# Question on 'penetration'

I'm having a hard time understanding the science behind "penetration". From what I can understand, a light particle (wave?) is emmited from one source and will travel in one direction at one speed until it is reflected or absorbed. If it is reflected, it slows down a tiny bit (still travels at light speed though? LOL)

My problem is that there seems to be sucha large dropoff in "penetration" which I take to be the amount of light particles or waves striking the leaf of a plant. So if I have 2 flourescent tubes 2 inches above a leaf structure lets say they get a light rating of 9 out of 10, but at 5 inches does it drop to 7 or does it drop to 3? or 1? What about at 10 inches?

And based on that info, if I created a four sided rectangle that I could put around the plants that extended all the way up to the light source, and painted the inside of that rectangle flat white, would I dramatically increase the amount of light getting down to the palnts? It seems to me that re-directing the available light to the plant is cheaper than adding more light to effectively, overpower the situation.

Can I have 95% of a 2 bulb flourescent get to a plant surface 10 inches away under such a setup? And if not? Can someone explain why? Doesn't a white surface reflect 95% of light? So if it is reflected and not absorbed by the walls of the rectangle, why wouldn't it continue bouncing down till it hits the leaves, dirt, container or table.

I am going to be building a small seed starting system and just want to optimize it for as little cost as I can. I am thinking of doing 2 four ft double shop lights with 6500K t8s. I'm going to be trying to grow salad greens after that in the setup.

Is my idea sound? Should I separate the 2 shop light systems and make 2 divided growing spaces, or surround both lights with the white walled reflective rectangel box thing.

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mistascott(7A VA)

Don't believe it. It is a myth. The farther the light is from the plant, the more the light disperses before reaching it. With fluorescents, the closer to the plant (without touching) the better. HIDs tend to produce more concentrated heat, so they are usually kept higher. The lighting is more concentrated under HID, which allows for more light to go to one place, whereas flouresent tubes are long and narrow and less "focused." There is a balancing act between proximity to the plant and total coverage -- the higher the light is, the wider the coverage area.

As for the box idea, I think the logic is sound however I question whether it would allow for adequate air circulation for the plants.

February 22, 2012 at 10:25PM
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penfold2(4b, MN)

The problem is that bulbs radiate light in all directions. The individual photons are not getting any weaker as they travel, but they are spreading further apart, reducing the density of light. This reduction is described by the inverse square law.

Reflectors are an attempt to correct this by focusing the light downwards rather than allowing it to spread out. Some reflectors are better at this than others, since a bit of spread may be desirable when illuminating a room, for example. A good reflector for our purposes, however, will wrap around the sides of the bulb, focusing the light as tightly as possible. The size of the bulb also makes a difference. With an infinitely small point source of light and a perfect parabolic reflector, all of the light could theoretically be directed straight down with no spread and no loss of intensity. Of course there is no such thing as an infinitely small point source of light or a perfect parabolic reflector. But still, the smaller the light source, the more accurately the reflector can do its job. A larger bulb also gets in the way, blocking some of its own reflected light.

Covering the growing area with reflective material will also help quite a bit. You will always lose a good deal of intensity in the first few inches from the fixture as light spreads outward, but once the light is high enough to evenly light the entire grow area, further increases in height should have a much smaller effect.

I think your plan should work well since your setup will be very efficient, and salad greens do not require particularly high light intensities. I would combine the two fixtures into one grow area if possible. It will require less reflective material which will also result in fewer reflections and therefore higher efficiency.

February 22, 2012 at 11:22PM
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Thanks Penfold,

A secondary question and then a hypothetical.

Is a flat white smooth surfave going to reflect more light than a mirror?

And then, what if I made a full box of mirrors and put a light inside the box, since the light would just be pretty much reflecting around in the box, and not be able to escape unless in hit the bulb, would the intesity in the box just continue to accumulate?

February 23, 2012 at 3:54PM
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penfold2(4b, MN)

"Is a flat white smooth surfave going to reflect more light than a mirror?"

I don't know the specific values for different materials, but I don't think mirrors are necessarily as reflective as many people assume they are. Just because they are smooth enough to reflect an image does not mean they have a high reflectivity. Motionless water can be smooth enough to reflect an image, but only a portion of it is being reflected while much of it is passing through the water. "And then, what if I made a full box of mirrors and put a light inside the box, since the light would just be pretty much reflecting around in the box, and not be able to escape unless in hit the bulb, would the intesity in the box just continue to accumulate?"

If you had 100% reflective surfaces, I would think so. But such a thing does not exist. Realistically, I think light intensity would accumulate to a point. But eventually (and by eventually I mean within a fraction of a second), the intensity would become high enough that even a 99% reflective surface would absorb the same amount of energy as the bulb is producing. At this point the intensity would remain the same and the energy being added would be dissipated as heat.

February 23, 2012 at 5:16PM
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Thanks Again, I'll try to do a little googling on what the most reflective surfaces are and then also see which ones don't cost a fortune.

As to the air movement, What if I left the ends of the reflective box open and had a fan blowing on them. I mean if I think about the shape of the bulb, the light is going to be dispersing mainly radially out from the bulb, so if I put two reflective surfaces lengthwise, I should reflect a high percentage of the light back towards the center.

February 24, 2012 at 9:39AM
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vorkus(z6 PA)

Depends on how much heat is being generated. If you use an HID light in an enclosed space it better be connected to a ventilation fan. Same is true for fluorecent lights. If you get enought wattage in a small space you are going to have heat issues. I use Mylar for a reflective surface. It works pretty well but my grow area isn't nearly completely enclosed. I'd fry my plants. If you use something like a grow tent, the you better be providing both cool air in and hot air out.

February 24, 2012 at 3:09PM
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penfold2(4b, MN)

He's just talking about 4 T8 bulbs. I think he'll be fine with a small fan blowing from one end to the other. Just the passive ventilation from leaving the ends open may be enough, but a fan is probably a good idea.

February 24, 2012 at 4:44PM
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vorkus(z6 PA)

Depends on how much heat is being generated. If you use an HID light in an enclosed space it better be connected to a ventilation fan. Same is true for fluorecent lights. If you get enought wattage in a small space you are going to have heat issues. I use Mylar for a reflective surface. It works pretty well but my grow area isn't nearly completely enclosed. I'd fry my plants. If you use something like a grow tent, the you better be providing both cool air in and hot air out.

February 24, 2012 at 10:05PM
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