To answer the mylar question at walmart
No we do not sell rolls of mylar, but as someone pointed out we DO sell "emergency" blankets or "space blankets" as others call them, which is just a mylar sheet, in my sporting goods department, the company who makes them that we have, is called Ozark Trail.
i use mylar on my tank and have not had a problem with it. its just a pain to keep straight and wrinkle/scuff free... but if you can manage to keep from moving it around or having prying hands play with it, it is great stuff...
Mylar burning cannabis, sorry but these plants are certainly not cannabis, which i have experience with growing (legally of course, when i was a designated grower for a family member i shall not name for security reasons). And even then, I never had a problem with mylar causing leaf burn on them, if the plants are kept well hydrated and the light source at the right distance, there should be no reason for the mylar itself to cause any form of leaf burn to the plants... more likely the problem would be the heat from the lights, and how mylar (holding in more light) will keep it a wee bit warmer, drying out the plants faster than normal and boom, brown leaves...
hmmm, I must have stepped on someones mylar....
I mean no disrespect here, and we all agree...anything is better than allowing the light to escape the imediate area.
In the words of the late "Sgt Joe Friday"...."just the facts Ma'am". Do some research on light reflectivity materials, you may be surprised.
Before my current setup, I used mylar. They sell different thicknesses, and I used a thicker kind. It let no light through that I could discern, plus it was very resistant to crinkling and was easy to work with. I got it for cheap from some online hydro store (don't remember who now..been a few years).
What properties do you want? In terms of reflectivity, mylar is probably the best choice, or foil of the two options you mention. If you want something that looks nice, a matte white is likely better. If you want something that will last a long time without problems, I suggest covering a hard surface like plywood with a product called panda film - it's bright white on one side, and black on the other, and is pest and disease resistant. If you're trying to improve light availability to your plants, a shiny metal surface is better, but you want something that's textured and not smooth, so that it will spread the light more evenly. In all likelihood, you'll need to go to a hydroponic supplier to do this right.
Wrinkling your reflective surface has the effect of making it a diffuser. Personally when i used mylar/foil/etc I went through great pains not to have any wrinkles.
IMHO the risk of burning from the reflective surface forming a paraboloidal shape with the angle of incidence equal to the angle of reflection is a non issue. The odds of that much light energy being focused on one tiny, highly concentrated spot is extrememly low to impossible without considerable effort in design.
Personally, I want the focal point to be concentrated across the plant surfaces, otherwise it is wasted energy. Especially with the low canopy height we typically deal with.
So IMHO focus those photons, I do...... and I think i get decent performance out of my lights ;-)
As far as mirrors go, most mirrors are "second surface" which means the reflective surface is on the back side. So when these are used the light energy has to travel through the glass twice before it can be used. This is why most high dollar lights use 95% effecient, highly polished faceted aluminum surfaces.
You can get first surface mirrors but the cost is very high and they are very susceptible to scratching. Telescopes and Binoculars are probably the most common application for those.
All of this is just my 2 cents of course, so your mileage may vary... I can only speak of what works for me.
Be aware that there is a whole lot of "tribal knowledge" floating around when it comes to plant lighting
no worries, I am very in tune to "screenitis" and take much of it in stride. The biggest point is, we are learning something in this thread. I appreciate all input, right, wrong, good or bad....it all leads to a better understanding of the subject at hand.
Here i got two different opinion about the reflection and the usage. As aluminum foil would reflect best.But white cardboard will probably look nicer and will be easier to manage. so depends upon your conditions and choice.