Mylar usage

[NS775]

Just thought I'd share something I thought of recently. Being my first time around I am still learning the ropes, so I dunno if this is common knowledge or not....

I keep seeing grow room pics where people are lining their walls with Mylar to boost reflectivity, which I have done as well. While examining my handiwork I started thinking that light degrades exponentially as a function of distance, so when you line your walls with Mylar the light has to travel out the bulb to the wall and then back to the plant. That is alot of distance, even in my cramped little closet. By reducing that distance by 1/2 you could exponentially increase the intensity of *some* of the light (the light you gained by initially putting up the Mylar.

To take advantage of this, what I did is to hang one side of the mylar off of the HPS lamp reflector housing, and then drape it over the far sides of the plants on all 4 sides. Its kind of hard to describe, but something to the effect of how mosquito nets are positioned over bed frames, where the bed frame is the actual plants/plant canopy. Its like using the Mylar as an extension of the reflector housing.

After doing this I've noticed the girls seemed to have noticed the overall increase in brightness. Especially on the outskirts of the canopy, and it also created 2 additional nice little high intensity spots where the light hits the reflector and then hits the Mylar on the opposite side. I don't know if anyone else does this, but it seems to make so much more sense than putting Mylar on the wall 1-2 ft. away from the canopy.

 

[Sam the Caveman]

There are some pictures of a grow tunnel on pg. 29 in Eminem's 30k watt grow diary in the hydro forum. The grow tunnel uses a linear parabolic shaped reflector the whole width of the grow room and extends however long you want. This same concentration of light along the sides you notice is shown in the pictures referenced above. There is also a long tutorial on CW for making one, but the pictures are gone. The reflector is made out of 1/8" masonite with mylar glued to the bottom.

I've been calculating a parabolic gull wing shaped reflector the width of my grow room that should provide very even light distribution without that lateral concentration.

 

[JustanotherDave]

^Are all the tunnel pics gone Sam?
I think I have them all saved on my comp somewhere...

NS...if the plants aren't already close enough to the wall, then yes...it would be best to get the reflection up close as opposed to relying on the wall alone.

 

[NS775]

Yea I've been playing w it the last couple of hours. Having the Mylar loose, I am able to bend it concaved/convexed to focus the light on the areas that need a little help. Is there any sort of software for modeling light distribution/concentration? Or do I need to actually learn the math/physics part?

 

[cashmunny]

The focal length is half the radius of curvature for a spherical mirror. So if you create a concave hemispherical shape, with a diameter of lets say 2 feet (radius of one foot) the focal point hot spot will be 6 inches off the mirror surface. With the caveat that the mirror shouldn't span a large angle and the distance of the light source is approx ten times the focal length.

But if you have a completely enclosed grow area, flat mylar is pretty much as good as curved mylar since light is energy and energy is conserved. All that light will just keep bouncing off the walls until it hits a plant surface and is absorbed. But every time it "bounces" off the mylar two things will happen: you will lose a little bit from absorbtion in the mylar and it will tend to scatter and become more evenly distributed.

You are correct though that you can focus the light and create a hot spot by creating concave curved surfaces with the mylar. But the hot spot you create is at the expense of subtracting a little bit from the overall diffuse ambient light. You can't get something for nothing. Theoretically at least you could start a fire if your input light is intense enough and the mirror shape is accurate enough. I very much doubt that's a serious concern for a diy concave mirror, but you could get a bright spot that would at least brown the leaves if you were able to create a well shaped mirror. It's the same principle as burning stuff with a magnifying glass and the sun.

A half cylinder mirror shape would focus the light into a line rather than a point also and would probably be easier to form by shaping some foil or mylar around a section of pipe. Some reflector hoods are designed like this with the bulb at the focal point such that the light goes from being a line source to being parallel rays coming off the reflector hood surface. Same principle as what you are trying to do but in reverse.

Google mirror equation if you are comfortable with a little math. It's not a bad idea at all if for example you wanted to focus light on a top cola.

Also I'll just add that for walls, it might be easier and just as effective to use ceiling white paint. Ask for white paint for ceilings at the paint store. It's designed for high reflectivity. That is if you don't mind having a room with a glaring white paint job when you are all done growing.

A large hemispherical stainless steel bowl from a cooking store would probably suffice for what you want to do. Or like I said wrap some foil on the inside surface of some large diameter pipe if you can find the right pipe. Left over construction pipe maybe? Might be able to do a fair approximation with some sheet metal and a curved cardboard template to guide you when you are bending. Ask for clad aluminum at the metal store. It has a thin layer of pure aluminum deposited on the surface that is very, very shiny.