of all, dimming hid ballasts can have a shift in emitted wavelengths. The reference to photons could become important because different wavelengths provide different amounts of energy. I’m not aware of any linear shift to say dimming is a “not as good” light spectrum. But I assume that if you get into a horticulture tuned bulb, it’s designed to run at nameplate power level.

The second part of this points out the flaw in something like a single point ppfd reading from any part of your canopy. Using your example, its absolutely possible to run at light at reduced power closer to the canopy and get an identical ppfd reading as running the same light at higher power further away from the canopy. Where you would run into differences is if you started taking a series of readings from one side of the canopy to the other. Let’s assume half the power is half the light, your light density over the entire canopy has been reduced. So while you can adjust to a similar reading at the center, you will absolutely see fall off somewhere.

This is what makes integrated sphere data so valuable in light selection. If you have total radimetric output of light running the numbers to determine your average light density is a piece of cake, amount of light over area in square meters. You’ll have ppfd average at that point, and should be able to move on with your grow accordingly.


Thanks very much. It’s on par with what I was thinking.

I realize my example was extremely vague/general, but that was on purpose to keep this a simple layperson-type discussion. Once we start getting into all the different types of lights, power sources, side reflectivity, etc. this goes down a rabbit hole that only the cerebrally masochistic want to read…


The inverse square law describes the intensity of light at different distances from a light source. Every light source is different, but the intensity changes in the same way. The intensity of light is inversely proportional to the square of the distance. This means that as the distance from a light source increases, the intensity of light is equal to a value multiplied by 1/d2,. The proportional symbol, , is used to show how these relate. The relationship between the intensity of light at different distances from the same light source can be found by dividing one from the other. The formula for this is shown below. Visible light is part of the electromagnetic spectrum, and the inverse square law is true for any other waves or rays on that spectrum, for example, radio waves, microwaves, infrared and ultraviolet light, x rays, and gamma rays. The intensity of visible light is measured in candela units, while the intensity of other waves is measured in Watts per meter squared (W/m2).


I = light intensity ( candela , W/m2)

means “is proportional to”

d = distance from a light source ( m )

Intensity at different distances:

I1 = light intensity at distance 1

I2 = light intensity at distance 2

d1 = distance 1 from light source ( m )

d2 = distance 2 from light source ( m )


I was about to say the identical thing.


At least I don’t have to be the one getting picked on for putting that up haha


I know; but it was easier than 'splaining lol.

@TommyBahama: a rough rule of thumb: double the distance between object and light and it’s (roughly) 1/4 the amount of light reaching that object. Conversely; half the distance; (roughly) 4 times the light.

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I purposely avoid inverse square “splaining” lol. The main reason is because of the effective footprint at different distances. Inverse square law explains what happens directly under light source. Changing height of light source also changes effective footprint. A measurement taken outside of effective footprint at say 12" height will be lower than a measurement taken at 18" height that is then within the effective footprint.

It’s yet another reason to find/buy lights you can determine total flux. The output of the light doesn’t change regardless of what height you’re at, simply up to the end user to determine height/coverage.

Information like this tells you nothing about performance in any particular space

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Absolutely! But it can give an operator an idea of options when hanging lights: higher up with a higher output to maximize footprint or down lower at a lower power output with a smaller footprint. Nice to have some rule of thumb method for doing that.

Great point on the footprint; something we all have to take into account.


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I agree. I just don’t wabt to be the one explaining why one thing is true in some circumstances but completely the opposite in other lol.

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You left out the third option. lower Lights on higher power. [quote=“Myfriendis410, post:9, topic:28588”]
when hanging lights: higher up with a higher output to maximize footprint or down lower at a lower power output with a smaller footprint. Nice t


I was trying to compare apples to apples. Yes that’s another option.


Putting a strobe light on em for last 3 days may be fun, lol


LED’s turned down ARE strobing (that’s how they lower power)