Lighting Gradients: The Inverse Square Law and You

[NorthernNevets]

So, I've been growing various plants under artificial lighting ever since I went to college and we had a growing season that was about 2 months. Jalapeno's, tomatoes, perennials, annuals- Pretty much anything green I could get my hands on I tried to see if I could make it grow in my dorm room. Currently, my house is probably is a fire hazard due to the excess oxygen. At least I would have plenty of dirt to throw at it, heh.

Haven't grown weed (well, I have grown *weeds* aka random green things I found in-between bricks, you get the point) but generally, herbaceous annuals tend to like more or less the same things when it comes to photons.

Ive been browsing these forums (and others) for a while and have seen various charts about lumen levels and lights and reflectors and so on. Particularly, people use the inverse square law to determine how much light their plants are getting based on the light output of their bulb.

You shouldn't. Ever. Period.

Why?

Because the inverse square law assumes that the light source is a point source. At the minimum, the diameter of the source needs to be very very small compared to the distance away from it.

If you are using one of those large 100w clfs that look like mini baseball bats and its sitting a few inches from your plants, the diameter of the light source could be larger than the distance. If your plant gets closer, it could actually get less effective light because the light coming from the outside of the bulb has a higher angle of incidence (probably not in reality, but still). The inverse square law does not even come close to giving you the right numbers- your measurements will be far lower than the formula would predict.

Now lets say you have a big 'ol 12 inch reflector set to a tight beam with a 400w mh or hps in it. Guess what that reflector does? It collimates the light- makes it all go in more or less the same direction. Now the inverse square law is broken twice- not only do you have a large source, but the light is collimated. The luminosity gradient will be, for a while, sub-linear!! Your light levels will be greater than you would otherwise think.

Now you get a quiz. Imagine a 5x5 array of CFLs all individually collimated with 6 inch reflectors. Which gets more light, a plant 2 inches away from the bottom of the middle reflector, or a plant 2 feet way?

 

[JamieShoes]

great post... I use a light meter these days... what I thought about my footprint and the reality were just too far apart..

 

[Rooibard]

I would loove to see how this apply on vertical barebulbs vs reflectors on diff canopy level... Anybody smarter than me pls?

Peace

 

[trichrider]

trick question, you gave answer already...sly.

 

[NorthernNevets]

Yea, the one farther away will have more light. This is really important for 2 reasons-

1) Obviously, more light is good, and distance from the heat source is also good.

2) Gradients. This is the real big one. It has SO many implications to growing plants. Plants pretty much do everything based on gradients in concentration, inside and out.

Inside the plant, gradients/flow of auxin and other hormones control what parts of the plant grow. Not simply concentration, because plants can't 'measure' the concentration of things, but they do 'compare' it.

Outside the plant, it responds to gradients of light and nutrients. Roots don't have nutrient radar, so they cant tell if there is more N or P to be had somewhere. What they can tell, however, is if this one side of the plant has more than the other. This nutrient gradient stimulates the roots that are in the higher concentration to grow. Now if all the roots are exposed to the same concentration, nutrients actually inhibit root growth.

Light is similar, and its called phototropism. Plants grow towards light. Who knew? (heh) Light inhibits stem elongation, and darkness promotes it. Seems backwards? Well, imagine a plant getting lit on only one side. We have a light gradient across the plant, and the plant relies on this gradient to 'know' what is light and what is dark. It compares the two.

The darkened side gets longer, bending the plant towards the light. Genuis!

But no one lights their plants just from one side. How does this help us?

It all comes down to gradients in your light source. Look at the sun. The sun is very, very bright and very, very far away. This means that the height differences in your plant are tiny compared to how far away the sun is, so a plant doesn't get more light simply by getting closer to the sun.

With artificial lights though, its different. Here is where the inverse square law does matter. As a plant grows closer to your lights, the closer part will generally get more light, especially with single light source setups that are very close. This encourages it to keep growing closer because your light source itself has a distance depended gradient on the scale of inches, unlike the sun.

This is bad. It causes excessive stem elongation and inhibits lateral growth. In extreme cases, your plant will try to kiss your light source. Ouch.

It also is the reason some light sources seem to have more 'penetration' than others. A single, uncollimated high intensity light will provide far less light to the lower parts of the plant than an array of lower intensity sources. The peak intensity might be lower, but the average intensity over the whole plant might be much much higher.

Why do HIDs seem to have more 'penetration' than other sources? Well, its not the 'type' of light. Its that HIDs are generally placed farther away (so changes in plant height are smaller compared to total distance from the light) and they usually come with big aluminum reflectors that collimate the light. This all reduces the gradient that the plants experience.

Second quiz time! Knowing that plants respond to a gradient of light, how would you set up your lights?

 

[trichrider]

ideally a light mover, but more practically, spinning each pot and leaving the lights up high.

jmo. this is quite interesting tho'. things i didn't know, thanks.

amirite?

btw...Welcome to ICMag!

 

[Payaso]

Welcome to the site NorthernNevets.

Your interesting notes about lighting are of interest to many.

Light metering will solve the problem for many, showing them the optimum distance when testing with he meter will answer many questions.

I do not know of a chart that represents this, I have only seen simplistic things. Sometimes a good infographic can tell the story better than words can.

 

[waveguide]

bit silly really, stick something under the light and have a look at what it looks like. it's a good way to tell.

pending your lights/exposed cfl bulb surface aren't blocking each other, it's straight up gonna be brighter with shorter distance. even if there's a theory on the internet that says different.

 

[NorthernNevets]

Welcome to the site NorthernNevets.

Your interesting notes about lighting are of interest to many.

Light metering will solve the problem for many, showing them the optimum distance when testing with he meter will answer many questions.

I do not know of a chart that represents this, I have only seen simplistic things. Sometimes a good infographic can tell the story better than words can.

Ill see if I can put together a little infographic. I got about a thousand words up there, so about right.

The real point of all this is to 'stretch' area that receives the optimal amount of light. Plants don't want to grow into your lights because they don't get exponentially more light as they get closer. The tops and bottom of the plant get almost the same amount of light. Plus, you can set your lights farther away (less heat) and in a really good setup, you might never have to move them.

bit silly really, stick something under the light and have a look at what it looks like. it's a good way to tell.

pending your lights/exposed cfl bulb surface aren't blocking each other, it's straight up gonna be brighter with shorter distance. even if there's a theory on the internet that says different.

Almost always, yea, unless you make it that on purpose!

My example with the array of CFLs was pretty much exactly what you are saying- the reflectors would be 'blocking' adjacent bulbs by directing the light straight down, and not to the side. Pretty contrived, but you can intentionally do something like that to prevent your plants from stretching towards the lights.

Ever seen something burn under a magnifying glass, and how the smoke makes this inverted cone of light visible? That's almost the exact opposite of a point source. You get less light as you get closer to the source, aka the lens of the magnifying glass.

The light is more or less traveling in a straight line as it enters the lens, so rays of light get bent inwards. Light rays coming from a light bulb are spreading out, so the same lens, instead of focusing the light rays to a point, would focus them into a column, where all the rays are parallel.

That's what tight beam reflectors try to do- make all the light go in the same direction. You get the (almost) the same light level reading 1 foot, 5 feet, or 10 feet away from a collimated/focused source like that.