Can I use foil to reflect light?
Yes, but you must use the dull side to reflect your light as the shinny side will produce concentrated hotspots. Using aluminum foil is a cheap alternative to using mylar or white paint, but it does not perform nearly as well.
Is a dimpled or polished reflector better at reflecting light?
A dimpled pattern will be better, as they produce balanced light footprints, the flat polished type can produce hot spots and uneven light distribution.
(snoofer) High quality reflectors have a 'hammer specular' finish (to distribute the light evenly) and titanium dioxide (for max. reflectiviety) paint. The surface feels slippery.
Are mirrors good for reflecting light?
Mirrors waste energy by reflecting only a small fraction of the light that falls on them.
When light, which, like radio waves, is a form of electromagnetic radiation, strikes a metallic mirror the electrons in the metal move just as they do when a radio signal strikes an antenna. Pushing electrons around takes energy, which dims the reflected image.
Metallic mirrors reflect infrared light (heat) and if your mirror has imperfections this will cause hot spots, which can burn plants. Please note hot spots also apply to Mylar and Foil and IMO are not applicable to growers using small amounts of fluorescent lighting.
By using a mirror to reflect your light on a wall, you can test for imperfections, if you see an uneven image, with focused beams (normally located at the edge of the reflected pattern) these are known as hot spots and depending on the wattage of your bulb, may burn your foliage.
Metallic mirrors should not be used as a reflector for your grow room as minimizing light loss is important.
What is black and white poly and how do I use it?
Black & White poly (6mil) is very tough, reflective (The white side reflects 90% of all light) and can be handled on a daily basis without damage or tearing. It is also a good choice for partitioning off spaces ensuring maximum efficiency of your lighting.
A "mil" is not a millimetre, it is another standard of measurement used to describe film thickness in the plastic film and poly bag business. Using a micrometer a mil measures @ .001 inch